Enhancementof Viral Gene Expressionin Friend ErythroleukemicCells by 12-O-Tetradecanoylphorbol-13-acetate 1

in vitro (1 4). While these cells are incapable of undergoing spontaneous differentiation in vitro, they can be induced to differentiate by several compounds, including Me2SO (for a review, see Ref. 18). In an earlier study (7), we demonstrated (a) that Me2SO treatment of Friend cells enhances virus expression and (b) that Me2SO plus 5'-bromo-2-deoxyuridine (known to inhibit Me2SO-induced differentiation) brings about an even greater increase in virus expression. In the present report, we examine the effect of TPA on the expression of the Friend virus genome. Our results show that this compound causes an increase in virus expression which correlates with an enhancement of plasminogen activator levels and an inhibition of the Me2SO induced differentiation.