Rapid, cell surface-initiated, pregenomic androgen actions have been described in various vertebrate cells, but the receptors mediating these actions remain unidentified. We report here the cloning and expression of a cDNA from Atlantic croaker (Micropogonias undulatus) ovaries encoding a 33-kDa, seven-transmembrane protein with binding and signaling characteristics of a membrane androgen receptor that is unrelated to any previously described steroid receptor. Instead, croaker membrane androgen receptor has 81–93% amino acid sequence identity with zinc transporter ZIP9 (SLC39A9) subfamily members, indicating it is a ZIP9 protein. Croaker ZIP9 is expressed in gonadal tissues and in brain and is up-regulated in the ovary by reproductive hormones. Croaker ZIP9 protein is localized to plasma membranes of croaker granulosa cells and human breast cancer (SKBR-3) cells stably transfected with ZIP9. Recombinant croaker ZIP9 has a high affinity (dissociation constant, K_d, 12.7 nM), limited capacity (maximal binding capacity 2.8 nM/mg protein), displaceable, single binding site-specific for androgens, characteristic of steroid receptors. Testosterone activates a stimulatory G protein coupled to ZIP9, resulting in increased cAMP production. Testosterone promotes serum starvation-induced cell death and apoptosis in transfected cells and in croaker ovarian follicle cells that is associated with rapid increases in intracellular free zinc concentrations, suggesting an involvement of zinc in this nonclassical androgen action to promote apoptosis. These responses to testosterone are abrogated by treatment with ZIP9 small interfering RNA. The results provide the first evidence that zinc transporter proteins can function as specific steroid membrane receptors and indicate a previously unrecognized signaling pathway mediated by steroid receptors involving alterations in intracellular zinc.

Several populations of the estuarine killifish, Fundulus heteroclitus, also known as the mummichog, exhibit characteristics of adaptation to priority pollutants. One such population of mummichog inhabits the Elizabeth River in Virginia at the Atlantic Wood site (AW), a U.S. Environmental Protection Agency (EPA) Superfund site heavily contaminated with creosote containing a mixture of polyaromatic hydrocarbons (PAHs). Although PAHs are known to be immunotoxic in experimental animals, resident AW mummichogs seem to thrive. Mummichogs from the AW site and a reference site were subsequently examined over a 2-yr period for total immunoglobin (IgM), as well as circulating antibody levels against 5 ubiquitous marine bacteria. Expression profiles of circulating and lymphoid lysozyme and lymphoid cyclooxygenase-2 (COX-2) were also examined. Compared to relatively high total IgM and specific antibody responses in reference fish, AW mummichogs had lower circulating IgM and lower specific antibody levels against all bacteria examined, however they had higher levels of circulating lysozyme. Lymphoid cells in the AW mummichogs also expressed higher levels of lysozyme, as well as COX-2, which may indicate a state of macrophage activation. Elevated COX-2 levels may be associated with enhanced metabolism of PAHs through cooxidation-peroxidase pathways. Additional studies attempted to immunize AW mummichogs reared in uncontaminated water to compare their antibody responses to that of reference fish. AW mummichogs did not survive 40 d post culture, while reference fish thrived. Our findings suggest that the chemical environment at the AW site may be vicariously enhancing components of innate immunity, probably through oxidative stress pathways, in resident mummichogs, while actively suppressing humoral immune responses.