HYDROCORTISONE INDUCES THE SYNTHESIS OF ALPHA-2-MACROGLOBULIN BY RAT MAMMARY MYOEPITHELIAL CELLS

The effects of lactogenic hormones on protein secretion by 25.5-G4.2.3 cells, a rat mammary myoepithelial cell line immortalised with a temp erature-sensitive T-antigen, were investigated. Insulin, prolactin, es tradiol and progesterone had no effect but hydrocortisone induced the secretion of two proteins with molecular masses of 175 kDa (p175) and 146 kDa (p146), 10-30-fold and 5-fold respectively. The induction of p 175 and p146 synthesis by hydrocortisone was greater at 39.5-degrees-C than at 33-degrees-C reflecting the increased differentiation of 25.5 -G4.2.3 cells at the higher temperature. Rat mammary epithelial cells did not synthesise p175. After addition of hydrocortisone to 25.5-G4.2 .3 cells, there was a lag phase of 10 h before the synthesis of p175 w as induced. Half-maximal induction of p175 synthesis required a hydroc ortisone concentration of 0.5 muM. p175 was identified as alpha2-macro globulin by N-terminal amino-acid sequence determination and immunopre cipitation with a specific antibody. Hydrocortisone induced a 5-kb alp ha2-macroglobulin-specific mRNA transcript in 25.5-G4.2.3 cells. Myoep ithelial cells are responsible for synthesising the basement membrane around the rapidly expanding mammary alveoli during pregnancy. Myoepit helial cells also secrete metalloproteinases which are probably involv ed in turnover of the basement membrane. We suggest that increased lev els of hydrocortisone during pregnancy induce the synthesis of alpha2- macroglobulin, which is believed to be a potent inhibitor of metallopr oteinases, by rat mammary myoepithelial cells to reduce proteolytic de gradation of the basement membrane.