We investigated the effects of tumor necrosis factor (TNF) α on the human megakaryocytic leukemic cell lines Mo7e, Meg-01, and Dami/HEL. Our data show that both type I and type II TNF receptors (TNF-RI and TNF-RII) are expressed on all of these cells, and TNF-α significantly stimulates the proliferation of growth factor-dependent Mo7e cells but not of Meg-01 or Dami/HEL cells, which grow in a factor-independent manner. TNF-α serves predominantly as a mitogen for Mo7e cell proliferation and does not induce Mo7e cell differentiation. Coincubation with both TNF-α and anti-TNF-α neutralizing antibody completely abolishes the TNF-α-induced proliferation of Mo7e cells. In bioassays, there is no detectable level of other stimulatory cytokines in conditioned medium from Mo7e cells previously stimulated by TNF-α, implying that the stimulatory effect of TNF-α on Mo7e cells is derived from the direct action of TNF-α rather than via the induction of secondary cytokines by TNF-α. Flow cytometric studies demonstrated that TNF-α binds to Mo7e cells that have been pretreated with either anti-TNF-RI or anti-TNF-RII neutralizing antibody, but TNF-α does not bind to cells pre-exposed to both receptor antibodies. However, the incubation of Mo7e cells with either TNF-RI or TNF-RII neutralizing antibodies or with either soluble TNF-RI or TNF-RII inhibits TNF-α-induced cell proliferation, indicating the requirement of interactions with both TNF receptors for the mitogenic activity of TNF-α. Furthermore, our data suggest that an alternative signaling pathway may be involved in TNF-α-induced Mo7e cell proliferation, because the common mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription (STAT) signaling pathways activated by other cytokines that induce Mo7e cell proliferation are not activated by TNF-α.

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Supported by National Cancer Institute Grant CA56072.

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