A growing body of information now supports the existence of a complete intraovarian insulin-like growth Factor I (IGF-I) systemreplete with ligands, receptors, and binding protein(s). However, studiesconcerned with the regulation of ovarian IGF-I gene expression remainscarce. It was thus the objective of this communication to evaluate theexpression of the IGF-I gene in the immature rat ovary under in vitroconditions. Whole ovarian dispersates or isolated granulosa cells werecultured for up to 96 h under serum-free conditions in the absence orpresence of the indicated experimental agents. Extracted total RNAwas subjected to a sensitive solution hybridization/RNase protectionassay using32P-labeled rat IGF-I and/or type I IGF receptor antisenseRNA probes. Cultured in the absence or presence of FSH (100 ng/ml), whole ovarian dispersates (or isolated granulosa cells) displayed time-dependent (FSH-independent) decrements in the relative abundanceof IGF-I transcripts apparent as early as 3 h after the onset of culture.No evidence of recovery was apparent by 96 h of culture. The apparentlack of an FSH effect did not reflect diminished biopotency as attestedto by the ability of the hormone to promote time-dependent incrementsin the accumulation of progesterone. Importantly, the apparent decrease in ovarian IGF-I gene expression proved to be IGF-I specific in that type I IGF receptor transcripts displayed a substantial and sustained (for up to 96 h) FSH-independent increase beginning at the 24-h time point. At no point were IGF-II transcripts detected. The apparent decrease in the expression of IGF-I did not reflect the lack ofextracellular matrix support in that neither laminin, collagen, norwhole serum supported sustained ovarian IGF-I gene expression. Treatment of whole ovarian dispersates with pharmacological concentrationsof either insulin (1 μg/ml) or dexamethasone (10-7 m) did not reversethe decline in IGF-I gene expression. Importantly, however, the combined application of both insulin and dexamethasone resulted in virtually complete preservation of IGF-I gene expression, the relativeabundance of the corresponding transcripts proving uniform throughout. Taken together, these in vitro observations reveal irreversible(FSH-independent) decrements in ovarian IGF-I (but not type I IGFreceptor) gene expression, the preservation of which required theconcurrent provision of both insulin and dexamethasone.
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