I:10.1371/journal.pone.0051320.gimplications of this interaction. Lipin 1 Methyl linolenate site significantly enhanced HNF4a-mediated activation of the human PPARa gene promoter-luciferase reporter and multimerized HNF4a-responsive AcadmTKLuc reporter CASIN construct (Figure 2B), suggesting that lipin 1 was acting in a feed forward manner to enhance HNF4a activity. Lipin 1 overexpression augmented the effects of HNF4a on the expression of Ppara and Acadm genes (Figure 2C) and rates 18325633 of fat catabolism (Figure 2D) in hepatocytes in an LXXIL-dependent manner. We also took a lipin 1 loss of function approach to evaluate the interaction between lipin 1 and HNF4a. Overexpression of similar amounts of HNF4a in hepatocytes from fld mice, which lack lipin 1, was less effective at inducing the expression of genes encoding PPARa and fatty acid oxidation enzymes (Cpt1a and Acadm) (Figure 3A). The increase in rates of fatty acid oxidation induced by HNF4a overexpression was blunted in fld hepatocytes compared to WT controls (Figure 3B). Basal rates of palmitate oxidation were also diminished in fld hepatocytes compared to WT controls (Figure 3B). Collectively, these data indicate that lipin 1 enhances the stimulatory effects of HNF4a on fatty acid oxidation.Lipin 1 Suppresses the Expression of Apoproteins that are Induced by HNF4aHNF4a is known to stimulate the expression of various genes involved in VLDL metabolism [29], whereas we have shown that lipin 1 suppresses the expression of these genes [2]. Lipin 1 overexpression suppressed the ability of HNF4a to induce the expression of Apoa4 and Apoc3 in an LXXIL motif-dependent manner (Figure 4A). HNF4a overexpression was also more potent at inducing the expression of Apoa4 and Apoc3 in fld hepatocytes compared to WT controls (Figure 4B). We also assessed rates of TG synthesis and secretion by isolated hepatocytes from WT and fld mice and found that, despite the role of lipin 1 in the TG synthesis pathway, rates of TG synthesis were not affected by lipin 1 deficiency or HNF4a overexpression (Figure 4C). Consistent with our previous work [12], rates of VLDL-TG synthesis were significantly increased in hepatocytes from fld mice 23727046 infected with GFP adenovirus (Figure 4C). However, HNF4a-stimulated secretion of newly synthesized VLDL-TG, which was strongly enhanced by HNF4a overexpression, was not affected by loss of lipin 1 (Figure 4C). This may be explained by the strong stimulation of microsomal triglyceride transfer protein (Mttp) expression by HNF4a, which is not affected by lipin 1 deficiencyFigure 5. Lipin 1 inhibits Apoc3/Apoa4 promoter activity in an HNF4a-dependent manner. [A] The schematic depicts the luciferase reporter construct under control of the intergenic region between the genes encoding ApoC3 and ApoA4 (Apoc3/Apoa4.Luc). The relative positions of two 
HNF4a response elements denoted as Apoc3 enhancer and Apoa4 enhancer are indicated. Graphs depict results of luciferase assays using lysates from HepG2 cells transfected with Apoc3/Apoa4.Luc reporter constructs and cotransfected with lipin 1 and/or HNF4a expression constructs as indicated. Apoc3/Apoa4.Luc constructs were either wild-type or contained mutations in the ApoC3 enhancer or ApoA4 enhancer HNF4a response elements. The results are the mean of 3 independent experiments done in triplicate. *p,0.05 versus pCDNA control. **p,0.05 versus vector control or lipin 1 cotransfection. [B] The schematic depicts the heterologous luciferase reporter construct driven by three.I:10.1371/journal.pone.0051320.gimplications of this interaction. Lipin 1 significantly enhanced HNF4a-mediated activation of the human PPARa gene promoter-luciferase reporter and multimerized HNF4a-responsive AcadmTKLuc reporter construct (Figure 2B), suggesting that lipin 1 was acting in a feed forward manner to enhance HNF4a activity. Lipin 1 overexpression augmented the effects of HNF4a on the expression of Ppara and Acadm genes (Figure 2C) and rates 18325633 of fat catabolism (Figure 2D) in hepatocytes in an LXXIL-dependent manner. We also took a lipin 1 loss of function approach to evaluate the interaction between lipin 1 and HNF4a. Overexpression of similar amounts of HNF4a in hepatocytes from fld mice, which lack lipin 1, was less effective at inducing the expression of genes encoding PPARa and fatty acid oxidation enzymes (Cpt1a and Acadm) (Figure 3A). The increase in rates of fatty acid oxidation induced by HNF4a overexpression was blunted in fld hepatocytes compared to WT controls (Figure 3B). Basal rates of palmitate oxidation were also diminished in fld hepatocytes compared to WT controls (Figure 3B). Collectively, these data indicate that lipin 1 enhances the stimulatory effects of HNF4a on fatty acid oxidation.Lipin 1 Suppresses the Expression of Apoproteins that are Induced by HNF4aHNF4a is known to stimulate the expression of various genes involved in VLDL metabolism [29], whereas we have shown that lipin 1 suppresses the expression of these genes [2]. Lipin 1 overexpression suppressed the ability of HNF4a to induce the expression of Apoa4 and Apoc3 in an LXXIL motif-dependent manner (Figure 4A). HNF4a overexpression was also more potent at inducing the expression of Apoa4 and Apoc3 in fld hepatocytes compared to WT controls (Figure 4B). We also assessed rates of TG synthesis and secretion by isolated hepatocytes from WT and fld mice and found that, despite the role of lipin 1 in the TG synthesis pathway, rates of TG synthesis were not affected by lipin 1 deficiency or HNF4a overexpression (Figure 4C). Consistent with our previous work [12], rates of VLDL-TG synthesis were significantly increased in hepatocytes from fld mice 23727046 infected with GFP adenovirus (Figure 4C). However, HNF4a-stimulated secretion of newly synthesized VLDL-TG, which was strongly enhanced by HNF4a 
overexpression, was not affected by loss of lipin 1 (Figure 4C). This may be explained by the strong stimulation of microsomal triglyceride transfer protein (Mttp) expression by HNF4a, which is not affected by lipin 1 deficiencyFigure 5. Lipin 1 inhibits Apoc3/Apoa4 promoter activity in an HNF4a-dependent manner. [A] The schematic depicts the luciferase reporter construct under control of the intergenic region between the genes encoding ApoC3 and ApoA4 (Apoc3/Apoa4.Luc). The relative positions of two HNF4a response elements denoted as Apoc3 enhancer and Apoa4 enhancer are indicated. Graphs depict results of luciferase assays using lysates from HepG2 cells transfected with Apoc3/Apoa4.Luc reporter constructs and cotransfected with lipin 1 and/or HNF4a expression constructs as indicated. Apoc3/Apoa4.Luc constructs were either wild-type or contained mutations in the ApoC3 enhancer or ApoA4 enhancer HNF4a response elements. The results are the mean of 3 independent experiments done in triplicate. *p,0.05 versus pCDNA control. **p,0.05 versus vector control or lipin 1 cotransfection. [B] The schematic depicts the heterologous luciferase reporter construct driven by three.
