He H, Herington A C, Roupas P
Centre for Hormone Research and Department of Clinical Biochemistry, Royal Children's Hospital Parkville, 3052, Victoria, Australia.
Endocrine. 1995 Feb;3(2):159-67. doi: 10.1007/BF02990068.
Insulin-like growth factor I (IGF-I) is able to stimulate ovarian granulosa cell steroidogenesis induced by gonadotopins. This gonadotropin-induced potentiation of IGF-I action appears to be due, at least in part, to a gonadotropin-induced increase in membrane-bound IGF-I receptor number and/or decrease in extracellular IGF binding proteins (IGFBPs). Protein kinase C (PKC) has recently been reported to inhibit gonadotropin-induced steroidogenesis in rat ovarian granulosa cells. The role of PKC in the effects of IGF-I on gonadotropin action, however, is unknown. In this study, the effects of phorbol 12-myristate 13-acetate (PMA, a PKC activator) and staurosporine (ST, a PKC inhibitor) on IGF-I action were studied using immature rat ovarian granulosa cells. Activation of PKC by PMA did not affect steroidogenesis or cAMP secretion in cells treated with or without IGF-I. On the other hand, inhibition of PKC by ST alone (10(-9)-10(-7)m) led to an increase in progesterone production in a dose- and time-dependent manner without affecting cAMP secretion. In the presence, but not absence, of ST, IGF-I was able to stimulate progesterone production in the absence of any gonadotropin. PMA decreased ST-induced steroidogenesis and essentially abolished ST-potentiated IGF-I stimulation of steroidogenesis, suggesting the effects of ST on IGF-I action involved a PKC-dependent mechanism. Unlike gonadotropin, ST did not change IGF-I receptor binding. However, ST significantly decreased a major IGF binding protein (IGFBP, ∼30kDa) which is likely to be IGFBP-5, whereas it increased a minor IGFBP (∼24kDa) which is likely to be IGFBP-4. Both effects of ST were dose- and time-dependent. Furthermore, ST inhibited the expression of mRNA for IGFBP-5 suggesting that ST decreased IGFBP-5 levels by inhibiting its transcription and/or decreasing the stability of its mRNA. Interestingly, ST also decreased mRNA levels of IGFBP-4 despite a significant increase in secreted IGFBP-4 levels. The mechanisms involved are not known. Activation of PKC by PMA had no acute effect on these IGFBPs. The regulation by ST of IGFBPs was not antagonized by PMA, and was not affected by PKC-down regulation. Thus, it is likely that ST induces granulosa cell steroidogenesis, potentiates the IGF-I stimulation of steroidogenesis and regulates IGFBP via both PKC-dependent and -independent pathways.
胰岛素样生长因子I（IGF-I）能够刺激促性腺激素诱导的卵巢颗粒细胞类固醇生成。促性腺激素诱导的IGF-I作用增强似乎至少部分归因于促性腺激素诱导的膜结合IGF-I受体数量增加和/或细胞外IGF结合蛋白（IGFBPs）减少。最近有报道称蛋白激酶C（PKC）可抑制大鼠卵巢颗粒细胞中促性腺激素诱导的类固醇生成。然而，PKC在IGF-I对促性腺激素作用的影响中的作用尚不清楚。在本研究中，使用未成熟大鼠卵巢颗粒细胞研究了佛波酯12-肉豆蔻酸酯13-乙酸酯（PMA，一种PKC激活剂）和星形孢菌素（ST，一种PKC抑制剂）对IGF-I作用的影响。PMA激活PKC对用或未用IGF-I处理的细胞中的类固醇生成或cAMP分泌没有影响。另一方面，单独使用ST（10^(-9)-10^(-7)m）抑制PKC会导致孕酮生成以剂量和时间依赖性方式增加，而不影响cAMP分泌。在存在但不是不存在ST的情况下，IGF-I能够在没有任何促性腺激素的情况下刺激孕酮生成。PMA降低了ST诱导的类固醇生成，并基本上消除了ST增强的IGF-I对类固醇生成的刺激，表明ST对IGF-I作用的影响涉及PKC依赖性机制。与促性腺激素不同，ST没有改变IGF-I受体结合。然而，ST显著降低了一种主要的IGF结合蛋白（IGFBP，30kDa），其可能是IGFBP-5，而增加了一种次要的IGFBP（24kDa），其可能是IGFBP-4。ST的这两种作用都是剂量和时间依赖性的。此外，ST抑制了IGFBP-5的mRNA表达，表明ST通过抑制其转录和/或降低其mRNA的稳定性来降低IGFBP-5水平。有趣的是，尽管分泌的IGFBP-4水平显著增加，但ST也降低了IGFBP-4的mRNA水平。其中涉及的机制尚不清楚。PMA激活PKC对这些IGFBPs没有急性影响。ST对IGFBPs的调节不受PMA拮抗，也不受PKC下调影响。因此，ST可能通过PKC依赖性和非依赖性途径诱导颗粒细胞类固醇生成，增强IGF-I对类固醇生成的刺激并调节IGFBP。
