Tumor necrosis factor α regulates collagenolytic activity in preovulatory ovine follicles: Relationship to cytokine secretion by the oocyte-cumulus cell complex

Abstract

The pleiotropic cytokine tumor necrosis factor (TNF)-α has been implicated in the mechanism of ovulation. Experiments were designed to test the hypothesis that TNF-α secreted from the oocyte-cumulus cell complex stimulates follicular collagenase production and thereby contributes to ovarian wall degradation and ovulatory rupture. Proestrous ewes were treated with GnRH to synchronize the onset of the gonadotropin surge; ovulation occurs approximately 24 h later. There was an increase in TNF-α (immunoassay) in antral fluid of preovulatory follicles at 18 h after GnRH, which was related to tissue collagenolytic bioactivity (radiolabeled type I substrate digestion by enzymatic extract) and collagen (hydroxyproline) depletion. Intrafollicular injection of TNF-α antibodies at 12 h after GnRH negated the rise in follicular collagenolytic bioactivity (and is known to block ovulation in the sheep). Moreover, collagenase production was enhanced when follicular tissues (0 h GnRH) were incubated (6 h) with recombinant TNF- α; this effect was abolished by the transcriptional inhibitor actinomycin D. Secretion of TNF-α by oocyte-cumulus cell complexes isolated from preovulatory follicles simulated the in vivo circumstance. Immunostaining indicated that TNF-α was confined mainly to the oocyte before GnRH administration, accumulated in cumulus cells during the mid-to-late preovulatory period, and was expended with the imminent approach of ovulation. To our knowledge, this is the first report specifying that up- regulation of collagenase expression is a target mode of TNF-α action in preovulatory follicles. The oocyte-cumulus cell complex is an apparent source of soluble TNF-α.