Inhibition of adrenocortical steroidogenesis by alpha 2-macroglobulin is caused by associated transforming growth...

alpha 2-Macroglobulin (alpha 2M) is the major protein secreted by bovine adrenocortical cells in primary culture and its synthesis is stimulated by transforming growth factor beta (TGF beta). We investigated here the effects of alpha 2M on adrenocortical steroidogenesis. We observed that commercial preparations of bovine plasma alpha 2M were able to mimic the inhibitory action of TGF beta on adrenocortical cortisol production, with the same specificity of action directed at the steroid 17 alpha-hydroxylation step. This inhibition was time-dependent and dose-dependent (50% inhibition observed with 2 mg/ml alpha 2M). Acid/ethanol extracts of alpha 2M appeared to retain the full inhibitory activity of alpha 2M. Anti-TGF beta antibodies could reverse the inhibition caused by the acid/ethanol extract but not that caused by native alpha 2M. Taken together, these results indicate that the inhibition of adrenocortical steroidogenesis induced by alpha 2M is caused by associated TGF beta. We estimated that 2 mg of alpha 2M contained approximately 0.1 ng of TGF beta, corresponding to a molar ratio of 1/700,000 between TGF beta and alpha 2M. These results also clearly indicate that the alpha 2M-TGF beta complexes are biologically active on adrenocortical cells, suggesting that these cells possess the enzymatic equipment that can activate the latent alpha 2M-TGF beta complexes.