GLUCOCORTICOIDS COORDINATELY DISRUPT A TRANSFORMING GROWTH FACTOR-ALPHA AUTOCRINE LOOP AND SUPPRESS THE GROWTH OF 13762NF-DERIVED CON8 RAT MAMMARY ADENOCARCINOMA CELLS

We have demonstrated previously that the synthetic glucocorticoid dexa methasone suppresses the growth of Con8 rat mammary tumor cells, which are derived from the 13762NF transplantable, hormone-responsive rat m ammary adenocarcinoma. Dexamethasone inhibited [H-3]thymidine incorpor ation into Con8 cells at high cell density under both serum and serum- free conditions. Fractionation in nonreducing sodium dodecyl sulfate-p olyacrylamide gels of proteins secreted from dexamethasone-treated and untreated Con8 mammary tumor cells revealed two size classes of gluco corticoid inhibited mitogenic activities; a larger M(r) 27,000-33,000 and a smaller M(r) 5,000-12,000 activity. Both size classes of mitogen s restimulated the growth of glucocorticoid-suppressed Con8 cells sugg esting that they can act in an autocrine fashion. The smaller mitogen was identified as transforming growth factor alpha (TGF-alpha) since t his activity competed with I-125-epidermal growth factor (EGF) for EGF receptor binding and was selectively immunodepleted with monoclonal T GF-alpha antibodies but not with EGF antibodies. Western blots and rad ioreceptor assay of Con8-secreted proteins revealed that glucocorticoi ds inhibited the production of a M(r) 5500 immunoreactive TGF-alpha pr otein by 10-fold. Consistent with a steroid effect on the level of TGF -alpha production, rather than on its activity, the specific mitogenic activities of the TGF-alphas secreted by dexamethasone-treated and un treated Con8 cells were identical to that of recombinant human TGF-alp ha. Treatment of intact cells with suramin, which dissociates ligand-r eceptor complexes, revealed that the EGF receptor-mediated mitogenic r esponse is functional in both glucocorticoid-treated and untreated cel ls. Taken together, our results demonstrate that glucocorticoids suppr ess Con8 mammary tumor cell growth and disrupt a potential TGF-alpha a utocrine loop which results in a dramatic reduction in the level of ex tracellular TGF-alpha.