DIVERGENT PATHWAYS REGULATE LIGAND-INDEPENDENT ACTIVATION OF ER-ALPHAIN SK-N-BE NEUROBLASTOMA AND COS-1 RENAL-CARCINOMA CELLS

The alpha-estrogen receptor (ER alpha) transcriptional activity can be regulated either by binding to the cognate ligand or by intracellular signaling pathways responsive to a variety of factors acting through cell membrane receptors. Studies carried out in HeLa and COS-1 cells d emonstrated that the cross-coupling between estrogen and growth factor receptors is mediated by p21ras and requires phosphorylation of a spe cific serine residue (Ser 118 in the human ER alpha and Ser 122 in mou se ER alpha) located in the ER alpha N-terminal activation function 1 (AF-1), Likewise, in the SK-N-BE neuroblastoma cell line p21ras is inv olved in the cross-coupling between insulin and ER alpha receptors. Ho wever, in this cell line Ser 122 is not necessary for insulin-dependen t activation of unliganded ER alpha. In addition, after insulin activa tion, the electrophoretic mobility associated to serine hyperphosphory lation of ER alpha in SK-N-BE and in COS-1 cells is different. Our stu dy rules out the possibility of tyrosine phosporylation in unliganded ER alpha activation by means of transactivation studies of ER alpha ty rosine mutants and analysis of Tyr phosphorylation immunoreactivity. T he two cofactors for steroid receptors RIP 140 and SRC-1 do not seem t o be specifically involved in the insulin-induced ER alpha transactiva tion. The present study demonstrates the possibility of an alternative , cell-specific pathway of cross-coupling between intracellular and me mbrane receptors, which might be of importance for the understanding o f the physiological significance of this mode of activation in the ner vous system.