Central role of protein kinase Cepsilon in constitutive activation of ERK1/2 and Rac1 in the malignant cells of hairy cell leukemia.

We have previously identified the presence of Ras/Raf-independent constitutive activation of extracellular signal-regulated kinase (ERK) in the hairy cells (HCs) of hairy cell leukemia. The aim of the present study was to characterize the signaling components involved in this activation and their relationship to the reported activation of Rac1. We found that both Rac1 and ERK activation in HCs are downstream of active Src and protein kinase C (PKC). Inhibition with toxin B showed that Rac1 plays no role in ERK activation in HCs. However, toxin B inhibited p60src and the Rac1-GEF Vav, demonstrating a positive feedback/activation of p60src by Rac1. Treatment with specific small interfering RNA for various PKC isoforms, or with PKC isoform-specific inhibitors, demonstrated a central role for PKCepsilon in the constitutive activation of Rac1 and ERK in HCs. PKCepsilon and active ERK were mutually associated and co-localized with mitochondria in HCs. Furthermore, active PKCepsilon was nitrated on tyrosine, pointing to a reactive oxygen species-dependent mechanism of activation. By being involved in activation of ERK and Rac1, PKCepsilon plays roles in both the survival of HCs and in the cytoskeletal dynamics responsible for the distinctive morphology and tissue homing of these cells. Our study therefore describes novel aspects of signaling important for the pathogenesis of hairy cell leukemia.