Effects of gender and GH secretory pattern on sterol regulatory element-binding protein-1c and its target genes in rat liver

We investigated whether the sexually dimorphic secretory pattern of growth hormone (GH) in the rat regulates hepatic gene expression of sterol regulatory element-binding protein-1c (SREBP-1c) and its target genes. SREBP-1c, fatty acid synthase (FAS), and glycerol-3-phosphate acyltransferase (GPAT) mRNA were more abundant in female than in male livers, whereas acetyl-CoA carboxylase-1 (ACC1) and stearoyl-CoA desaturase-1 (SCD-1) were similarly expressed in both sexes. Hypophysectomized female rats were given GH as a continuous infusion or as two daily injections for 7 days to mimic the female- and male-specific GH secretory patterns, respectively. The female pattern of GH administration increased the expression of SREBP-1c, ACC1, FAS, SCD-1, and GPAT mRNA, whereas the male pattern of GH administration increased only SCD-1 mRNA. FAS and SCD-1 protein levels were regulated in a similar manner by GH. Incubation of primary rat hepatocytes with GH increased SCD-1 mRNA levels and decreased FAS and GPAT mRNA levels but had no effect on SREBP-1c mRNA. GH decreased hepatic liver X receptor-alpha (LXRalpha) mRNA levels both in vivo and in vitro. Feminization of the GH plasma pattern in male rats by administration of GH as a continuous infusion decreased insulin sensitivity and increased expression of FAS and GPAT mRNA but had no effect on SREBP-1c, ACC1, SCD-1, or LXRalpha mRNA. In conclusion, FAS and GPAT are specifically upregulated by the female secretory pattern of GH. This regulation is not a direct effect of GH on hepatocytes and does not involve changed expression of SREBP-1c or LXRalpha mRNA but is associated with decreased insulin sensitivity.     

Selective binding of sterol regulatory element-binding protein isoforms and co-regulatory proteins to promoters for lipid metabolic genes in liver

Mice were subjected to different dietary manipulations to selectively alter expression of hepatic sterol regulatory element-binding protein 1 (SREBP-1) or SREBP-2. mRNA levels for key target genes were measured and compared with the direct binding of SREBP-1 and -2 to the associated promoters using isoform specific antibodies in chromatin immunoprecipitation studies. A diet supplemented with Zetia (ezetimibe) and lovastatin increased and decreased nuclear SREBP-2 and SREBP-1, respectively, whereas a fasting/refeeding protocol dramatically altered SREBP-1 but had modest effects on SREBP-2 levels. Binding of both SREBP-1 and -2 increased on promoters for 3-hydroxy-3-methylglutaryl-CoA reductase, fatty-acid synthase, and squalene synthase in livers of Zetia/lovastatin-treated mice despite the decline in total SREBP-1 protein. In contrast, only SREBP-2 binding was increased for the low density lipoprotein receptor promoter. Decreased SREBP-1 binding during fasting and a dramatic increase upon refeeding indicates that the lipogenic "overshoot" for fatty-acid synthase gene expression known to occur during high carbohydrate refeeding can be attributed to a similar overshoot in SREBP-1 binding. SREBP co-regulatory protein recruitment was also increased/decreased in parallel with associated changes in SREBP binding, and there were clear distinctions for different promoters in response to the dietary manipulations. Taken together, these studies reveal that there are alternative molecular mechanisms for activating SREBP target genes in response to the different dietary challenges of Zetia/lovastatin versus fasting/refeeding. This underscores the mechanistic flexibility that has evolved at the individual gene/promoter level to maintain metabolic homeostasis in response to shifting nutritional states and environmental fluctuations.     

Regulation of SREBP-1 expression and transcriptional action on HKII and FAS genes during fasting and refeeding in rat tissues

The sterol regulatory element binding protein 1 (SREBP-1) is regarded as a major factor involved in the nutritional regulation of lipogenesis. The aim of the present work was to demonstrate its involvement in the response of key genes of glucose and lipid metabolism in liver, adipose tissue, and skeletal muscle during fasting and refeeding. The regulation of hexokinase-2 (HKII) was investigated as a marker of the glucose metabolic pathway and that of FAS was investigated as a marker of the lipogenic pathway. The in vivo association of SREBP-1 with the promoter regions of these genes was determined in the different tissues using chromatin immunoprecipitation assays. Fasting decreased, and refeeding restored, FAS and HKII mRNA and protein levels in each tissue. The concomitant measurement of SREBP-1a and SREBP-1c mRNA levels, of mature SREBP-1 protein abundance in nuclear extracts, and of SREBP-1 interaction with target promoters led to the conclusion that SREBP-1 plays a major role in the response of FAS and HKII genes to nutritional regulation in rodents. These data elucidate the important role of SREBP-1 not only in the regulation of lipid metabolism but also of glucose metabolism and energy homeostasis.     

Alternative splicing produces a constitutively active form of human SREBP-1

We identified a novel alternative splicing event that constitutively produces a truncated active form of human sterol regulatory element-binding protein 1 (SREBP-1). A cDNA of this splicing variant (named SREBP-1Δ) contains a translational stop codon-encoding exon sequence between exons 7 and 8. It produces SREBP-1aΔ (470 a.a.) and SREBP-1cΔ (446 a.a.) proteins that lack transmembrane and C-terminal regulatory sequences necessary for localization of SREBP-1 to the endoplasmic reticulum. A luciferase reporter assay showed that SREBP-1aΔ and SREBP-1cΔ transactivated lipogenic gene promoters to the same extent as that induced by N-terminal active fragments of SREBP-1a and SREBP-1c, respectively. SREBP-1Δ mRNA is expressed in human cell lines as well as adipose and liver tissues. Expression levels ranged from 5% to 16% of total SREBP-1 expression. The ratio of SREBP-1Δ expression to total SREBP-1 expression in HepG2 cells was not affected by either insulin or high glucose treatment.