Impaired protein tyrosine phosphatase PTPRR reduces response to corticosteroid in mononuclear cells

Introduction: We have recently found that impaired nuclear translocation of glucocorticoid receptor (GR) due to its hyperphosphorylation at Ser226 via c-Jun N-terminal kinase 1 (JNK1) is one of the causes of corticosteroid (CS) insensitivity. Protein tyrosine phosphatases (PTPs) are known as critical regulators of physiological signalling molecules such as mitogen-activated protein kinases (MAPKs). Aims: To elucidate the involvement of MAPK-PTPs (PTPRR, PTPN5 and PTPN7), which have the potential to dephosphorylate MAPKs, in regulation of CS sensitivity. Methods: Effects of MAPK-PTPs RNAi were evaluated in U937 cells. CS sensitivity was determined by the dexamethasone ability to inhibit TNFα-induced IL-8 and to translocate GR from cytoplasm into nucleus (NT). Phosphorylation levels of GR-Ser226 and JNK1, and protein expression of PTPRR were evaluated by western-blot in U937 cells and/or peripheral blood mononuclear cells (PBMCs). Phosphatase activity of immunopurified PTPRR was analyzed by fluorescence-based assay. Results: PTPRR reduction (but not PTPN5, nor PTPN7) attenuated CS sensitivity in line with enhancement of GR-Ser226 phosphorylation. In addition, PTPRR reduction caused less NT of GR. Under IL-2/IL-4-induced CS insensitive condition, PTPRR expression/activity were reduced and PTPRR was dissociated from JNK1/GR. In PBMCs from severe asthmatics, PTPRR expression correlated negatively with JNK1/GR-Ser226 phosphorylation levels. Conclusion: Reduction of PTPRR leads to lower dephosphorylation of GR-Ser226 via JNK1 and to lower NT of GR, resulting in CS insensitivity. This novel mechanism may be one of the causes of CS insensitivity in patients with severe asthma.