Differential expression and phosphorylation of distinct STAT3 proteins during granulocytic differentiation

External stimuli act in concert with intracellular signals to regulate a cell's genetic program, activating genes important in granulocytic lineage commitment, proliferation, and maturation. Signal transducer and activator of transcription 3 (STAT3), a transcription factor, has been implicated in mediating granulocytic differentiation. We have examined the role of STAT3 as a physiologic mediator of granulocytic kinetics. Distinct isoforms the long form STAT3α, the truncated forms STAT3β and STAT3γ, and a putative novel form STAT3δ-were expressed and activated in a maturation stage-specific manner. With the progression of differentiation, the ratio of isoforms shifted from predominantly STAT3α to STAT3β. The kinetics of STAT3γ, generated through proteolytic cleavage of STAT3α, coincided with but were inverse to those of STAT3α, STAT3δ was expressed at low levels and decreased with differentiation but was preferentially phosphorylated during an intermediate stage of maturation. Under different culture conditions (pH, O₂ tension [pO₂], IL-3), we found that the expression and phosphorylation status of the different STAT3 isoforms displayed unique kinetic patterns that correlated with the effects on granulocyte differentiation. The evidence suggests that signals triggered by pH, pO₂, and IL-3 each converge on STAT3 through independent mechanisms, exploiting the flexibility granted by the diversity in expression and phosphorylation of the different STAT3 isoforms, to regulate distinct granulocytic cell responses. The selective expression of STAT3 isoforms and their activation is a major determinant of granulocytic cell development and provides a molecular basis for evaluating the effects of various environmental factors on the STAT3-mediated signaling pathway.