High-affinity L-aspartate transporter in prostate epithelial cells that is regulated by testosterone

The prostate gland produces and secretes extraordinarily high levels of citrate. Studies with rat ventral prostate (VP) have demonstrated that aspartate can serve as a four-carbon source of oxalacetate in the synthesis of citrate. To achieve this, prostate secretory epithelial cells must contain a transport system for the active uptake of aspartate from circulation. The present studies with VP epithelial cells confirm the existence of a Na(+)-dependent high-affinity L-aspartate transporter. The transporter has an optimal pH approximately 7.5 and is temperature dependent. It appears to be an anionic amino acid transporter capable of transporting L-glutamate but not basic or neutral amino acids. The transporter is inhibited by ATPase inhibitors, thereby indicating its dependency on a Na+ gradient. The characteristics of the high-affinity L-aspartate transporter are consistent with its operation at the basilar membrane for the transport of circulating aspartate into the cell. Castration (24 hr) resulted in a significant decrease in the ability of VP epithelial cells to transport L-aspartate. The administration of testosterone to castrated rats completely restored L-aspartate transport. In addition, in vitro testosterone addition (10(-8) M for 30 min) to isolated prostate epithelial cells markedly increased L-aspartate transport. Both cycloheximide and actinomycin inhibited the testosterone effect. The studies reveal that testosterone is a regulator of this Na(+)-dependent high-affinity L-aspartate transporter. The mechanism of this testosterone effect appears to involve both RNA and protein synthesis. We now have a model system to elucidate this novel effect of testosterone.