Expression and localization of insulin-like growth factor system in corpus luteum during different stages of estrous cycle in water buffaloes (Bubalus bubalis) and the effect of insulin-like growth factor I on production of vascular endothelial growth factor and progesterone in luteal cells cultured in vitro

Abstract

This study investigated the expression and localization of insulin-like growth factor (IGF) system at different stages of buffalo CL and the role of IGF-I in stimulating vascular endothelial growth factor (VEGF) and progesterone (P4) production in cultured luteal cells. The mRNA expression of IGF system, VEGF, steroidogenic acute regulatory protein, P450scc, and hydroxysteroid dehydrogenase (HSD) was investigated by quantitative real-time polymerase chain reaction (PCR). Protein expression of IGF was demonstrated by Western blot and localization by immunohistochemistry. Progesterone and VEGF production was assayed using RIA and ELISA. A relatively high mRNA expression of IGF-I and IGF-II in early, mid- and late luteal phases with immunoreactivity mostly restricted to cytoplasm of large luteal cells indicates their autocrine role, whereas very weak immunoreactivity in endothelial cells during the mid-luteal phase indicates their paracrine role. Insulin-like growth factor receptors, IGF-IR and IGF-IIR, were restricted to large luteal cells with high mRNA and protein expressions in the mid-luteal phase. The significantly higher expression of insulin-like growth factor binding protein (IGFBP)-1, -3, -5, and -6 in the early or mid-luteal phase suggested their stimulatory role, whereas that of IGFBP-2 and -4 in mid-, late, and regressive luteal stages implied their inhibitory role. The mRNA expressions of key steroidogenic factors and VEGF were significantly higher (P < 0.05) when the culture medium was supplemented with 100 ng/mL of IGF-I for 72 hours. Moreover, IGF-I at a dose of 100 ng/mL increased P4 and VEGF production (P < 0.05). It can be concluded that IGF family members via their autocrine and paracrine effect play significant roles in promoting angiogenesis through the production of VEGF in luteal cells and steroid synthesis through the production of key steroidogenic factors.