Effects of insulin-like growth factor I on steroidogenic enzyme expression levels in mouse Leydig cells

The role of IGF-I in Leydig cell maturation was studied by evaluation of: 1) steady state levels for nine mRNA species expressed specifically in Leydig cells of 35- and 50-d-old IGF-I-null mice and wild-type controls; 2) protein levels for 17alpha-hydroxylase/ C17-20 lyase, cholesterol side-chain cleavage, and type I 5alpha-reductase (5alphaR-1) in Leydig cells by immunocytochemistry; and 3) serum testosterone ( T) and testicular interstitial fluid IGF-I levels. Expression levels of all mRNA species associated with T biosynthesis were lower in the absence of IGF-I stimulation. In contrast, androgen-metabolizing enzyme mRNA species had either normal (3alpha-hydroxysteroid dehydrogenase) or higher expression (5alphaR-1) levels in IGF-I-null mice ( P < 0.05) relative to wild-type controls. None of the mRNA species studied changed developmentally in the mutant, whereas there were increases or decreases between d 35 and 50 in normal controls. Parallel trends were observed for average Leydig cell 5 alpha R-1 immunostaining intensity. T levels in mutants were initially higher during d 14 - 21, equivalent to normal on d 28, and then failed to increase pubertally, remaining at 30% of control levels ( P < 0.01) in 90-d-old adult animals. In normal wild-type mice, interstitial fluid and plasma IGF-I levels were highest (P < 0.05) on d 24, indicating that the action of this growth factor on the testis peaks during pubertal development. These results show that in the absence of IGF-I, there is a failure of adult Leydig cells to mature, and that the reduced capacity for T production is caused by disproportionate expression of T biosynthetic and metabolizing enzymes.