Cyclic adenosine 3',5'-monophosphate regulation of corticotropin-releasing hormone promoter activity in AtT-20 cells and in a transformed hypothalamic cell line

The regulation of CRH promoter activity by cAMP was studied in two cell lines, the pituitary corticotroph cell line AtT-20 and the immortalized hypothalamic cell line 4B, which expresses CRH and vasopressin. In 4B cells transfected with a CRH promoter-luciferase construct, the adenylyl cyclase stimulator, forskolin, increased luciferase activity in parallel with increases in intracellular cAMP. In 4B cells, however, the phosphodiesterase inhibitor, isobutylmethylxanthine, potentiated forskolin-stimulated cAMP without affecting further increases in luciferase activity. In AtT-20 cells, forskolin plus isobutylmethylxanthine elevated cAMP only slightly, but increased luciferase activity to levels similar to those observed in 4B cells. AtT-20 cells were also unresponsive to 8-bromo-cAMP, due in part to higher phosphodiesterase (PDE) activities. Although both cells contained PDE1, -3, and -4, inhibition of either PDE4 or PDE1 potentiated luciferase activity stimulated by submaximal forskolin concentrations in 4B cells, while only simultaneous inhibition of PDE3 and PDE4 was effective in AtT-20 cells. The data show that minor elevations in intracellular cAMP are sufficient for full stimulation of CRH promoter activity regardless of the cell line. Furthermore, poor CRH promoter activation in AtT-20 cells appears to result from deficient cAMP production and rapid cAMP degradation by PDE.     

Promoter activity of sea lamprey proopiocortin and proopiomelanotropin genes in AtT-20/D16v cells

Adrenocorticotropic hormone (ACTH) and melanophore-stimulating hormone (MSH) are produced in the pars distalis and pars intermedia, respectively, throughout vertebrates. These hormones together with beta-endorphin are encoded on a single gene proopiomelanocortin (POMC) in gnathostomes, but in the sea lamprey, an agnathan, ACTH and MSH are encoded on two separate genes, proopiocortin (POC) and proopiomelanotropin (POM), respectively. Moreover, the nucleotide sequences of 5'-flanking regions of the POC and POM genes are significantly different from each other. To investigate the potential promoter activities of the POC and POM genes, we constructed promoter reporter plasmids by fusing the 5' flanking sequences (nucleotides -1151 to +31 and -2510 to +51, respectively) to a firefly luciferase gene. Transient transfection studies in AtT-20/D16v cells, which derived from a mouse pituitary tumor cell line, revealed that the 5'-flanking sequence of the POC gene did not exhibit promoter activity, whereas that of the POM gene showed the activity at high levels nearly equivalent to SV40 promoter. Analysis of a series of the 5'-deleted reporter for the POM gene in the AtT-20/D16v cells demonstrated that the 422 bp 5'-flanking sequence was sufficient for promoter activity, while the sequence from -853 to -574 may contain negatively acting regulatory elements. Because the POC and POM genes are supposed to have differentiated from a common ancestor, during evolution, the POC gene may lack essential element(s) for expression in the AtT-20/D16v cells.     

Function of a truncated glucocorticoid receptor form at a negative glucocorticoid response element in the proopiomelanocortin gene

ACTH-producing tumors of nonpituitary origin characteristically exhibit insensitivity to the negative feedback effects of glucocorticoids. In the DMS-79 cell line derived from an ACTH-producing small cell lung cancer we have previously identified an aberrantly spliced glucocorticoid receptor (GRDelta) that lacks a ligand-binding domain. We examined the interactions of this truncated form of GR with the proximal human proopiomelanocortin (POMC) promoter. In electrophoretic mobility shift assays GRDelta bound to the negative glucocorticoid response element (nGRE) at position -78 to -50 in the human POMC promoter. Nur77, an orphan nuclear receptor that exerts positive regulatory effects on the POMC gene is also known to bind to this DNA element. The functional properties of GR and GRDelta binding to this DNA element were examined in transient transfection experiments in murine AtT-20 corticotroph tumor cells. Reporter gene expression under the control of proximal POMC promoter elements was stimulated by addition of forskolin to the culture medium or by transfection with expression constructs for human Nak1, the human homologue of Nur77. Treatment of transfected cells with dexamethasone resulted in suppression of forskolin- or Nak1-stimulated POMC-reporter gene expression in the presence of co-transfected GR but not with GRDelta. The experiments indicate that in the human POMC promoter GRDelta is capable of binding to the nGRE but cannot effect trans-repression of POMC-reporter gene expression.     

Analysis of the 5'-upstream region of mouse P/Q-type Ca2+ channel alpha1A subunit gene for expression in pancreatic islet beta cells using transgenic mice and HIT-T15 cells

The omega-agatoxin-IVA-sensitive P/Q-type Ca(2+) channel plays a role in insulin release from the pancreatic islets of beta cells. To dissect the molecular mechanisms underlying beta cell expression of the P/Q-type channel, we characterized the 5'-upstream region of the mouse alpha(1A) subunit gene using transgenic mice and HIT insulinoma cells. The E. coli lacZ reporter gene was expressed in pancreatic acini and islets in transgenic mice carrying the 6.3 kb or 3.0 kb of the 5'-upstream region, although those with 1.5 kb or 0. 5 kb of the 5'-upstream region failed to show reporter expression on histological examination. As the expression of alpha(1A)subunit gene could not be detected in acini using RT-PCR analysis, the reporter expression in acini might have been ectopic expression. When linked to the placental alkaline phosphatase reporter gene to examine promoter activity for beta cell expression, the 6.3 kb and 3.0 kb fragment of the 5'-upstream region, but not the smaller 1.5 kb fragment, were able to drive reporter gene expression in HIT cells. The sequence between 3.0 and 1.5 kb upstream of the start codon enhanced thymidine kinase promoter activity in HIT cells, but not in fibroblast NIH3T3 cells. These results suggested that the beta cell-specific elements of the alpha(1A) subunit gene are likely to be located in the distal upstream region (-3021 to-1563) of the 5'-upstream sequence and that the 6.3 kb fragment of the 5'-upstream region alone might be a lack of a negative cis-regulatory element(s) to suppress the alpha(1A) subunit gene expression in acini.