VARIATION IN THE END-PRODUCTS OF ANDROGEN BIOSYNTHESIS AND METABOLISMDURING POSTNATAL DIFFERENTIATION OF RAT LEYDIG-CELLS

The amount of testosterone (T) secreted by Leydig cells is determined by a balance between T biosynthetic and metabolizing enzyme activities . It has been established that 5 alpha-androstan-3 alpha,17 beta-diol (3 alpha-DIOL) is the predominant androgen secreted by the testes of i mmature rats during days 20-40 postpartum, whereas T is the major andr ogen by day 56. However, the underlying changes in T biosynthetic and metabolizing enzymes during Leydig cell development and their magnitud es have remained unclear. The aim of the present study was to define t he developmental trends for T biosynthetic and metabolizing enzymes in Leydig cells at three distinct stages of pubertal differentiation: me senchymal-like progenitors on day 21, immature Leydig cells on day 35, and adult Leydig cells on day 90. Production rates for precursor andr ogen (androstenedione), T, and 5 alpha-reduced androgens [androsterone (AO) and 3 alpha-DIOL] were measured in progenitor, immature, and adu lt Leydig cells in spent medium after 3 h in vitro. Steady state messe nger RNA (mRNA) levels and enzyme activities of biosynthetic and metab olizing enzymes were measured in fractions of freshly isolated cells a t each of the three stages. Unexpectedly, progenitor cells produced si gnificant amounts of androgen, with basal levels of total androgens (a ndrostenedione, AO, T, and 3 alpha-DIOL) 14 times higher than those of T alone. However, compared with immature and adult Leydig cells, the capacity for steroidogenesis was lower in progenitor cells, with a LH- stimulated production rate for total androgens of 84.33 +/- 8.74 ng/10 (6) cells 3 h (mean +/- SE) US. 330.13 +/- 44.19 in immature Leydig ce lls and 523.23 +/- 67.29 in adult Leydig cells. The predominant androg en produced by progenitor, immature, and adult Leydig cells differed, with AO being released by progenitor cells (72.08 +/- 9.02% of total a ndrogens), 3 alpha-DIOL by immature Leydig cells (73.33 +/- 14.52%), a nd T by adult Leydig cells (74.38 +/- 14.73%). Further examination ind icated that changes in the predominant androgen resulted from differen tial gene expression of T biosynthetic and metabolizing enzymes. Low l evels of type III 17 beta-hydroxysteroid dehydrogenase (17 beta HSD) m RNA and enzyme activity were present in progenitor cells compared with immature and adult Leydig cells. In contrast, levels of type I 5 alph a-reductase (5 alpha R) and 3 alpha-hydroxysteroid dehydrogenase (3 al pha HSD) mRNA and enzyme activities were dramatically lower in adult L eydig cells compared with those in progenitor and immature Leydig cell s. Several T biosynthetic enzymes attained equivalent levels in immatu re and adult Leydig cells, but T was rapidly metabolized in the former to 3 alpha DIOL by high 5 alpha R and 3 alpha HSD activities, which w ere greatly reduced in the latter. Therefore, declines in 5 alpha R an d 3 alpha HSD activities are hypothesized to be a major cause of the a scendancy of T as the predominant androgen end product produced by adu lt Leydig cells. These results indicate that steroidogenic enzyme gene expression is not induced simultaneously, but through sequential chan ges in T biosynthetic and metabolizing enzyme activities, resulting in different androgen end products being secreted by Leydig cells during pubertal development.