Protein Kinase Cζ Attenuates Hypoxia-induced Proliferation of Fibroblasts by Regulating MAP Kinase Phosphatase-1 Expression

Abstract

We have previously found that hypoxia stimulates proliferation of vascular fibroblasts through Galphai-mediated activation of ERK1/2. Here, we demonstrate that hypoxia also activates the atypical protein kinase Czeta (PKCzeta) isozyme and stimulates the expression of ERK1/2-specific phosphatase, MAP kinase phosphatase-1 (MKP-1), which attenuates ERK1/2-mediated proliferative signals. Replication repressor activity is unique to PKCzeta because the blockade of classical and novel PKC isozymes does not affect fibroblast proliferation. PKCzeta is phosphorylated upon prolonged (24 h) exposure to hypoxia, whereas ERK1/2, the downstream kinases, are maximally activated in fibroblasts exposed to acute (10 min) hypoxia. However, PKCzeta blockade results in persistent ERK1/2 phosphorylation and marked increase in hypoxia-induced replication. Similarly prolonged ERK1/2 phosphorylation and increase in hypoxia-stimulated proliferation are also observed upon blockade of MKP-1 activation. Because of the parallel suppressive actions of PKCzeta and MKP-1 on ERK1/2 phosphorylation and proliferation, the role of PKCzeta in the regulation of MKP-1 expression was evaluated. PKCzeta attenuation reduces MKP-1 expression, whereas PKCzeta overexpression increases MKP-1 levels. In conclusion, our results indicate for the first time that hypoxia activates PKCzeta, which acts as a terminator of ERK1/2 activation through the regulation of downstream target, MKP-1 expression and thus serves to limit hypoxia-induced proliferation of fibroblasts.

Hypoxia-Induced Pulmonary Vascular Remodeling

“…20,170 PA adventitial fibroblasts derived from chronically hypoxic animals exhibit heightened growth responses to a number of growth promoting stimuli including hypoxia as well as alterations of proliferation associated signaling pathways, compared with fibroblasts derived from normoxic animals. 161,162,171,172 For instance, the atypical PKC isozyme acts as a proproliferative kinase in fibroblasts from chronically hypoxic animals, whereas it exhibits antiproliferative actions in fibroblasts from normoxic animals. 172 These observations raise the possibility that chronic hypoxia leads to the emergence of fibroblasts in the adventitia that have lost their ability to limit stimulus-induced proliferation.…”

“…161,162,171,172 For instance, the atypical PKC isozyme acts as a proproliferative kinase in fibroblasts from chronically hypoxic animals, whereas it exhibits antiproliferative actions in fibroblasts from normoxic animals. 172 These observations raise the possibility that chronic hypoxia leads to the emergence of fibroblasts in the adventitia that have lost their ability to limit stimulus-induced proliferation.…”

Involvement of MEK, ERK, PKC and GSK3B in Maintaining the Mitotic Spindle

“…Results from this study indicated that p(ser9)GSK3β and pPKCζ are possibly involved in maintaining an intact mitotic spindle independent of the MEK/ERK pathway in mouse 3T3 fibroblasts, although earlier studies have indicated that p(ser9)GSK3β can act downstream of the MAPK pathway in other cells and is involved in maintaining the dynamic instability of microtubules (Goold et al 1995;Goold et al, 1999;Scales et al, 2009). PKCζ also has been implicated by other studies to be upstream of the MAPK pathway (Berra et al, 1995;Short et al, 2006). We speculated that these kinases may independently regulate specific components of the spindle since the absence of MEK/ERK activity and the presence of p(ser9)GSK3β and pPKCζ at the centrosomes resulted in a partially intact spindle.…”

FIH-1-Mint3 Axis Does Not Control HIF-1α Transcriptional Activity in Nucleus Pulposus Cells

“…The repression is possibly mediated by HIF-2 but more likely is through a HIF-independent hypoxic pathway (e.g. through PKC, which is both activated by hypoxia (30) and capable of modulating FIH-1 expression through Cut-like homeodomain protein (CDP/Cut) (31)). …”