A variant of estrogen receptor-α, hER-α36: Transduction of estrogen- and antiestrogen-dependent membrane-initiated mitogenic signaling

Abstract

The status of the 66-kDa human estrogen receptor-␣ (hER-␣66) is a critical determinant in the assessment of the prognosis and in the design of treatment strategies of human breast cancer. Recently, we cloned the cDNA of an alternatively spliced variant of hER-␣66, termed hER-␣36; the predicted protein lacks both transcriptional activation domains of hER-␣66 but retains its DNA-binding domain, partial dimerization, and ligand-binding domains and three potential myristoylation sites located near the N terminus. These findings thus predict that hER-␣36 functions very differently from hER-␣66 in response to estrogen signaling. We now demonstrate that hER-␣36 inhibits the estrogen-dependent and estrogen-independent transactivation activities of hER-␣66 and hER-␤. We further demonstrate that hER-␣36 is predominantly associated with the plasma membrane where it transduces both estrogen-and antiestrogen-dependent activation of the mitogen-activated protein kinase͞extracellular signal-regulated kinase signaling pathway and stimulates cell growth. We conclude that hER-␣36 is a predominantly membrane-based, unique alternatively spliced variant of hER-␣66 that acts as a dominant-negative effector of both estrogen-dependent and estrogen-independent transactivation functions signaled through hER-␣66 and ER-␤; it also transduces membrane-initiated estrogen-dependent activation of the mitogenactivated protein kinase͞extracellular signal-regulated kinase mitogenic signaling pathway. The estrogen and antiestrogen signaling pathways mediated by hER-␣36 provide an alternative explanation for why some human breast cancers are resistant to and others are worsened by antiestrogen therapy; the data suggest that hER-␣36 also may be an important marker to direct therapy in human breast cancers, and perhaps hER-␣36 also may transduce estrogen-dependent signaling in other estrogen target tissues.breast cancer ͉ mitogen-activated protein kinase͞extracellular signal-regulated kinase (MAPK͞ERK) ͉ membrane antiestrogen signaling ͉ membrane estrogen signaling B reast cancer is the second-most-common cause of mortality in women in the United States; the toll in human life is particularly devastating because breast cancer strikes women at times of highest productivity and in the years of childbearing and rearing of family. This toll on morbidity and human life continues despite great progress in the diagnosis and development of treatment strategies in human breast cancer, leaving as a high priority the solution to the many remaining questions in understanding the mechanisms that drive breast cancer.

The MPA mouse breast cancer model: evidence for a role of progesterone receptors in breast cancer

“…Estrogens, unlike what happens in the parental tumors (both in vivo and in primary cultures), where they behave as inhibitory agents, may stimulate cell proliferation. Even though preliminary data of our laboratory suggest that this difference is not related to the expression of specific ERa or ERb isoforms, we cannot rule out a role for the ERa splice variant ER 36 (Wang et al 2006). This variant has been demonstrated to be involved in cell proliferation, and the fact that it may be active only in the cell lines remains to be explored.…”

Induction of estrogen receptor α-36 expression by bone morphogenetic protein 2 in breast cancer cell lines

“…A previous study on the induction of ERα and ERβ in granulosa cells by activin also showed that BMP-2 increased ERα mRNA levels by approximately 50% (23), consistent with our results. Wang et al (21) reported that ERα-36 lacks both transcriptional activation domains of ERα-66, retains portions of the DNA-binding domain, the partial dimerization and ligand-binding domains, and possesses a unique 27 amino acid domain that replaces the last 138 amino acid of ERα-66. Moreover, ERα-36 can inhibit the transactivation of both ERα-66 and ERβ, stimulate the MAPK signaling pathway and induce cell growth in breast cancer cell lines.…”

“…Recent studies have indicated that the efficacy of endocrine therapy cannot be simply explained by the expression levels of ERα, but may be determined by the expression profiles of both ERα and ERβ, as well as their various splice variants (14,21,24,25). Furthermore, to understand the meaning of the demonstrated change in ERα-36 expression induced by BMP2 in breast cancer cells, additional research should be carried out to determine whether the ERα-36 reported in the present study is the same as that reported by Wang et al (21).…”

“…The protein concentration was quantified using a BCA™ Protein Assay kit (Pierce). Equal amounts of protein (20 µg) were subjected to 12% SDS-PAGE and western blot analysis, as described previously (21). Immunoreactive bands were visualised by an enhanced chemiluminescence reaction kit (Thermo Fisher Scientific).…”

Dysregulation of microRNA expression in acquired endocrine-resistant breast cancer.

“…However, others have reported that an ERa, variant called ERa,36, and not GPR30, mediates non-genomic ER signaling, including ICI agonist activity [305]. ERa,36 arises from a promoter in the first exon of ERa" but lacks both the N-and C terminal transcription activation domains, AF-I and AF-2, respectively, of full-length wild type ERa,66 [306,307]. Further studies would be required to examine ERa,36 expression in L Y2 cells.…”