MT1-MMP-dependent and -independent regulation of gelatinase A activation in long-term, ascorbate-treated fibroblast cultures: regulation by fibrillar collagen

The synergistic effect of low oxygen tension and macromolecular crowding in the development of extracellular matrix-rich tendon equivalents

Cellular therapies play an important role in tendon tissue engineering, with tenocytes being the most prominent and potent cell population available. However, for the development of a rich extracellular matrix tenocyte-assembled tendon equivalent, prolonged in vitro culture is required, which is associated with phenotypic drift. Recapitulation of tendon tissue microenvironment in vitro with cues that enhance and accelerate extracellular matrix synthesis and deposition, whilst maintaining tenocyte phenotype, may lead to functional cell therapies. Herein, we assessed the synergistic effect of low oxygen tension (enhances extracellular matrix synthesis) and macromolecular crowding (enhances extracellular matrix deposition) in human tenocyte culture. Protein analysis demonstrated that human tenocytes at 2% oxygen tension and with 50 μg ml^-1 carrageenan (macromolecular crowder used) significantly increased synthesis and deposition of collagen types I, III, V and VI. Gene analysis at day 7 illustrated that human tenocytes at 2% oxygen tension and with 50 μg ml^-1 carrageenan significantly increased the expression of prolyl 4-hydroxylase subunit alpha 1, procollagen-lysine 2- oxoglutarate 5-dioxygenase 2, scleraxis, tenomodulin and elastin, whilst chondrogenic (e.g. runt-related transcription factor 2, cartilage oligomeric matrix protein, aggrecan) and osteogenic (e.g. secreted phosphoprotein 1, bone gamma-carboxyglutamate protein) trans-differentiation markers were significantly down-regulated or remained unchanged. Collectively, our data clearly illustrates the beneficial synergistic effect of low oxygen tension and macromolecular crowding in the accelerated development of tissue equivalents.

Low, but not too low, oxygen tension and macromolecular crowding accelerate extracellular matrix deposition in human dermal fibroblast culture

A key challenge of in vitro organogenesis is the development in timely manner tissue equivalents. Herein, we assessed the simultaneous effect of oxygen tension (0.5%, 2% and 20%), foetal bovine serum concentration (0.5% and 10%) and macromolecular crowding (75μg/ml carrageenan) in human dermal fibroblast culture. Our data demonstrate that cells cultured at 2% oxygen tension, in the presence of carrageenan and at 0.5% serum concentration deposited within 3days in culture more extracellular matrix than cells grown for 14days, at 20% oxygen tension, 10% serum concentration and in the absence of carrageenan. These data suggest that optimal oxygen tension coupled with macromolecular crowding are important in vitro microenvironment modulators for accelerated development of tissue-like modules in vitro.

STATEMENT OF SIGNIFICANCE

To enable clinical translation and commercialisation of in vitro organogenesis therapies, we cultured human dermal fibroblast at 2% oxygen tension, under macromolecular crowding conditions (75μg/ml carrageenan) and at low foetal bovine serum concentration (0.5%). Within 3days in culture, more extracellular matrix was deposited under these conditions than cells grown for 14days, at 20% oxygen tension, 10% FBS concentration and in the absence of crowding agents. These data bring us closer to the development of more clinically relevant tissue-like modules.

Fibroblast-Derived MMP-14 Regulates Collagen Homeostasis in Adult Skin

Proteolytic activities in the extracellular matrix by the matrix metalloproteinase (MMP)-14 have been implicated in the remodeling of collagenous proteins during development. To analyze the function of fibroblast-derived MMP-14 in adult skin homeostasis, we generated mice with inducible deletion of MMP-14 in the dermal fibroblast (MMP-14^Sf–/–). These mice are smaller and display a fibrosis-like phenotype in the skin. The skin of these mice showed increased stiffness and tensile strength but no altered collagen cross-links. In vivo, we measured a significantly increased amount of collagen type I accumulated in the skin of MMP-14^Sf–/– mice without an increase in collagen fibril diameters. However, bleomycin-induced fibrosis in skin proceeded in a comparable manner in MMP-14^Sf+/+ and MMP-14^Sf–/– mice, but resolution over time was impaired in MMP-14^Sf–/– mice. Increased accumulation of collagen type I was detected in MMP-14^Sf–/– fibroblasts in culture without significant enhancement of collagen de novo synthesis. This points to a degradative but not synthetic phenotype. In support of this, MMP-14^Sf–/– fibroblasts lost their ability to process fibrillar collagen type I and to activate proMMP-2. Taken together, these data indicate that MMP-14 expression in fibroblasts plays a crucial role in collagen remodeling in adult skin and largely contributes to dermal homeostasis underlying its pathogenic role in fibrotic skin disease.

Src-dependent Tks5 phosphorylation regulates invadopodia-associated invasion in prostate cancer cells

BACKGROUND

The Src tyrosine kinase substrate and adaptor protein Tks5 had previously been implicated in the invasive phenotype of normal and transformed cell types via regulation of cytoskeletal structures called podosomes/invadopodia. The role of Src-Tks5 signaling in invasive prostate cancer, however, had not been previously evaluated.

METHODS

We measured the relative expression of Tks5 in normal (n = 20) and cancerous (n = 184, from 92 patients) prostate tissue specimens by immunohistochemistry using a commercially available tumor microarray. We also manipulated the expression and activity of wild-type and mutant Src and Tks5 constructs in the LNCaP and PC-3 prostate cancer cell lines in order to ascertain the role of Src-Tks5 signaling in invadopodia development, matrix-remodeling activity, motility, and invasion.

RESULTS

Our studies demonstrated that Src was activated and Tks5 upregulated in high Gleason score prostate tumor specimens and in invasive prostate cancer cell lines. Remarkably, overexpression of Tks5 in LNCaP cells was sufficient to induce invadopodia formation and associated matrix degradation. This Tks5-dependent increase in invasive behavior further depended on Src tyrosine kinase activity and the phosphorylation of Tks5 at tyrosine residues 557 and 619. In PC-3 cells we demonstrated that Tks5 phosphorylation at these sites was necessary and sufficient for invadopodia-associated matrix degradation and invasion.

CONCLUSIONS

Our results suggest a general role for Src-Tks5 signaling in prostate tumor progression and the utility of Tks5 as a marker protein for the staging of this disease.

Functional interactions between matrix metalloproteinases and glycosaminoglycans

Similar to most proteinases, matrix metalloproteinases (MMP) do not recognize a consensus cleavage site. Thus, it is not surprising that, in a defined in vitro reaction, most MMPs can act on a wide range of proteins, including many extracellular matrix proteins. However, the findings obtained from in vivo studies with genetic models have demonstrated that individual MMPs act on just a few extracellular protein substrates, typically not matrix proteins. The limited, precise functions of an MMP imply that mechanisms have evolved to control the specificity of proteinase:substrate interactions. We discuss the possibility that interactions with the glycosaminoglycan chains of proteoglycans may function as allosteric regulators or accessory factors directing MMP catalysis to specific substrates. We propose that understanding how the activity of specific MMPs is confined to discreet compartments and targeted to defined substrates via interactions with other macromolecules may provide a means of blocking potentially deleterious MMP-mediated processes at the same time as sparing any beneficial functions.

Chymase-mediated intestinal epithelial permeability is regulated by a protease-activating receptor/matrix metalloproteinase-2-dependent mechanism

Mast cells regulate intestinal barrier function during disease and homeostasis. Secretion of the mast cell-specific serine protease chymase regulates homeostasis. In the present study, we employ in vitro model systems to delineate the molecular pathways involved in chymase-mediated intestinal epithelial barrier dysfunction. Chymase stimulation of intestinal epithelial (Caco-2 BBe) cell monolayers induced a significant reduction in transepithelial resistance, indicating decreased intestinal epithelial barrier function. The chymase-induced intestinal epithelial barrier dysfunction was characterized by chymase-induced protease-activated receptor (PAR)-2 activation and matrix metalloproteinase (MMP)-2 expression and activation. Consistent with this observation, in vitro analysis revealed chymase-induced PAR-2 activation and increased MAPK activity and MMP-2 expression. Pharmacological and small interfering RNA-mediated antagonism of PAR-2 and MMP-2 significantly attenuated chymase-stimulated barrier dysfunction. Additionally, the chymase/MMP-2-mediated intestinal epithelial dysfunction was associated with a significant reduction in the tight junction protein claudin-5, which was partially restored by MMP-2 inhibition. Finally, incubation of Caco-2 BBe cells with chymase-sufficient, but not chymase-deficient, bone marrow-derived mast cells decreased barrier function, which was attenuated by the chymase inhibitor chymostatin. Collectively, these results suggest that mast cell/chymase-mediated intestinal epithelial barrier function is mediated by PAR-2/MMP-2-dependent pathways.

The relationship between the MMP system, adrenoceptors and phosphoprotein phosphatases

The MMPs and their inhibitors [tissue inhibitor of MMPs (TIMPs)] form the mainstay of extracellular matrix homeostasis. They are expressed in response to numerous stimuli including cytokines and GPCR activation. This review highlights the importance of adrenoceptors and phosphoprotein phosphatases (PPP) in regulating MMPs in the cardiovascular system, which may help explain some of the beneficial effects of targeting the adrenoceptor system in tissue remodelling and will establish emerging crosstalk between these three systems. Although α- and β-adrenoceptor activation increases MMP but decreases TIMP expression, MMPs are implicated in the growth stimulatory effects of adrenoceptor activation through transactivation of epidermal growth factor receptor. Furthermore, they have recently been found to catalyse the proteolysis of β-adrenoceptors and modulate vascular tone. While the mechanisms underpinning these effects are not well defined, reversible protein phosphorylation by kinases and phosphatases may be key. In particular, PPP (Ser/Thr phosphatases) are not only critical in resensitization and internalization of adrenoceptors but also modulate MMP expression. The interrelationship is complex as isoprenaline (ISO) inhibits okadaic acid [phosphoprotein phosphatase type 1/phosphoprotein phosphatase type 2A (PP2A) inhibitor]-mediated MMP expression. While this may be simply due to its ability to transiently increase PP2A activity, there is evidence for MMP-9 that ISO prevents okadaic acid-mediated expression of MMP-9 through a β-arrestin, NF-κB-dependent pathway, which is abolished by knock-down of PP2A. It is essential that crosstalk between MMPs, adrenoceptors and PPP are investigated further as it will provide important insight into how adrenoceptors modulate cardiovascular remodelling, and may identify new targets for pharmacological manipulation of the MMP system.

Physiological type I collagen organization induces the formation of a novel class of linear invadosomes

This study shows that fibrillar collagen I is the physiological inducer of a novel class of invadosomes, which we named “linear invadosomes.” They are dependent on the scaffold protein Tks5 and are able to degrade extracellular matrix elements. Moreover, we demonstrate that they are β1- and β3-integrin independent, unlike classical invadosomes.

Invadosomes are F-actin structures capable of degrading the matrix through the activation of matrix metalloproteases. As fibrillar type I collagen promotes pro-matrix metalloproteinase 2 activation by membrane type 1 matrix metalloproteinase, we aimed at investigating the functional relationships between collagen I organization and invadosome induction. We found that fibrillar collagen I induced linear F-actin structures, distributed along the fibrils, on endothelial cells, macrophages, fibroblasts, and tumor cells. These structures share features with conventional invadosomes, as they express cortactin and N-WASP and accumulate the scaffold protein Tks5, which proved essential for their formation. On the basis of their ability to degrade extracellular matrix elements and their original architecture, we named these structures “linear invadosomes.” Interestingly, podosomes or invadopodia were replaced by linear invadosomes upon contact of the cells with fibrillar collagen I. However, linear invadosomes clearly differ from classical invadosomes, as they do not contain paxillin, vinculin, and β1/β3 integrins. Using knockout mouse embryonic fibroblasts and RGD peptide, we demonstrate that linear invadosome formation and activity are independent of β1 and β3 integrins. Finally, linear invadosomes also formed in a three-dimensional collagen matrix. This study demonstrates that fibrillar collagen I is the physiological inducer of a novel class of invadosomes.

Overexpression of proto-oncogene FBI-1 activates membrane type 1-matrix metalloproteinase in association with adverse outcome in ovarian cancers

Background

FBI-1 (factor that binds to the inducer of short transcripts of human immunodeficiency virus-1) is a member of the POK (POZ and Kruppel) family of transcription factors and play important roles in cellular differentiation and oncogenesis. Recent evidence suggests that FBI-1 is expressed at high levels in a subset of human lymphomas and some epithelial solid tumors. However, the function of FBI-1 in human ovarian cancers remains elusive.

Results

In this study, we investigated the role of FBI-1 in human ovarian cancers, in particularly, its function in cancer cell invasion via modulating membrane type 1-matrix metalloproteinase (MT1-MMP). Significantly higher FBI-1 protein and mRNA expression levels were demonstrated in ovarian cancers samples and cell lines compared with borderline tumors and benign cystadenomas. Increased FBI-1 mRNA expression was correlated significantly with gene amplification (P = 0.037). Moreover, higher FBI-1 expression was found in metastatic foci (P = 0.036) and malignant ascites (P = 0.021), and was significantly associated with advanced stage (P = 0.012), shorter overall survival (P = 0.032) and disease-free survival (P = 0.016). In vitro, overexpressed FBI-1 significantly enhanced cell migration and invasion both in OVCA 420 and SKOV-3 ovarian carcinoma cells, irrespective of p53 status, accompanied with elevated expression of MT1-MMP, but not MMP-2 or TIMP-2. Moreover, knockdown of MT1-MMP abolished FBI-1-mediated cell migration and invasion. Conversely, stable knockdown of FBI-1 remarkably reduced the motility of these cells with decreased expression of MT1-MMP. Promoter assay and chromatin immunoprecipitation study indicated that FBI-1 could directly interact with the promoter spanning ~600bp of the 5'-flanking sequence of MT1-MMP and enhanced its expression in a dose-dependent manner. Furthermore, stable knockdown and ectopic expression of FBI-1 decreased and increased cell proliferation respectively in OVCA 420, but not in the p53 null SKOV-3 cells.

Conclusions

Our results suggested an important role of FBI-1 in ovarian cancer cell proliferation, cell mobility, and invasiveness, and that FBI-1 can be a potential target of chemotherapy.

Effects of ovariectomy and resistance training on MMP-2 activity in rat calcaneal tendon

Tendon remodeling relies on extracellular matrix (ECM) restructuring by the matrix metallopeptidases (MMPs). The aim of this study was to investigate MMP-2 activity in different regions of the calcaneal tendon (CT) after resistance training (RT) in ovariectomized rats. Wistar adult female rats were grouped into sedentary (Sed-Intact), ovariectomized sedentary (Sed-Ovx), acute exercise (AcuteEx-Intact), ovariectomized acute exercise (AcuteEx-Ovx), resistance trained (ChronicEx-Intact), and ovariectomized resistance trained (ChronicEx-Ovx) (n = 10 each group). The RT protocol required the animals to climb a 1.1-m vertical ladder with weights attached to their tail. The sessions were performed once every 3 days with 4-9 climbs and 8-12 dynamic movements per scaling. The acute groups performed one session and the chronic groups underwent 12 weeks of RT. There was an increase in total MMP-2 activity in Sed-Ovx, AcuteEx-Intact, and ChronicEx-Intact compared with that in Sed-Intact in the proximal region of CT. AcuteEx-Ovx exhibited higher total MMP-2 than Sed-Ovx and AcuteEx-Intact in the distal region of CT. Chronic-Ovx presented lower total MMP-2 activity than Sed-Ovx and Chronic-Intact in the distal region of tendon. The active MMP-2 was higher for the AcuteEx-Ovx than Sed-Ovx and AcuteEx-Intact in proximal region of tendon. There was higher active MMP-2 in the distal region of tendon in the Acute-Ovx than in the Sed-Ovx and AcuteEx-Intact. Ovariectomy and resistance exercise modulate MMP-2 activity according to specific tendon region, indicating a differentiated tissue remodeling.

Regulation of ATPase activity of transglutaminase 2 by MT1-MMP: implications for mineralization of MC3T3-E1 osteoblast cultures

A pro-mineralization function for transglutaminase 2 (TG2) has been suggested in numerous studies related to bone, cartilage, and vascular calcification. TG2 is an enzyme which can perform protein crosslinking functions, or act as a GTPase/ATPase depending upon different stimuli. We have previously demonstrated that TG2 can act as an ATPase in a Ca(2+)-rich environment and that it can regulate phosphate levels in osteoblast cultures. In this study, we investigate the role MT1-MMP in regulating the ATPase activity of TG2. We report that proteolytic cleavage of TG2 by MT1-MMP in vitro results in nearly a 3-fold increase in the ATPase activity of TG2 with a concomitant reduction in its protein-crosslinking activity. We show that MC3T3-E1 osteoblasts secreted full-length TG2 and major smaller fragments of 66 and 56 kDa, the latter having ATP-binding abilities. MT1-MMP inhibition by a neutralizing antibody suppressed mineralization of osteoblast cultures to 35% of control, and significantly reduced phosphate levels in conditioned medium (CM). Furthermore, MT1-MMP inhibition abolished two of TG2 fragments in the cultures, one of which, the 56-kDa fragment, has ATPase activity. Neutralization of MT1-MMP at early phases of mineralization significantly reduced mineral deposition, but had no effect in later phases implying MT1-MMP and TG2 might contribute to the initiation of mineralization. The cleavage of TG2 by MT1-MMP likely occurs on the cell surface/pericellular matrix where MT1-MMP and TG2 were co-localized. Based on these data, we propose that MT1-MMP modulates the extracellular function TG2 as part of a regulatory mechanism activates the pro-mineralization function of TG2.

Different activation forms of MMP-2 oppositely affect the fate of endothelial cells

Detachment of endothelial cells (ECs) from the extracellular matrix (ECM) is required not only for angiogenesis, but also for EC apoptosis. Matrix metalloproteinase (MMP)-2 plays a major role in the degradation of the ECM, supporting an essential role for this enzyme in both survival (angiogenesis) and death of ECs. Our aim was to study these seemingly paradoxical effects of MMP-2. We rationalized that inhibiting apoptosis would drive MMP-2 toward a prosurvival activity, clarifying the mechanisms involved. By employing specific inhibitors to two major apoptotic pathways in ECs, caspases and p38 MAPK (p38), we demonstrated that they differently affected EC behavior as well as MMP-2 expression. The p38 pathway appears to enhance MMP-2 synthesis, its partial ("intermediate") and its full activation, probably via membrane type (MT)1-MMP, while caspases enhance MMP-2 synthesis and full activation but reduce MT1-MMP and MMP-2 intermediate form. Evaluation of the reciprocal influences of MMP-2 on ECs showed that the intermediate form supported survival and migration, and the fully active form led to cell death. In addition, a pro- and intermediate form-rich environment, even in the presence of the fully active form, exerted protective effects. Thus the seemingly conflicting effects of MMP-2 on EC survival may be explained by the ratio between the MMP-2 activation forms. A regulatory loop between active MMP-2 and p38 but not between MMP-2 and caspases was also observed, suggesting that MMP-2 is downstream to caspases where it serves as an "exterminator" molecule. Altogether, modification of caspase and p38 pathways, via changes of local MMP-2, affect survival and angiogenic steps in ECs.

Mouse models of MMP and TIMP function

As their name implies, matrix metalloproteinases (MMPs) are thought to be responsible for the turnover of connective tissue proteins, a function that is indeed performed by some family members. However, matrix degradation is possibly not the predominant function of these enzymes. Several studies have demonstrated that MMPs also act on a variety of non-matrix extracellular proteins, such as cytokines, chemokines, receptors, junctional proteins, and antimicrobial peptides, to mediate a wide range of biological processes, such as repair, immunity, and angiogenesis. Our understanding of the many, diverse and, at times, unexpected functions of MMPs largely arose from the use of gene-targeted mice. In this chapter, we discuss the phenotypes of some MMP-deficient and TIMP-null mice and strategies and pitfalls in targeted mutagenesis.

Membrane type-1 matrix metalloprotease-independent activation of pro-matrix metalloprotease-2 by proprotein convertases

Matrix metalloprotease-2 is implicated in many biological processes and degrades extracellular and non-extracellular matrix molecules. Matrix metalloprotease-2 maintains a latent state through a cysteine-zinc ion pairing which, when disrupted, results in full enzyme activation. This pairing can be disrupted by a conformational change or cleavage within the propeptide. The best known activation mechanism for pro-matrix metalloprotease-2 occurs via cleavage of the propeptide by membrane type-1 matrix metalloprotease. However, significant residual activation of pro-matrix metalloprotease-2 is seen in membrane type-1 matrix metalloprotease knockout mice and in fibroblasts treated with metalloprotease inhibitors. These findings indicate the presence of a membrane type-1 matrix metalloprotease-independent activation mechanism for pro-matrix metalloprotease-2 in vivo, which prompted us to explore an alternative activation mechanism for pro-matrix metalloprotese-2. In this study, we demonstrate membrane type-1 matrix metalloprotease-independent propeptide processing of matrix metalloprotease-2 in HEK293F and various tumor cell lines, and show that proprotein convertases can mediate the processing intracellularly as well as extracellularly. Furthermore, processed matrix metalloprotease-2 exhibits enzymatic activity that is enhanced by intermolecular autolytic cleavage. Thus, our experimental data, taken together with the broad expression of proprotein convertases, suggest that the proprotein convertase-mediated processing may be a general activation mechanism for pro-matrix metalloprotease-2 in vivo.

Collagen I regulates matrix metalloproteinase-2 activation in osteosarcoma cells independent of S100A4

This work investigates the effect of cell-collagen I interactions on the synthesis and activation of MMP-2, as well as synthesis of MT1-MMP and TIMP-1, by using an in vitro model with 3D fibrillar and 2D monomeric collagen. In order to reveal whether the metastasis-associated protein S100A4 can influence the cell's response to the two forms of collagen, osteosarcoma cell lines with high and low endogenous levels of S100A4 were used. Attachment of osteosarcoma cells to 3D fibrillar and 2D monomeric collagen resulted in opposite effects on MMP-2 activation. Attachment to 3D fibrillar collagen decreased activation of proMMP-2, with a corresponding reduction in MT1-MMP. By contrast, attachment to monomeric collagen increased the amount of fully active MMP-2. This was caused by a reduction in TIMP-1 levels when cells were attached to monomeric 2D collagen. The effect of collagen on proMMP-2 activation was independent of endogenous S100A4 levels, whereas synthesis of TIMP-1 was dependent on S100A4. When cells were attached to monomeric collagen, cells with a high level of S100A4 showed a greater reduction in the synthesis of TIMP-1 than did those with a low level of S100A4. Taken together, this study shows that synthesis and activation of MMP-2 is affected by interactions between osteosarcoma cells and collagen I in both fibrillar and monomeric form.

Characterization of the transcriptional regulation of the human MT1-MMP gene and association of risk reduction for focal-segmental glomerulosclerosis with two functional promoter SNPs

BACKGROUND

The matrix metalloproteinase MT1-MMP (MMP-14) is an important player in wound healing, bone development, angiogenesis, inflammation and tumour invasion. MT1-MMP also plays an important role in the development and resolution of experimental kidney diseases. The role of MT1-MMP was investigated for distinction between minimal-change glomerulonephritis (MCGN) and focal-segmental glomerulosclerosis (FSGS) that can sometimes be difficult due to sampling error in renal biopsy.

METHODS

We defined the transcriptional regulation of the human MT1-MMP and the influence of single nucleotide polymorphisms (SNPs) within its promoter region in renal mesangial cells with reporter gene constructs and gel sift analysis. Genomic DNA from healthy blood donors (n = 500) and from kidney biopsies with defined renal diseases (MCGN: n = 189, FSGS: n = 311) was screened for MT1-MMP promoter SNPs.

RESULTS

Transcription of MT1-MMP is regulated by two enhancers, an Sp1 binding site and a regulatory region 1 (RR1). RR1 contains an Ets site binding the transcription factors Elf-1 and E1AF but not NFAT. The MT1-MMP promoter contains two SNPs (-378 T/C and -364 G/T) in close vicinity to the RR1. Occurrence of the SNP variant -378 C leads to strong inhibition of nuclear protein binding to the RR1 reducing its enhancer function. Appearance of either variant -378 C or variant -364 T in at least one copy of the MT1-MMP promoter was associated with a significant risk reduction for the development of FSGS (P < 0.048).

CONCLUSION

Genetic testing for MT1-MMP promoter SNPs could put renal biopsy results into new perspective. An independent study will be required to verify these findings and their possible diagnostic value for differentiation between certain renal diseases.

Control of matrix metalloproteinase catalytic activity

As their name implies, MMPs were first described as proteases that degrade extracellular matrix proteins, such as collagens, elastin, proteoglycans, and laminins. However, studies of MMP function in vivo have revealed that these proteinases act on a variety of extracellular protein substrates, often to activate latent forms of effector proteins, such as antimicrobial peptides and cytokines, or to alter protein function, such as shedding of cell-surface proteins. Because their substrates are diverse, MMPs are involved in variety of homeostatic functions, such as bone remodeling, wound healing, and several aspects of immunity. However, MMPs are also involved in a number of pathological processes, such as tumor progression, fibrosis, chronic inflammation, tissue destruction, and more. A key step in regulating MMP proteolysis is the conversion of the zymogen into an active proteinase. Several proMMPs are activated in the secretion pathway by furin proprotein convertases, but for most the activation mechanisms are largely not known. In this review, we discuss both authentic and potential mechanisms of proMMP activation.

Effect of a cleavage-resistant collagen mutation on left ventricular remodeling

Matrix metalloproteinase-mediated degradation of type I collagen may play a role in cardiac remodeling after strain or injury. To explore this hypothesis, we used mice homozygous (r/r) for a targeted mutation in Col1a1; these mice synthesize collagen I that resists collagenase cleavage at Gly975-Leu976. A total of 64 r/r and 84 littermate wild-type mice (WT) underwent experimental pressure overload by transverse aortic constriction (TAC) or myocardial infarction (MI). Echocardiographic, hemodynamic, and histological parameters were evaluated up to 12 weeks after TAC or 21 days after MI. At 4 weeks after TAC, collagen levels, wall thickness, and echocardiographic parameters were similar in the 2 groups. At 12 weeks after TAC, r/r mice had smaller LV dimensions (ESD: 2.7+/-0.2 mm WT versus 1.7+/-0.2 mm r/r, P<0.013; EDD: 3.8+/-0.2 mm WT versus 3.1+/-0.1 mm r/r, P<0.013); better fractional shortening (30+/-2% WT versus 46+/-4% r/r; P<0.013); and lower LV/body weight ratios (7.3+/-0.6 WT and 5.1+/-0.5 r/r; P<0.013). Surprisingly, these differences were not accompanied by differences in collagen accumulation, myocyte cross-sectional areas, wall thickness, or microvessel densities. Furthermore, no differences in LV remodeling assessed by echocardiography, fibrosis, or hemodynamic parameters were found between r/r and WT mice after MI. Thus, a mutation that encodes a collagenase cleavage-resistant collagen I does not affect early LV remodeling after TAC or MI, suggesting that collagen cleavage at this site is not the mechanism by which metalloproteinases mediate LV remodeling. Collagen cleavage could, however, have a role in preservation of cardiac function in late remodeling by mechanisms independent of collagen accumulation. We were not able to detect collagen cleavage fragments, and could not, therefore, rule out the possibility of collagen cleavage at additional sites.