N-cadherin-mediated cell contact inhibits granulosa cell apoptosis in a progesterone-independent manner

Previous studies have shown that both progesterone and cell contact inhibit granulosa cell (GC) apoptosis in vitro. Since the progesterone concentration associated with aggregated GCs may be higher than that of single GCs, experiments were conducted to differentiate progesterone's action from that of cell contact. For these studies, GCs were isolated from immature rats. Large GCs were collected after Percoll gradient centrifugation and placed in serum-free culture for 24 h. These studies confirmed that the rate of apoptosis was 2-3 times higher for single GCs than for aggregated GCs. This relationship was observed in the presence of aminoglutethimide, where progesterone concentrations were 3 ng/ml or less. A dose-response studied revealed that a minimum of 100 ng/ml progesterone were required to suppress apoptosis of single GCs. In addition, a single cell contact was shown to be sufficient to suppress apoptosis, with a small nonsteroidogenic GC being as effective as a large steroidogenic GC. Taken together, these data support the concept that cell contact blocks apoptosis in a progesterone-independent manner. GC contact is due to the presence of gap and adhesion-type junctions. To assess which, if either, of these junctions is involved in mediating the antiapoptotic action of cell contact, cocultures were set-up between GCs and R2C cells. Contact with R2C cells inhibits GC apoptosis, but does not result in the formation of functional gap junctions. This demonstrates that gap junctions are not essential to maintain GC viability. Adhesion-type junctions result from a homophilic binding of N-cadherin, which is expressed by both GCs and R2C cells. When this binding is inhibited by treatment with either an antibody to N-cadherin or a synthetic N-cadherin peptide, cell aggregation is attenuated. For those cells that form cell contacts in the presence of these N-cadherin-binding inhibitors, the percentage of apoptotic cells is increased compared to that in controls. These observations suggest that homophilic binding of N-cadherin molecules on the surface membranes of adjacent GCs initiates a signal transduction cascade that ultimately inhibits apoptosis.