Protein kinase C epsilon activates lens mitochondrial cytochrome c oxidase subunit IV during hypoxia

Protein kinase C (PKC) isoforms have been identified as major cellular signaling proteins that act directly in response to oxidation conditions. In retina and lens two isoforms of PKC respond to changes in oxidative stress, PKCγ and PKCɛ, while only PKCɛ is found in heart. In heart the PKCɛ acts on connexin 43 to protect from hypoxia. The presence of both isoforms in the lens led to this study to determine if lens PKCɛ had unique targets. Both lens epithelial cells in culture and whole mouse lens were examined using PKC isoform-specific enzyme activity assays, co-immunoprecipitation, confocal microscopy, immunoblots, and light and electron microscopy. PKCɛ was found in lens epithelium and cortex but not in the nucleus of mouse lens. The PKCɛ isoform was activated in both epithelium and whole lens by 5% oxygen when compared to activity at 21% oxygen. In hypoxic conditions (5% oxygen) the PKCɛ co-immunoprecipitated with the mitochondrial cytochrome c oxidase IV subunit (CytCOx). Concomitant with this the CytCOx enzyme activity was elevated and increased co-localization of CytCOx with PCKɛ was observed using immunolabeling and confocal microscopy. In contrast, no hypoxia-induced activation of CytCOx was observed in lenses from the PKCɛ knockout mice. Lens from 6-week-old PKCɛ knockout mice had a disorganized bow region which was filled with vacuoles indicating a possible loss of mitochondria but the size of the lens was not altered. Electron microscopy demonstrated that the nuclei of the PCKɛ knockout mice were abnormal in shape. Thus, PKCɛ is found to be activated by hypoxia and this results in the activation of the mitochondrial protein CytCOx. This could protect the lens from mitochondrial damage under the naturally hypoxic conditions observed in this tissue. Lens oxygen levels must remain low. Elevation of oxygen which occurs during vitreal detachment or liquification is associated with cataracts. We hypothesize that elevated oxygen could cause inhibition of PKCɛ resulting in a loss of mitochondrial protection.