To better understand the multiple hormonal control of the expression of apolipoprotein D (apo-D) and gross cystic disease fluid protein-15 (GCDFP-15, also designated prolactin-inducible protein), which are 2 major proteins found in benign breast-disease fluid, we investigated their regulation by interleukin-1 alpha (IL-1 alpha) in the presence or absence of steroid hormones in ZR-75-1 human breast cancer cells. Exposure of these cells to IL-1 alpha decreased basal cell proliferation by half and markedly reduced the mitogenic action of 17 beta-estradiol (E2), the half-maximal inhibitory effect being exerted at 1.5 pM. In parallel, IL-1 alpha stimulated apo-D and GCDFP-15 secretion with a similar potency. The antiproliferative effect of IL-1 alpha was additive to the inhibition of cell proliferation caused by dihydrotestosterone (DHT) or the glucocorticoid dexamethasone (DEX). In parallel, IL-1 alpha-induced stimulation of apo-D and GCDFP-15 secretion was additive to that exerted by DHT or DEX. The sensitivity of the apo-D and GCDFP-15 responses to the stimulatory action of DHT or DEX was not changed by the presence of IL-1 alpha. IL-1 alpha also increased apo-D and GCDFP-15 mRNA levels. The present findings demonstrate the potent stimulatory effect of IL-1 alpha on basal as well as androgen- and glucocorticoid-induced apo-D and GCDFP-15 expression. The present data strongly suggest that IL-1 alpha and steroids may modulate the secretion of these 2 proteins through different transduction pathways.