Involvement of interferon-system in the regulation of cell growth and differentiation.

In this report we review the current knowledge on the involvement of the interferon (IFN) system in the regulation of cell growth and differentiation. We also summarize our own data which provide evidence for the strong correlation between IFN-mediated growth-arrest of transformed cells and the elevated enzymatic activity of an IFN-induced protein. Similarly, it is demonstrated that elevated levels of IFN-induced proteins accompany the early phases of in-vitro cell differentiation. IFN-treatment of NIH/3T3 mouse fibroblasts transformed by Moloney-murine sarcoma virus (MSV) resulted in a significant reduction in the rates of cell growth, protein synthesis and cloning efficiency. In parallel, 2-5A-synthetase activity was induced ten-fold above the background level. Treatment of these cells for 3 days with 450 international units (IU)/ml of IFN followed by its removal, resulted in a gradual increase in all parameters associated with cell growth while the 2-5A-synthetase activity was reduced to its normal level. However, almost no recovery occurred when cells were treated with 1,800 IU/ml. In parallel, 2-5A-synthetase activity remained highly elevated even at 3 days after the removal of IFN. In these cells, the expression of both c-myc and v-mos was reduced rapidly following IFN treatment. Upon removal of IFN after 24 h of treatment, the expression of both genes was resumed but with a different kinetics, suggesting that different mechanisms are responsible for the reduction in gene expression. In rat skeletal muscle cultures which differentiate to form myotubes, the level of both 2-5A-synthetase and protein kinase activities was transiently elevated, reaching a peak at 3 days followed by a decrease to background levels. This peak activity precedes the appearance of the major muscle differentiating proteins.