Neuronal protein kinase C epsilon–regulated endoplasmic reticulum stress and autophagic formation mediated neuropathic manifestation in diabetic neuropathy

Abstract In chronic diabetic neuropathy (DN), the cellular mechanisms of neuropathic manifestation remain unclear. Protein kinase C epsilon (PKCε) is an intracellular signaling molecule that mediates chronic pain. This paper addresses the DN-associated chronic neuropathic manifestations linked to long-term upregulated PKCε–mediated endoplasmic reticulum (ER) stress and autophagic formation. We found that the course of thermal hyperalgesia and mechanical allodyniawas associated with PKCε upregulation after DN, but not skin denervation. Pathologically, PKCε upregulation was associated with the expression of inositol-requiring enzyme 1α (IRE1α; ER stress–related molecule) and ubiquitin D (UBD), which are involved in the ubiquitin-proteasome system (UPS)-mediated degradation of misfolded proteins under ER stress. Manders coefficient analyses revealed an approximately 50% colocalized ratio for IRE1α(+):PKCε(+) neurons (0.34–0.48 for M1 and 0.40–0.58 for M2 Manders coefficients). The colocalized coefficients of UBD/PKCε increased (M1: 0.33 ± 0.03 vs. 0.77 ± 0.04, p < 0.001; M2: 0.29 ± 0.05 vs. 0.78 ± 0.04; p < 0.001) in the acute DN stage. In addition, the regulatory subunit p85 of phosphoinositide 3-kinase, which is involved in regulating insulin signaling, exhibited similar expression patterns to those of IRE1α and UBD; for example, it had highly colocalized ratios to PKCε. Upregulated PKCε–mediated neuropathic manifestation as well as ER stress and UPS-related molecule upregulation was reversed by PKCεv1-2, a PKCε specific inhibitor. The ultrastructural examination further confirmed that autophagic formation was associated functional PKCε. Collectively, this finding confirmed the pivotal roles of PKCε in DN-associated neuropathic manifestation and neuropathology.