Roles of PTP&phis; and Cbl in CSF-1 receptor signal transduction

Abstract

Colony stimulating factor-1 (CSF-1) is a homodimeric glycoprotein that is the major regulator of the survival, proliferation and differentiation of cells of the mononuclear phagocytic lineage. The effects of CSF-1 are mediated by the CSF-1 receptor (CSF-1R) tyrosine kinase. The ligand-activated CSF-1R induces transient tyrosine phosphorylation of cellular proteins, which is partially dependent on intact cytoskeleton and is tightly regulated by protein tyrosine phosphatases (PTPs).;In an investigation of the role of PTPs in CSF-1R signaling, three cDNAs encoding protein tyrosine phosphatase-phi (PTP&phis;) were cloned from the murine macrophage cell line BAC1.2F5. The corresponding mRNAs encode two membrane-spanning isoforms, p47PTP&phis; and p43PTP&phis;, and a putative cytosolic form p33PTP&phis;, only detectable at the mRNA level, that arise by alternative splicing. A GST-p47PTP&phis; fusion protein displayed PTP activity that was lost upon mutation of the catalytic cysteine residue to serine. Neither of the membrane-spanning PTP&phis;s are tyrosine phosphorylated. However, PTP&phis; expression was downregulated by CSF-1 treatment, but was elevated as BAC1.2F5 macrophages approached confluence. This was in contrast to the low PTP&phis; expression in undifferentiated M1 myeloid precursor cells where PTP&phis; was rapidly upregulated during interleukin-6-induced differentiation to macrophage-like cells. To address the function of PTP&phis;, wild type (p47PTP&phis;WT) and catalytically inactive (p47PTP CS) PTP&phis; were overexpressed 5--10-fold in BAC1.2F5 macrophages. Compared to the control cells, the p47PTP&phis;WT overexpressing cells were more rounded, while the p47PTPCS overexpressing cells displayed enhanced spreading. Overexpression of either PTP&phis; did not have any major effect on basal or CSF-1-induced protein tyrosine phosphorylation. Furthermore, no tyrosine phosphorylated protein could be specifically co-precipitated with PTP&phis; in anti-PTP&phis; immunoprecipitates. However, several tyrosine phosphorylated proteins bound to GST-p47PTP&phis;CS in vitro and three were identified as Pyk2, paxillin and MAYP. Immunoprecipitation of these proteins from BAC1.2F5 cells overexpressing p47PTP&phis;WT , p47PTP&phis;CS and from control cells did not reveal any reproducible difference in their specific tyrosine phosphorylation, suggesting that either a small fraction of their total cellular contents is regulated by PTP&phis; or that they are not direct in vivo substrates of PTP&phis;. Nevertheless, the elevated levels of MAYP in p47PTP&phis;CS cells were correlated with their enhanced cell spreading. Further identification of the tyrosine phosphorylated proteins that bind to GST-p47PTP&phis; CS should help to elucidate PTP&phis; function in macrophages.