Protein Kinase C Isoforms ζ and ι Mediate Collagenase Expression and Cartilage Destruction via STAT3- and ERK-dependent c-fos Induction

Abstract

The protein kinase C (PKC) signaling pathway is a major regulator of cellular functions and is implicated in pathologies involving extracellular matrix remodeling. Inflammatory joint disease is characterized by excessive extracellular matrix catabolism, and here we assess the role of PKC in the induction of the collagenases, matrix metalloproteinase (MMP)-1 and MMP-13, in human chondrocytes by the potent cytokine stimulus interleukin-1 (IL-1) in combination with oncostatin M (OSM). IL-1 ؉ OSM-stimulated collagenolysis and gelatinase activity were ameliorated by pharmacological PKC inhibition in bovine cartilage, as was collagenase gene induction in human chondrocytes. Small interfering RNA-mediated silencing of PKC gene expression showed that both novel (nPKC␦, nPKC) and atypical (aPKC, aPKC) isoforms were involved in collagenase induction by IL-1. However, MMP1 and MMP13 induction by IL-1 ؉ OSM was inhibited only by aPKC silencing, suggesting that only atypical isoforms play a significant role in complex inflammatory milieus. Silencing of either aPKC led to diminished IL-1 ؉ OSM-dependent extracellular signal-regulated kinase (ERK) and signal transducer and activator of transcription (STAT) 3 phosphorylation, and c-fos expression. STAT3 gene silencing or ERK pathway inhibition also resulted in loss of IL-1 ؉ OSM-stimulated c-fos and collagenase expression. Silencing of c-fos and c-jun expression was sufficient to abrogate IL-1 ؉ OSM-stimulated collagenase gene induction, and overexpression of both c-fos and c-jun was sufficient to drive transcription from the MMP1 promoter in the absence of a stimulus. Our data identify atypical PKC isozymes as STAT and ERK activators that mediate c-fos and collagenase expression during IL-1 ؉ OSM synergy in human chondrocytes. aPKCs may constitute potential therapeutic targets for inflammatory joint diseases involving increased collagenase expression.Inflammation is a major characteristic of joint diseases such as rheumatoid arthritis and osteoarthritis (OA), 3 during which inflammatory mediators released by infiltrating immune cells as well as resident joint cells induce alterations in gene expression that can lead to ECM degradation (1, 2). Cytokines such as interleukin (IL-)1, IL-17, and tumor necrosis factor ␣ (TNF␣) are key mediators thought to be involved in promoting inflammatory responses in such destructive joint diseases (3, 4). We have shown that the IL-6 family cytokines oncostatin M (OSM) and IL-6 markedly exacerbate the catabolic potential of these mediators, synergistically promoting cartilage ECM catabolism both in vitro and in vivo (5-9).Chondrocytes are the only resident cell type in normal articular cartilage and function to preserve homeostasis. This is achieved by balancing the expression of ECM components with catabolic factors such as the matrix metalloproteinases (MMPs), which collectively can degrade all the ECM macromolecules. During inflammatory joint diseases, chondrocytes are stimulated to secrete elevated levels of MMPs that, once activated, media...

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