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Center for Biomedical Research, The Population Council, and Rockefeller University (G.M.W., M.P.H.), New York, New York 10021; Division of Veterinary Preclinical Study, University of Glasgow Veterinary School (P.J.O.), Glasgow, Scotland G61 1QH, United Kingdom; Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center (C.C.), Dallas, Texas 75390; and Department of Pharmacology and Therapeutics, McGill University (B.R.), Montréal, Québec, Canada H3G 1Y6
Address all correspondence and requests for reprints to: Matthew P. Hardy, Ph.D., The Population Council, 1230 York Avenue, New York, New York 10021. E-mail: mhardy{at}popcbr.rockefeller.edu.
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 17
-hydroxylase/C17–20 lyase, cholesterol side-chain cleavage, and type I 5-reductase (5R-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 (3-hydroxysteroid dehydrogenase) or higher expression (5R-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 5R-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.
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