Testosterone regulates mitochondrial aspartate aminotransferase gene expression in rat ventral prostate

Abstract

One of the physiological functions of the rat ventral prostate, like human prostate, is to secrete an extraordinarily high quantity of citrate. This unique characteristic is under the influence of testicular androgen. The continuous secretion of citrate from prostate results in loss of a 6-carbon compound from the metabolic pool which must be replenished. Citrate is synthesized from the concentration of acetyl-CoA and oxaloacetate. Acetyl-CoA may come from various sources. However, recent studies indicated that the possible available source for oxaloacetate is the transamination of aspartate. Mitochondrial aspartate amino-transferase (mAAT) is an important enzyme which regulates citrate production in rat ventral prostate. Testosterone, as a major testicular androgen, stimulates citrate production and mAAT activity in a similar pattern. The hypothesis in this dissertation is that the ability of testosterone to increase citrate production and mAAT activity is the result of stimulation of mAAT gene expression. To verify this hypothesis, numerous in vivo and in vitro studies were utilized to analyze the steady-state level of mAAT mRNA in response to testosterone depletion and repletion. The rats were castrated, followed by testosterone or oil vehicle injection. Twenty-four (or forty-eight) hours later all animals were killed, prostates removed, RNA and nuclei isolated. In vitro studies were performed using primary cultured pig prostate cells. The quantity of mAAT mRNA was measured using northern hybridization. The rates of transcription and degradation of mAAT mRNA were determined by in vitro transcription assay and pulse chase labeling assay, respectively. Results show that castration caused a significant decrease in the content of mAAT mRNA and the transcription rate of the mAAT gene. However, testosterone administration reversed these hormonal depletion effects. Moreover, testosterone administration also prolonged the half life of mAAT mRNA. These results support the hypothesis that one of the major physiological functions of testosterone is to stimulate mAAT mRNA gene expression, which in turn enhances citrate production in rat ventral prostate. Furthermore, these results confirm that the increase in the steady-state level of mAAT mRNA in response to testosterone administration results from stimulation of mAAT gene transcription and inhibition of mAAT mRNA degradation.