Altered redox homeostasis in steroid-depleted follicles attenuates hCG regulation of follicular events: Cross-talk between endocrine and IGF axis in maturing oocytes

Abstract

Steroids and insulin-like growth factors (Igfs) are indispensable for folliculogenesis and reproductive fitness in the vertebrate ovary. The intrafollicular redox balance is also of immense importance for ovarian follicles wherein low levels of ROS are being utilized for cell signalling and regulation of gene expression; its excess may interfere with normal physiological processes leading to ovotoxicity. However, the functional relevance of ovarian steroidogenesis in maintaining the follicular microenvironment with coordinated redox homeostasis and intra-ovarian growth factors axis is relatively less understood. Using zebrafish full-grown (FG) ovarian follicles in vitro, our study shows that blocking steroid biosynthesis with anti-steroidal drugs, DL-aminoglutethimide (AG) or Trilostane (Trilo), prevents hCG (LH analogue)-induced StAR expression concomitant with a robust increase in intrafollicular ROS levels. Congruent with heightened intracellular levels of superoxide anions (O2•–) and hydrogen peroxide (H2O2), priming with AG or Trilo abrogates the transcript abundance of major antioxidant enzyme genes (SOD1, SOD2, and CAT) in hCG-stimulated follicles. Significantly, blocking steroidogenesis attenuates transcript abundance of HSP70 but elevates NOX4 expression potentially through ERα-mediated pathway. Importantly, disrupted redox balance in AG/Trilo pre-incubated FG follicles negatively impacts hCG-mediated activation of PKA/CREB signaling and transcriptional activation of igf ligands. Elevated ROS attenuation of antioxidant defense parameters and impaired endocrine and autocrine/paracrine homeostasis converge upon reduced p34cdc2 (Thr-161) phosphorylation, a reliable marker for MPF activation, and resumption of meiotic G2-M1 transition in hCG-treated follicles. Collectively, altered redox homeostasis in steroid-depleted follicles has a significant negative influence on GTH (LH) regulation of follicular events, specifically Igf synthesis, meiotic maturational competence and ovarian fitness.