The effect of transforming growth factor-β (TGF-β) on relaxin release by porcine large luteal cells (LLC) was examined by use of a reverse haemolytic plaque assay. In this assay, mixed luteal cells were co-cultured in monolayers with protein A-coupled sheep erythrocytes. In the presence of complement and porcine relaxin antiserum, a zone of haemolysis (a plaque) developed around relaxin-releasing LLCs. The rate of plaque development in time-course experiments and the average size of plaque areas were used to monitor the rate of relaxin release and cumulative amounts of hormone respectively. Monolayers were bathed in medium containing TGF-β alone, or in the co-presence of a stimulatory secretagogue (prostaglandin E2; PGE2).
Exposure of luteal cell-containing monolayers to TGF-β (1 ng/1–100 μg/l) elicited a dose-related inhibition in the rate of basal relaxin release. Minimal and maximal concentrations were approximately 10 ng/l and 10 μg/l respectively. Treatment with 1 μg TGF-β/l reduced the cumulative amount of relaxin released to 63 ± 6% of control values (mean ± s.d., P < 0·05, n = 6; averaged over the whole course of the experimental incubation). Exposure of monolayers treated with TGF-β to the relaxin-stimulatory secretagogue PGE2 (0·1 μmol) resulted in a significant (P < 0·05) increase in the amount of relaxin released by TGF-β-suppressed LLCs, and restored rates of hormone release to control levels. This is evidence that TGF-β and PGE2 interact antagonistically in the modulation of relaxin. The effect of TGF-β was strictly time-dependent. A period of at least 16 h of treatment with TGF-β was required to induce an inhibitory effect on relaxin.
These results implicate TGF-β as a novel inhibitor of relaxin release, and the presence of TGF-β in porcine luteal tissue suggests that such regulation may be achieved via paracrine or autocrine routes. It is possible that TGF-β interacts functionally during luteal life with other secretagogues to achieve integrated control of hormone release.
Journal of Endocrinology (1992) 135, 543–550
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