Glucocorticoids Coordinately Disrupt a Transforming Growth Factor α Autocrine Loop and Suppress the Growth of 13762NF-derived Con8 Rat Mammary Adenocarcinoma Cells

Abstract We have demonstrated previously that the synthetic glucocorticold dexamethasone suppresses the growth of Con8 rat mammary tumor cells, which are derived from the 13762NF transplantable, hormone-responsive rat mammary adenocarcinoma. Dexamethasone inhibited [ 3 H]thymidine incorporation into Con8 cells at high cell density under both serum and serum-free conditions. Fractionation in nonreducing sodium dodecyl sulfate-polyacrylamide gels of proteins secreted from dexamethasone-treated and untreated Con8 mammary tumor cells revealed two size classes of glucocorticoid 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 mitogens restimulated the growth of glucocorticoid-suppressed Con8 cells suggesting that they can act in an autocrine fashion. The smaller mitogen was identified as transforming growth factor α (TGF-α) since this activity competed with 125 I-epidermal growth factor (EGF) for EGF receptor binding and was selectively immunodepleted with monoclonal TGF-α antibodies but not with EGF antibodies. Western blots and radioreceptor assay of Con8-secreted proteins revealed that glucocorticoids inhibited the production of a M r 5500 immunoreactive TGF-α protein by 10-fold. Consistent with a steroid effect on the level of TGF-α production, rather than on its activity, the specific mitogenic activities of the TGF-αs secreted by dexamethasone-treated and untreated Con8 cells were identical to that of recombinant human TGF-α. Treatment of intact cells with suramin, which dissociates ligand-receptor complexes, revealed that the EGF receptor-mediated mitogenic response is functional in both glucocorticoid-treated and untreated cells. Taken together, our results demonstrate that glucocorticoids suppress Con8 mammary tumor cell growth and disrupt a potential TGF-α autocrine loop which results in a dramatic reduction in the level of extracellular TGF-α.