Increase in ALK1/ALK5 Ratio as a Cause for Elevated MMP-13 Expression in Osteoarthritis in Humans and Mice

Abstract

During osteoarthritis (OA) chondrocytes show deviant behavior resembling terminal differentiation of growth-plate chondrocytes, characterized by elevated MMP-13 expression. The latter is also a hallmark for OA. TGF-beta is generally thought to be a protective factor for cartilage, but it has also displayed deleterious effects in some studies. Recently, it was shown that besides signaling via the ALK5 (activin-like kinase 5) receptor, TGF-beta can also signal via ALK1, thereby activating Smad1/5/8 instead of Smad2/3. The Smad1/5/8 route can induce chondrocyte terminal differentiation. Murine chondrocytes stimulated with TGF-beta activated the ALK5 receptor/Smad2/3 route as well as the ALK1/Smad1/5/8 route. In cartilage of mouse models for aging and OA, ALK5 expression decreased much more than ALK1. Thus, the ALK1/ALK5 ratio increased, which was associated with changes in the respective downstream markers: an increased Id-1 (inhibitor of DNA binding-1)/PAI-1 (plasminogen activator inhibitor-1) ratio. Transfection of chondrocytes with adenovirus overexpressing constitutive active ALK1 increased MMP-13 expression, while small interfering RNA against ALK1 decreased MMP-13 expression to nondetectable levels. Adenovirus overexpressing constitutive active ALK5 transfection increased aggrecan expression, whereas small interfering RNA against ALK5 resulted in increased MMP-13 expression. Moreover, in human OA cartilage ALK1 was highly correlated with MMP-13 expression, whereas ALK5 correlated with aggrecan and collagen type II expression, important for healthy cartilage. Collectively, we show an age-related shift in ALK1/ALK5 ratio in murine cartilage and a strong correlation between ALK1 and MMP-13 expression in human cartilage. A change in balance between ALK5 and ALK1 receptors in chondrocytes caused changes in MMP-13 expression, thereby causing an OA-like phenotype. Our data suggest that dominant ALK1 signaling results in deviant chondrocyte behavior, thereby contributing to age-related cartilage destruction and OA.

Differential Signalling Through ALK-1 and ALK-5 Regulates Leptin Expression in Mesenchymal Stem Cells

“…Indeed, recent works have shown that leptin inhibits chondrogenesis [26]. Blaney Davidson et al showed an increased ALK-1/ALK-5 ratio as a cause of deviant chondrocyte behavior (elevated MMP-13 expression), which contributes to age-related cartilage destruction and osteoarthritis in humans [27,28]. Our results suggest that leptin may be one of the aspects of aberrant chondrocytes differentiation that results from changes in TGF-b signalling related to age or osteoarthritis.…”

The Effects of Aging on the Development of Osteoarthritis

“…In addition to the possible role of the SASP and oxidative stress/damage, other changes occur: an accumulation of advanced glycation end-products that make the tissue more brittle; a reduction in the size of aggrecan with resultant decreased hydration that would reduce cartilage resiliency; an increase in collagen cleavage that would reduce tensile strength; increased matrix calcification that would alter mechanical properties and activate inflammatory pathways; and a reduced response to growth factors, including insulin-like growth factor 1 (IGF-1) and transforming growth factor-β (TGF-β) that would reduce repair [10]. The altered response to TGF-β appears to occur at the receptor level due to changes in the ratio of ALK1 to ALK5 promoting more catabolic relative to anabolic activity [1,14], and for IGF-1, an altered balance of MAP kinase and Akt signaling due to ROS may be playing a role [15]. Other findings on OA chondrocytes that also might be linked to aging include reduced levels of Sirt1, a potential positive regulator of cartilage matrix gene expression [4,6], and a decline in AMPK activity, which could also promote a pro-catabolic state [13].…”

Tackling chondrocyte hypertrophy with multifunctional nanoparticles

“…Nanoparticles targeted to hypertrophic chondrocytes could also be used to improve the clinical translatability of resveratrol (a SIRT1 activator), rapamycin (a suppressor of mammalian target for rapamycin (mTOR), a negative regulator of autophagy) and Z-VAD-FMK (a pan-caspase inhibitor), in OA. [19][20][21] As an alternative to inhibiting transcription factors or activating bioenergy sensors in chondrocytes, blocking upstream signalling through receptors that have a part in chondrocyte hypertrophy, for example, activin-like kinase 1 (ALK1), 11,12 discoidin domain receptor-2 (DDR2), 22 syndecan-4 (SYND4), 23 and toll-like receptor 2 and 4 (TLR2/4), 24 by means of specific receptor antagonists can interfere with OA progression. Novel OA therapies aimed at interfering with the signalling through these receptors would benefit from using degradable nanoparticles targeted to damaged cartilage components.…”

“…5 Many transcription factors, including runt-related transcription factor-2 (RUNX2), 7 HIF-2α, 8,9 CCAAT/enhancer-binding protein-β (C/EBPβ) 10 and small mothers against decapentaplegic 1/5/8 (Smad1/5/8), 11,12 to name a few, have been identified as targets and their inhibition has been regarded as an ideal therapeutic strategy for OA. However, the activation of many of these transcription factors is not constant during OA development-for example, HIF-2α is overexpressed in early and progressive stages of OA but is downregulated in the late stages of OA 9 -which makes the time window for the therapeutic use of gene inhibitors potentially narrow.…”

“…By providing proof-of-principle that chondrocytes can be therapeutically targeted in a surgical OA mouse model through the use of nanoparticles, this investigation provides a key step in the development of novel OA approaches aimed at blocking chondrocyte hypertrophy. Nevertheless, many questions remain to be addressed before such approaches can be translated into the clinic, as we capsulize below.A pragmatic approach to tackle chondrocyte hypertrophy through multifunctional nanoparticles involves the identification of a molecular target involved in chondrocyte hypertrophy along with the assembly of a suitable combination of components in order to maximize the accumulation of drugs inside, or in proximity to, chondrocytes, while avoiding off-targeting.5 Many transcription factors, including runt-related transcription factor-2 (RUNX2), 7 HIF-2α, 8,9 CCAAT/enhancer-binding protein-β (C/EBPβ) 10 and small mothers against decapentaplegic 1/5/8 (Smad1/5/8), 11,12 to name a few, have been identified as targets and their inhibition has been regarded as an ideal therapeutic strategy for OA. However, the activation of many of these transcription factors is not constant during OA development-for example, HIF-2α is overexpressed in early and progressive stages of OA but is downregulated in the late stages of OA 9 -which makes the time window for the therapeutic use of gene inhibitors potentially narrow.…”