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 [3H]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 Mr 27,000–33,000 and a smaller Mr 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 125I-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 Mr 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.

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This work was supported by USPHS Grant CA-05388 awarded by the National Cancer Institute and in part by funds awarded from the Lucille P. Markey Program in Biomolecular Structure and Design.

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