Objective and design: We designed a study to detect downstream phosphorylation targets of PKCβ in MCP-1-induced human monocytes.
Methods: Two-dimensional gel electrophoresis was performed for monocytes treated with MCP-1 in the presence or absence of PKCβ antisense oligodeoxyribonucleotides (AS-ODN) or a PKCβ inhibitor peptide, followed by phospho- and total protein staining. Proteins that stained less intensely with the phospho-stain, when normalized to the total protein stain, in the presence of PKCβ AS-ODN or the PKCβ inhibitor peptide, were sequenced.
Results: Of the proteins identified, vimentin was consistently identified using both experimental approaches. Upon (32)P-labeling and vimentin immunoprecipitation, increased phosphorylation of vimentin was observed in MCP-1 treated monocytes as compared to the untreated monocytes. Both PKCβ AS-ODN and the PKCβ inhibitor reduced MCP-1-induced vimentin phosphorylation. The IP of monocytes with anti-vimentin antibody and immunoblotting with a PKCβ antibody revealed that increased PKCβ becomes associated with vimentin upon MCP-1 activation. Upon MCP-1 treatment, monocytes were shown to secrete vimentin and secretion depended on PKCβ expression and activity.
Conclusions: We conclude that vimentin, a major intermediate filament protein, is a phosphorylation target of PKCβ in MCP-1-treated monocytes and that PKCβ phosphorylation is essential for vimentin secretion. Our recently published studies have implicated vimentin as a potent stimulator of the innate immune receptor Dectin-1 as reported by Thiagarajan et al. (Cardiovasc Res 99:494-504, 2013). Taken together our findings suggest that inhibition of PKCβ regulates vimentin secretion and, thereby, its interaction with Dectin-1 and downstream stimulation of superoxide anion production. Thus, PKCβ phosphorylation of vimentin likely plays an important role in propagating inflammatory responses.