Bovine granulosa cells express extracellular matrix proteins and their regulators during luteinization in culture

This study investigated the ability of bovine granulosa cells to express and secrete collagen, metalloproteinase (MMP) activity and a tissue inhibitor of metalloproteinase (TIMP-1) during luteinization in vitro. Cells from mature (1-2-mL fluid volume) bovine follicles were cultured over 4 days in serum-free medium. Their luteinization during culture was confirmed by a 10-fold increase in progesterone secretion. Samples of cell extracts, culture media and follicular fluid were subjected to Western blotting to identify secreted proteins and to gelatin zymography to detect enzyme activity. Poly A⁺ RNA, isolated from cells before and after culture, was probed to detect expression of collagen α1(I), collagen α3(IV) and TIMP-1. The results revealed that: (1) the collagen α1(I) subunit gene was expressed in cells before culture but with greater intensity by Day 4 of culture; collagen I protein, on the other hand, was not detectable in culture medium; (2) the collagen α3(IV) subunit gene was expressed at a low level in uncultured cells and could be detected on Day 4 of culture; low amounts of the protein were detected in medium; (3) a 92-kDa band of gelatinase activity (presumed MMP-9) was present in all medium samples, together with bands of unidentified activity; and (5) the TIMP-1 gene was expressed in uncultured cells but its expression increased markedly up to Day 4 of culture. These results show that granulosa luteinization is associated with an increase in the expression of collagen, collagen-degrading enzymes and TIMP-1. Collagen protein, however, may be only poorly synthesized in this culture model. The results suggest that granulosa-derived cells are a likely source of components of the extracellular matrix during post-ovulatory remodelling of early luteal tissue.