Arthritis & Rheumatism, Volume 62,
November 2010 Abstract Supplement

Abstracts of the American College of
Rheumatology/Association of Rheumatology Health Professionals
Annual Scientific Meeting
Atlanta, Georgia November 6-11, 2010.


MAPK Kinase 3 (MKK3) Regulates Osteoclast Differentiation and Activation.

Boyle3,  David L., Edgar4,  Meghan, Zaiss1,  Mario M., Schett2,  Georg, Firestein3,  Gary S.

Department of Internal Medicine 3, University of Erlangen-Nuremberg
Friedrich Alexander Univ, Erlangen, Germany
UCSD School of Medicine, La Jolla, CA
UCSD School of Medicine

Purpose:

p38 MAPK inhibitors have modest efficacy in diseases like rheumatoid arthritis (RA), in part because exposure is limited by side effects. Two upstream kinases regulate p38 function, namely MKK3 and MKK6. These two MKKs have non-redundant functions and control expression of complementary subsets of pro-inflammatory cytokines. Deficiency of either MKK decreases inflammation in the passive K/BxN model of arthritis. To explore distinct functions of MKK3 and MKK6, we examined their relative contributions to joint damage and bone resorption by evaluating the differentiation and function of MKK deficient osteoclasts. The data could support developing MKK inhibitors for bone protection in RA and in osteoporosis.

Methods:

BM monocytes from wild type (WT), MKK3-/-, and MKK6-/- mice were differentiated with M-CSF (30ng/ml) and non-adherent cells subsequently cultured with M-CSF (30ng/ml) and RANKL (50ng/ml) for 5 days in the Osteologic Bone Cell Culture System. Osteoclast number and matrix resorption were determined by TRAP staining followed by light microscopy and automated image analysis. SB203580 was used at 3mg/ml. Gene expression and p38 phosphorylation were determined by qPCR and Western Blot analysis, respectively.

Results:

The number of osteclasts generated from BM monocytes was similar in WT and MKK6-/- mice. Surprisingly, MKK3-/- osteoclast differentiation was lower than WT or MKK6-/- (n=9, 33% inhibition, P<0.004). The p38 inhibitor SB203580 inhibited osteoclast differentiation by 74% (P=0.001). In vitro matrix resorption by osteoclasts was decreased by 46% by MKK3-/- osteoclasts (P=0.002), while MKK6 deficiency had no effect compared with WT cells. p38 activation, as measured by determining relative P-p38/p38 ratios, for WT, MKK6-/-, and MKK3-/- osteoclasts was 0.71±0.09, 0.54±0.05, and 0.09±0.02, respectively (n=3, P=0.001 for MKK3-/-). MMP13 expression was similar in all genotypes, but expression of the key bone resorbing proteinase cathepsin K was significantly lower in MKK3-/- osteoclasts (67% inhibition; P=0.0001) but not in MKK6-/- cells.

Conclusions:

MKK3, but not MKK6, is required for osteoclast differentiation and activation, p38 phosphorylation, and cathepsin K gene expression. Selective MKK3 inhibitors could be targeted for RA in order to suppress bone erosions and inflammation while sparing p38 functions that might contribute to toxicity. In addition, an MKK3 inhibitor could have utility as an oral therapy for osteoporosis.

To cite this abstract, please use the following information:
Boyle, David L., Edgar, Meghan, Zaiss, Mario M., Schett, Georg, Firestein, Gary S.; MAPK Kinase 3 (MKK3) Regulates Osteoclast Differentiation and Activation. [abstract]. Arthritis Rheum 2010;62 Suppl 10 :639
DOI: 10.1002/art.28407

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