tokens: A 40 - fold stimulation of chloramphenicol acetyltransferase activity mediated by four tandem repeats of the SNE could be induced within 2 h ( and up to 250 - fold within 6 h ) after addition of TPA in DNA - transfected U - 937 cells , indicating that the stimulation appeared likely to be a true protein kinase C - mediated signal transduction event rather than a differentiation response .