S not clear. In this work, we showed that increased lipin 1 availability affected HNF4a activity in a pathway-specific manner, suggesting that the activation of lipin 1 serves to feed forward and modulate HNF4a activity. Lipin 1 enhanced HNF4a-mediated activation of fatty acid oxidation while abrogating the ability of HNF4a to induce Apoa4 and Apoc3 gene expression. In the nucleus, lipin 1 can function as either a coactivator or corepressor depending upon the context of its transcription factor partner. Lipin 1 is most likely a molecular scaffold that recruits histone acetyltransferases or deacetylases to enhance or repress transcription depending upon the transcription factor partner [10,11]. However, since ChIP analyses did not detect the presence of lipin 1 on the Apoc3 enhancer and Gal4-HNF4a activity, a measure of intrinsic activity independent of promoter binding, was enhanced by lipin 1, 23148522 lipin 1 is probably not inhibiting HNF4a activity by an active repression mechanism. Rather, we surmise that lipin 1 may be mediating this effect by binding to HNF4a and directing its binding to the promoter of one gene versus another. Other coregulatory proteins that act in a promoter-specific manner have been reported. For example, in adipocytes, PGC-1a strongly coactivates PPARc on the Ucp1 promoter, but does not enhance expression of Fabp4 [37], which is also a robust and primary PPARc target gene [38].Although it is still unclear how promoter PS-1145 web selectivity by coregulatory proteins is mediated, it is possible that the presence of other response elements and DNA-bound transcription factors on certain promoters is required to influence occupancy. In many ways, the present studies clarify several mechanistic questions from our previous work. For example, although PPARa is coactivated by lipin 1, PPARa deficiency did not affect the transcriptional effects of lipin 1 in liver [10]. The present data indicate that this observation can be explained by the HNF4alipin 1 interaction. We also previously reported that Apoa4 is markedly overexpressed in liver of fld mice [39], lipin 1 overexpression suppressed the expression of Apoc3 and Apoa4, and the transcriptional activity of lipin 1 was required for this repressive effect [2]. In this work, we provide a more detailed mechanistic explanation for this observation using the Apoc3/Apoa4 gene cluster, which is well-characterized as an HNF4a target [16,31,40,41,42,43]. However, it is unlikely that the modulation of Apoc3/Apoa4 expression per se is sufficient to explain our previous report that lipin 1inhibits hepatic VLDL-TG secretion [44,45,46]. HNF4a overexpression was equally efficacious at increasing VLDL-TG secretion in WT and fld hepatocytes, likely 125-65-5 because of a strong induction of Mttp, which is known to be sufficient to stimulate VLDL secretion. The identification of the gene targets mediating this response will be the subject of future inquiry. In conclusion, we demonstrate herein that the gene encoding lipin 1 is direct target gene of HNF4a that feeds forward to modulate HNF4a activity, seemingly in a promoter-specific manner. Whereas lipin 1 promotes the expression of genes encoding fatty acid oxidation enzymes in response to HNF4a overexpression, lipin overexpression impedes the induction of apolipoprotein gene expression by this nuclear receptor. These data suggest 
that lipin 1 functions to promote the catabolic actions of HNF4a, which fits with the induction of lipin 1 in liver of starved mice w.S not clear. In this work, we showed that increased lipin 1 availability affected HNF4a activity in a pathway-specific manner, suggesting that the activation of lipin 1 serves to feed forward and modulate HNF4a activity. Lipin 1 enhanced HNF4a-mediated activation of fatty acid oxidation while abrogating the ability of HNF4a to induce Apoa4 and Apoc3 gene expression. In the nucleus, lipin 1 can function as either a coactivator or corepressor depending upon the context of its transcription factor partner. Lipin 1 is most likely a molecular scaffold that recruits histone acetyltransferases or deacetylases to enhance or repress transcription depending upon the transcription factor partner [10,11]. However, since ChIP analyses did not detect the presence of lipin 1 on the Apoc3 enhancer and Gal4-HNF4a activity, a measure of intrinsic activity independent of promoter binding, was enhanced by lipin 1, 23148522 lipin 1 is probably not inhibiting HNF4a activity by an active repression mechanism. Rather, we surmise that lipin 1 may be mediating this effect by binding to HNF4a and directing its binding to the promoter of one gene versus another. Other coregulatory proteins that act in a promoter-specific manner have been reported. For example, in adipocytes, PGC-1a strongly coactivates PPARc on the Ucp1 promoter, but does not enhance expression of Fabp4 [37], which is also a robust and primary PPARc target gene [38].Although it is still unclear how promoter selectivity by coregulatory proteins is mediated, it is possible that the presence of other response elements and DNA-bound transcription factors on certain promoters is required to influence occupancy. In many ways, the present studies clarify several mechanistic questions from our previous work. For example, although PPARa is coactivated by lipin 1, PPARa deficiency did not affect the transcriptional effects of lipin 1 in liver [10]. The present data indicate that this observation can be explained by the HNF4alipin 1 interaction. We also previously reported that Apoa4 is markedly overexpressed in liver 
of fld mice [39], lipin 1 overexpression suppressed the expression of Apoc3 and Apoa4, and the transcriptional activity of lipin 1 was required for this repressive effect [2]. In this work, we provide a more detailed mechanistic explanation for this observation using the Apoc3/Apoa4 gene cluster, which is well-characterized as an HNF4a target [16,31,40,41,42,43]. However, it is unlikely that the modulation of Apoc3/Apoa4 expression per se is sufficient to explain our previous report that lipin 1inhibits hepatic VLDL-TG secretion [44,45,46]. HNF4a overexpression was equally efficacious at increasing VLDL-TG secretion in WT and fld hepatocytes, likely because of a strong induction of Mttp, which is known to be sufficient to stimulate VLDL secretion. The identification of the gene targets mediating this response will be the subject of future inquiry. In conclusion, we demonstrate herein that the gene encoding lipin 1 is direct target gene of HNF4a that feeds forward to modulate HNF4a activity, seemingly in a promoter-specific manner. Whereas lipin 1 promotes the expression of genes encoding fatty acid oxidation enzymes in response to HNF4a overexpression, lipin overexpression impedes the induction of apolipoprotein gene expression by this nuclear receptor. These data suggest that lipin 1 functions to promote the catabolic actions of HNF4a, which fits with the induction of lipin 1 in liver of starved mice w.