Copies of the Apoc3 enhancer HNF4a response element. Graphs depict results of luciferase assays using lysates from HEK293 cells transfected with Apoc3 enhancer.3X.TKLuc and cotransfected with empty vector (pcDNA and pMT), lipin 1, and/or HNF4a expression constructs as indicated. 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. doi:10.1371/journal.pone.0051320.gLipin 1 and HNFWe sought to explore the molecular mechanism for the crosstalk between lipin 1 and HNF4a using the Apoc3 and Apoa4 genes as a model system. These two genes are located adjacent to 12926553 one another on human chromosome 11 and are oriented in opposing directions so that the promoters and critical regulatory elements that control transcription of both genes are located in a 6 kB intergenic region [30]. HepG2 cells were transfected with a luciferase promoter construct driven by the entire intergenic region between the human Apoc3 and Apoa4 genes [17] in the presence or absence of expression constructs for HNF4a and/or lipin 1. As previously reported [16], HNF4a enhanced Apoc3/ Apoa4 promoter activity compared to empty vector control (Figure 5A). Co-transfection of the lipin 1 expression vector significantly repressed basal and HNF4a-induced Apoc3/Apoa4 promoter activity (Figure 5A). A site-directed mutation that abrogates binding of HNF4a and other nuclear receptors to a nuclear receptor response element (NRRE) proximal to the Apoc3 gene (“Apoc3 enhancer”; [16]) prevented both the lipin 1-mediated suppression and the HNF4ainduced purchase BIBS39 activation of the Apoc3/Apoa4 promoter (Figure 5A). In contrast, a mutation in another predicted HNF4aRE [16] proximal to the Apoa4 gene (“Apoa4 enhancer”) did not influence the effect of either lipin 1 or HNF4a (Figure 5A). The robust HNF4a-mediated activation of a heterologous reporter containing 3 copies of the “Apoc3 enhancer” was also attenuated by cotransfection of lipin 1b expression vector in HEK-293 cells (Figure 5B).Lipin 1 is not Associated with Chromatin in the Apoc3 PromoterWe sought to further dissect the transcriptional regulatory mechanisms mediating the divergent effects of lipin 1 on HNF4a activity. Consistent with the gene expression and promoter assays above, chromatin immunoprecipitation (ChIP) analyses demonstrated that HNF4a occupancy of the Apoc3 promoter was diminished by lipin 1 overexpression, whereas HNF4a occupancy of the Ppara promoter was significantly increased by lipin 1 (Figure 6A). However, ChIP analyses Biotin-NHS web utilizing an antibody to the HA epitope tag of lipin 1 did not detect a significant interaction between lipin 1 and chromatin in the Apoc3 promoter (Figure 6A). In contrast, significant 15755315 cross-linking of lipin 1 to the Ppara promoter was detected. To examine the effects of lipin 1 on HNF4a intrinsic activity in a promoter-independent fashion, the activity of a Gal4-HNF4a fusion construct on a multimerized Gal4-response element-driven luciferase reporter (UAS-TKLuc) was examined. Lipin 1 overexpression enhanced Gal4-HNF4a activity by more than 3-fold in this mammalian two-hybrid system (Figure 6B). We propose that the suppression of Apoc3/Apoa4 promoter activity is not mediated via an active repression mechanism and that lipin 1 may influence HNF4a promoter occupancy by directing it towards promoters of genes encoding proteins that affect fatty acid oxidation.Figure 6. Lipin 1 influences HNF4a promot.Copies of the Apoc3 enhancer HNF4a response element. Graphs depict results of luciferase assays using lysates from HEK293 cells transfected with Apoc3 enhancer.3X.TKLuc and cotransfected with empty vector (pcDNA and pMT), lipin 1, and/or HNF4a expression constructs as indicated. 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. doi:10.1371/journal.pone.0051320.gLipin 1 and HNFWe sought to explore the molecular mechanism for the crosstalk between lipin 1 and HNF4a using the Apoc3 and Apoa4 genes as a model system. These two genes are located adjacent to 12926553 one another on human chromosome 11 and are oriented in opposing directions so that the promoters and critical regulatory elements that control transcription of both genes are located in a 6 kB intergenic region [30]. HepG2 cells were transfected with a luciferase promoter construct driven by the entire intergenic region between the human Apoc3 and Apoa4 genes [17] in the presence or absence of expression constructs for HNF4a and/or lipin 1. As previously reported [16], HNF4a enhanced Apoc3/ Apoa4 promoter activity compared to empty vector control (Figure 5A). Co-transfection of the lipin 1 expression vector significantly repressed basal and HNF4a-induced Apoc3/Apoa4 promoter activity (Figure 5A). A site-directed mutation that abrogates binding of HNF4a and other nuclear receptors to a nuclear receptor response element (NRRE) proximal to the Apoc3 gene (“Apoc3 enhancer”; [16]) prevented both the lipin 1-mediated suppression and the HNF4ainduced activation of the Apoc3/Apoa4 promoter (Figure 5A). In contrast, a mutation in another predicted HNF4aRE [16] proximal to the Apoa4 gene (“Apoa4 enhancer”) did not influence the effect of either lipin 1 or HNF4a (Figure 5A). The robust HNF4a-mediated activation of a heterologous reporter containing 3 copies of the “Apoc3 enhancer” was also attenuated by cotransfection of lipin 1b expression vector in HEK-293 cells (Figure 5B).Lipin 1 is not Associated with Chromatin in the Apoc3 PromoterWe sought to further dissect the transcriptional regulatory mechanisms mediating the divergent effects of lipin 1 on HNF4a activity. Consistent with the gene expression and promoter assays above, chromatin immunoprecipitation (ChIP) analyses demonstrated that HNF4a occupancy of the Apoc3 promoter was diminished by lipin 1 overexpression, whereas HNF4a occupancy of the Ppara promoter was significantly increased by lipin 1 (Figure 6A). However, ChIP analyses utilizing an antibody to the HA epitope tag of lipin 1 did not detect a significant interaction between lipin 1 and chromatin in the Apoc3 promoter (Figure 6A). In contrast, significant 15755315 cross-linking of lipin 1 to the Ppara promoter was detected. To examine the effects of lipin 1 on HNF4a intrinsic activity in a promoter-independent fashion, the activity of a Gal4-HNF4a fusion construct on a multimerized Gal4-response element-driven luciferase reporter (UAS-TKLuc) was examined. Lipin 1 overexpression enhanced Gal4-HNF4a activity by more than 3-fold in this mammalian two-hybrid system (Figure 6B). We propose that the suppression of Apoc3/Apoa4 promoter activity is not mediated via an active repression mechanism and that lipin 1 may influence HNF4a promoter occupancy by directing it towards promoters of genes encoding proteins that affect fatty acid oxidation.Figure 6. Lipin 1 influences HNF4a promot.