Identification of an additional isoform of STAT5 expressed in immature macrophages.

We are interested in understanding the molecular basis of macrophage (Mphi) differentiation and activation by cytokines. Recent reports have suggested that the transcription factor STAT5 may play a role in Mphi differentiation. In the experiments described here, we assessed the expression of STAT5-related molecules in three Mphi cell lines, RAW 264.7, WEHI-3, and WEHI-3D+, which represent different stages of Mphi maturation, and also in primary peritoneal and bone marrow Mphi from BALB/c mice. The studies revealed that the previously characterized STAT5a and STAT5b isoforms are detectable at both the mRNA and protein levels in these Mphi populations. Additional STAT5-related proteins were detected by immunoblot analysis and were preferentially expressed in both the immature WEHI-3 cell population and the adherent bone marrow population containing immature Mphi. These results identify new isoforms of STAT5 and demonstrate that distinct patterns of expression of STAT5-related proteins are observed in Mphi at different stages of maturation.