Membrane Androgen Receptor ZIP9 Induces Croaker Ovarian Cell Apoptosis via Stimulatory G Protein Alpha Subunit and MAP Kinase Signaling
Recent studies show that androgen-induced apoptosis in Atlantic croaker primary granulosa and theca (G/T) cells and in human breast and prostate cancer cell lines is mediated by the membrane androgen receptor ZIP9, which belongs to the SLC39A zinc transporter family. However, the apoptotic signaling pathways remain unclear because ZIP9 activates an inhibitory G protein in human cancer cells, whereas recombinant croaker ZIP9 activates a stimulatory G protein (Gs) in transfected cancer cells. We investigated androgen-dependent apoptotic pathways to identify the signaling pathways regulated through wild-type croaker ZIP9 in ovarian follicle cells. We show that the ZIP9-mediated apoptotic signaling pathway in croaker G/T cells shares several proapoptotic members with those in human cancer cells, but is activated through a Gsα subunit-dependent pathway. Testosterone treatment of croaker G/T cells increased intracellular zinc levels, mitogen-activated protein (MAP) kinase activity, caspase 3 activity, messenger RNA levels of proapoptotic members Bax, p53, and c-Jun N-terminal kinase, and the incidence of apoptosis, similar to findings in mammalian cancer cells, but also increased cyclic adenosine monophosphate concentrations. Transfection with small interfering RNA targeting croaker ZIP9 blocked testosterone-induced increase in bax, p53, and jnk expression. Testosterone-induced apoptosis and caspase 3 activation depended on the presence of extracellular zinc and were effectively blocked with cotreatment of inhibitors of the Gsα subunit, adenylyl cyclase, protein kinase A, and MAP kinase (Erk1/2) activation. These results indicate that ZIP9-mediated testosterone signaling in croaker G/T cells involves multiple pathways, some of which differ from those activated through ZIP9 in human cancer cells even though a similar apoptotic response is observed.