RAPID INDUCTION OF PIM-1 EXPRESSION BY PROLACTIN AND INTERLEUKIN-2 INRAT 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 t o PRL-stimulated cell cycle progression. Expression of pim-1, a protoo ncogene that encodes a conserved cytosolic serine/threonine protein ki nase, is rapidly induced in hematopoietic cells upon mitogen stimulati on and is thought to be important for lymphocyte activation. The prese nt study was conducted to determine whether mitogen stimulation in Nb2 -11 or lactogen-independent Nb2-SFSCD1 cells provokes pim-1 gene expre ssion. The pim-1 transcript was undetectable in control growth-arreste d 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 expres sed in control cultures and augmented by PRL stimulation. Results from stability studies indicated that the t1/2 values for the pim-1 transc ript 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 e xpression of the pim-1 protein was evaluated in [S-35]methionine-label ed cells by immunoprecipitation and sodium dodecyl sulfate-polyacrylam ide gel electrophoresis. In Nb2-11 cells, enhanced [S-35]pim-1 express ion paralleled its messenger RNA transcription through 8 h. Elevated [ S-35]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 Nb 2-11 cells. Its initial peak of transcription occurs early during G(1) cell cycle progression, whereas a second elevation is coincident with the G(1)/S transition. These results demonstrate that mitogen-induced expression of pim-1 is a rapid event in Nb2 lymphoma cells and sugges t that it may be associated with cell cycle progression.