Several different proto-oncogenes have been shown to influence cellular differentiation. One of the most widely studied model systems has been the Friend murine erythroleukemia cell (F-MELC) line, which can be induced to undergo erythroid differentiation by a variety of chemical agents. Constitutive overexpression of either the c-myc or c-myb proto- oncogenes has been previously shown to inhibit F-MELC differentiation, whereas c-myc antisense sequences accelerate the process. To investigate the potential involvement of other proto-oncogenes and immediate early response genes in F-MELC differentiation, we studied the expression of the three known members of the jun family as well as another gene, egr-1, which, like the jun family members, is expressed as an immediate early response gene in growth factor-stimulated quiescent cells. All four genes were expressed in F-MELC, although the levels of expression and modes of regulation differed. Transfection with amplifiable c-jun, junB, or junD expression plasmids inhibited differentiation, whereas transfection with an egr-1 expression plasmid was without effect. These results indicate that jun family members play a role in mediating F-MELC differentiation. The known inhibitory effect of phorbol ester tumor promoters on F-MELC differentiation may be the result of their known stimulation of jun expression.

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