Glucocorticoid-prolactin interactions in Nb2 lymphoma cells: antiproliferative versus anticytolytic effects

The interaction between the immunosuppressive effects of glucocorticoids and the mitogenic effects of prolactin (PRL) were examined in Nb2 lymphoma cells, a pre-T cell line. The synthetic glucocorticoid, dexamethasone (Dex), caused a concentration-dependent (6.25-200 nM) inhibition of basal and ovine PRL (oPRL)-stimulated Nb2 cell proliferation. Although Dex was antiproliferative, the steroid had no effect on cell viability in the presence of PRL. However, when PRL was omitted from the medium, Dex increased the proportion of dead Nb2 cells by 24 hr in a concentration (25-200 nM)-dependent fashion without affecting total cell number. The antiproliferative and cytolytic effects of Dex were mimicked by other corticosteroids (cortisol, corticosterone, aldosterone, and deoxycorticosterone) in the expected order of glucocorticoid potency, but not by other steroids (17-beta-estradiol, progesterone, testosterone, 5-alpha-dihydrotestosterone, and dehydroepiandrosterone) or triiodothyronine. In addition, the antiproliferative and cytolytic effects of glucocorticoids were antagonized by the glucocorticoid receptor antagonist RU 486. Since corticosteroid-induced cytolysis was apparent only in the absence of mitogen, the anticytolytic effects of oPRL were tested. In the presence of Dex (100 nM), oPRL (25-1600 pg/ml) caused a concentration-dependent inhibition of cytolysis without changing cell number. Other lactogenic hormones (human growth hormone, human placental lactogen, rat PRL), but not trophic nonlactogenic hormones (rat growth hormone, human chorionic gonadotropin, ACTH), also inhibited Dex (100 nM)-induced cytolysis. Agarose gel electrophoresis of DNA extracted from Nb2 cells revealed that within 12 hr, 100 nM Dex induced DNA fragmentation, indicative of programmed cell death or apoptosis. Coincubation of cells with Dex and oPRL (1 ng/ml) inhibited Dex-induced fragmentation of Nb2 cell genomic DNA. These studies reveal a complex interaction between glucocorticoids and PRL in Nb2 cells. Although a glucocorticoid receptor-mediated antiproliferative effect is evident, PRL (at concentrations that usually stimulate cell proliferation) has the capacity to protect the cell against glucocorticoid-receptor-mediated induction of apoptosis.