ALTERATIONS IN PIM-1 AND C-MYC EXPRESSION ASSOCIATED WITH SODIUM BUTYRATE-INDUCED GROWTH-FACTOR DEPENDENCY IN AUTONOMOUS RAT NB2 LYMPHOMA-CELLS

Whereas Nb2-11 lymphoma cells critically require prolactin (PRL) for g rowth, NbP-SFJCD1 subline cells are growth factor independent, Treatme nt with the differentiating agent, sodium butyrate (NaBT), has been de monstrated previously to lead to growth arrest of Nb2-SFJCD1 cells and a transient reversion to PRL growth requirement following removal of NaBT. In the present study, the relation of NaBT-induced growth arrest to the cell cycle was examined using flow cytometry, and the effect o f PRL on expression of the immediate-early proto-oncogenes, pim-1 and c-myc, in NaBT-pretreated cultures was evaluated. Treatment of Nb2-SFJ CD1 cells with 2 mM NaBT for 72 h caused growth arrest in the majority of the cells in the G(1) phase of the cell cycle, an effect similar t o that produced by lactogen deprivation in PRL-dependent Nb2 cultures. The addition of PRL stimulated a concentration-dependent re-entry int o the cell cycle. In other experiments, NaBT treatment significantly r educed the steady-state levels of pim-1 and c-myc mRNA. Stimulation wi th PRL induced a rapid and concentration-dependent biphasic accumulati on of each mRNA with similar kinetics, Maximal expression of both prot o-oncogenes occurred within 2-4 h and after 12 h. Results from mRNA st ability studies suggest that the observed increases in expression of p im-1 and c-myc most likely do not reflect increased stability of the t ranscripts. The results indicate that NaBT-induced differentiation in autonomous Nb2-SFJCD1 causes growth arrest of the cells in the G(1) ph ase of the cell cycle and reduces the basal levels of pim-1 and c-myc mRNAs. Mitogenic stimulation with PRL reinitiates cell cycle progressi on characterized by biphasic expression of each proto-oncogene. It is suggested that NaBT is a useful tool for investigation of the malignan t progression from growth factor dependency to autonomy in the Nb2 lym phoma paradigm.