2,3,7,8-Tetrachlorodibenzo-p-dioxin inhibits steroidogenesis in the rat testis by inhibiting the mobilization of cholesterol to cytochrome P450scc.

Testosterone synthesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats is decreased because pregnenolone production by the testis is inhibited. This inhibition can only be caused by a reduction in the activity of the mitochondrial enzyme which converts cholesterol into pregnenolone (cytochrome P450scc), and/or by an impairment in the multistep process by which luteinizing hormone (LH) stimulates the mobilization of cholesterol to this enzyme. Seven days after rats were treated with 100 micrograms TCDD/kg, testicular cytochrome P450scc activity (assayed with 20 alpha-hydroxycholesterol as substrate) was decreased to 45% of control. If this decrease were responsible for the inhibition of testicular steroidogenesis in vivo, substrate pools for cytochrome P450scc in the testis would be increased. Yet TCDD decreased the amount of cholesterol that was readily available to cytochrome P450scc in isolated testis mitochondria (the reactive cholesterol pool), even when steroidogenesis was maximally stimulated in vivo with the LH analogue human chorionic gonadotropin (hCG). These decreases in substrate pools were not due to a reduction in mitochondrial capacity for reactive cholesterol. We conclude that the 55% decrease in cytochrome P450scc activity is not severe enough to inhibit testicular steroidogenesis in vivo. Instead, TCDD must act by inhibiting the LH-stimulated mobilization of cholesterol to cytochrome P450scc. This conclusion is supported by two observations. First, when pregnenolone formation was blocked by treating rats with the cytochrome P450scc inhibitor aminoglutethimide, TCDD greatly reduced the rate at which hCG caused reactive cholesterol to accumulate in testis mitochondria in vivo. Second, TCDD inhibited both testosterone synthesis and the mobilization of cholesterol to cytochrome P450scc within 1 day. The steroidogenic inhibition does not appear to be due to an LH receptor defect, because TCDD inhibited dibutyryl cAMP- and hCG-stimulated steroid secretion by isolated perfused testes to comparable extents. We conclude that TCDD inhibits testicular steroidogenesis predominantly if not exclusively by inhibiting the mobilization of cholesterol to cytochrome P450scc, and that this inhibition occurs subsequent to cAMP formation.