Complex hormone response unit regulating transcription of the phosphoenolpyruvate carboxykinase gene: from metabolic pathways to molecular biology

The orphan receptors COUP-TF and HNF-4 serve as accessory factors required for induction of phosphoenolpyruvate carboxykinase gene transcription by glucocorticoids

Glucocorticoids stimulate hepatic phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) gene expression, thereby increasing the rate of gluconeogenesis. The effect of glucocorticoids on PEPCK gene expression is mediated by a set of promoter elements collectively referred to as the glucocorticoid response unit. The response unit spans a 100-bp segment and includes two glucocorticoid receptor binding sites (GR1 and GR2) and two accessory factor binding sites (AF1 and AF2), all of which are required for a maximal glucocorticoid response. The AF1 element also serves as a retinoic acid response element and may be involved in developmental and tissue-specific expression of the gene. In this study we report that COUP-TF and HNF-4, two orphan members of the nuclear receptor superfamily, bind to the AF1 element and function as accessory factors for the glucocorticoid response of the PEPCK gene.

Expression of the phosphoenolpyruvate carboxykinase gene in 3T3-F442A adipose cells: effects of retinoic acid and differentiation

3T3-F442A adipocytes express the gene encoding cytosolic phosphoenolpyruvate carboxykinase (GTP) (PEPCK). Retinoic acid (RA) caused a 5-fold induction of PEPCK mRNA within 6 h in these cells with a half-maximal effective concentration of approximately 75 microM. This effect was independent of cycloheximide and inhibited by actinomycin D. In vitro run-on experiments using isolated nuclei confirmed that the RA-induced increase was mainly due to an increased rate of transcription of the gene. Stable transfectants bearing either the region of the PEPCK promoter from -2100 to +69 fused to the chloramphenicol acetyltransferase (CAT) gene (pPL1-CAT) or -600 to +69 fused to CAT (pPL9-CAT) were used to study PEPCK gene regulation during differentiation. The same transfected cells were used to analyse the RA effect. Preadipocytes containing pPL1-CAT expressed a much lower level of CAT activity than did adipocytes. pPL9-CAT was not expressed in either preadipocytes or adipocytes. RA induced the expression of CAT activity in preadipocytes and adipocytes transfected with pPL1-CAT, but had no effect in cells transfected with pPL9-CAT. These results suggest that one or more DNA sequences located between -2100 and -600 bp of the PEPCK promoter is required for adipocyte-specific expression of this gene. RA action is independent of the state of differentiation and appears to require different elements in fat cells from those required in liver.

Glucocorticoids activate somatostatin gene transcription through co-operative interaction with the cyclic AMP signalling pathway

The somatostatin (SS) gene is transcriptionally regulated via the cyclic AMP (cAMP) response element (CRE), located in the proximal promoter (-41 to -48 bp). We have previously reported that glucocorticoids induce dose-dependent cell-specific alterations in the steady-state SS mRNA level. Here we have investigated direct transcriptional control of the SS gene by glucocorticoids. We have examined transcriptional interaction between glucocorticoids and the cAMP signalling pathway and mapped the 5' upstream regulatory region of the SS gene involved in glucocorticoid transactivation. Transcriptional regulation was determined by analysis of chloramphenicol acetyltransferase (CAT) activity in PC12 rat pheochromocytoma cells and A126-1B2 (protein kinase A-deficient mutant PC12) cells, by acute transfection of 5' flanking SS DNA (- 750, -250 and -71 bp) ligated to the reporter (CAT) gene. Dexamethasone (DEX) induced a dose-dependent 2.2-fold stimulation of SS gene transcription in PC12 cells, but not in A126-1B2 cells. Other steroid and thyroid hormones tested, and retinoic acid, were ineffective, while cAMP and forskolin stimulated gene transcription 4-5-fold in PC12 cells but not in A126-1B2 cells. DEX exerted an additive effect on cAMP-induced gene transcription. Deletion of the promoter from -750 to -71 bp (but not from -750 to -250 bp) abolished all stimulatory effects of DEX without affecting cAMP responsiveness. Mutation of the CRE abrogated both DEX- and cAMP-dependent gene enhancement. Gel electrophoretic mobility shift assays confirmed that the -250 to -71 bp region of the SS promoter (but not the -71 to +55 bp domain) binds specifically to a glucocorticoid response element-sensitive nuclear protein(s) from PC12 cells, suggesting a putative glucocorticoid receptor interaction with SS promoter DNA. We conclude that glucocorticoids regulate SS gene transcription positively. Glucocorticoid-induced transactivation shows dependence on protein kinase. A activity, and may be mediated via protein-protein interaction between the glucocorticoid receptor and the CRE binding protein. DNA sequences upstream from the CRE between -250 and -71 bp in the SS promoter appear to be the target of glucocorticoid action.

Activation of the phosphoenolpyruvate carboxykinase gene retinoic acid response element is dependent on a retinoic acid receptor/coregulator complex

The accessory factor 1 (AF1) element is an upstream transcriptional control region that plays a role in the response of the phosphoenolpyruvate carboxykinase (PEPCK) gene to both glucocorticoids and retinoic acid. We demonstrate here that retinoic acid receptor alpha (RAR alpha) binds to a sequence within the AF1 element, TGACCT (site B), that is a consensus retinoic acid response element (RARE) half-site. A similar DNA sequence, TGGCCG (site C), located 1 bp downstream of site B, is not involved in the binding of RAR alpha monomers or dimers but is required for the constitution of a functional RARE. Site C is also required for the formation of a complex involving RAR alpha and a liver nuclear factor designated CR, for coregulator. Mutational analysis of the AF1 element shows that the RAR alpha/CR complex is the trans-acting unit that mediates the retinoic acid response of the PEPCK gene. Another member of the retinoid receptor family, retinoid X receptor alpha (RXR alpha), can also form a complex with RAR alpha and the AF1 element. Several observations, including the observation that RXR alpha antibody interacts with CR, indicate that RXR alpha and CR are identical or closely related proteins. Through RXR alpha forms a complex with RAR alpha and the AF1 element, we demonstrate that the AF1 element is functionally distinguishable from a retinoid X response element. Taken together, our results show that the AF1 element contains an RARE that mediates a retinoic acid response by binding an RAR alpha/coregulator complex; this coregulator is presumably RXR alpha.

Activation of the phosphoenolpyruvate carboxykinase gene retinoic acid response element is dependent on a retinoic acid receptor/coregulator complex

The accessory factor 1 (AF1) element is an upstream transcriptional control region that plays a role in the response of the phosphoenolpyruvate carboxykinase (PEPCK) gene to both glucocorticoids and retinoic acid. We demonstrate here that retinoic acid receptor alpha (RAR alpha) binds to a sequence within the AF1 element, TGACCT (site B), that is a consensus retinoic acid response element (RARE) half-site. A similar DNA sequence, TGGCCG (site C), located 1 bp downstream of site B, is not involved in the binding of RAR alpha monomers or dimers but is required for the constitution of a functional RARE. Site C is also required for the formation of a complex involving RAR alpha and a liver nuclear factor designated CR, for coregulator. Mutational analysis of the AF1 element shows that the RAR alpha/CR complex is the trans-acting unit that mediates the retinoic acid response of the PEPCK gene. Another member of the retinoid receptor family, retinoid X receptor alpha (RXR alpha), can also form a complex with RAR alpha and the AF1 element. Several observations, including the observation that RXR alpha antibody interacts with CR, indicate that RXR alpha and CR are identical or closely related proteins. Through RXR alpha forms a complex with RAR alpha and the AF1 element, we demonstrate that the AF1 element is functionally distinguishable from a retinoid X response element. Taken together, our results show that the AF1 element contains an RARE that mediates a retinoic acid response by binding an RAR alpha/coregulator complex; this coregulator is presumably RXR alpha.