Rapid induction of pim-1 expression by prolactin and interleukin-2 in rat Nb2 lymphoma cells

The lactogen-dependent Nb2 lymphoma line (Nb2-11) represents a useful pre-T cell model for investigation of early molecular events coupled to PRL-stimulated cell cycle progression. Expression of pim-1, a protooncogene that encodes a conserved cytosolic serine/threonine protein kinase, is rapidly induced in hematopoietic cells upon mitogen stimulation and is thought to be important for lymphocyte activation. The present study was conducted to determine whether mitogen stimulation in Nb2-11 or lactogen-independent Nb2-SFJCD1 cells provokes pim-1 gene expression. The pim-1 transcript was undetectable in control growth-arrested Nb2-11 cultures; however, PRL rapidly stimulated its expression in a biphasic manner. Peak expression occurred within 2-4 h (40-fold) and was followed by a second elevation at 12 h. The effect of PRL and IL-2 to induce pim-1 at 2 h was concentration dependent and not inhibited by cycloheximide. In Nb2-SFJCD1 cells, pim-1 messenger RNA was expressed in control cultures and augmented by PRL stimulation. Results from stability studies indicated that the t1/2 values for the pim-1 transcript were 79 and 81 min in PRL-stimulated Nb2-11 cells at 2 and 12 h. However, in the lactogen-treated Nb2-SFJCD1 line, it was nearly 3-fold more stable (219 min) at 2 h compared to that determined at either 12 h or in unstimulated cultures. In other experiments, PRL-stimulated expression of the pim-1 protein was evaluated in [35S]methionine-labeled cells by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In Nb2-11 cells, enhanced [35S]pim-1 expression paralleled its messenger RNA transcription through 8 h. Elevated [35S]pim-1 was detected within 1 h and peaked by 2-4 h. Therefore, pim-1 represents an immediate early gene induced by PRL stimulation in Nb2-11 cells. Its initial peak of transcription occurs early during G1 cell cycle progression, whereas a second elevation is coincident with the G1/S transition. These results demonstrate that mitogen-induced expression of pim-1 is a rapid event in Nb2 lymphoma cells and suggest that it may be associated with cell cycle progression.