Protein kinase Cδ oxidation contributes to ERK inactivation in lupus T cells

Objective

CD4+ T cells from patients with active lupus have impaired ERK pathway signaling that decreases DNA methyltransferase expression, resulting in DNA demethylation, overexpression of immune genes, and autoimmunity. The ERK pathway defect is due to impaired phosphorylation of T⁵⁰⁵ in the protein kinase Cδ (PKCδ) activation loop. However, the mechanisms that prevent PKCδ T⁵⁰⁵ phosphorylation in lupus T cells are unknown. Others have reported that oxidative modifications, and nitration in particular, of T cells as well as serum proteins correlate with lupus disease activity. We undertook this study to test our hypothesis that nitration inactivates PKCδ, contributing to impaired ERK pathway signaling in lupus T cells.

Methods

CD4+ T cells were purified from lupus patients and controls and then stimulated with phorbol myristate acetate (PMA). Signaling protein levels, nitration, and phosphorylation were quantitated by immunoprecipitation and immunoblotting of T cell lysates. Transfections were performed by electroporation.

Results

Treating CD4+ T cells with peroxynitrite nitrated PKCδ, preventing PKCδ T⁵⁰⁵ phosphorylation and inhibiting ERK pathway signaling similar to that observed in lupus T cells. Patients with active lupus had higher nitrated T cell PKCδ levels than did controls, which correlated directly with disease activity, and antinitrotyrosine immunoprecipitations demonstrated that nitrated PKCδ, but not unmodified PKCδ, was refractory to PMA‐stimulated T⁵⁰⁵ phosphorylation, similar to PKCδ in peroxynitrite‐treated cells.

Conclusion

Oxidative stress causes PKCδ nitration, which prevents its phosphorylation and contributes to the decreased ERK signaling in lupus T cells. These results identify PKCδ as a link between oxidative stress and the T cell epigenetic modifications in lupus.