Matrix metalloproteinases (MMPs) are a family of zinc- and
calcium-dependent proteolytic enzymes. In humans, 23
MMPs are described, and they are either membrane-
anchored or secreted [1]. These enzymes are made up of different domains, modules, and motifs which are the basis
for their classification and are important for their substrate
specificity, their localization, and interactions with other mac-
romolecules. All MMPs contain an N-terminal signal peptide
which directs the enzymes to the secretory pathway, a pro-
domain that interacts with the active site and thereby confers
the latency of the enzymes and a catalytic domain with a zinc
ion in the active site. Most of the MMPs also contain a C-
terminal hemopexin-like domain that is linked to the catalytic
domain through a hinge region. The membrane-bound MMPs
and three of the secreted MMPs (MMP-11, MMP-21,
and MMP-28) contain a short basic motif at the end of
their pro-domain and can be activated by the serine
protease furin prior to secretion [2, 3].
Together, the MMPs can degrade most extracellular matrix
(ECM) proteins as well as regulate the activity of other pro-
teinases, growth factors, cytokines, chemokines, and cell re-
ceptors [4]. By cleaving ECM proteins and other molecules,
MMPs can modify signaling pathways and are therefore cen-
tral regulators of cell function under both physiological and
pathological conditions. Examples of biological activities me-
diated by MMPs are cell migration, differentiation, prolifera-
tion, apoptosis, inflammatory reactions, angiogenesis, and
platelet aggregation [2, 5].
As MMPs are capable of degrading a broad spectrum of
substrates, their synthesis and activity are kept under tight
control at various levels including transcription, activation,
inhibition, complex formation, and localization. At the tran-
scriptional level, the synthesis of MMPs is regulated by the
cells’ interactions with other cells, ECM molecules, growth
factors, cytokines, and chemokines [6]. As most of the MMPs
are synthesized in latent form, they need to be activated after
their secretion from the cells. This activation usually involves
proteolytic removal of the pro-domain or autoactivation in-
duced by binding to compounds such as reactive oxygen
species, chaotropic agents, or organomercurials. The activity
of MMPs is also controlled by inhibitors in the ECM and in the circulation such as the four tissue inhibitors of MMPs
(TIMP-1–4) and α2-macroglobulin [2].
In addition to MMP inhibition, some of the TIMPs may
form complexes with the pro-forms of MMP-2 and MMP-9.
The pro-MMP-2–TIMP-2 complex is essential for the acti-
vation of MMP-2, showing a dual role for TIMP-2 in the
modulation of MMP-2 activity. The TIMPs can also pro-
mote cell proliferation and can inhibit apoptosis. This is
mediated by cell-signaling events and is not related to the
regulation of MMP activity [7, 8].
The MMPs are multifunctional enzymes and have compli-
cated and sometimes opposing roles in the development of
various diseases, such as cancer [9]. In cancer tissue, the
expression of many members of the MMP family is induced
both in the cancer cells as well as in the surrounding stroma.
The enzymes can affect many of the central processes involved
in tumorigenesis and cancer progression, such as growth,
survival, angiogenesis, invasion, and regulation of the immune
response and thus can be important modulators of tumor
progression. The same MMPs may be involved in several of
these processes, where they can have contradicting effects
dependent on which substrates they act on [10–14], as illus-
trated in Fig. 1. An example of this is the role of MMP-9 in
angiogenesis, a fundamental process in cancer progression.
The enzyme can exert pro-angiogenic activities by releasing
and activating pro-angiogenic factors such as vascular endo-
thelial growth factor, basic fibroblast growth factor, and
transforming growth factor beta [15, 16]. MMP-9 can also
promote migration of endothelial cells and recruitment of
pericytes, which are necessary for the stabilization of the blood
vessels [17]. On the other hand, MMP-9 can exert anti-
angiogenic activities by formation of anti-angiogenic factors
which are often fragments of larger ECM molecules produced
by proteolysis of the ECM or the basement membrane of
vessels. Examples of such factors are endostatin, tumstatin,
canstatin, and arrestin [12, 17].
Many MMP members have overlapping substrate specific-
ity, illustrating a high degree of redundancy among them. This
makes it difficult to predict the actual function of a specific
MMP in cancer. The use of genetically engineered mice has
offered some more insight into this complex field [18, 19].
Mice over-expressing various MMPs have been employed in
different cancer models. Over-expression of MMP-7 or MMP-
14 can increase tumor incidence, indicating that these enzymes
have tumor-promoting functions [20, 21], whereas over-
expression of MMP-3 can either induce or decrease breast
cancer incidence, which is dependent on the cancer model
employed [22, 23]. Knockout of either MMP-2, MMP-7, or
MMP-9 is found to decrease tumorigenesis [24–27],
supporting their role as cancer-promoting enzymes, while
cancer-protecting effects of MMP-3, MMP-8, and MMP-12
are indicated by the increased tumorigenesis and metastases
seen in mice where these enzymes have been knocked out [28–30]. Hence, an increased amount of one or several MMPs
in cancer tissue does not necessarily indicate poor prognosis,
and it should therefore be carefully evaluated in all cases
whether an MMP should be regarded as a target or an anti-
target for pharmacological agents.