Role of ErbB-3 and MAPK in the mechanism of heregulin-induced progesterone receptor transcriptional activation

PV, PROIETTI CJ, LABRIOLA L, SALATINO M, CHARREAU EH

Washington DC, USA.

Congreso; Annual Meeting of the American Association for Cancer Research; 2003

The present study addresses the capacity of heregulin (HRG), ligand of the type I receptor tyrosine kinases (RTKs), to transactivate the progesterone receptor. For this purpose, we studied on one hand, an experimental model of hormonal carcinogenesis, in which the synthetic progestin medroxyprogesterone acetate (MPA) induced mammary adenocarcinomas in female Balb/c mice, and on the other hand, the human breast cancer cell line T47D. HRG was able to exquisitely regulate biochemical attributes of PR in a way that mimicked PR activation by progestins. Thus, HRG treatment of primary cultures of epithelial cells of the progestin-dependent C4HD murine mammary tumor line and of T47D cells, induced a decrease of protein levels of PRA and B isoforms and the down-regulation of progesterone-binding sites. HRG also promoted a significant increase in the percentage of PR localized in the nucleus in both cell types. DNA mobility shift assay revealed that HRG was able to induce PR binding to a progesterone response element (PRE) in C4HD and T47D cells. Transient transfections of C4HD and T47D cells with a plasmid containing a PRE upstream of a chloranfenicol acetyltransferase (CAT) gene demonstrated that HRG promoted a significant increase in CAT activity. In order to assess molecular mechanisms underlying PR transactivation by HRG, we blocked ErbB-2 expression in C4HD and T47D cells by using antisense oligodeoxynucleotides to ErbB-2 mRNA, which resulted in the abolishment of HRG capacity to induce PR binding to a PRE, as well as CAT activity in the transient transfection assays. Whereas inhibition of HRG binding to ErbB-3 by an anti-ErbB-3 monoclonal antibody suppressed HRG-induced PR activation, abolishment of HRG binding to ErbB-4 had no effect on HRG activation of PR. To investigate the role of mitogen-activated protein kinases (MAPK), we used the selective MEK1/MAPK inhibitor PD98059. Blockage of MAPK activation resulted in complete abrogation of HRG capacity to induce PR binding to a PRE, and CAT activity.  Finally, we have herein for the first time demonstrated that HRG-activated MAPK can phosphorylate both human and mouse PR in vitro.