Expression of PKC Iota Affects Neuronal Differentiation of PC12 Cells at Least Partly Independent of Kinase Function

Abstract

Atypical PKC (aPKC) plays a role in establishing cell polarity and has been indicated in neuronal differentiation and polarization, including neurite formation in rat pheochromocytoma PC12 cells, albeit by unclear mechanisms. Here, the role of the aPKC isoform, PKC iota (PKCι), in the early neuronal differentiation of PC12 cells was investigated. NGF-treated PC12 cells with stably expressed exogenous wild-type PKCι showed decreased expression of a neuroendocrine marker, increased expression of a neuronal marker, and increased neurite formation. Stable expression of a kinase- inactive PKCι, but not constitutively active PKCι lacking a regulatory domain, had similar although less potent effects. Pharmacological inhibition of endogenous aPKC kinase activity in parental PC12 cells did not inhibit neurite formation, suggesting that some of the observed effects of PKCι expression on neuronal differentiation are kinase- independent. Interestingly, exogenous expression of wild-type and kinase-inactive PKCι had little effect on overall PKCι activity, but caused a decrease in PKC zeta (PKCζ) kinase activity, suggesting an interplay between the two isoforms that may underlie the observed results. Overall, these findings suggest that in PC12 and perhaps other neuroendocrine precursor cells, PKCι influences an early differentiation decision between the neuroendocrine (chromaffin) and sympathetic neuron cell lineages, potentially by affecting PKCζ function.

aPKCι promotes gallbladder cancer tumorigenesis and gemcitabine resistance by competing with Nrf2 for binding to Keap1

“…All abovementioned studies focused on the regulation of cell polarization and kinase activity of aPKCι, while interestingly, we found that aPKCι functioned as an antioxidative factor in a kinase-independent manner. Our findings were supported by other studies that aPKCι promoted neuronal differentiation in a manner that did not depend on kinase activity [39]. More importantly, we further demonstrated that this function of aPKCι was involved in drug resistance and cell growth.…”

“…Although previous studies have reported that phosphorylation of Nrf2 by PKC facilitated the dissociation of Nrf2 from Keap1 [19,46], the main enzyme and molecular mechanism underlying the interaction were not investigated. In contrast, the kinase-independent functions of aPKC, including aPKCι, have been reported in multiple signaling pathways [39,[47], [48], [49]]. In this study, we have investigated that aPKCι can promote nuclear translocation of Nrf2, but not phosphorylated Nrf2, to activate cytoprotective gene expression.…”

Insulin stimulates atypical protein kinase C‐mediated phosphorylation of the neuronal adaptor FE65 to potentiate neurite outgrowth by activating ARF6‐Rac1 signaling

“…Indeed, suppressing the expression of PKCζ, another aPKC isoform, results in a reduction in neurite outgrowth in PC12 cells 76 . Conversely, overexpression of either PKCζ or PKCι/λ potentiate neurite outgrowth in nerve growth factor‐treated PC12 cells 8,77 . Additionally, PKCι/λ is implicated in Wnt3a‐dependent neurite outgrowth through an association with Dvl2 9 .…”

aPKC in neuronal differentiation, maturation and function

“…Although kinase-independent functions have been described for aPKC [23,24], most studies ascribe aPKC function to its kinase activity. However, the rate of in vitro enzymatic activity of aPKCs is slow compared with that of other PKCs-the catalytic activity of PRKCZ was calculated to be 5 mol of phosphate/min/mol of kinase, while that of PKC βII was 200 mol of phosphate/min/mol of kinase [25][26][27], and the mechanism of activation of its enzymatic kinase activity is still under scrutiny.…”

“…In in vitro experiments, PRKCI expression favors neuronal differentiation of PC12 (pheochromocytoma 12 cell line) cells after NGF (nerve growth factor) treatment [23]. Other experiments suggested that PRKCZ binds a neuronal differentiation factor, TRIM32, and retains it in the cytoplasm, thus preventing differentiation of mouse neural stem cell cultures [90].…”