Inhibition of Hormone-Induced Steroidogenesis during Cell Proliferation in Serum-Free Cultures of Rat Granulosa Cells*

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Abstract
Long term cultures of rat granulosa cells were grown in serum-free medium, consisting of Dulbecco''s modified Eagle''s medium mixed 1:1 with Ham''s nutrient F-12 medium and supplemented with insulin, transferrin, hydrocortisone and fibronectin (4F medium). In sparse cultures (104 cells/cm2), the granulosa cells were steriodogenically responsive to ovine FSH (NIADDK-oFSH-15) during days 1-2 and 10-14 (responsive periods). The major steroids produced were 20.alpha.-hydroxyprogesterone (20.alpha.-OH-P) and 5.alpha.-pregnane-3.alpha.,20.alpha.-diol (pregnanediol). However, as of day 3, the cells gradually lost their steriodogenic responsiveness which was inhibited by 88% at day 7 (refractory period). Nevertheless, from day 8 onward, the cells regained their responsiveness which was fully restored at day 12. The transient loss of responsiveness was uniquely associated with progestin biosynthesis, since FSH-induced aromatase activity declined to background levels within 12 days and was never restored again. The loss of progestin responsiveness was not due to lack of cAMP because FSH induced increasing levels of cAMP accumulation, reaching maximal values on day 7 in culture. The onset of the refractory period occurred concomitantly with the entry of the cultured cells into a synchronous proliferation phase, during which the cell population doubled. Thereafter, as DNA synthesis ceased, the cells regained their steroidogenic responsiveness. A deliberate arrest of cell replication, in the presence of excess thymidine or in high density cultures, prevented the temporal loss of activity. The data presented favor the notion that cell proliferation and expression of differentiated functions are inversely related. Apparently, growth-related processes suppress steroidogenesis by an as yet unknown mechanism.