Transforming growth factor-beta stimulates the expression of alpha 2-macroglobulin by cultured bovine adrenocortical cells.

Abstract

Adrenocortical cell major secreted protein was purified from the conditioned medium of primary cultures of bovine adrenocortical (BAC) cells. Immunochemical analysis and N-terminal sequencing of the purified protein identified it to alpha 2-macroglobulin (alpha 2-M). It appeared that 15 out of the 17 N-terminal amino acids were conserved between adrenocortical cell major secreted protein and human alpha 2-M. Study of alpha 2-M production by BAC cells revealed that its secretion was stimulated severalfold by transforming growth factor-beta 1 (TGF-beta 1). The stimulation occurred in a time-dependent (reaching a plateau at 24 h) and dose-dependent (ED50 = 0.1 ng/ml TGF-beta 1) manner. It was blocked when BAC cells were exposed to 5,6-dichlorobenzimidazole riboside, a potent inhibitor of RNA polymerase II, suggesting that TGF-beta 1 acts as an activator of alpha 2-M gene expression at the transcriptional level. Northern blot analysis confirmed that the alpha 2-M mRNA level was increased (4-fold) in BAC cells following TGF-beta 1 treatment. TGF-beta 2, TGF-beta 1,2, basic fibroblast growth factor, and angiotensin II also appeared able to stimulate alpha 2-M secretion in BAC cells, whereas adrenocorticotropin was strongly inhibitory. Given the previous reports that TGF-beta 1 is a potent inhibitor of adrenocortical steroidogenesis (Feige J.J., Cochet, C., Rainey, W.E., Madani, C., and Chambaz, E. M. (1987) J. Biol. Chem. 262, 13491-13495) and that alpha 2-M is a TGF-beta 1-binding protein, these observations suggest that alpha 2-M may play an important role in conjunction with hormones and growth factors in the homeostatic regulation of adrenocortical functions.