Glucocorticoids are widely used for the treatment of malignancy-associated hypercalcemia to delay the occurrence of an escape phenomenon inherent in calcitonin therapy. Using parathyroid hormone-related protein (PTHrP)-producing squamous carcinoma cells (T3M-1 and EC-GI) established in our laboratory, we investigated the in vitro effects of glucocorticoids and calcitonin on PTHrP mRNA expression in the cells and release of PTHrP into the culture medium. The PTHrP gene was constitutively expressed in the logarithmic growth phase in both squamous carcinoma cell lines. When these cells became superconfluent, PTHrP mRNA expression was greatly diminished in T3M-1 cells but was not distinctly diminished in EC-GI cells. Hydrocortisone inhibited the PTHrP mRNA expression in T3M-1 cells and EC-GI cells in a dose-dependent manner. In accordance with the decreased expression of PTHrP mRNA, the release of immunoreactive as well as bioactive PTHrP also decreased in the conditioned medium of glucocorticoid-treated cells. The minimal effective concentration of prednisolone was about 10(-7) M, which is readily attainable in the serum of patients treated with the agent. Calcitonin and indomethacin did not affect the PTHrP mRNA expression or PTHrP release into the medium. Calcitonin did not modulate the hydrocortisone-induced inhibition of PTHrP production. These in vitro findings suggest that the combined use of glucocorticoids and calcitonin plays a beneficial role in the treatment of malignancy-associated hypercalcemia, since the steroid hormone can suppress PTHrP mRNA expression and release of bioactive PTHrP in certain PTHrP-producing tumors.