Abstract
Macrophages are professional phagocytic cells, and express pattern recognition receptors such as C-type lectins and integrins for the detection of invading pathogens. Both Dectin-1 (a C-type lectin) and complement receptor 3 (CR3, a β2-integrin) are expressed on innate immune cells including macrophages, neutrophils, and dendritic cells. Dectin-1 stimulation by b-glucan-containing particles (zymosan) and CR3 stimulation by serum opsonized zymosan (SOZ) activate Erk- and Akt-dependent signaling resulting in phagocytosis and production of an oxidative burst.
Shp2, a protein tyrosine phosphatase encoded by Ptpn11, promotes activation of Ras-Erk and PI3K-Akt signaling, supports hematopoietic development, and is commonly mutated in juvenile myelomonocytic leukemia (JMML). However, no studies have examined the role of Shp2 in Dectin-1- or CR3-stimulated NADPH oxidase activation or ROS production. As activation of Erk and Akt stimulates NADPH oxidase by phosphorylating p47phox, we hypothesized that Shp2 positively regulates ROS production in response to Dectin-1 or CR3 stimulation.
Using murine peritoneal exudate macrophages (PEMs), both zymosan and SOZ exposure induced maximal ROS production 10 minutes post-stimulation, which corresponded to maximal induction of Shp2 phosphorylation (Y580, proposed to promote Shp2 phosphatase activity) and Erk phosphorylation. Using bone marrow derived macrophages (BMMs) from mice bearing a conditionally deleted allele of Ptpn11 (Shp2flox/flox;Mx1Cre+), ROS production was significantly reduced in response to zymosan and SOZ in Shp2flox/flox;Mx1Cre+ BMMs compared to control Shp2flox/flox;Mx1Cre- BMMs. Notably, the phagocytic index of the Shp2flox/flox;Mx1Cre+ and Shp2flox/flox;Mx1Cre- BMMs was similar, and protein components of the NADPH oxidase complex (p40phox, p67phox, and p47phox) were expressed at similar levels.
To define the biochemical role of Shp2 in ROS production, we generated yellow fluorescent protein (YFP)-tagged Shp2 constructs bearing mutation of the N-SH2 (R32K) or phosphatase (C463A) domain and retrovirally expressed these constructs in murine BMMs. When subjected to zymosan or SOZ stimulation, mutation of either the N-SH2 or phosphatase domain resulted in reduced ROS production. Using time-lapse confocal videomicroscopy, we found that Shp2-R32K-YFP failed to translocate to the phagosome in SOZ-stimulated BMMs; however, phosphatase dead Shp2-C463A-YFP strongly translocated to the phagosome despite producing lower ROS levels. These findings specifically pointed to Shp2 phosphatase function as crucial in positively regulating NADPH oxidase and ROS production. Accordingly, we reasoned that macrophages expressing JMML-associated gain-of-function (GOF) Shp2 mutants, characterized to have increased phosphatase activity, would produce elevated ROS levels. As anticipated, BMMs retrovirally expressing GOF Shp2-D61Y or GOF Shp2-E76K and PEMs from mice bearing a conditionally induced gain-of-function allele of Ptpn11 (Shp2D61Y/+;Mx1Cre+) similarly produced significantly elevated levels of zymosan- and SOZ-stimulated ROS compared to WT Shp2-expressing BMMs or PEMs, respectively.
Given the positive role of Shp2 phosphatase in promoting zymosan- and SOZ-stimulated ROS production, we investigated putative Shp2 substrates in response to zymosan stimulation. SHPS-1 (SH2 domain-containing protein tyrosine phosphatase substrate 1) is a myeloid inhibitory immunoreceptor expressed on macrophages, requires tyrosine phosphorylation to exert its inhibitory effect, and has been shown to be de-phosphorylated by Shp2. Consistent with its potential function in regulation ROS production, SHPS-1 is strongly associated with phagosomes in zymosan-stimulated PEMs. In immunoblot analysis, reduced phospho-SHPS-1 levels kinetically correlated with maximal zymosan-stimulated Shp2 phosphorylation and ROS production, and increased levels of phospho-SHPS-1 were found in BMMs expressing phosphatase dead Shp2-C463A compared to cells expressing WT Shp2. Collectively, these findings indicate that Shp2 phosphatase function positively regulates Dectin-1- and CR3-stimulated NADPH oxidase activation and ROS production in macrophages, and that mechanistically, Shp2 may exert its positive effect by de-phosphorylating and thus negatively regulating the inhibitory function of SHPS-1.
Disclosures
No relevant conflicts of interest to declare.
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