Two Domains of the Progesterone Receptor Interact with the Estrogen Receptor and Are Required for Progesterone Activation of the c-Src/Erk Pathway in Mammalian Cells

Abstract

Steroid hormones influence a plethora of cellular functions, depending on the nature of the target cell and the constellation of signals impinging on the cell at a given time. To achieve the necessary coordination with other signaling pathways in the complex intracellular space, steroid hormones likely use a variety of mechanisms. Until very recently, attention has mainly been focused on the transcriptional effects of steroid hormones. These responses are mediated by the intracellular hormone receptors, which participate in multiple interactions with DNA, other sequence-specific transcription factors, transcriptional coregulators, and the general transcriptional machinery (3). In the last few years, a great effort has been devoted to understanding the nature of the transcriptional coregulators and how they mediate the interaction of the hormone receptors with chromatin remodelling complexes and the transcriptional apparatus (18). Considerable progress has been achieved, leading to the recognition of covalent and conformational chromatin changes as key steps in transcriptional regulation by steroid hormone receptors and other transcription factors (5).In addition to their direct transcriptional effects, steroid hormones have been found to influence the activity of many other signaling pathways by so-called "nongenomic mechanisms" (25,34,38). These effects are mediated by interactions at the membrane or cytoplasmic level and offer a possibility for integration of the steroid hormone signals at the entry site of many other physiological signals acting via membrane receptors (39). Very often, these nongenomic effects have been attributed to poorly characterized receptors, whose relationship with the conventional nuclear receptors remains unclear (38). In the case of the ovarian hormones estrogens and progestins, cross talk with a number of other signaling pathways has been described, including cyclic AMP (1, 16), Ca-calmodulin (15), the G protein-coupled receptors (20), and the mitogen-activated protein (MAP) kinase pathway (25,26). In breast cancer cells, estrogens stimulate cell proliferation, and this effect can be blocked by inhibitors of the MAP kinase signaling pathway (30) or by intracellular calcium chelators (22). This pathway is activated by estrogens through an interaction of the classical estrogen receptor alpha (ER␣) with c-Src, which can be detected by coimmunoprecipitation (30). c-Src activity is enhanced 2 min after addition of 17␤-estradiol, reaches a peak after 5 min, and returns to basal levels after 15 to 30 min. Activation of c-Src can be inhibited by steroidal and nonsteroidal antiestrogens and is followed by transient activation of Ras, Raf, and Erk1/2 (7, 31). The participation of the classical ER␣ in activation of the mitogenic pathway was demonstrated in gene transfection studies in COS-7 cells (31). Activation of the cascade in these cells by estrogens was strictly dependent on transfection of ER␣, was inhibited by antiestrogens, and led to the formation of a complex of ER␣ and c-Src. Simi...

Progestin-induced caveolin-1 expression mediates breast cancer cell proliferation

“…The requirement of Src activity for caveolin-1 activation has already been acknowledged (Li et al, 1996;Lee et al, 2000;Volonte et al, 2001). In addition, accumulating data (Migliaccio et al, 1998;Castoria et al, 1999;Boonyaratanakornkit et al, 2001;Ballare et al, 2003), including our own findings (Proietti et al, 2005), have evidenced progestins ability to induce c-Src phosphorylation by a nongenomic mechanism in mammary tumor cells. Thus, we have recently shown that MPA treatment of C4HD cells for 2-5 min induced strong c-Src tyrosine phosphorylation, which was significantly inhibited by the selective Src kinase family inhibitor PP2 (Proietti et al, 2005).…”

Integration of progesterone receptor action with rapid signaling events in breast cancer models

“…In their model, activation of cSrc and the MAPK pathway by progestins depends upon the presence of unliganded ERα, which interacts constitutively with PR-B via two domains that flank the proline-rich sequence of PR. Deletion of either of these two ER-interacting domains in PR-B blocked c-Src/MAPK activation by progestins in the presence of ERα [61]. Mutation of PR-B's PXXP domain had no effect.…”

“…Ballare et al [61] reported that MAPK activation by progestins is blocked by antiprogestins and antiestrogens in COS-7 cells transfected with both PR and ERα. They propose that c-Src/ MAPK activation by PR is mediated indirectly by the interaction of the Src-homology two (SH2) domain of c-Src with phosphotyrosine 537 of ERα [61].…”

“…In addition, progestins activated c-Src in PR-null MCF12A cells transduced with wt PR but not PXXP-mutant PR adenoviruses. Both groups found that ERα interacts with the SH2-domain of c-Src, but neither group tested the effects of estrogen on the ability of progesterone to activate c-Src or MAPKs [59,61].…”

“…They propose that c-Src/ MAPK activation by PR is mediated indirectly by the interaction of the Src-homology two (SH2) domain of c-Src with phosphotyrosine 537 of ERα [61]. In their model, activation of cSrc and the MAPK pathway by progestins depends upon the presence of unliganded ERα, which interacts constitutively with PR-B via two domains that flank the proline-rich sequence of PR.…”