Induction of alpha-caveolin-1 (alphaCAV1) expression in bovine granulosa cells in response to an ovulatory dose of human chorionic gonadotropin.

Caveolins are implicated in endocytosis, cholesterol trafficking and signal transduction. A cDNA fragment corresponding to caveolin-1 (CAV1) was identified in a mRNA profiling expression study in bovine granulosa cells (GC) following human chorionic gonadotropin (hCG)-induced ovulation. Thus, we have characterized CAV1 cDNA and studied its spatio-temporal expression pattern in bovine ovarian follicles. The full-length bovine alphaCAV1 cDNA was cloned and encodes a putative 22 kDa protein. Expression of alphaCAV1 was studied in bovine GC obtained from follicles at different developmental stages: small follicles (SF: 2-4 mm), dominant follicles (DF), ovulatory follicles (OF: 24 hr post-hCG), and corpus luteum (CL). Semiquantitative RT-PCR analysis showed a 6.5-fold increase in alphaCAV1 mRNA in GC of OF versus DF (P < 0.0001), whereas CAV2 mRNA was increased by only twofold (P < 0.0007). Temporal expression of alphaCAV1 mRNA from OF recovered at 0, 6, 12, 18, and 24 hr after hCG injection showed an 8.5-fold increase of alphaCAV1 mRNA after 24 hr compared to 0 hr (P < 0.0018) whereas no significant variation was detected for CAV2. Immunoblot demonstrated an initial increase in alphaCAV1 protein level 12 hr post-hCG, reaching a maximum at 24 hr. Immunohistochemical localization of CAV1 was observed in GC of OF isolated 18 and 24 hr after hCG injection, whereas no signal was detected in GC of DF and SF. The induction of alphaCAV1 in GC of OF suggests that alphaCAV1 likely contributes to control the increase in membrane signaling that occurs at the time of ovulation and luteinization.