Báo cáo khoa học: Roles of matrix metalloproteinases in cancer progression and their pharmacological targeting pdf

Roles of matrix metalloproteinases in cancer progression and their pharmacological targeting Chrisostomi Gialeli1, Achilleas D.. In this regard, their activity plays a pivotal role in tu

Roles of matrix metalloproteinases in cancer progression and their pharmacological targeting

Chrisostomi Gialeli1, Achilleas D Theocharis1and Nikos K Karamanos1,2

1 Department of Chemistry, Laboratory of Biochemistry, University of Patras, Greece

2 Institute of Chemical Engineering and High-Temperature Chemical Processes (FORTH ⁄ ICE-HT), Patras, Greece

Introduction

Cancer is one of the leading causes of disease and

mortality worldwide [1] As a result, the past two

dec-ades of biomedical research have yielded an enormous

amount of information on the molecular events that

take place during carcinogenesis and the signaling

pathways participating in cancer progression The

molecular mechanisms of the complex interplay

between the tumor cells and the tumor

microenviron-ment play a pivotal role in this process [2]

Studies conducted over more than 40 years have revealed mounting evidence supporting that extracellu-lar matrix remodeling proteinases, such as matrix metalloproteinases (MMPs), are the principal media-tors of the alterations observed in the microenviron-ment during cancer progression [2,3] MMPs belong to

a zinc-dependent family of endopeptidases implicated

in a variety of physiological processes, including wound healing, uterine involution and organogenesis,

Keywords

angiogenesis; invasion and metastasis;

matrix metalloproteinase; matrix

metalloproteinase inhibitor; pharmacological

target

Correspondence

N Karamanos, Laboratory of Biochemistry,

Department of Chemistry, University of

Patras, 26110 Patras, Greece

Fax: +30 2610 997153

Tel: +30 2610 997915

E-mail: n.k.karamanos@upatras.gr

(Received 20 June 2010, revised 20 August

2010, accepted 18 October 2010)

doi:10.1111/j.1742-4658.2010.07919.x

Matrix metalloproteinases (MMPs) consist of a multigene family of zinc-dependent extracellular matrix (ECM) remodeling endopeptidases implicated

in pathological processes, such as carcinogenesis In this regard, their activity plays a pivotal role in tumor growth and the multistep processes of invasion and metastasis, including proteolytic degradation of ECM, alteration of the cell–cell and cell–ECM interactions, migration and angiogenesis The under-lying premise of the current minireview is that MMPs are able to proteolyti-cally process substrates in the extracellular milieu and, in so doing, promote tumor progression However, certain members of the MMP family exert con-tradicting roles at different stages during cancer progression, depending among other factors on the tumor stage, tumor site, enzyme localization and substrate profile MMPs are therefore amenable to therapeutic intervention

by synthetic and natural inhibitors, providing perspectives for future studies Multiple therapeutic agents, called matrix metalloproteinase inhibitors (MMPIs) have been developed to target MMPs, attempting to control their enzymatic activity Even though clinical trials with these compounds do not show the expected results in most cases, the field of MMPIs is ongoing This minireview critically evaluates the role of MMPs in relation to cancer pro-gression, and highlights the challenges, as well as future prospects, for the design, development and efficacy of MMPIs

Abbreviations

ADAM, a disintegrin and metalloproteinase; ADAMTS, a disintegrin and metalloproteinase with thrombospondin motifs; bFGF, basic fibroblast growth factor; ECM, extracellular matrix; EGFR, epidermal growth factor receptor; EMT, epithelial to mesenchymal transition; GAG, glycosaminoglycan; HB-EGF, heparin-binding epidermal growth factor; IGF, insulin-like growth factor; MMP, matrix metalloproteinase; MMPI, metalloproteinase inhibitor; MT-MMP, membrane-type matrix metalloproteinase; NK, natural killer; siRNA, small interfering RNA; TGF, transforming growth factor; TIMP, tissue inhibitor of metalloproteinase; VEGF, vascular endothelial growth factor.

as well as in pathological conditions, such as

inflamma-tory, vascular and auto-immune disorders, and

carcino-genesis [3–6] MMPs have been considered as potential

diagnostic and prognostic biomarkers in many types

and stages of cancer [7] The notion of MMPs as

thera-peutic targets of cancer was introduced 25 years ago

because the metastatic potential of various cancers was

correlated with the ability of cancer cells to degrade the

basement membrane [8] Subsequently, a growing

num-ber of MMP inhibitors (MMPIs) have been developed

and evaluated in several clinical trials

A zinc-dependent family of proteinases related to

the MMPs is represented by a disintegrin and

metallo-proteinase (ADAM), which includes two subgroups:

the membrane-bound ADAM and a disintegrin and

metalloproteinase with thrombospondin motifs

(AMTS) Recent studies show that ADAM and

AD-AMTS present altered expression in diverse tumor

types, suggesting that these proteins are involved in

different steps of cancer progression including

carcino-genesis [9,10] ADAM molecules are implicated in

tumor cell prolireration⁄ apoptosis, cell

adhe-sion⁄ migration and cell signaling In particular, they

exhibit proteolytic activity like MMPs, although their

main roles focus on ectodomain shedding and

nonpro-teolytic functions, such as binding to adhesion

mole-cules, integrins and interacting with phosphorylation

sites for serine⁄ threonine and tyrosine kinases, thus

contributing to cancer development [11]

Roles of MMPs in cancer progression

During development of carcinogenesis, tumor cells

participate in several interactions with the tumor

microenvironment involving extracellular matrix

(ECM), growth factors and cytokines associated with

ECM, as well as surrounding cells (endothelial cells,

fibroblasts, macrophages, mast cells, neutrophils,

pericytes and adipocytes) [2,10,12] Four hallmarks of

cancer that include migration, invasion, metastasis and

angiogenesis are dependent on the surrounding

micro-environment Critical molecules in these processes are

MMPs because they degrade various cell adhesion

molecules, thereby modulating cell–cell and cell–ECM

interactions (Fig 1) Key MMPs in relation to the

stages of cancer progression, their activity and their

effects are summarized in Table 1, as they are depicted

in the text

The emerging view, reflected by several studies,

reveals that the expression and role of MMPs and

their natural inhibitors [i.e tissue inhibitor of

metallo-proteinases (TIMP)] is quite diverse during cancer

development The over-expression of MMPs in the

tumor microenvironment depends not only on the can-cer cells, but also on the neighboring stromal cells, which are induced by the cancer cells in a paracrine manner Cancer cells stimulate host cells such as fibro-blasts to constitute an important source of MMPs through the secretion of interleukins and growth fac-tors and direct signaling through extracellular MMP inducer [10] The cellular source of MMPs can there-fore have critical consequences on their function and activity For example, in this regard, neutrophils express MMP-9 free of TIMP-1, which results in acti-vation of the proteinase more readily [13]

Recent studies show that members of the MMP family exert different roles at different stages during cancer progression In particular, they may promote or inhibit cancer development depending among other factors on the tumor stage, tumor site (primary, metas-tasis), enzyme localization (tumor cells, stroma) and substrate profile For example, MMP-8 provides a pro-tective effect in the metastatic process, decreasing the metastatic potential of breast cancer cells when it is over-expressed [14] Similarly, MMP-8 expression in squamous cell carcinoma of the tongue is correlated with improved survival of patients and it is proposed that this protective action is probably correlated with the role of estrogen in the growth of tongue squamous cell carcinomas [12,15] On the other hand, MMP-9 might function as tumor promoter in the process of carcinogenesis as well as an anticancer enzyme at later stages of the disease in some specific situations This dual role is based on the findings in animal models, where it observed that MMP-9 knockdown mouse models exhibited decreased incidence of carcinogenesis, whereas tumors formed in MMP-9 deficient mice were significantly more aggressive [12]

Similarly, ADAMTS exhibits some contradictive outcomes because ADAMTS-12 and ADAMTS-1 dis-play anti-angiogenic and antimetastatic properties One possible explanation to consider, especially for ADAMTS-1, is that this molecule undergoes auto-pro-teolytic cleavage or even proauto-pro-teolytically impairment of its catalytic site that can account for these outcomes [11,16] In both cases, the story will mature over the next few years because much research is in progress within this field

MMPs and cancer cell invasion The ECM is a dynamic structure that orchestrates the behavior of the cells by interacting with them The proteolytic activity of MMPs is required for a cancer cell to degrade physical barriers during local expansion and intravasation at nearby blood vessels, extravasation

and invasion at a distant location (Fig 1) During

invasion, the localization of MMPs to specialized cell

surface structures, called invadopodia, is requisite for

their ability to promote invasion These structures

rep-resent the site where active ECM degradation takes

place Invadopodia utilize transmembrane

invadopodi-a-related proteinases, including MMP-14

[membrane-type (MT)1-MMP], several members of the ADAM

family, as well as secreted and activated MMPs at the

site, such as MMP-2 and -9, to degrade a variety of

ECM macromolecules and facilitate cell invasion [17]

The contribution of MMP activities to several critical

steps of cancer progression is described below

MMPs and cancer cell proliferation There are several mechanisms by which MMPs con-tribute to tumor cell proliferation In particular, they can modulate the bioavailability of growth factors and the function of cell-surface receptors The above pro-cess also involves the ADAM family Members of the MMP and ADAM families can release the cell-mem-brane-precursors of several growth factors, such as insulin-like growth factors (IGFs) and the epidermal growth factor receptor (EGFR) ligands that promote proliferation Several MMPs (MMP-1, -2, -3, -7, -9, -11 and -19) and ADAM12 cleave IGF-binding

Fig 1 Pivotal roles of MMPs in cancer progression Cancer progression involves different stages, including tumor growth and the multistep processes of invasion, metastasis and angiogenesis, all of which can be modulated by MMPs The expression of MMPs in the tumor micro-environment depends not only on the cancer cells, but also on the neighboring stromal cells MMPs exert their proteolytic activity and degrade the physical barriers, facilitating angiogenesis, tumor cells invasion and metastasis Tumor growth and angiogenesis also depend on the increased availability of signaling molecules, such as growth factors and cytokines, by MMPs making these factors more accessible to the cancer cells and the tumor microenvironment This occurs by liberating them from the ECM (IGF, bFGF and VEGF) or by shedding them

by from the cell surface (EGF, TGF-a, HB-EGF) Angiogenesis is also tightly modulated by the release of negative regulators of angiogenesis, such as angiostatin, tumstatin, endostatin and endorepellin MMPs also modulate the cell–cell and cell–ECM interactions by processing E-cadherin and integrins, respectively, affecting both cell phenotype (EMT) and increasing cell migration.

proteins that regulate the bioavailability of the growth

factor [18,19] EGFR, mediator of cell proliferation, is

implicated in cancer progression because it is

over-expressed in more than one-third of all solid tumors

[20] During cancer progression, increased shedding of

the membrane-anchored ligands of EGFR, including

heparin-binding EGF (HB-EGF), transforming growth

factor (TGF)-a and amphiregulin, was observed with

the action of MMP-3, -7, ADAM17 or ADAM10

[21,22] MMPs and ADAM also control proliferation

signals through integrins because the shedding of E-cadherin results in b-catenin translocation to the nucleus, leading to cell proliferation [23] It is worth noting that the inactive proform of TGF-b, an impor-tant biomolecule in cancer, is proteolytically activated

by MMP-9, -2, -14 in a similar way [24,25]

One of the key observations that has emerged from several studies is the pivotal role of the interactions between glycosaminoglycans (GAGs)-MMPs-GFs, leading to the activation of the proMMPs and their

Table 1 Key matrix metalloproteinases in relation with the stages of cancer progression, their activity and effect.

Cancer cell invasion

Several MMPs such as MT1-MMP,

MMP-2 and MMP-9

Several members of the ADAM family

Cancer cell proliferation

MMP-1, -2, -3, -7, -9, -11, -19, ADAM12 Cleavage of IGF-binding proteins Proliferation

MMP-3, -7, ADAM17, ADAM10 Shedding of membrane-anchored ligands of EGFR

(HB-EGF, TGF-a and amphiregulin)

Cancer cell apoptosis

histocompatibility proteins complex class-I Several MMPs and ADAMs Indirect activation of Akt through activation of

EGFR and IGFR Tumor angiogenesis and vasculogenesis

Several MMPs (including MMP-2,

-9 MMP-3, -10, -11 MMP-1, -8, -13)

Degradation of COL-IV, perlecan; release of VEGF and bFGF, respectively

Up-regulation of angiogenesis Degradation of COL-IV, COL-XVIII, perlecan;

generation of tumstatin, endostatin, angiostatin and endorepellin, respectively

Down-regulation of angiogenesis

Cell adhesion, migration, and epithelial to mesenchymal transition

peptides

Promote migration

peptides

ADAM10

MMP-1, -7

Shedding of E-cadherin

cell migration Immune surveillance

T-lymphocytes surface

Suppress T-lymphocyte proliferation

against cancer cells

to NK cells

of their mobilization

Affect leukocyte infiltration and migration

subsequent proliferative effects Notably, GAGs chains

can recruit MMPs to release growth factors from the

cell surface and, as a result, induce cancer cell

prolifer-ation For example, MMP-7 exerts high affinity for

heparan sulfate chains On the basis of this notion,

heparan sulfate chains on cell surface receptors, such

as some variant isoforms of CD44, anchor the

proteo-lytically active MMP-7, resulting in the cleavage of

HB-EGF [26] The above findings may explain the

diverse proliferative outcomes of the various GAG

types in human malignant mesothelioma cell lines,

as well as indicating a structure–function

relation-ship [27]

MMPs and cancer cell apoptosis

Matrix-degrading enzymes confer both apoptotic and

anti-apoptotic actions MMPs and ADAMs, especially

MMP-7 and ADAM10, confer anti-apoptotic signals

to cancer cells by cleaving Fas ligand, a

transmem-brane stimulator of the death receptor Fas, from the

cell surface This proteolytic activity inactivates Fas

receptor and induces resistance to apoptosis and

chemoresistance to the cancer cells or promotes

apop-tosis to the neighboring cells depending on the system

[28–30] Moreover, proteolytic shedding of

tumor-associated major histocompatibility proteins complex

class-I related proteins by ADAM17 may suppress

natural killer (NK) cell-mediated cytotoxicity toward

cancer cells [31] Notably, MMPs may contribute to

the anti-apoptotic effect by activating indirectly the

serine⁄ threonine kinase Akt ⁄ protein kinase B through

the signaling cascades of EGFR and IGFR [20,32]

MMPs also promote apoptosis, most likely indirectly

by changing the ECM composition; for

exam-ple, by cleaving laminin, which influences integrin

signaling [33]

MMPs and tumor angiogenesis and

vasculogenesis

MMPs display a dual role in tumor vasculature

because they can act both as positive and negative

reg-ulators of angiogenesis depending on the time point of

expression during tumor angiogenesis and

vasculogene-sis as well as the availability of the substrates The key

players of the MMP family that participate in tumor

angiogenesis are mainly MMP-2, -9 and MMP-14,

and, to a lesser extent, MMP-1 and -7 [34]

For cancer cells to continue to grow and start

migrating, it is necessary to form new blood vessels

The first step in this process is to eliminate the physical

barriers by ECM degradation and, subsequently, to

generate pro-angiogenic factors Indeed, MMP-9 par-ticipates in the angiogenic switch because it increases the biovailability of important factors in this process, such as the vascular endothelial growth factor (VEGF), which is the most potent mediator of tumor vasculature, and basic fibroblast growth factor (bFGF), by degradation of extracellular components, such as collagen type IV, XVIII and perlecan, respec-tively [35–38]

The angiogenic balance is tightly regulated by MMPs because they can also down-regulate blood ves-sel formation through the generation of degradation fragments that inhibit angiogenesis Such molecules include tumstatin, endostatin, angiostatin and endore-pellin, which are generated via cleavage of type IV, XVII collagen, plasminogen, an inactive precursor of a serine proteinase plasmin, and perlecan [38–41]

MMPs and cell adhesion, migration, and epithelial to mesenchymal transition Cell movement is highly related to the proteolytic activity of MMPs and ADAMs, regulating the dynamic ECM–cell and cell–cell interactions during migration Initially, the generation of cryptic peptides via degradation of ECM molecules, such as collagen type IV and laminin-5, promotes the migration of can-cer cells [35,42] Several integrins play an active role in regulation of cell migration because they can serve as substrates for MMPs [43]

Over-expression of several MMPs (MMP-2, -3, -9, -13, -14) has been associated with epithelial to mesen-chymal transition (EMT), a highly conserved and fundamental process of morphological transition [5]

In particular, during this event, epithelial cells actively down-regulate cell–cell adhesion systems, lose their polarity, and acquire a mesenchymal phenotype with reduced intercellular interactions and increased migra-tory capacity [44] The communication between the cells is disrupted by the shedding of E-cadherin by ADAM10, leading to disrupted cell adhesion and induction of EMT, followed by increased cell migra-tion [23] MMP-1 and -7 also appear to contribute to this morphological transition by cleaving E-cadherin [45] Recent studies indicate the implication of

MMP-28 in the proteolytic activation of TGF-b, a powerful inducer of EMT, leading to EMT [46,47]

It is worth noting that the interaction between hyal-uronan and its major cell surface receptor, CD44, results in the activation of signaling molecules such as Ras, Rho, PI-3 kinases and AKT, consequently promoting cancer progression A recent study reported that hyaluronan promotes cancer cell migration and

increased matrix metalloproteinase secretion,

specifi-cally the increased active form of MMP-2, through

Rho kinase-mediated signaling [48]

MMPs and immune surveillance

The host immune system is capable of recognizing and

attacking cancer cells by recruiting tumor-specific

T-lymphocytes, NK cells, neutrophils and

macrophag-es By contrast, cancer cells evolve escaping

mecha-nisms using MMPs to acquire immunity

MMPs shed interleukin-2 receptor-a by the cell

sur-face of T-lymphocytes, thereby suppressing their

prolif-eration [49] In addition, TGF-b, a significant

suppressor of T-lymphocyte reaction against cancer

cells, is released as a result of MMP activity [50]

Simi-larly, MMPs decrease cancer-cell sensitivity to NK

cells by generating a bioactive fragment from

a1-pro-teinase inhibitor [51] A number of studies have also

shown the ability of MMPs to efficiently cleave several

members of the CC (b-chemokine) and CXC

(a-chemokine) chemokine subfamilies or to regulate

their mobilization, affecting leukocyte infiltration and

migration [52,53]

Pharmacological targeting of matrix

metalloproteinases

On the basis of the pivotal roles that MMPs and

ADAMs play in several steps of cancer progression,

the pharmaceutical industry has invested considerable

effort over the past 20 years aiming to develop safe

and effective agents targeting MMPs In this regard, multiple MMPIs have been developed, in an attempt

to control the synthesis, secretion, activation and enzy-matic activity of MMPs

Several generations of synthetic MMPIs were tested

in phase III clinical trials in humans, including pepti-domimetics, nonpeptidomimetics inhibitors and tetra-cycline derivatives, which target MMPs in the extracellular space [54] In addition, various natural compounds have been identified as inhibiting MMPs [55] Other strategies of MMP inhibition in develop-ment involve antisense and small interfering RNA (siRNA) technology Antisense strategies are directed selectively against the mRNA of a specific MMP, resulting in decrease of RNA translation and down-regulation of MMP synthesis [55–57] Despite the noted low toxicity of these strategies, they are still immature with respect to the effectiveness of the tar-geted delivery of oligonucleotides or siRNA to tumor cancer cells Categories of the potential matrix metallo-proteinase inhibitors and their specificities are summa-rized in Table 2

Peptidomimetic MMPIs The first geneneration of MMPIs introduced com-prised the peptidomimetic These pseudopeptide deriv-atives mimic the structure of collagen at the MMP cleavage site, functioning as competitive inhibitors, and chelating the zinc ion present at the activation site [58] On the basis of the group that binds and chelates the zinc ion, peptidomimetis are subdivided into

Table 2 Potential matrix metalloproteinase inhibitors.

Synthetic inhibitors

Off-target inhibitors

Natural inhibitors

hydroxamates, carboxylates, hydrocarboxylates,

sul-fhydryls and phosphoric acid derivatives The earliest

representative of this generation and the first MMPI

that entered clinical trials is batimastat (BB-94), a

hy-droxymate derivative with low water solubility and a

broad spectrum of inhibition [59] To overcome the

solubility factor, marimastat, another

hydroxymate-based inhibitor, was introduced for oral administered

However, it was also associated with musculoskeletal

syndrome, probably as a result of the broad spectrum

of inhibition [60,61] In addition, in vitro studies with

batimastat and marimastat showed that they can act

synergistically with TIMP-2 in the promotion of

proMMP-2 activation by MT1-MMP, increasing overall

pericellular proteolysis [62]

Nonpeptidomimetic MMPIs

To improve specificity and oral bioavailability, the

nonpeptidomimetic MMPIs were synthesized on the

basis of the current knowledge of the 3D conformation

of the MMP active site This generation comprises of

BAY12-9566, prinomastat (AG3340), BMS-275291

and CGS27023A [63] The latter agent was aborted as

a result of limited efficacy and musculoskeletal side

effects in phase I clinical trials [64] Musculoskeletal

toxicity has also been reported in clinical trials with

prinomastat and BMS-275291 [65,66]

Chemically modified tetracyclines

Another generation of MMPIs, tetracycline derivatives,

inhibit both the enzymatic activity and the synthesis of

MMPs via blocking gene transcription Chemically

modified tetracyclines, lacking antibiotic activities, may

inhibit MMPs by binding to metal ions such as zinc and

calcium This family of inhibitors, including metastat

(COL-3), minocycline and doxycycline, cause limited

systemic toxicity compared to regular tetracyclines The

chemically modified tetracycline, doxycycline, is

cur-rently the only Food and Drug Administration

approved MMPI for the prevention of periodontitis,

whereas metastat has entered phase II trials for Kaposi’s

sarcoma and brain tumors [67]

Novel mechanism-based inhibitors

A novel inhibitor, SB-3CT, was designed aiming to

selectively bind to the active site of gelatinases (MMP-2

and MMP-9) and reform the proenzyme structure

Specifically, the fundamental step in the inhibition of

gelatinases by SB-3CT is an enzyme-catalyzed ring

opening of the thirane, giving a stable zinc-thiolate

spe-cies It was reported to inhibit liver metastasis and increase survival in mouse models [68]

On the basis of the importance of the ADAM family

in cancer progression, small molecule inhibitors have been developed, such as INCB7839, and are currently being tested in clinical trials [69] Such agents may be administered as single agents or in combination with agents that block the EGFR family at EGFR-depen-dent tumors [70]

Off-target inhibitors of MMPs There are several other drugs that have been shown to influence MMPs and other ECM molecules in a benefi-cial way beyond their primary target This is the case for bisphosphonates, analogs of PPi, which inhibit the function of the mevalonate pathway Besides the inhi-bition of osteoclast activity and bone resorption, bis-phosphonates inhibit the enzymatic activity of various MMPs [71] According to data obtained in our labora-tory (P.G Dedes and N.K Karamanos, unpublished data), certain bisphosphonates show beneficial effects

as a result of altering the expression pattern of MMPs⁄ TIMPs by inhibiting and increasing the gene and protein expression of several MMPs and TIMPs, respectively, in breast cancer cells

Another agent that has exhibited inhibitory effects

on MMPs is letrozole, a reversible nonsteroidal inhibi-tor of P450 aromatase In particular, letrozole prevents the aromatase from converting androgens to estrogens, the most crucial step in the estrogen synthesis pathway

in post-menopausal women, by binding to the heme of its cytochrome P450 unit In addition, the gelatinases (MMP-2 and -9) released by breast cancer cells, as well

as functional invasion in vitro, are considerably sup-pressed by letrozole in a dose-dependent fashion, limit-ing the metastatic potential of these cells [72] The above observation is in accordance with the results obtained in the British International Group 1-98 study showing that letrozole lowers the occurrence of distant metastases [73]

It is worth noting that estrogen receptor-a suppres-sion with siRNA in breast cancer cells lines abolishes the ability of estradiol to up-regulate the expression of MMP-9, highlighting the importance of signaling by estrogen receptors in the expression pattern of MMPs and therefore their potential pharmacological targeting [74]

Natural inhibitors of MMPs TIMPs, the natural inhibitors of MMPs, were also used to block MMPs activity Although they have

demonstrated efficacy in experimental models, TIMPs

may exert MMP-independent promoting effects [75]

To avoid the negative results and toxicity issues

raised by the use of synthetic MMPIs, one answer

was provided by the field of natural compounds One

compound taken into consideration was extracted

from shark cartilage Oral administration of a

stan-dardized extract, neovastat, exerts anti-angiogenic and

anti-metastatic activities and these effects depend not

only on the inhibition of MMPs enzymatic activity,

but also on the inhibition of VEGF [76] Another

natural agent that has anticancer effects is genistein,

a soy isoflavonoid structurally similar to estradiol

Apart from its estrogening and anti-estrogenic

prop-erties, genistein confers tumor inhibition growth

and invasion effects, interfering with the expression

ratio and activity of several MMPs and TIMPs

[77,78]

Challenges and future prospects

MMPs have well-established complex and key roles in

cancer progression However, in most cases, the agents

targeting MMPs exhibited poor performance in clinical

trials, in contrast to their promising activity in many

preclinical models [79] There are several possible

explanations for these contradictive outcomes First,

the failure observed in phase III clinical trials with

respect to MMPIs reaching the endpoints of

progres-sion-free survival and overall survival may be

attrib-uted to no proper subgroup selection, with mostly

endstage disease patients [80] As is the case for many

anticancer agents, the administration of MMPIs

should be made after thorough consideration of the

specific cancer-types and stages of disease In

particu-lar, for certain cancer types, especially those where the

stroma is an essential player in carcinogenesis, the

inhi-bition of MMPs is proven to be more effective [81] In

addition, the timeframe of targeting MMPs differs,

depending on the stage of cancer, because the

expres-sion profile, as well as the activity of MMPs, is not the

same in the early stage compared to advanced cancer

disease Recent studies show that members of the

MMP family exert different roles at different steps of

cancer progression As a consequence, the use of

broad-spectrum MMPI raises concerns when certain

MMPs that exert anticancer effects are inhibited In

this regard, the use of such MMPIs may lead to

unsat-isfying clinical outcomes as a result of the wide range

of MMPs that are inhibited [82] In addition, toxicity

effects, such as muscolosceletal syndrome, have limited

the maximum-tolerated dose of certain MMPIs, thus

limiting drug efficacy The challenge is to distinguish

the specific role of individual enzymes in each case using both widespread gene and tissue microarrays [83]

Considering all of the above, one of the major challenges for the future is the development of inhib-itors or monoclonal antibodies that bind to the active site of the enzyme and are specific for certain MMPs, showing little or no cross-reaction with other MMPs [81] In this respect, a potent and highly selective antibody, DX-2400, against the catalytic domain of MMP-14 was designed with high binding affinity [84,85] To further increase the specificity of MMPIs, the future of drug development comprises the use of drugs targeting specific exosites [86] Exo-sites are binding Exo-sites outside the active domain of the MMPs and are related to substrate selection of MMPs [87] Therefore, future drugs targeting less conserved exosites rather than the catalytic domain will result in drugs that are both MMP- and sub-strate-specific In this respect, a new class of selec-tives MMPIs, triple-helical transition state analogs, is introduced, modulating the collagenolytic activity of MMPIs [88]

In addition, the molecular complexity of cancer progression suggests that the appropriate combination

of MMPIs with other chemotherapeutic or molecular targeted agents may play an important role with respect to increasing drug efficacy Last, but not least, imaging activity of specific MMPs in vivo with probes will make it possible to evaluate the therapeu-tic efficacy of MMPIs, as well as their activity, at dif-ferent stages of cancer progression in certain tumors [89]

Taking into consideration the data presented in the present minireview, the minireview by Murphy and Nagase in this same series [90], and knowledge that enhanced MMP activity may be required to counter-balance excessive ECM deposition by myofibroblasts

in the tumor microenvironment, as well as the findings

of a recent study [91] reporting amoeboid-like nonpro-teolytic cell invasion may affect the action of MMPI,

it is concluded that that the pharmacological targeting

of cancer by the development of a new generation of effective and selective MMPIs is an emerging and promising area of research

Acknowledgements

We thank Professor G N Tzanakakis (University of Crete, Greece) and Dr D Kletsas (NCSR ‘Demokri-tos’, Greece) for their critical reading and valuable advice We apologize to the authors whose work could not be cited as a result of space limitations

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