A fraction of the nuclear estrogen receptor ␣ (ER␣) is localized to the plasma membrane region of 17␤-estradiol (E2) target cells. We previously reported that ER␣ is a palmitoylated protein. To gain insight into the molecular mechanism of ER␣ residence at the plasma membrane, we tested both the role of palmitoylation and the impact of E2 stimulation on ER␣ membrane localization. The cancer cell lines expressing transfected or endogenous human ER␣ (HeLa and HepG2, respectively) or the ER␣ nonpalmitoylable Cys447Ala mutant transfected in HeLa cells were used as experimental models. We found that palmitoylation of ER␣ enacts ER␣ association with the plasma membrane, interaction with the membrane protein caveolin-1, and nongenomic activities, including activation of signaling pathways and cell proliferation (i.e., ERK and AKT activation, cyclin D 1 promoter activity, DNA synthesis). Moreover, E2 reduces both ER␣ palmitoylation and its interaction with caveolin-1, in a time-and dose-dependent manner. These data point to the physiological role of ER␣ palmitoylation in the receptor localization to the cell membrane and in the regulation of the E2-induced cell proliferation. INTRODUCTIONThe sex steroid 17␤-estradiol (E2) acts by binding to its nuclear receptors (i.e., ER␣ and ER␤) that then transactivate target genes. In addition, E2 induces rapid, nongenomic actions involving plasma membrane-associated signaling that require a membrane ER (Coleman and Smith, 2001;Kelly and Levin, 2001;Jakacka et al., 2002;Marino et al., 2002). Although different structural and functional properties have been reported for the membrane-associated ER by comparison with nuclear ER␣ and ER␤ (Ropero et al., 2002;Toran-Allerand et al., 2002;Deecher et al., 2003), immunocytochemical studies revealed the presence of a significant fraction of nuclear ER also on the plasma membrane (Pappas et al., 1995;Norfleet et al., 1999;Dan et al., 2003;Razandi et al., 2003;Arvanitis et al., 2004;Song et al., 2004). In addition, a single mRNA originates a similarly sized nuclear and membrane ER in ER␣-transfected Chinese hamster ovary and HeLa cells (Razandi et al., 1999;Marino et al., 2002Marino et al., , 2003. Thus, ER␣ localizes to both the nucleus and the plasma membrane. Moreover, the membrane ER␣ is emerging as the primary endogenous mediator of E2 rapid responses important in cell proliferation (Marino et al., 1998(Marino et al., , 2002Castoria et al., 1999Castoria et al., , 2001Razandi et al., 1999Razandi et al., , 2000Lobenhofer et al., 2000;Acconcia et al., 2004a;Fernando and Wimalasena, 2004).Debate is open regarding the structural bases and the mechanisms for ER␣ maintenance at and translocation to the plasma membrane. ER␣ does not display any intrinsic transmembrane domain (Song et al., 2004); thus, ER␣ interaction with specific membrane proteins have been proposed to explain its membrane localization Migliaccio et al., 2002;Razandi et al., 2002Razandi et al., , 2003Toran-Allerand et al., 2002;Arvanitis et al., 2004). In particular, the Ser522 re...

The capability of 17beta-estradiol (E2) to induce the non-genomic activities of its receptors (ER alpha and ER beta) and to evoke different signaling pathways committed to the regulation of cell proliferation has been analyzed in different cell cancer lines containing transfected (HeLa) or endogenous (HepG2, DLD1) ER alpha or ER beta. In these cell lines, E2 induced different effects on cell growth/apoptosis in dependence of ER isoforms present. The E2-ER alpha complex rapidly activated multiple signal transduction pathways (i.e., ERK/MAPK, PI3K/AKT) committed to both cell cycle progression and apoptotic cascade prevention. On the other hand, the E2-ER beta complex induced the rapid and persistent phosphorylation of p38/MAPK which, in turn, was involved in caspase-3 activation and cleavage of poly(ADP-ribose)polymerase, driving cells into the apoptotic cycle. In addition, the E2-ER beta complex did not activate any of the E2-ER alpha-activated signal molecules involved in cell growth. Taken together, these results demonstrate the ability of ER beta isoform to activate specific signal transduction pathways starting from plasma membrane that may justify the effect of E2 in inducing cell proliferation or apoptosis in cancer cells. In particular this hormone promotes cell survival through ER alpha non-genomic signaling and cell death through ER beta non-genomic signaling.

Bisphenol A (BPA) exposure has been associated with serious endocrine-disrupting effects in humans and wildlife. Toxicological and epidemiological studies evidenced that BPA increases body mass index and disrupts normal cardiovascular physiology by interfering with endogenous hormones in rodents, nonhuman primates, and cell culture test systems. The BPA concentration derived from these experiments were used by government regulatory agencies to determine the safe exposure levels of BPA in humans. However, accumulating literature in vivo and in vitro indicate that at concentrations lower than that reported in toxicological studies, BPA could elicit a different endocrine-disrupting capacity. To further complicate this picture, BPA effects rely on several and diverse mechanisms that converge upon endocrine and reproductive systems. If all or just few of these mechanisms concur to the endocrine-disrupting potential of low doses of BPA is at present still unclear. Thus, taking into account that the incidence and/or prevalence of health problems associated with endocrine disruption have increased worldwide, the goal of the present review is to give an overview of the many mechanisms of BPA action in order to decipher whether different mechanisms are at the root of the effect of low dose of BPA on endocrine system.

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