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Little information exists concerning the role of guanine nucleotide-binding proteins (GNBP) in hematopoietic progenitor cell proliferation. We hypothesized that GNBP-mediated activation of adenylate cyclase plays an important role in factor-driven hematopoietic cell proliferation. Using cholera toxin and other probes of the cyclase system, we observe that cyclase activation results in a lineage-specific amplification of megakaryocytic progenitor cell numbers, an intermediate effect on erythroid progenitors, and, conversely, an inhibition of granulocytic colony formation. The effect of GNBP activation is synergistic with, and dependent upon, concomitant activation of the cells with phorbol diesters. This suggests a role for both calcium-dependent mechanisms, such as protein kinase C activation, and for other processes mediated by GNBP and cyclic AMP. The use of an intracellular calcium antagonist (Quin-2) partially abrogates the GNBP-mediated response, confirming that the effects of GNBP activation involve both calcium-dependent and calcium-independent processes. We conclude that hematopoietic progenitor cells are influenced by lineage-specific alterations in GTP-binding protein function, which affects both adenylate cyclase activity and calcium homeostasis.

Many studies report the presence of platelet-megakaryocyte-associated antigens on human immature and mature megakaryocytes as well as progenitor cells of this lineage. However, studies of purified megakaryocytes or megakaryocyte colonies yield little information on the effect(s) of growth factors in regulating antigenic expression, the kinetics of expression, or the relative content of antigens expressed. We report the detection of human megakaryocyte antigens using a solid-phase radioimmunoassay. The assay is linear, specific, and detects a range of approximately 300-10,000 megakaryocytes per aliquot. Analyses of unstimulated and stimulated suspension cultures indicate that activities known to influence megakaryocyte development in vitro also increase total megakaryocyte antigenic content. Additionally, studies on human erythroleukemia cells show that those cells constitutively express megakaryocyte antigens and increase these antigens when stimulated. Finally, since the specificity of the assay is that of the primary antibody, this procedure is easily extended to analysis of antigenic expression in other lineages.