Contact-dependent cell interactions determine hormone responsiveness and desensitization in rat granulosa cells.
The maintenance of associations between granulosa cells (GCs) is necessary for FSH-stimulated induction of LH receptors. In cultures in which these associations have been disrupted, FSH fails to induce LH receptors. As FSH exerts its action in GCs via cAMP, we have examined if the aggregation state of GCs plays a role in modulating FSH-stimulated cAMP production. GCs were obtained from the ovaries of diethylstilbestrol-primed immature rats. Cells were prepared as aggregate or dispersed populations by isolating GCs in either the presence or absence of Ca2+. Nonviable cells were removed by a brief exposure to trypsin. We have shown previously that trypsin treatment in the absence of Ca2+ removes a class of cell adhesion molecules, termed cadherins, from the plasma membranes of GCs. Hence, the dispersed GCs are incapable of reaggregating. Dispersed or aggregate GC preparations were incubated with different doses of human FSH (0-1 microg) for 0-60 min in the presence of isobutylmethylxanthine, a phosphodiesterase inhibitor. Incubations were terminated, and the cAMP accumulated was measured using a specific RIA. As desensitization to hormonal stimuli is a characteristic property of many G protein-coupled response systems, cAMP production of cell aggregates and dispersed cells in response to a repeated stimulation with FSH was assessed. Our results indicate that aggregate GCs have a significantly attenuated cAMP response to all doses of FSH compared with dispersed GC preparations. Changing cell densities did not alter the nature of these responses, indicating that nonspecific cell interactions were not responsible for this difference. The number of FSH receptors and their affinity were unaltered in the two cell preparations. Cholera toxin- and forskolin-stimulated cAMP production were similar in the two preparations, demonstrating that the changes in responsiveness did not arise from alterations in G protein activation or adenylate cyclase activity. Only the aggregate GCs could be desensitized. The dispersed cells displayed undiminished cAMP responsiveness to a second FSH stimulation. Finally, culture of the GC preparations with cholera toxin induced LH receptors in GC aggregates only. LH receptor induction in dispersed cell cultures required the addition of estradiol. These results indicate that contact-dependent cell interactions can modulate GC cAMP production in response to FSH. cAMP responses, however, were not the sole determinant of cell differentiation, as assessed by LH receptor induction. We speculate that cell-cell interactions within the follicular epithelium are important determinants for cell differentiation leading to follicle selection for ovulation or atresia.