Hematopoiesis is a profound example of cell homeostasis that is regulated throughout life. This process requires the participation of many factors, including positive and negative regulators of growth and differentiation, that determine survival, growth stimulation, differentiation, functional activation, and programmed cell death. Understanding the effects of multiple stimuli on specific cells at the molecular and cellular level is crucial towards understanding how the population of blood cells maintains a homeostatic state. Two appropriate stimuli for analysis, both of which are found in bone marrow, are differentiation-inducing cytokines, which induce terminal differentiation associated with growth arrest, ultimately culminating in programmed cell death, and transforming growth factor-beta 1 (TGF- beta 1), which induces rapid growth arrest and apoptosis of hematopoietic cells. Previously, we have shown, using M1 myeloblastic leukemic cells as a model system, that differentiation-inducing cytokines induce terminal differentiation associated with growth arrest and, only after 5 to 7 days, apoptosis, whereas TGF-beta 1 induces rapid growth arrest and apoptosis. In this report, we show that M1 myeloid leukemic cells treated concomitantly with the differentiation inducer interleukin-6 and TGF-beta 1 undergo terminal differentiation, in which modulators of the MyD118 gene product, previously shown to be a positive regulator of TGF-beta 1-induced apoptosis, are implicated to play a role in protecting the cells from TGF-beta 1-induced apoptosis. Furthermore, using M1 cell variants blocked at different stages after induction of differentiation, including M1myb and M1myc, as well as conditionally blocked M1mycer, it has been shown that the dominance of interleukin-6 to TGF-beta 1-induced apoptosis is dependent on the progression of the differentiation program. Further studies with M1 and the genetically engineered M1 cell variants will be instrumental towards molecularly dissecting the interaction of hematopoietic differentiation with a variety of apoptotic pathways.

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