Source: MICHIGAN STATE UNIV submitted to
DIETARY FAT REGULATION OF HEPATIC GENE EXPRESSION
Sponsoring Institution
National Institute of Food and Agriculture
Project Status
TERMINATED
Funding Source
Reporting Frequency
Annual
Accession No.
0179154
Grant No.
(N/A)
Project No.
MICL01892
Proposal No.
(N/A)
Multistate No.
(N/A)
Program Code
(N/A)
Project Start Date
Jul 1, 2003
Project End Date
Jun 30, 2008
Grant Year
(N/A)
Project Director
Jump, D.
Recipient Organization
MICHIGAN STATE UNIV
(N/A)
EAST LANSING,MI 48824
Performing Department
PHYSIOLOGY
Non Technical Summary
Dietary fat has a significant impact on human health. Understanding the molecular basis for dietary fatty acid regulation of cell function and its contribution to human health is a central issue in modern nutrition research. This project will provide a better understanding into the metabolic factors that contribute to the regulation of intracellular fat metabolites, which in turn control the activity and/or abundance of key transcription factors regulating lipid and cholesterol metabolism.
Animal Health Component
(N/A)
Research Effort Categories
Basic
100%
Applied
(N/A)
Developmental
(N/A)
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
7023840101050%
7023840104050%
Goals / Objectives
Understanding the molecular basis for dietary fatty acid regulation of cell function and its contribution to human health is a central issue in modern nutrition research. The effects of dietary fat on cell function are very complex, occurring at multiple levels of control. A significant revelation over the last several years is that dietary fat, a macronutrient, can significantly affect hepatic gene transcription. Such effects contribute to the normal performance of the many tissues, but also contribute to the onset and progression of chronic diseases, e.g., insulin resistance, obesity, atherosclerosis. Peroxisome proliferator activated receptor (PPAR sterol regulatory element binding proteins (SREBP-1c and SREBP-2) and the liver X receptor (LXRa) have emerged as metabolic sensors ("Lipo-Stats") for intracellular lipid (fatty acid and cholesterol). Fatty acid and cholesterol serve as regulators of major metabolic pathways through these transcription factors. A change in the activity or abundance of these transcription factors leads to major changes in intracellular as well as whole body lipid and cholesterol levels. Our studies focus on the control of PPAR, LXR and SREBP-1c by fatty acids. A fundamental gap in our understanding of fatty acid regulation of gene transcription is how fatty acid metabolism contributes to the control of transcription factor regulatory networks. Our studies focus on non-esterified fatty acids (NEFA) and fatty acyl CoA thioesters (FaCoA) because the response profile of both the PPAR and SREBP-1c regulatory networks correlates with fatty acid flux through the NEFA/FACoA pools rather than storage of fatty acids as neutral lipids or phospholipids. Understanding the factors that influence fatty acid flux through the NEFA and/or FACoA pools is critical to understanding how these regulatory networks are affected by lipid.
Project Methods
We hypothesize that fatty acid structure along with specific enzymes, like acyl CoA synthetase, fatty acid elongase, fatty acyl CoA thioesterase and peroxisomal beta-oxidation; play a central role in controlling hepatic transcription factor activity (PPAR and LXR) and abundance (SREBP-1c). Accordingly, changes in the activity of these key enzymes will impact the PPAR and SREBP-1c regulatory networks. In Aim 1, we will determine how fatty acid structure and specific metabolic pathways affect PPAR , and SREBP-1c regulatory networks in rat primary hepatocytes. This aim will use primary hepatocytes and will involve a combination of gene expression and metabolic labeling analyses to examine the dynamic relationship between changes in intracellular NEFA and FaCoA levels and PPAR and SREBP-1c regulated transcription. In Aim 2, we will develop adenovirus-mediated gene transfer technology to alter hepatocellular levels of NEFA and FaCoA. As in Aim 1, will use use a combination of gene expression and metabolic analyses to evaluate how changes in hepatic lipid metabolism induced by over expression of key lipid handling enzymes affect the PPAR and SREBP-1c regulatory networks.

Progress 07/01/03 to 06/30/08

Outputs
Fatty acid elongases (Elovl) are required for long chain saturated, mono- and polyunsaturated fatty acid synthesis in mammals. Our preliminary studies prompted us to hypothesize that hepatic fatty acid elongase not only regulated hepatic fatty acid composition, but also had effects on the function of fatty acid-regulated transcription factors, like PPAR-alpha & SREBP-1. The lack of information on mammalian fatty acid elongases prompted the following studies: 1) to define the metabolic regulation of hepatic fatty acid elongases; 2) to define the role of PPAR-alpha and SREBP-1 in the regulation of fatty acid elongases; 3) to determine how over expression of specific fatty acid elongases affected hepatic function and blood lipid composition. The key findings are as follows. We cloned and sequenced 7 fatty acid elongase (Elovl 1-7) subtypes; these enzymes display differential expression in rodent tissues. Using recombinant adenovirus approaches, we established that the hepatic elongases (Elovl-1, -2, -5 and -6) have unique preference for specific fatty acid substrates. The hepatic elongases displayed differential regulation during postnatal development and by specific nutrients. Throughout these studies we compared the expression of fatty acid elongases to the expression of fatty acid desaturases (delta-5, D5D; delta-6, D6D and delta-9 desaturase, D9D). The expression of fatty acid elongases and desaturases is regulated by several key transcription factors. PPAR-alpha controls the expression of Elovl-5 & Elovl-6, D5D, D6D and D9D. SREBP-1 controls the expression of Elovl-6, D5D, D6D and D9D. Expression of Elovl-6 and D9D was controlled by ChREBP and MLX. LXR agonist regulated D9D, but had no effect on any elongase. Hepatic expression of Elovl-1 and Elovl-2 is constitutive. Diet-induced insulin resistance suppressed Elovl-5, Elovl-6 and D5D, D6D and D9D expression in mouse liver. In contrast, leptin-deficient obesity had the opposite effect on Elovl-5, Elovl-6 and D9D. A recombinant adenovirus approach was used to over express Elovl-5 in mouse liver. Elevated hepatic Elovl-5 activity (4-fold) altered hepatic and plasma lipid profiles, as well as promoting changes in the expression of PPAR-alpha-regulated genes. Elevated Elovl-5 activity had no significant effect on HNF4-alpha-, LXR-, SREBP-1-, ChREBP- or MLX-regulated genes. Of all the elongases and desaturases expressed in liver, only Elovl-6 and D9D were coordinately regulated during postnatal development, by diet and metabolic disease and in response to various transcription factors. Taken together, these studies establish fatty acid elongases as having dual role in hepatic function: 1) to control hepatic and blood lipid composition, and 2) to regulate gene expression through effects on at least one fatty acid regulated transcription factor, i.e., PPAR-alpha. Finding that both elongases and desaturases are regulated in chronic metabolic disorders, like diabetes and obesity, suggest these enzymes play a role in mediating changes in blood lipid profiles associated with these diseases.

Impacts
These studies represent the first detailed analysis of factors controlling fatty acid elongase function in the liver. The outcome of these studies indicates that elongases, like desaturases, are highly regulated and play a key role in determining cellular fatty acid composition. Finding that specific elongases and desaturases are regulated in chronic disease further strengthens this concept. The capacity to alter elongase (or desaturase) expression in vivo provides a novel approach to assess the impact these metabolic pathways have on cell function and the onset and progression of disease.

Publications

  • Chen, W., Esselman, W.J., Jump, D.B. and Busik, J.V. (2007). Modification of Caveolae/lipid Rafts by Docosahexaenoic Acid Inhibits Cytokine Signaling in Human Retinal Endothelial Cells Investigative Opthalomalogy and Visual Sciences 48: 18-26.
  • Jump, D.B. (2004) Unsaturated fatty acid regulation of gene transcription. Critical Reviews in Clinical Laboratory Science 41: 41-78.
  • Jump, D.B., Botolin, D., Wang, Y., Xu, J., Christian, C. and Demeure, O. (2005) Fatty acid regulation of hepatic gene transcription. Journal of Nutrition 135: 2503-2506.
  • Jump, D.B., Botolin, D., Wang, Y., Xu, J. and Christian, B. (2006) Fatty Acids and Gene Transcription. Scandinavian Journal of Food and Nutrition. 50 (suppl 2): 5-10.
  • Wang, Y., Botolin, D., Christian, B., Xu, J., Busik, J. and Jump, D.B. (2005)Tissue specific, nutritional and developmental regulation of rat fatty acid elongases. Journal of Lipid Research 46: 706-715.
  • Boverhof, D.R., Burgoon, L.D., Tashiro, C., Chittim, B., Harkema, J.R., Jump, D.B., Zacharewski, T.R. (2005) Temporal and dose-dependent hepatic gene expression patterns in mice provide new insights into TCDD-mediated hepatotoxicity. Toxicological Sciences 85: 1048-1062.
  • Chen, W., Esselman, W.J., Jump, D.B. and Busik, J.V. (2005) Anti-inflammatory effect of docosahexaenoic acid on cytokine-induced adhesion molecule expression in human retinal vascular endothelial cells. Investigative Opthalomalogy and Visual Sciences 46:4342-4347.
  • Botolin, D., Wang, Y., Christian, B., and Jump, D.B. (2006) Docosahexaneoic acid [22:6,n-3] regulates rat hepatocyte sterol regulatory element binding protein-1 (SREBP-1) nuclear abundance by Erk-and 26S proteasome-dependent pathways. Journal of Lipid Research 47: 181-192.
  • Xu, J., Christian, B. and Jump, D.B. (2006) Regulation of rat hepatic L-pyruvate kinase promoter composition and transcription by glucose, n-3 PUFA and peroxisome proliferator activated receptor-alpha agonist. Journal of Biological Chemistry 281, 18351-18362.
  • Wang, Y., Botolin, D, Xu, J., Christian, B, Mitchell, E., Jayaprakasam, B., Nair, M., Peters, J., Busik, J., Olson, L.K., and Jump, D.B. (2006) Regulation of Hepatic Fatty Acid Elongase and Desaturase Expression in Diabetes and Obesity. Journal of Lipid Research 47: 2020-2048.


Progress 01/01/06 to 12/31/06

Outputs
We hypothesized that hepatic enzymes involved in fatty acid elongation: a) regulate hepatic & extrahepatic (aortic and blood) fatty acid composition; b) regulate key transcription factors and c) influence the onset or progression of chronic disease, e.g., diabetes, obesity or atherosclerosis. The specific aims are: 1. Define the metabolic regulation of hepatic fatty acid elongases. 2. Define the role of PPARalpha and SREBP-1 in the regulation of fatty acid elongases. 3. Define the effects of fatty acid elongases on hepatocyte lipid composition and gene expression. 4. Determine how over expression of specific fatty acid elongases affect hepatic function, blood lipid composition and the onset of chronic disease. We decided to include in our analysis an examination of both fatty acid elongases and desaturases. This combined analysis is justified since both enzyme families are involved in the synthesis of long chain unsaturated fatty acids. We have identified the key transcriptional regulatory networks controlling hepatic elongase and desaturase expression. The key transcription factors regulating these enzyme include, SREBP1, the ChREBP/MLX heterodimer, PPAR-alpha and HNF-4alpha. These studies also established that chronic diseases, like diabetes and obesity, have associated changes in fatty acid elongase and desaturase expression. Changes in elongase expression correlate with changes in key transcription factor (SREBP-1, ChREBP or MLX) function and hepatic and plasma lipid composition. To determine the impact elongases have on hepatic function, we over expressed Elovl-2 and Elovl-5 in livers of C57BL/6 male mice using a recombinant adenovirus approach. Infection of mice with a control virus, (luciferase, Ad-Luc) had no effect on hepatic or plasma lipid content, hepatic carbohydrate or protein content or PPARalpha-, SREBP1-, ChREBP/MLX-regulated genes. Infection of mice with Ad-Elovl-2 selectively increased (5-fold) C20 PUFA elongation activity in the liver which correlated with altered hepatic and plasma lipid composition. Elevated Elovl-2 activity induced significant changes in the expression of genes regulated by PPARalpha, SREBP-1 & ChREBP/MLX as well as Akt, Erk and Gsk3beta phosphorylation status. Infection of mice with Elovl-5 (elongates C16-20 mono- and polyunsaturated fatty acids) increased fatty acid elongation rate 6-fold. This level of elongation is comparable to that seen in livers of leptin-deficient obese mice. Elovl-5 over expression had many of the same effects as over expressed Elovl-2. The exception was that over expressed Elovl-5 significantly lowered hepatic glycogen content. The mechanism for this effect may be due to a decline in hepatic glucose transport since Glut2 mRNA abundance was suppressed without effects on glucokinase expression. These results provide a "proof of concept" that endogenous pathways for fatty acid metabolism (in this case elongation) likely contribute to some of the pathology associated with chronic disease.

Impacts
These studies represent the first detailed analysis of factors controlling fatty acid elongase function in the liver. The outcome of these studies indicates that elongases, like desaturases, are highly regulated and play a key role in determining cellular fatty acid composition. Finding that specific elongases and desaturases are regulated in chronic disease further strengthens this concept. The capacity to alter elongase (or desaturase) expression in vivo provides a novel approach to assess the impact these metabolic pathways have on cell function and the onset and progression of disease.

Publications

  • Xu, J., Christian, B. and Jump, D.B. (2006) Regulation of rat hepatic L-pyruvate kinase promoter composition and transcription by glucose, n-3 PUFA and peroxisome proliferator activated receptor- agonist. Journal of Biological Chemistry 281, 18351-18362.
  • Wang, Y., Botolin, D, Xu, J., Christian, B, Mitchell, E., Jayaprakasam, B., Nair, M., Peters, J.M., Busik, J.V., Olson, L.K., and Jump, D.B. (2006) Regulation of Hepatic Fatty Acid Elongase and Desaturase Expression in Diabetes and Obesity. Journal of Lipid Research 47: 2020-2048.
  • Jump, D.B., Botolin, D., Wang, Y., Xu, J. and Christian, B. (2006) Fatty Acids and Gene Transcription. Scandinavian Journal of Food and Nutrition. In press.


Progress 01/01/05 to 12/31/05

Outputs
We hypothesize that hepatic enzymes involved in fatty acid elongation: a) regulate hepatic & extrahepatic (aortic and blood) fatty acid composition; b) regulate key transcription factors and c) influence the onset or progression of chronic disease, e.g., diabetes, obesity or atherosclerosis. The specific aims are: 1. Define the metabolic regulation of hepatic fatty acid elongases. 2. Define the role of PPARalpha and SREBP-1 in the regulation of fatty acid elongases. 3. Define the effects of fatty acid elongases on hepatocyte lipid composition and gene expression. 4. Determine how over expression of specific fatty acid elongases affect hepatic function, blood lipid composition and the onset of chronic disease. In Aim 1, we compared the tissue-specific, nutritional and developmental regulation of fatty acid elongase and desaturase gene expression. Seven fatty acid elongases are expressed in the rat, mouse and human genomes. Four fatty acid elongases (Elovl-1, Elovl-2, Elovl-5 and Elovl-6) and 3 fatty acid desaturases (delta5, delta6 and delta9) are expressed in rat, mouse and human liver. In rats, overnight starvation and fish oil enriched diets repressed hepatic elongase activity. Nutritional regulation of hepatic fatty acid elongase activity correlated with Elovl-5 and Elovl-6 mRNA abundance. Adult rats fed the PPARalpha agonist, WY14,643, have elevated hepatic elongase activity, Elovl-1, Elovl-5, Elovl-6, delta5, delta6 & delta9 desaturase mRNA abundance, and 20:3,n-9 in both liver and plasma. 20:3n9 is a product of 18:1,n-9 elongation and desaturation. Thus, PPARalpha agonists impact both fatty acid elongation and desaturation pathways. Hepatic fatty acid elongase activity is low in rat fetal liver, but increased significantly after birth. Developmental changes in hepatic elongase activity parallel changes in Elovl-5 mRNA abundance, along with the PPARalpha-regulated transcripts, delta5 and delta6 desaturase and CYP4A. In contrast, Elovl-6, delta9 desaturase and FAS mRNA abundance paralleled changes in hepatic SREBP-1 nuclear content. SREBP-1 is present in fetal liver nuclei, absent from nuclei immediately after birth and reappeared in nuclei at weaning, 21 days postpartum. In conclusion, Elovl-5 mRNA is the most abundant elongase subtype expressed in rat liver. Hepatic Elovl-5, delta5 and delta6 desaturase are coordinately regulated by diet and during postnatal development. Changes in Elovl-5 expression may account for much of the nutrition and developmental control of fatty acid elongation activity in the rat liver. In Aim 2, we have identified a role for several transcription factors controlling hepatic elongase and desaturase expression. These transcription factors include SREBP-1, PPAR-alpha, ChREBP & MLX, LXR, Ah-receptor. In Aim 3, we successfully developed recombinant adenovirus expressing functional elongases and desaturases. In Aim 4, we have initiated the development of transgenic mice over expressing specific fatty acid elongases.

Impacts
Our findings have broad implications for metabolic regulation, particularly with regard to chronic diseases like diabetes, obesity and atherosclerosis, where clinical data links fatty acid metabolites to disease progression. In contrast to the fatty acid desaturases, there remains a poor understanding of the role fatty acid elongases play in hepatic and whole body physiology. Our studies will critically evaluate the role hepatic fatty acid elongases and desaturases play in both hepatic and whole body lipid metabolism.

Publications

  • Wang, Y, Botolin, D., Christian, B., Xu, J., Busik, J. and Jump, D.B. (2005) Tissue specific, nutritional and developmental regulation of rat fatty acid elongases. Journal of Lipid Research 46: 706-715.
  • Botolin, D., Wang, Y., Christian, B., and Jump, D.B. (2006) Docosahexaneoic acid [22:6,n-3] regulates rat hepatocyte sterol regulatory element binding protein-1 (SREBP-1) nuclear abundance by Erk- and 26S proteasome-dependent pathways. Journal of Lipid Research 47: 181-192.
  • Jump, D.B., Botolin, D., Wang, Y., Xu, J., Christian, C. and Demeure, O. (2005) Fatty acid regulation of hepatic gene transcription. Journal of Nutrition 135: 2503-2506.


Progress 01/01/04 to 12/31/04

Outputs
Our studies have focused on dietary polyunsaturated fatty acid (PUFA) control of 3 hepatic transcription factors: SREBP-1c, PPARalpha and LXRalpha. These transcription factors are linked to specific chronic diseases (atherosclerosis, obesity and diabetes) or risk factors for chronic disease (dyslipidemia & inflammation) through their regulation of carbohydrate, fatty acid, cholesterol and apolipoprotein metabolism. We have established that fatty acid structure and cell-specific fatty acid metabolism are major determinants controlling these transcription factors. While our focus has been on exogenous fatty acid regulation of these transcription factors, we have reason to believe that the capacity of cells to modify fatty acid structure is equally important in this regulatory scheme. We are particularly interested in pathways for fatty acid remodeling, including fatty acid elongation, desaturation and peroxisomal beta-oxidation. Products of these pathways include oleic acid (18:1n9), arachidonic acid (20:4n6) and docosahexaenoic acid (22:6n3). 20:4n6 and 22:6n3, are the predominant C20-22 PUFA in membrane lipids; 20:4n6 is a well-established precursor of bioactive lipids (eicosanoids) that control multiple homeostatic processes. Changes in 20:4n6 and 22:6n3 tissue levels, as well as blood profiles of their precursors are linked to chronic diseases, e.g., diabetes and atherosclerosis. Our interest in this pathway is based on finding that C20 and C22 PUFA (N3 & N6) have differential effects on PPARalpha and LXRalpha activity and SREBP-1 nuclear content. Using a recombinant adenovirus approach, we have over expressed a fatty acid elongase (Elovl-2) in primary hepatocytes. Over expression of Elovl-2 enhances C20 conversion to C22-24 PUFA and significantly affects fatty acid control of PPARalpha target genes and SREBP-1c nuclear content. Moreover, the liver expresses 4 elongase subtypes; several are regulated during postnatal development and by dietary factors as well as PPARalpha activators. Finally, a PPARalpha activator induces all elongases & desaturases in rat liver leading to the accumulation of 20:3n9 (mead acid) in the liver and blood. 20:3n9 accumulates in essential fatty acid deficiency (EFAD) and replaces 20:4n6 in membranes affecting eicosanoid production and signaling. These studies indicate that the elongases, like desaturases, are highly regulated and under specific circumstances generate fatty acids that do not normally accumulate in cells; these rare fatty acids impact cell signaling by affecting eicosanoid production.

Impacts
Our findings have broad implications for metabolic regulation, particularly with regard to chronic diseases like diabetes, obesity and atherosclerosis, where clinical data links fatty acid metabolites to disease progression. In contrast to the fatty acid desaturases, there remains a poor understanding of the role fatty acid elongases play in hepatic and whole body physiology. Our future studies will critically evaluate the role hepatic fatty acid elongases play in both hepatic and whole body lipid metabolism.

Publications

  • No publications reported this period


Progress 01/01/03 to 12/31/03

Outputs
Hypothesis: PUFA control of SREBP-1c, a key regulator of hepatic lipid synthesis and storage, requires microsomal fatty acid oxidation. One goal of these studies was to evaluate the role PUFA played in the transcriptional control of SREBP-1. These studies focused on two lines of investigation: 1) the role LXR played in the PUFA control of SREBP-1; and 2) the development of the chromatin immunopercipititation (CHIP) assay to assess PUFA effects on SREBP-1 promoter composition in vivo. Oxysterols acting through LXR induce SREBP-1 gene transcription. We and others reported that oxysterol regulation of LXR (alpha) was antagonized by PUFA in Hek293 cells. However, we found no evidence for PUFA control in liver (in vivo), in primary hepatocytes or a cell line of rat hepatoma cells (FTO2B). These findings suggest that if SREBP-1 gene transcription is sensitive to PUFA control, this mechanism does not involve LXR. The second line of investigation used the CHIP assay to monitor changes in promoter composition following activation by glucose/insulin and inhibition by PUFA. These studies provided convincing evidence for rapid effects of PUFA on SREBP-1 acetylated histone H4 composition. These findings argue for rapid effects of PUFA on SREBP-1 gene transcription. A second goal of this funding period was to examine the postnatal control of SREBP-1 and SREBP-2. These studies revealed reciprocal control of SREBP-1 and -2 and their target genes during postnatal development. Essentially, prior to weaning, SREBP-1 is not in hepatic nuclei and its target genes are poorly expressed. At weaning to a chow diet (low fat diet), nuclear SREBP-1 levels increase dramatically along with the expression of its target genes. In contrast, SREBP-2 was present in hepatic nuclei in suckling animals and its target genes were well expressed. The failure of SREBP-1 to appear in the nucleus during the suckling phase was due to a failure to proteolytically convert the SREBP-1 precursor to its mature form. This effect on processing was attributed to the high fat milk diet, a diet that suppresses blood insulin levels. This major switch in SREBP-1 and -2 expression from birth to adult raises fundamental questions regarding the regulation of key metabolic pathways controlled by SREBP-1 and -2 during postnatal development. A third goal during this funding period was on PUFA metabolism. In the absence of detailed metabolic studies it was impossible to gain any insight into fatty acid regulation of gene expression. Accordingly, detailed studies on PUFA metabolism were preformed in parallel to the PUFA control of PPAR-alpha and SREBP-1. These studies revealed, for the first time, the dynamics of PUFA metabolism and their effects on gene expression. The outcome of these studies argued against our original hypothesis and ruled out oxidized lipids as mediators of PUFA control of PPARalpha, LXRalpha and SREBP-1. The studies argue for enzymes associated with endogenous PUFA synthesis pathway as controllers of fatty acid structure and lipid-mediators controlling PPARalpha,LXRalpha and SREBP-1.

Impacts
These findings have provided novel information on the role dietary fat plays in the control of major regulatory networks in the neonate and the adult. Clearly, the capacity of cells to metabolize exogenous fatty acids is an important consideration in understanding how dietary fat controls gene expression.

Publications

  • Pawar, A. and Jump, D.B. (2003). Unsaturated fatty acid regulation of PPARalpha in rat primary hepatocytes. Journal of Biological Chemistry 278: 35931-35939
  • Pawar, A., Botolin, D., Mangelsdorf, D.J., and Jump, D.B. (2003) The role of liver X receptor-alpha (LXRalpha) in the fatty acid regulation of hepatic gene expression. Journal of Biological Chemistry 278: 40736-40743.
  • Jump, D.B. (2004) Unsaturated fatty acid regulation of gene transcription. Critical Reviews in Clinical Laboratory Science: In press.
  • Parameswaran, N, Hall, C.S., Bomberger, J.M., Sparks, H.V., Jump, D.B. and Spielman, W.S. (2003)Negative growth effects of ciglitazone on kidney interstitial fibroblasts: Role of PPARgamma. Kidney and Blood Pressure Research 26: 2-9.
  • Chen, W., Jump, D.B., Grant, M.B., Esselman, W.J. and Busik, J.V. (2003) Dyslipidemia, but not hyperglycemia, induces pro-inflammatory adhesion molecules in human retinal vascular endothelial cells. Investigative Ophthalmology and Visual Sciences. 44:5016-5022.


Progress 01/01/02 to 12/31/02

Outputs
The liver plays a central role in whole body lipid synthesis and metabolism. Dietary fat has significant effects on gene expression leading to changes in hepatic lipid metabolism. Our studies have focused on defining the molecular basis of fatty acid regulation of transcription of genes encoding proteins involved in fatty acid synthesis and oxidation. During this past year, we have focused on two issues: 1) the role of fatty acid metabolism in the control of transcription factor activity and abundance; and 2) defining the molecular basis of the ontogeny of lipogenic gene expression in rodent liver. These studies have led to new insight into cellular mechanisms for fatty acid control of gene expression. In the first case, we focused on the fatty acid control of PPAR-alpha and liver X receptors (LXR-alpha and beta) in an established cell line, Hek293 cells, and in rat liver and rat primary hepatocytes. LXRs control the expression of genes encoding proteins involved in bile acid synthesis, cholesterol transport and lipoprotein clearance from the circulation as well as lipogenesis. These studies indicated that the rate of assimilation of exogenous fatty acids and their metabolites into complex lipids plays an important role in regulating the activity of both PPAR and LXR as well as the abundance of SREBP-1c in the nucleus. One of our key findings is that while LXR-alpha (but not LXR-beta) is regulated by fatty acids in certain established cell lines, there is no fatty acid control of either LXR receptor in primary hepatocytes or rat liver. In fact, feeding studies clearly show that PPAR-alpha and SREBP-1c regulated pathways, but not LXR-alpha regulated pathways, are sensitive to PUFA regulation in vivo. Thus, in vivo hepatic pathways that are sensitive to PUFA regulation include fatty acid oxidation and synthesis, but not bile acid metabolism or cholesterol efflux. The second line of investigation examined the ontogeny of lipogenesis. Lipogenic gene expression is low in suckling rats and increases dramatically at weaning. Based on our understanding of PUFA control of SREBP-1c and the role SREBP-1c plays in lipogenic gene expression, we expected that during the suckling phase, SREBP-1c would be suppressed in liver and a pre-translational mechanism for account for this control. We found no evidence for pre-translational suppression of hepatic SREBP-1c, i.e., both the SREBP-1c mRNA and its precursor 125 kd protein were abundant in livers of preweaned animals. However, nuclear SREBP-1c (65 kd) levels were suppressed. The mechanism accounting for this control involved abrogated proteolytic processing of SREBP-1c. In contrast to SREBP-1c, SREBP-2 was not subject to this same regulatory control. In fact, there was ample SREBP-2 in the nucleus as well as expression of SREBP-2-regulated genes, e.g., HMG CoA reductase. Thus, during postnatal development a selective proteolytic processing of rat hepatic SREBP-1 versus SREBP-2 leads to suppression of lipogenic gene expression and favors robust expression of genes regulated by SREBP-2 (cholesterol synthesis, LDL receptor).

Impacts
These studies illustrate the importance of lipid metabolism in the control of hepatic transcription factor activity and abundance, which in turn affect lipid synthesis and oxidation. Moreover, these studies have identified a novel mechanism for selective control of lipid versus cholesterol homeostasis in the neonatal liver, a mechanism that differs from that seen in the adult animal.

Publications

  • Jump, D.B. (2002) Polyunsaturated fatty acids and the regulation of gene expression. Current Opinions in Lipidology. 13: 155-164.
  • Jump, D.B., (2002) Dietary polyunsaturated fatty acid regulation of hepatic gene transcription. Scandinavian Journal of Nutrition. 46: 59-67.
  • Pawar, A., Xu, J., Jerks, E., Mangelsdorf, D.J., and Jump, D.B. (2002) Fatty acid regulation of liver X receptors (LXR) and peroxisome proliferator activated receptor-alpha (PPAR-alpha) in HEK 293 cells. Journal of Biological Chemistry 277: 39243-39250.
  • Botolin, D. and Jump, D.B., (2002) Selective proteolytic processing of hepatic SREBP-1 and SREBP-2 during postnatal development. Journal of Biological Chemistry, In press, Dec. 17, 2002.


Progress 01/01/01 to 12/31/01

Outputs
The quantity and type of dietary fat we ingest contributes to our normal health as well as the onset and progression of several chronic diseases. The liver plays a central role in whole body lipid synthesis and metabolism. Dietary fat has significant effects on gene expression leading to changes in hepatic lipid metabolism. Our studies have focused on defining the molecular basis of fatty acid regulation of transcription of genes encoding proteins involved in fatty acid synthesis and oxidation. These studies have revealed several distinct mechanisms for fatty acid control of hepatic lipid metabolism: A: the peroxisome proliferator-activated receptor-alpha is required for highly unsaturated n-3 fatty acids to induce the expression of genes involved in peroxisomal and microsomal fatty acid oxidation; B: n-3 and n-6 PUFA suppress the nuclear content of the sterol regulatory element binding protein-1c (SREBP1c). SREBP1c is involved in the insulin-mediated induction of several lipogenic genes. C: conversion of n-6 PUFA to prostanoids activates G-protein linked receptors in adipocytes that affect adipocyte levels of mRNAs encoding specific lipogenic genes. Hepatic parenchymal cells do not express COX activity, but do express prostaglandin receptors. Lipogenic gene expression can be controlled by specific prostaglandins acting on parenchymal cells. A fourth pathway discovered by others involves the Liver X receptors, LXRa and b. LXR is important in this regard because LXR along with insulin control the transcription of the SREBP-1c gene. We have established that fatty acids have a suppressive effect on oxysterol induction of LXRa activity in some, but not all cell lines. Moreover, we find no fatty acid regulation of LXRa in primary hepatocytes. These studies reveal an important facet of PUFA control of transcription factor function where cell-specific fatty acid metabolism impacts transcription factor function. A second line of investigation has focused on the role SREBP-1c plays in promoters. SREBP-1c is a helix-loop-helix transcription factors that has a reputation for being a weak transcription factor and for interacting poorly with co-activators. However, in the context of certain promoters, SREBP-1c can exert strong effects on gene transcription. We recently reported that SREBP-1c functionally interacts with T3-nuclear receptors in the control of gene transcription. This interaction is through co-activators. Our studies suggest that ligand activated nuclear receptors recruit co-activators to promoters, but SREBP-1c and a second ancillary transcription factor (NFY) are required to confer high levels of transcription to the gene. SREBP-1c interaction with NFY may be required for nuclear receptors that bind distal regulatory elements to interact with the basal transcription machinery. Many lipogenic genes controlled by SREBP-1c are also regulated by nuclear receptors, e.g. LXR, TR, GR, ER. These findings suggest that SREBP-1c plays an important role in the hormonal control of lipogenic gene transcription.

Impacts
These studies have revealed an important role for cell-specific metabolism in the control of gene transcription. This not only impacts our fundamental understanding of fatty acid regulation of gene transcription but also addresses how changes in metabolism seen in different cells or with disease might affect cell signaling through transcription factor regulation.

Publications

  • Jump, D.B. 2002. Dietary polyunsaturated fatty acids and regulation of gene transcription. Current Opinions in Lipidology 13: In Press.
  • Jump, D.B., Thelen, A.P., and Mater, M.K. 2001. Functional interaction between sterol regulatory element binding protein-1c, nuclear factor Y and 3,5,3'-triiodothyronine nuclear receptors. Journal of Biological Chemistry 276: 34419-34427.
  • Jump, D.B. 2002. The biochemistry of N3-polyunsaturated fatty acids. Journal of Biological Chemistry 277: In Press.


Progress 01/01/00 to 12/31/00

Outputs
The quantity and type of dietary fat we ingest contributes to our normal health as well as the onset and progression of several chronic diseases. The liver plays a central role in whole body lipid synthesis and metabolism. Dietary fat has significant effects on gene expression leading to changes in hepatic lipid metabolism. Our studies have focused on defining the molecular basis of fatty acid regulation of transcription of genes encoding proteins involved in fatty acid synthesis and oxidation. These studies have revealed 3 distinct mechanisms for fatty acid control of hepatic lipid metabolism: A: highly unsaturated n-3 fatty acids activate the peroxisome proliferator activated receptor, PPAR-alpha, and induce the expression of genes involved in peroxisomal and microsomal fatty acid oxidation; B: n-3 and n-6 polyunsaturated fatty acids, PUFA, suppress the nuclear content of the sterol response element binding protein, SREBP1c. SREBP1c plays an important role in the control of lipid synthesis and storage. C: conversion of n-6 PUFA to prostanoids activates G-protein linked receptors in parenchymal cells and cultured adipocytes to suppress mRNAs encoding specific lipogenic genes. Of these 3 pathways, PUFA control of nuclear SREBP1c, nSREBP1c, levels appear central to the regulation of hepatic lipid synthesis. Our studies have shown that unsaturated fatty acids rapidly (within hours) suppress the hepatic levels of SREBP1c mRNA. Moreover, we have recently established that SREBP1c mRNA in livers of obese animals is resistant to PUFA control. This change in PUFA sensitivity is associated with a change in microsomal fatty acid metabolism. Additional studies with primary hepatocytes have suggested that cytochrome P450 mediated oxidation systems are involved in the control of SREBP1c mRNA levels.

Impacts
SREBP1c is a major transcription factor controlling hepatic lipid synthesis. Our studies suggest that hepatic microsomal contributes to the regulation of hepatic SREBP1c levels. Understanding how microsomal metabolism contributes to the control of SREBP1c in normal and obese animals will have important implications for human health.

Publications

  • Pan, D.A., Mater, M.K., Thelen, A. P., Peters, J.M., Gonzalez, F.J. Jump, D.B. 2000. Evidence against the peroxisome proliferator alpha as a mediator of PUFA suppression of L-pyruvate kinase gene transcription. Journal of Lipid Research: 41: 742-751.
  • Jump, D.B., Pawar, A, Thelen, A.P. and Romsos, D. 2001. Fatty Acid Regulation of SREBP1c in Livers of Lean and Obese (ob/ob) Mice. Abstract submitted to FASEB (Experimental Biology).


Progress 01/01/99 to 12/31/99

Outputs
The quantity and type of dietary fat we ingest contributes to our normal health as well as the onset and progression of several chronic diseases. The liver plays a central role in whole body lipid synthesis and metabolism. Dietary fat has significant effects on gene expression leading to changes in hepatic lipid metabolism. Our studies have focused on defining the molecular basis of fatty acid regulation of transcription of genes encoding proteins involved in fatty acid synthesis and oxidation. These studies have revealed 3 distinct mechanisms for fatty acid control of hepatic lipid metabolism: 1: highly unsaturated n-3 fatty acids activate the peroxisome proliferator activated receptor, PPAR-alpha, and induce the expression of genes involved in peroxisomal and microsomal fatty acid oxidation; 2: n-3 and n-6 polyunsaturated fatty acids, PUFA, suppress the nuclear content of the sterol response element binding protein, SREBP1c. SREBP1c plays an important role in the control of lipid synthesis and storage. 3: conversion of n-6 PUFA to prostanoids activates G-protein linked receptors in parenchymal cells and cultured adipocytes to suppress mRNAs encoding specific lipogenic genes. Of these 3 pathways, PUFA control of nuclear SREBP1c, nSREBP1c, levels appears central to the regulation of hepatic lipid synthesis. However, the molecular basis for this control is not well understood.

Impacts
These studies are important for understanding the role of diet on gene expression, particularly in regard to risk factors for disease, such as obesity. Obesity is a growing human health issue worldwide and a risk factor for chronic diseases, like hypertension, insulin resistance, heart disease and cancer. PPAR alpha and SREBP1c are key hepatic transcription factors involved in partitioning lipid between synthesis/storage and oxidation. Understanding how these factors participate in this process in normal and obese animals will have important implications for human health.

Publications

  • Mater, M.K., Thelen, A.T., Jump, D.B. 1999. Arachidonic acid and PGE2 regulation of hepatic lipogenic gene expression. Journal of Lipid Research 40: 1045-1052.
  • Jump, D.B., Thelen, A.P., Ren, B., Mater, M.K. 1999. Multiple mechanisms for polyunsaturated fatty acid regulation of hepatic gene transcription. Prostaglandins, Leukotrienes and Essential Fatty Acids 60: 345-349.
  • Jump, D.B., Thelen, A.P., Mater, M.K. 1999. Dietary polyunsaturated fatty acids and hepatic gene expression. Lipids 34: S209-S212.
  • Mater, M.K., Thelen, A.P., Pan, D.A., Jump, D.B. 1999. Sterol response element binding protein 1C, SREBP1c, is involved in the polyunsaturated fatty acid suppression of hepatic S14 gene transcription. Journal of Biological Chemistry 274: 32725-32732.
  • Jump, D.B., Clarke, S.D. 1999. Regulation of gene expression by dietary fat. Annual Reviews of Nutrition 19: 63-90.


Progress 01/01/98 to 12/31/98

Outputs
Our studies focus on defining the molecular basis of dietary fat regulation of hepatic gene expression. One of the model genes we use is the hepatic L-pyruvate kinase gene (LPK). LPK gene transcription, mRNA and enzymatic activity are suppressed in livers of rats fed diets supplemented with fish oil versus rats fed diets supplements with olive oil. Our recent studies focus on define the cis- and trans-acting factors involved in this suppression mechanisms. Accordingly, two aims are proposed (see below). The progress on those aims is briefly described. Aim 1: Define the cis- and trans-regulatory factors within the L-pyruvate kinase (LPK) polyunsaturated fatty acid response region (PUFA-RR) that are targeted by polyunsaturated fatty acids (PUFA). Insulin/glucose-mediated transactivation of the L-PK gene requires at least two trans-acting factors interacting with the carbohydrate response region (CHO-RR, -170/-120 bp). While the precise biochemical identity of these factors has not been established, the CHO-RR cis-regulatory elements suggest that one factor is an E-box binding factor. We have established that the transcription factor, sterol response element binding protein 1c (SREBP1c), binds the LPK promoter. Interestingly, overexpression of SREBP1c in primary hepatocytes co-transfected with LPK-reporter genes and SREBP1c expression vectors indicates that SREBP1c is a weak activator of LPK promoter activity. The mRNA encoding SREBP1c is downregulated by PUFA. PUFA suppression of LPK promoter activity is unaffected by overexpression of SREBP1c. Thus, the PUFA control of LPK probably does not involve PUFA control of SREBP1c. Aim 2: Define how PUFA regulates the activity of specific trans-acting factors controlling L-PK gene transcription. Studies have been initiated to examine how fatty acid metabolism in primary cultures of hepatic parenchymal cells contributes to the regulation of LPK gene transcription.

Impacts
(N/A)

Publications

  • Mater, M.K., Thelen, A.T., Jump, D.B. 1998. Arachidonic acid and PGE2 regulation of hepatic lipogenic gene expression. Journal of Lipid Research (Submitted Oct. 1998).
  • Jump, D.B., Clarke, S.D., 1999. Regulation of Gene Expression by Dietary Fat. Annual Review of Nutrition 19: In Press.