Function of the chondrocyte PI-3 kinase-Akt signaling pathway is stimulus dependent

M.A Greene,R.F Loeser

doi:10.1016/j.joca.2015.01.014

M.A Greene, R.F Loeser

Open Access

https://doi.org/10.1016/j.joca.2015.01.014

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Abstract

OBJECTIVEThe PI-3 kinase-Akt pathway plays a role in cartilage anabolic as well as catabolic processes in response to activation by insulin-like growth factor-1 (IGF-1) and the pro-inflammatory cytokines interleukin-1β (IL-1β) and oncostatin M (OSM). The goal of this study was to determine how PI-3 kinase-Akt signaling regulates these seemingly opposing functions.DESIGNMonolayer cultures of primary human articular chondrocytes were treated with IGF-1, IL-1β, OSM, or the combination of IL-1β and OSM in time course experiments. Activation of signaling proteins and MMP production were measured by immunoblotting. Cells were pre-treated with chemical inhibitors to block MAP kinases, PI-3 kinase, or JAK/STAT pathway activation. Constitutively active Akt1 and Akt3 were expressed to study stimulus-independent activation of Akt.RESULTSIGF-1, OSM, and the combination of IL-1β and OSM but not IL-1β alone, stimulated phosphorylation of Akt which was sustained longer with IGF-1. IL-1β plus OSM, but not IGF-1, increased chondrocyte MMP-13 production which was inhibited with either a general PI-3 kinase inhibitor or specific inhibition of the PI-3 kinase-γ isoform. Akt1 or Akt3 activity alone was not sufficient to increase production of MMP-13. IL-1β/OSM induced MMP-13 production required activation of the MAP kinases, JNK and p38, as well as the JAK-STAT pathway which were activated by IL-1β plus OSM but not by IGF-1.CONCLUSIONSThe chondrocyte integrates signals from the PI-3 kinase-Akt pathway with signals from MAP kinases and the JAK-STAT pathway to allow for a differential response to a pro-anabolic (IGF-1) and a pro-catabolic (IL-1β plus OSM) stimulus.

Highlights

The activity of intracellular signaling pathways in chondrocytes is regulated by soluble mediators and by changes in the extracellular matrix[1,2]
1b/oncostatin M (OSM) in stimulation of MMP-13 is due to differential activation of mitogen activated protein (MAP) kinases and STAT3. These findings suggest that MAP kinase or JAK-STAT pathways would be more appropriate as a therapeutic target given the pro-anabolic and cell survival role of Akt
Primary articular chondrocytes were treated with insulin-like growth factor 1 (IGF-1), IL-1b, OSM, or the combination of IL-1b and OSM at increasing time points from five to 60 min and relevant signaling proteins were evaluated for phosphorylation status which, for the chosen proteins, correlates with activation

Summary

Introduction

The activity of intracellular signaling pathways in chondrocytes is regulated by soluble mediators and by changes in the extracellular matrix[1,2]. Major pathways that have been shown to be active in OA chondrocytes include the mitogen activated protein (MAP) kinase family, the Wnts, Smads, the JAK-STAT pathway, toll-like receptor pathways, and the NF-kB pathway[1,3]. These pathways mediate both anabolic and catabolic functions in cartilage and likely work in concert to determine the overall balance of matrix synthesis and degradation. Akt isoforms 1 and 3 have been shown to be necessary for the IL-1b/OSM stimulated MMP-13 production[7] These findings describing the role of the PI-3 kinase-Akt pathway in MMP-13 production have led to the suggestion that this pathway would be an effective target in the treatment of OA10

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