We examined the expression of N-myc, c-myc, and c-src in four embryonic carcinoma (EC) cell lines during different states of cell growth and following induction of in vitro differentiation. N-myc mRNA was detected in undifferentiated cells of four EC cell lines (PCC7, PCC3, PCC4, F9) neither of which showed N-myc gene amplification. No N-myc transcripts could be detected in mRNA prepared from a murine neuroblastoma cell line and from a murine fibroblast line. The level of N-myc mRNA decreased by 85% when PCC7 EC cells were induced by retinoic acid and CAMP treatment to form nerve-like cells. Six days after induction, the PCC7 cells changed into aggregates of neurofilament positive cells with massive neurite outgrowths. At this stage DNA replication had been reduced by more than 95%.The decreased N-myc expression in induced PCC7 cells was parallelled by 300-500% increase in csrc expression. Slowing of cell multiplication by serum starvation, on the other hand, did not affect t h e level of N-myc or c-src mRNA levels in PCC7 cells. C-myc was expressed in all EC lines except PCC7, which surprisingly did not express c-myc even at an exponential rate of proliferation. Chemical induction of F9 EC cells to form visceral endoderm or parietal endoderm resulted in markedly reduced (85%) levels of N-myc transcripts. A similar decline in c-myc expression was found in differentiated F9 cells. No c-src transcripts were detected in proliferating or differentiated F9 cells. These results suggest that N-myc may be expressed not only in neural development, but also in very early, undetermined embryonic cells. The activation of c-src expression when PCC7 EC cells differentiate into nerve-like cells shows that the pattern of proto-oncogene expression may change during a differentiation process, some proto-oncogenes increasing, others decreasing their representation in t h e mRNA pool.
Proto-oncogenes (c-onc), cellular sequences homologous to the transforming retroviral oncogenes (v-onc), may be of key importance in the control of cell proliferation and differentiation (Bishop, 1983). Recently a hu-