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 growth, Nb2-SFJCD1 subline cells are growth factor independent. Treatment with the differentiating agent, sodium butyrate (NaBT), has been demonstrated 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 of PRL on expression of the immediate-early proto-oncogenes, pim-1 and c-myc, in NaBT-pretreated cultures was evaluated. Treatment of Nb2-SFJCD1 cells with 2 mM NaBT for 72 h caused growth arrest in the majority of the cells in the G1 phase of the cell cycle, an effect similar to that produced by lactogen deprivation in PRL-dependent Nb2 cultures. The addition of PRL stimulated a concentration-dependent re-entry into the cell cycle. In other experiments, NaBT treatment significantly reduced the steady-state levels of pim-1 and c-myc mRNA. Stimulation with PRL induced a rapid and concentration-dependent biphasic accumulation of each mRNA with similar kinetics. Maximal expression of both proto-oncogenes occurred within 2-4 h and after 12 h. Results from mRNA stability studies suggest that the observed increases in expression of pim-1 and c-myc most likely do not reflect increased stability of the transcripts. The results indicate that NaBT-induced differentiation in autonomous Nb2-SFJCD1 causes growth arrest of the cells in the G1 phase of the cell cycle and reduces the basal levels of pim-1 and c-myc mRNAs. Mitogenic stimulation with PRL reinitiates cell cycle progression characterized by biphasic expression of each proto-oncogene. It is suggested that NaBT is a useful tool for investigation of the malignant progression from growth factor dependency to autonomy in the Nb2 lymphoma paradigm.