Prolactin (PRL)-dependent rat pre-T Nb2 (Nb2-11) cell lines serve as a useful model for investigation of mechanisms underlying lactogen-mediated suppression of apoptosis. Glucocorticoids, such as dexamethasone (DEX), induce apoptosis in Nb2-11 cells; the addition of PRL abrogates the cytolytic actions of DEX in this model, presumably because of increased expression of survival genes. In the present study, we investigated whether inhibition of DEX-induced apoptosis by PRL in Nb2-T cells was accompanied by altered expression of Bcl-2 family members, mcl-1, bad or bcl-x(L) determined by Northern and immunoblot analysis. The results indicated that a 0.9 kb bcl-x(L) transcript was rapidly induced by PRL. It reached maximal levels within 2 to 4 h (>20-fold) before declining toward basal values. Similar results were obtained in primary cultures of mouse thymocytes exposed to DEX in combination with PRL. In addition to increasing its mRNA expression, PRL also increased Bcl-xL protein levels by 6 h. Moreover, the effect of PRL to increase bcl-x(L) appeared to reflect direct and indirect mechanisms, since it was attenuated by the inhibition of protein synthesis. Results from other experiments suggest that PRL signaling to bcl-x(L) expression was independent of the Jak2/Stat pathway but appeared to require activation of a Src tyrosine kinase. In contrast, while a 1.1 kb mcl-1 transcript was detected in proliferating and quiescent cells, PRL did not alter its expression at either mRNA or protein levels. Moreover, neither bad mRNA nor its protein product were detectable under any of the experimental conditions evaluated. We have concluded that bad and mcl-1 are unlikely candidates for apoptosis regulatory genes modulated by PRL. However, the kinetic pattern of PRL-provoked bcl-x(L) expression is consistent with its playing a role as an apoptosis suppressor in Nb2-T cells and primary cultures of mouse thymocytes exposed to glucocorticoids.
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