The etiologies of RA and OA are quite different, in that RA is caused by immune dysfunction and chronic inflammation, while OA is the consequence of years of mechanical stress on the articular cartilage. A common feature, however, of these two diseases is the proteolytic degradation and ultimate destruction of articular cartilage, which results in loss of joint function. In RA, inflammatory cytokines such as IL-1 and tumor necrosis factor-α (TNF-α) are produced by activated macrophages in the synovium. These cytokines stimulate connective tissue cells such as synovial fibroblasts and articular chondrocytes to produce MMP-1 and MMP-13 [15
]. In OA, the mechanical insult causes cytokine expression by articular chondrocytes, with subsequent autocrine MMP expression [14
]. Upon ligand binding, IL-1 and TNF-α receptors each recruit a unique set of receptor-associated proteins that transduce the stimulus into the cell. Beyond the particularities of their receptors, however, IL-1 and TNF-α elicit a series of shared phosphorylation events within the cells that facilitate transcriptional induction of MMPs.
One group of proteins that mediate some of these phosphorylation events is the mitogen-activated protein kinase (MAPK) group [19
]. The MAPK family of serine/threonine kinases consists of the c-Jun N-terminal kinases (JNKs), the extracellular signal-regulated kinases (ERKs) and the p38 kinases. The JNKs and p38 kinases are activated in response to inflammatory cytokines, osmotic stress and apoptotic signals [20
], while the ERKs respond to cytokines, growth factors and phorbol esters [19
] (Fig. ). These stimuli first activate a group of protein kinases (MAPK kinase kinases [MAPKKKs]) that phosphorylate other kinases (MAPK kinases [MAPKKs]), which in turn are responsible for phosphorylation and activation of MAPK. Upon activation by MAPKKs, MAPKs translocate to the nucleus to phosphorylate and activate various transcription factors. Of particular relevance to MMP transcription, JNKs and ERKs phosphorylate and activate the activating protein-1 (AP-1) family member c-Jun [22
], which dimerizes with c-Fos to drive transcription of multiple MMP genes. In addition to c-jun
, the ERK pathway regulates the activity of erythroblastosis twenty-six (Ets) transcription factors [24
], which cooperate with AP-1 proteins in multiple MMP promoters. To date, there are no known targets of p38 that directly regulate MMP promoters. However, p38 phosphorylates activating transcription factor-2, which then drives both the c-jun
promoter and the ternary complex factor Elk-1, which activates the c-fos
]. Thus, by promoting expression of AP-1 genes, p38 may indirectly contribute to MMP transcription.
Figure 1 Activation of mitogen-activated protein kinase (MAPK; shown in yellow) pathways by IL-1. Stimulation by IL-1 activates the MAPK kinase kinases (MAPKKKs; shown in blue), transforming-growth-factor-β-activated kinase-1 (TAK 1) and Raf, which then (more ...)
Another major cytokine-induced signaling pathway involves translocation of nuclear factor-κB (NF-κB) family members from the cytoplasm to the nucleus (Fig. ). Upon binding of IL-1 to its cognate receptor, transforming-growth-factor-β-activated kinase becomes active, leading to the activation of the NF-κB-inducing kinase (NIK) [26
]. In turn, NIK is responsible for the phosphorylation and activation of the inhibitor of κB (IκB) kinases (IKKs), which then phosphorylate IκB [27
]. In resting cells, IκB binds to, and sequesters, dimers of the NF-κB1/p50 and c-rel
-related factor A (RelA)/p65 NF-κB subunits in the cytoplasm. Upon phosphorylation, however, IκB becomes ubiquitinated and is targeted for proteosome-mediated degradation. Loss of IκB leaves the p50/p65 dimers free to translocate to the nucleus and transactivate several genes including those for some MMPs. Indeed, when NF-κB is maintained in the cytoplasm by constitutive levels of IκB, reduced expression of MMP-1, MMP-3 and MMP-13 is observed in cytokine-stimulated cells [7
Figure 2 Activation of the NF-κB pathway by IL-1. IL-1 binds to its receptor (IL-1R1) and receptor-associated protein (IL-1RAcP), causing conformational changes in multiple receptor-bound proteins (MyD88, IRAK, TRAF6, TAB2; see Figure (more ...)
In their latent forms, some NF-κB family members function as IκB proteins. NF-κB1 is a 105 kDa protein that has its carboxyl terminus cleaved to yield the 50 kDa NF-κB subunit (p50). NF-κB2 is a 100 kDa protein that is cleaved similarly to yield the 52 kDa NF-κB subunit (p52) [27
]. In their latent states, both NF-κB1/p105 and NF-κB2/p100 can sequester p50 and p52 in the cytoplasm. Recent evidence suggests that the NIK and IKK activation leads to phosphorylation, ubiquitination and degradation of NF-κB1 and NF-κB2 [31
]. This process results in the release of p50 and p52, so that they can translocate to the nucleus. Heissmeyer et al.
reported, however, that IKK-dependent degradation of NF-κB1 is independent of NF-κB1 processing [32
], so that changes in the total amount of p50 and p52 may be controlled by a different mechanism. The functional consequence of this alternative pathway is not completely understood, since liberation of p50 from p105 leads to the association of p50 homodimers in the nucleus [34
], and p50 homodimers can repress NF-κB-dependent transcription by p50/p65 heterodimers [35
Transcriptional regulation by dimers of NF-κB containing p50 and/or p52 appears to require an IκB-related protein, Bcl-3. Following degradation of p105, Bcl-3 promotes p50 homodimer formation by creating a stable p50/p50/ Bcl-3 trimeric complex [34
]. Bcl-3 can then act as a coactivator molecule for p50 and directly contribute to transcriptional activation by p50 homodimers. Alternatively, Bcl-3 can inhibit the binding of p50 homodimers to certain promoter elements, and this frees these sites for transactivation by p50/p65 heterodimers [36
The MAPK and NF-κB pathways are coordinately activated by IL-1 and TNF-α, and are central pathways in RA and OA pathogenesis. While these kinase cascades lead to the transcription of an array of inflammatory genes, their direct regulation of MMP transcription is just beginning to be elucidated. In the remainder of this review, we address how these pathway-specific signals lead to the recruitment of a cohort of transcription factors that cooperate to initiate MMP-1 and MMP-13 transcription.