Regulation of Jak2 tyrosine kinase by protein kinase C during macrophage differentiation of IL-3–dependent myeloid progenitor cells

Abstract

Differentiation of macrophages from myeloid progenitor cells depends on a discrete balance between cell growth, survival, and differentiation signals. Interleukin-3 (IL-3) supports the growth and survival of myeloid progenitor cells through the activation of Jak2 tyrosine kinase, and macrophage differentiation has been shown to be regulated by protein kinase C (PKC). During terminal differentiation of macrophages, the cells lose their mitogenic response to IL-3 and undergo growth arrest, but the underlying signaling mechanisms have remained elusive. Here we show that in IL-3–dependent 32D myeloid progenitor cells, the differentiation-inducing PKC isoforms PKC- and PKC-δ specifically caused rapid inhibition of IL-3–induced tyrosine phosphorylation. The target for this inhibition was Jak2, and the activation of PKC by 12-O-tetradecanoyl-phorbol-13-acetate treatment also abrogated IL-3–induced tyrosine phosphorylation of Jak2 in Ba/F3 cells. The mechanism of this regulation was investigated in 32D and COS7 cells, and the inhibition of Jak2 required catalytic activity of PKC-δ and involved the phosphorylation of Jak2 on serine and threonine residues by the associated PKC-δ. Furthermore, PKC-δ inhibited the in vitro catalytic activity of Jak2, indicating that Jak2 was a direct target for PKC-δ. In 32D cells, the inhibition of Jak2 either by PKC-δ, tyrosine kinase inhibitor AG490, or IL-3 deprivation caused a similar growth arrest. Reversal of PKC-δ–mediated inhibition by the overexpression of Jak2 promoted apoptosis in differentiating 32D cells. These results demonstrate a PKC-mediated negative regulatory mechanism of cytokine signaling and Jak2, and they suggest that it serves to integrate growth-promoting and differentiation signals during macrophage differentiation.