In the present study using tissues from two different mouse models of oxidative stress, namely, the
Sod2+/− and
Sod1−/− mice and their age-matched
WT controls, we demonstrate that the level of MMP-13 immunoreactive protein increased with age in all tissues studied. Renal MMP-13 levels were particularly sensitive to the level of oxidative stress in the old
Sod1−/−(). The heart and kidney tissues from the old
Sod2+/− and
WT animals showed a significant increase in the level of MMP-13 over their respective young controls (). However, no statistically significant differences in MMP-13 expression were observed between the old
Sod2+/− and
WT Though both the
Sod2+/− and the
Sod1−/− mice reportedly show increases in levels of oxidative stress in tissues (
Muller et al., 2006;
Van Remmen et al., 2003), it is interesting that only the kidneys from the old
Sod1−/− mice showed an oxidant-stress dependent increase in MMP-13 expression though a trend exists in the old
Sod2+/− animals. In addition, the skeletal muscles from the old
Sod1−/− mice showed a significant increase in MMP-13 levels over their age-matched controls. It is interesting to note here that even at a comparatively early age (12–14 months), the old
Sod1−/− mice show a significant increase in MMP-13 levels in their skeletal muscles compared to their age-matched
WT controls () suggesting that oxidative stress also exacerbates age-dependent MMP-13 expression in this tissue.
The
Sod1−/− mice have a shortened lifespan compared to their
WT controls while a single functional
Sod2 allele is sufficient to maintain a normal lifespan (
Muller et al., 2006;
Van Remmen et al., 2003). In addition, loss of single
Sod2 allele leads to oxidative damage only to the DNA, while homozygous loss of
Sod1 damages all major biomolecules (
Muller et al., 2007). Oxidative stress is manifested in terms of increased incidence of cancer with no other differences in other ageing markers in the
Sod2+/− while the
Sod1−/− show more severe pathological outcomes including increased cataract development (
Olofsson et al., 2005), early hearing loss (
Keithley et al., 2005), skeletal muscle atrophy and increased incidence of cancer (
Muller et al., 2006). Thus, the Sod1 dismuting activity is critical for maintenance of redox homeostasis in cells and tissues. There is evidence to show that loss of Sod1 leads to significant damage to the mitochondria even though Sod2 levels in the organelle are intact (
Aquilano et al., 2006). Sod1 deficiency is associated with decreases in mitochondrial membrane potential and ATP synthesis, both of which are critical for maintenance of mitochondrial integrity. Overall, these observations suggest that the
Sod1−/− mice are likely exposed to high levels of oxidative damage and may explain the more robust increase in the expression of MMP-13, relative to animals with a partial deficiency in Sod2 activity.
In spite of the differences seen between the two mouse models, both displayed an age-dependent increase in MMP-13, which appear to be exacerbated by oxidative stress. The age-associated increase in MMP-13 was associated with an increase in phosphorylation of MAPKs – Erk, JNK and p38 () all of which contribute to regulating MMP expression (
Wu et al., 2004). Furthermore, MAPK phosphorylation was increased by Sod deficiency in a tissue-specific manner (). We have previously established that JNK plays an important role in the redox-responsiveness of the human MMP-1 promoter (
Nelson et al., 2006). Analysis of JNK abundance revealed that JNK2 was prominently upregulated in the aged Sod1-deficient mice. Work by Firestein and coworkers defined the importance of both JNK1 and JNK2 in human fibroblast-like synoviocytes and in a murine model of rheumatoid arthritis (
Han et al., 2001). In both cases the JNK inhibitor blocked hMMP-1 or mMMP-13 expression
in vitro and
in vivo, respectively. In their model system, JNK2 is the dominant JNK protein and is likely the most physiologically relevant as it binds c-Jun with greater avidity than the other JNKs. In addition, the IL-1 dependent expression of mMMP-13 is more severely impaired in JNK2- than in JNK1-deficient MEFs. It is possible that this age-dependent enhancement of JNK2 expression in the Sod1-deficient animals may participate in the increased expression of MMP-1 in the renal tissue.
The PI3K pathway is also known to regulate MMP expression (
Lechuga et al., 2004). Phosphorylation of Akt, a downstream target of PI3K, showed similar patterns in the two mouse models. While the
Sod2+/− and
WT mice showed significant age-dependent increase in Akt phosphorylation () this differences was not as striking when comparing the
Sod1−/− mice and their aged matched controls. The less striking age-dependent increase in Pi-Akt in the
Sod1−/− relative to
Sod2+/− is likely attributed to distinct chronological age of the two cohorts (
Sod1−/−, 16–18 months vs
Sod2+/−, 26 months).
A high level of interstitial collagenase in humans and rodents is associated with pathologies in diverse organs. Our studies were restricted to liver, kidney, heart and skeletal muscle tissues in either one or both murine models. These organs have been shown to be prone to MMP-dependent age related pathologies and our findings suggest that aberrant collagenase production may exacerbate the disease conditions in sensitized/susceptible individuals.
Kidneys lose function in an age-dependent manner that involves loss of renal mass and increased fibrosis both of which involve augmented MMP activity (
Ahmed et al., 2007). We have demonstrated that MMP-13 levels in kidneys seem to be particularly sensitive to increased oxidative stress (). High level of MMP-1 expression appears to be involved in renal fibrosis (
Catania et al., 2007), a major cause of dialysis and kidney transplants, as well as in initiating glomerular remodeling in progressive kidney scarring (
Denzinger et al., 2007). Our findings suggest that collagen deposition appears to dissipate in the Bowmans capsule. This cup-like structure houses the glomerular vessels and forms the filtration barrier with basement membrane (BM). This region is composed of laminin and type IV collagen is crucial for filtration. MMPs degrade extracellular matrix components such collagen, gelatin, fibronectin and laminins and it is possible that uncontrolled expression of MMPs in the aged and SOD deficient mice might affect filtration impairing renal function. This is clearly evident in Alport syndrome where mutations impacting collagen synthesis hamper production and assembly of collagen and composition of basement membranes. Associated with this disorder is a decline in renal function typically leading to renal failure. MMPs are also involved in other kidney pathologies such as acute kidney injury, chronic allograft nephropathy, diabetic nephropathy and polycystic kidney disease (
Hirata et al., 2004). Apart from fibrosis MMPs also play an important role in the progression of renal cell carcinoma that appears to have a higher tumor grade and poor survival with age in humans (
Nikkari et al., 1995) and is dependent on the MMP-1 and MMP-3 haplotype (
Brown et al., 1995).
One of the phenotypes of the
Sod1−/− mice is their loss of skeletal muscle tissue (
Muller et al., 2006). MMP-13 profiles in the skeletal muscle of
WT mice show age-dependent increases (). MMPs are associated with various muscle-related dystrophies and muscle degeneration(
Carmeli et al., 2004). Whether increased MMP-13 in skeletal muscles contributes to sarcopenia in aged
Sod1−/− animals remains to be determined.
MMPs are also important in other pathologies such as cancer and arthritis (
Tasci et al., 2008). High levels of MMP-13 have been reported in rat and murine models of arthritis (
Woo et al., 2007) Aberrant MMP-1 expression has been linked with bladder cancer (
Zhu et al., 2001) and colorectal cancer (
Zhu et al., 2001). MMP-1 is thought to contribute to tumor initiation and development (
Lee et al., 2005) promoting a tissue microenvironment that is permissive to tumor growth. MMPs can act on more than just the components of the ECM, including growth factors, cytokines and chemokines. MMP-dependent cleavage of these factors has also been shown to activate and increase their bioavailability (
Churg et al., 2003). Cardiac hypertrophy in response to increased IGF signaling has also been linked to increased MMP-1 and MMP-2 activity during β-adrenergic stimulation that is blocked by MMP inhibition (
Pardo and Selman, 2005). MMPs can also cleave and release membrane bound TNF-α making it available for signaling as observed in cigarette-smoke induced inflammation (
Churg et al., 2003). This latter scenario may increase the pathology of lung emphysema associated with smoking, which is also exacerbated with age. MMP-1 can also cleave and activate other MMPs serving to aggravate ECM degradation process. Thus, high endogenous levels of MMP-13 in aged murine tissues and that of the different collagenase in humans might serve to make the tissue microenvironment more susceptible to damage, decrease the threshold for disease induction and increase the pathology of diseases.
To summarize, the current work demonstrates that MMP-13 increases with age in murine tissues and can be used as a biomarker for ageing and potentially oxidative insult. It is important to emphasize that the fold increase in age-dependent induction of MMP-13, ERK1/2, p38, JNK and Akt remains relatively constant between the SOD genotypes studied. However, the amplitude of the signals driving their induction is redox-sensitive. Thus, increases in MMP-13 expression with age are likely responsible for aberrant extracellular matrix degradation and may be exacerbated by conditions that augment oxidant production.