ECTOPIC ICER EXPRESSION IN PITUITARY CORTICOTROPH ATT20 CELLS - EFFECTS ON MORPHOLOGY, CELL-CYCLE, AND HORMONAL PRODUCTION

The products of the cAMP response element modulator (CREM) gene play a n important role in the transcriptional response to cAMP in endocrine cells. By virtue of an alternative, intronic promoter within the gene, the inducible cAMP early repressor (ICER) isoform is generated. ICER was shown to act as a dominant negative regulator and to be cAMP-induc ible in various neuroendocrine cells and tissues. ICER negatively auto regulates its own expression and has been postulated to participate in the molecular events governing oscillatory hormonal regulations. To e lucidate ICER function in pituitary physiology, we have generated AtT2 0 corticotroph cell lines expressing the sense or antisense ICER trans cript under the control of the cadmium-inducible human methallothionei n IIA promoter. Here we demonstrate that changes in the regulated leve ls of ICER have drastic consequences on the physiology of the corticot rophs. Ectopic ICER expression induces remarkable modifications in AtT 20 morphology. Cells with persistent, nonregulated high levels of ICER are blocked in the G2/M phase of the cell cycle, while the opposite e ffect is obtained in cells expressing an antisense ICER transcript. We show that the effect of ICER on the AtT20 cell cycle is correlated to a direct down-regulation of the cyclin A gene promoter by ICER. Final ly, we show that ACTH hormonal secretion from the corticotrophs is com pletely blocked by ICER ectopic expression. Interestingly, this effect is not due to a direct regulation of the POMC gene, but is mediated b y a transcriptional control of the prohormone convertase 1 gene. These results point to a key regulatory function of CREM in pituitary physi ology.