Effects of hypoxia exposure on apoptosis and expression of membrane steroid receptors, ZIP9, mPRα, and GPER in Atlantic croaker ovaries

Hypoxia exposure causes endocrine disruption in croaker resulting in impairment of ovarian growth and oocyte development, but the effects of hypoxia on non-classical steroid actions in fish ovaries in vivo remain largely unknown. Membrane androgen receptor (ZIP9) enhances apoptosis of cultured Atlantic croaker ovarian follicle cells via upregulation of p53 and Bax, and increased caspase 3 activity through non-classical steroid signaling. Conversely, apoptosis is inhibited in these cells by non-classical steroid mechanisms through membrane progestin receptor alpha (mPRα) and G protein-coupled estrogen receptor (GPER). Apoptosis and the expression of ZIP9, mPRα, and GPER were investigated in ovaries of croakers that had been exposed to normoxia (7.0 mg DO/L) or hypoxia (2.0 mg DO/L) for 6 weeks during their period of gonadal recrudescence. The proportion of tertiary yolk stage oocytes was decreased, and the proportions of atretic and apoptotic ovarian follicles were increased in ovaries from hypoxia-exposed fish compared to controls. Ovarian expression of all three receptors was altered after hypoxia exposure. Expression of mPRα and GPER mRNA and protein levels were decreased after hypoxia exposure, consistent with a decline in anti-apoptosis. In contrast, ZIP9 mRNA and protein levels were upregulated 4-fold in hypoxia-exposed fish compared to normoxic controls. The enhanced apoptosis in ovaries of hypoxia-exposed fish was associated with increased caspase-3/7 activity and elevated expression of the pro-apoptotic genes, Bax and p53. The finding that ZIP9 expression was increased in ovaries of croaker exposed to hypoxia provides in vivo evidence supporting ZIP9's proposed function in mediating ovarian follicle cell apoptosis.