Species-Specific Mechanism in Rat Leydig Cell Tumorigenesis by Procymidone

To clarify the mechanism of species difference in the induction of testicular interstitial cell tumor (ICT, Leydig cell tumor) between rats and mice, male Sprague-Dawley rats and ICR mice were fed procymidone at dietary concentrations of 700, 2000 or 6000 ppm and 1000, 5000, or 10,000 ppm, respectively, for 3 months. The Leydig cell functions were evaluated by serum testosterone and luteinizing hormone (LH) levels, testosterone levels in the testis, LH levels in the pituitary, the capacity of the testis to respond to gonadotropin stimulation, i.e., the production of testosterone in vitro, and by the testicular binding of labeled human chorionic gonadotropin (hCG). Measurement of testosterone and LH levels in rat serum, the testis, or the pituitary showed that both hormones were enhanced throughout the 3-month treatment period. The hypergonadotropism was associated with the increase of interstitial cell response to hCG in vitro for up to 3 months. As with rats, both serum and pituitary LH were increased in mice at 4 weeks but not at 13 weeks. However, in contrast to rats, no significant increase in testosterone was observed in mice either in vivo or ex vivo during the course of the study. This suggests a difference between the rat and mouse in the response of the Leydig cell to the LH stimulation associated with procymidone administration. These differences in the response of interstitial cells to procymidone may be the basis for the distinct species responses to procymidone-induced Leydig cell tumorigenesis. The sustained response of the Leydig cells to stimulation in the rat results in chronic hyperplasia and subsequent benign tumor formation, while the attenuated response of Leydig cells in the mouse is associated with neither hyperplasia nor neoplasia.