We have studied the expression of G protein subtypes and the role of G protein-dependent signaling in two subclones of RED-1 cells, an erythropoetin(Epo)-sensitive, murine erythroleukemia cell line. Clone 6C8 showed terminal erythroid differentiation in response to a combined treatment with Epo and dimethylsulfoxide. Clone G3 was resistant to these inducers, but responded to Epo with enhanced proliferation. We measured G protein alpha subunit levels by toxin-catalyzed adenosine diphosphate (ADP)-ribosylation with [32P]-nicotinamide adenine dinucleotide (NAD) and by semiquantitative immunoblotting with specific antisera. Native RED-1 cells expressed G alpha i2, alpha i3, alpha s, and alpha q/11, but not alpha i1 and alpha o. Terminal differentiation was associated with a selective loss (approximately 80%) of G alpha i3 and an increase in a truncated cytosolic form of G alpha i2, while the membrane levels of alpha i2, alpha q/11, and alpha s did not change significantly. Treatment of G3 cells with the inducers was without effect on G protein abundance. However, except for alpha s, G3 cells contained significantly higher levels of the different G protein alpha subunits tested. Stimulation of G protein-coupled receptors by thrombin and ADP caused a pertussis toxin (PTX)-inhibitable transient increase in intracellular Ca2+ that was markedly reduced in differentiated cells. In G3 cells, but not in 6C8 cells, thrombin also caused a PTX- sensitive inhibition of isoprenaline-stimulated cyclic 3',5'-adenosine monophosphate (cAMP) formation. Our results show that specific alterations in G protein expression and function are associated with erythroid differentiation of erythroleukemia cells but do not prove a causal relationship. The loss of G alpha i3 may affect cellular responses that are mediated via P2T purine or thrombin receptors.

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