Regulation of MCL1 through a serum response factor/Elk-1-mediated mechanism links expression of a viability-promoting member of the BCL2 family to the induction of hematopoietic cell differentiation.

Proliferation, differentiation, and apoptosis are tightly regulated during hematopoiesis, allowing amplification along specific lineages while preventing excessive proliferation of immature cells. The MCL1 member of the BCL2 family is up-regulated during the induction of monocytic differentiation (approximately 10-fold with 12-O-tetradecanoylphorbol 13-acetate (TPA)). MCL1 has effects similar to those of BCL2, up-regulation promoting viability, but differs from BCL2 in its rapid inducibility and its pattern of expression. Nuclear factors that regulate MCL1 transcription have now been identified, extending the previous demonstration of signal transduction through mitogen-activated protein kinase. A 162-base pair segment of the human MCL1 5'-flank was found to direct luciferase reporter activity, allowing approximately 10-fold induction with TPA that was suppressible upon inhibition of the extracellular signal-regulated kinase (ERK) pathway. Serum response factor (SRF), Elk-1, and Sp1 bound to cognate sites within this segment, SRF and Elk-1 acting coordinately to affect both basal activity and TPA inducibility, whereas Sp1 affected basal activity only. Thus, the mechanism of the TPA-induced increase in MCL1 expression seen in myelomonocytic cells at early stages of differentiation involves signal transduction through ERKs and transcriptional activation through SRF/Elk-1. This finding provides a parallel to early response genes (e.g. c-FOS and EGR1) that affect maturation commitment in these cells and therefore suggests a means through which enhancement of cell viability may be linked to the induction of differentiation.