ABSTRACT
Although much is known about the expression insulin-like growth factors (IGF) and their receptors in the murine oviduct, significantly less is known about the expression of IGF binding proteins (IGFBPs). To fill this gap in our knowledge, we identified and characterized the tissue specific expression of IGFBP-1 to-6 in rat oviducts over the estrous cycle byin situ hybridization and immunocytochemistry. Tissues were analysed on proestrus (P1000 h, P2000 h), estrus (E0200, E1000 h), and diestrus I and II (DI 1100 h, DII 1100 h). IGFBP-1 was undetectable in the oviduct over the cycle. IGFBP-2 was selectively expressed in the luminal epithelium. The mRNA levels were high between P2000 h and E1000 h but low or undetectable thereafter. Immunoreactive IGFBP-2 was strong to very strong in these cells over most of the cycle. IGFBP-3 mRNA was undetectable in the oviduct; however, strong hybridization and immunoreactive signals were present in the mesosalpinx and mesotubarium, particularly at DI and DII. IGFBP-4 mRNA was not detected in the oviduct. In contrast, immunoreactive IGFBP-4 was observed in the luminal epithelium and the intensity was very strong after ovulation (E1000 h, DI and DII). IGFBP-5 and-6 mRNAs were selectively expressed in circular smooth muscle cells. Hybridization signals were evident over the cycle, but were greatest at estrus. By comparison, IGFBP-5 and-6 proteins were essentially undetectable in these cells except at DII 1100 h when immunostaining was moderate to high. Luminal epithelial cells were weakly positive for IGFBP-5 and-6. However, intense immunostaining was associated with the ciliated border and the luminal fluid juxtaposed to these cells during the cycle. The oocyte-cumulus complexes were immunostained intensely for IGFBP-2,-4,-5 and-6, but their mRNAs were undetectable. The signals were strongest in degenerating cumulus cells suggesting a potential role for these IGFBPs in cumulus apoptosis. These results demonstrate that the estrous cycle is accompanied by major changes in the pattern of expression of IGFBP-2,-4,-5 and-6 in the rat oviduct. We therefore conclude that the regulated production of these particular IGFBPs may be functionally important in modulating IGF activities in the oviduct, oocyte cumulus complexes, and perhaps the preimplantation embryo as well.
ABSTRACT
An intrinsic insulin-like growth factor (IGF) system, complete with IGF ligands, receptors, and biological responses, is present in the rat uterus, where it is thought to regulate uterine homeostasis by autocrine/paracrine mechanisms. It is known that IGF binding proteins (IGFBP) modulate IGF-I and IGF-II action, but very little information is available concerning their cellular localization in the uterus. Therefore, we have employed in situ hybridization to localize IGFBP-1, -2, -3, -4, -5, and -6 mRNAs in the adult rat uterus during the estrous cycle. IGFBP-1 was undetectable in all uteri examined. IGFBP-2 mRNA was localized only in the luminal epithelium of the endometrium. It was abundant during proestrus (P1000 h, P2000 h) and early estrus (E0200 h), but was relatively low at other stages of the cycle. IGFBP-3 mRNA was localized to the stroma cells juxtaposed to the endometrium. A weak signal was detected on estrus morning (E0200 h, E1000 h), but high levels of IGFBP-3 mRNA were observed in the stroma cells on Day 12 of pregnancy. IGFBP-4 mRNA was localized only in the luminal epithelium of the endometrium. It was moderately abundant at diestrus I and II, but the signal was very low or absent at other times in the cycle. IGFBP-5 mRNA was localized in the circular and longitudinal muscle layers of the myometrium. The IGFBP-5 hybridization signal was maximal at diestrus, weak on proestrus, and moderate during estrus. IGFBP-6 mRNA was also expressed in the myometrium. The signal was strong on estrus morning (E0200 h and E1000 h) and low or absent at other times in the cycle. These results provide the first direct evidence that the genes encoding the six IGFBP are expressed in a tissue-specific manner in the adult rat uterus. Equally important, the levels of the mRNA for each IGFBP appear to change throughout the estrous cycle, but not in a parallel fashion. These results support the hypothesis that inducible and tissue-specific expression of IGFBP-2 to -6 may be involved in modulating the activity of the IGF ligands during the proliferative and secretory phases of the uterine cycle.