Cyclic Nucleotide Phosphodiesterase Inhibitor, 3-Isobutyl-1-Methylxanthine, Induces Cytodifferentiation of Follicular Granulosa Cells Cultured in Serum-Free Medium*

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Abstract
Cultured granulosa cells from intact immature rats produced large amounts of progestins in response to phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (MIX). MIX alone stimulated up to 9 time higher amounts of 20.alpha.-hydroxy-4-pregnen-3-one (20.alpha.-OH-P) when compared with FSH (100 ng/ml)-stimulated steroidogenesis. Combined action of MIX and FSH did not yield more activity than MIX action alone. MIX activity was demonstrable exclusively in serum-free culture medium (Dulbecco modified Eagle''s medium/F-12) supplemented with insulin, transferrin, hydrocortisone and fibronectin. Serum severely inhibited MIX induction of 20.alpha.-OH-P production. Mix also caused dramatic cell shape changes which occurred within 14 h of exposure to the drug. The cells assumed a spherical shape and remained so as long as MIX was maintained in the culture medium. On removal of the drug, the cells respread and acquired the morphology of luteinized cells. In contrast to FSH action, MIX failed to induce the appearance of functional LH receptors. However, similar to FSH effect on immature granulosa cells, MIX successfully induced aromatase enzyme throughout 12 days in culture. MIX alone caused only a minute increase in the accumulation of cAMP. cAMP content in cells induced with FSH (100 ng/ml) was 46 times higher than the cyclic nucleotide levels measured under maximal effective doses of MIX. The discrepancy between the intensive steroidogenic activity of MIX and its inability to substantially raise cAMP content suggests a possible cAMP-independent mechanism for steroid production in the ovarian cells.