On 2015 Jan 14, Richard Schulz commented:
This interesting paper challenges the traditional and widely held view of matrix metalloproteinases (MMPs) as secreted proteases which cleave extracellular proteins after activation in the pericellular space by the proteolytic removal of their inhibitory propeptide. It adds another novel entry to the growing number of intracellular functions performed by this family of 23 human enzymes [1,2].
Yet some important findings regarding MMP localization, activation mechanisms and nuclear targets have been overlooked in this report. For example, Marchant et al wrote that “the intracellular transport and nuclear targeting mechanisms of these proteins are unknown, as are their intracellular roles”. However, MMP-2 does possess a canonical nuclear localization sequence [3], and MMP-2 and MMP-9 were identified a decade ago in purified nuclear fractions from heart and liver extracts, and visualized in the nuclei of cardiomyocytes by immunogold electron microscopy [3]. Furthermore, the nuclear proteins PARP1 [3] and XRCC1 [4] have both been shown to be proteolysed by MMPs.
This paper ascribed the nuclear localization of MMP-12 to an unknown mechanism where it is secreted by macrophages and then taken up by the target cell and enters the nucleus. In contrast, there are at least 3 bona fide intracellular isoforms of MMP-2 [5,6]. MMP-2 has an N-terminal signal sequence that only inefficiently targets it to the secretory pathway, resulting in approximately half of it being retained in the cytosol [5]. In addition, two MMP-2 isoforms lacking the signal sequence (and hence non-secreted) are expressed in cardiomyocytes [5,6]. Intracellular MMP-2 [7] and other MMPs [8] can be activated by post-translational modifications triggered by oxidants such as peroxynitrite, without proteolytic removal of the inhibitory propeptide. Regarding these supposedly unknown “intracellular roles”, one pathological role of intracellular MMP-2 in cardiomyocytes is its cleavage of specific sarcomeric proteins such as troponin I, which results in acute contractile dysfunction in ischemia-reperfusion injury [1,2,9].
In addition to MMP-12 and MMP-2, at least 6 other nuclear MMPs have already been reported [2]. Unbiased high throughput screens to identify putative MMP targets [2] suggest that several more nuclear protein targets will come to light, and unveil, as reported by Marchant et al for MMP-12, more nuclear functions for these proteases. In this respect, it is worth noting that the authors did not directly assess the potential contribution of MMP-12 to the transcription of endogenous genes but instead measured gene products (mRNA and protein). This is notable because the localization that they observe in HeLa cells reveals several large foci located in chromatin-depleted regions of the nucleus, which is not typical for a protein that binds to chromatin or regulates RNA polymerase II transcription, but shows some similarity to proteins involved in mRNA processing [10].
A better understanding of how MMPs act within the cell will be key to the development of better targeted MMP inhibitors for the treatment of viral infections, cancer, inflammation and heart disease.
This comment was written as a collaboration by Bryan G. Hughes, Sabina Baghirova, Michael J. Hendzel and Richard Schulz
References
1.Schulz R. Intracellular Targets of Matrix Metalloproteinase-2 in Cardiac Disease: Rationale and Therapeutic Approaches. Annual Review of Pharmacology and Toxicology 2007;47:211-242.
Schulz R, 2007
2.Cauwe B, Opdenakker G. Intracellular substrate cleavage: a novel dimension in the biochemistry, biology and pathology of matrix metalloproteinases. Crit Rev Biochem Mol Biol 2010;45:351-423.<br>
Cauwe B, 2010
3.Kwan JA, Schulze CJ, Wang W, Leon H, Sariahmetoglu M, Sung M, Sawicka J, Sims DE, Sawicki G, Schulz R. Matrix metalloproteinase-2 (MMP-2) is present in the nucleus of cardiac myocytes and is capable of cleaving poly (ADP-ribose) polymerase (PARP) in vitro. The FASEB Journal 2004;18:690-692.<br>
Kwan JA, 2004
4.Yang Y, Candelario-Jalil E, Thompson JF, Cuadrado E, Estrada EY, Rosell A, Montaner J, Rosenberg GA. Increased intranuclear matrix metalloproteinase activity in neurons interferes with oxidative DNA repair in focal cerebral ischemia. Journal of Neurochemistry 2010;112:134-149.<br>
Yang Y, 2010
5.Ali MAM, Chow AK, Kandasamy AD, Fan X, West LJ, Crawford BD, Simmen T, Schulz R. Mechanisms of cytosolic targeting of matrix metalloproteinase-2. Journal of Cellular Physiology 2012;227:3397-3404.<br>
Ali MA, 2012
6.Lovett DH, Mahimkar R, Raffai RL, Cape L, Maklashina E, Cecchini G, Karliner JS. A Novel Intracellular Isoform of Matrix Metalloproteinase-2 Induced by Oxidative Stress Activates Innate Immunity. PLoS One 2012;7:e34177.<br>
Lovett DH, 2012
7.Viappiani S, Nicolescu AC, Holt A, Sawicki G, Crawford BD, León H, van Mulligen T, Schulz R. Activation and modulation of 72 kDa matrix metalloproteinase-2 by peroxynitrite and glutathione. Biochemical Pharmacology 2009;77:826-834.<br>
Viappiani S, 2009
8.Okamoto T, Akaike T, Sawa T, Miyamoto Y, van der Vliet A, Maeda H. Activation of Matrix Metalloproteinases by Peroxynitrite-induced Protein S-Glutathiolation via Disulfide S-Oxide Formation. Journal of Biological Chemistry 2001;276:29596-29602.<br>
Okamoto T, 2001
9.Wang W, Schulze CJ, Suarez-Pinzon WL, Dyck JRB, Sawicki G, Schulz R. Intracellular Action of Matrix Metalloproteinase-2 Accounts for Acute Myocardial Ischemia and Reperfusion Injury. Circulation 2002;106:1543-1549.<br>
Wang W, 2002
10.Hendzel MJ, Kruhlak MJ, Bazett-Jones DP. Organization of Highly Acetylated Chromatin around Sites of Heterogeneous Nuclear RNA Accumulation. Molecular Biology of the Cell 1998;9:2491-2507.<br>
Hendzel MJ, 1998
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