Dynamic regulation of estrogen receptor-α isoform expression in the mouse fallopian tube: mechanistic insight into estrogen-dependent production and secretion of insulin-like growth factors

Abstract

Estrogen receptors (ERs) are members of the nuclear receptor superfamily and are involved in regulation of fallopian tube functions (i.e., enhancement of protein secretion, formation of tubal fluid, and regulation of gamete transport). However, the ER subtype-mediated mechanisms underlying these processes have not been completely clarified. Recently, we identified ER␤ expression and localization in rat fallopian tubes, suggesting a potential biological function of ER␤ related to calcium-dependent ciliated beating. Here we provide for the first time insight into the less studied ER␣ isoforms, which mediate estrogen-dependent production and secretion of IGFs in vivo. First, Western blot studies revealed that three ER␣ isoforms were expressed in mouse fallopian tubes. Subsequent immunohistochemical analysis showed that ER␣ was detected in all cell types, whereas ER␤ was mainly localized in ciliated epithelial cells. Second, ER␣ isoform levels were dramatically downregulated in mouse fallopian tubes by treatment with E2 or PPT, an ER␣ agonist, in a time-dependent manner. Third, the presence of ICI 182,780, an ER antagonist, blocked the E2-or PPT-induced downregulation of tubal ER␣ isoform expression in mice. However, alteration of ER␣ immunoreactivity following ICI 182,780 treatment was only detected in epithelial cells of the ampullary region. Fourth, changes in ER␣ isoform expression were found to be coupled to multiple E2 effects on tubal growth, protein synthesis, and secretion in mouse fallopian tube tissues and fluid. In particular, E 2 exhibited positive regulation of IGF-I and IGF-II protein levels. Finally, using growth hormone receptor (GHR) gene-disrupted mice, we showed that regulation by E 2 of IGF production was independent of GHinduced GHR signaling in mouse fallopian tubes in vivo. These data, together with previous studies from our laboratory, suggest that the long-term effects of estrogen agonist promote IGF synthesis and secretion in mouse tubal epithelial cells and fallopian tube fluid via stimulation of ER␣.estrogen receptor-␣ isoforms; tubal epithelial cells A CRITICAL FUNCTION of the mammalian fallopian tube (24) is to provide an optimal microenvironment for the necessary transport and maturation of gametes and the establishment of pregnancy. It excretes the intraluminal tubal fluid, containing a number of growth factors (1,4,29,30,34). Compelling evidence suggests that estrogen may be involved in the regulation of biological processes in the fallopian tube. Studies have shown that ovarian-derived estradiol (E 2 ), the principal bioactive estrogen, is involved in the regulation of tubal protein secretion in human fallopian tubes in vivo (34) and in vitro (63). Insulin-like growth factors (IGFs) have been identified in human and rodent fallopian tube fluid (4). Accumulating data suggest that IGF-I may support embryonic development and survival (1). Moreover, E 2 has been shown to increase endogenous IGF-I mRNA levels in rat fallopian tube and uterus (5, 46), and estrogen-responsive elements are...

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