The role of matrix metalloproteinases in pathogenesis, diagnostics, and treatment of human prostate cancer

The prostate gland is an odd, walnut-sized organ located under the bladder in the small pelvis of men. The prostate gland is affected by three basic pathological processes: prostate cancer (PCa), benign prostatic hyperplasia (BPH), and acute or chronic prostatitis [1,2]. The prostate gland belongs to the male reproductive system and its function is the production of fluid, which, as part of the ejaculate, contributes to increasing male fertility. The prostate gland is highly susceptible to oncogenic transformation, many times more than other sex tissues such as seminal vesicles [2].

Natural extracellular matrix metalloproteinases (MMPs) are a family of proteolytic enzymes called endopeptidases and play an important role in numerous physiological and pathological processes, including proliferation, migration, invasion, cell differentiation, and angiogenesis. MMPs may also play an important role in the diagnosis of many diseases related to extracellular matrix (ECM) remodeling, including neoplastic diseases such as PCa. The activity of MMPs is regulated by their natural tissue inhibitors (TIMPs), which, by binding to the active center of the enzyme, block the attachment of substrates [3,4,5].

There are also synthetic matrix metalloproteinase inhibitors (s-MMP-Is), mainly synthesized for use as medicaments. S-MMPIs have been studied in disseminated tumors originating in the ovary, breast, and prostate gland, however, due to the lack of survival benefit, clinical studies were discontinued [4].

To review the evidence for the literature search, use was made of Google Scholar and included articles published between 2000–2021. Combinations of the following keywords were used: “matrix metalloproteinases,” “prostate cancer.” From this search, a total of 29,600 potentially relevant articles were identified. This number was reduced to 951 articles after screening titles and abstracts. The studies were mainly in English. After reaching a list of potentially relevant articles, the full text of each paper was appraised, with particular emphasis on articles presenting the characteristics of MMPs and TIMPs, the epidemiology of PCa, and the role of MMPs in the pathogenesis, diagnosis, and treatment of PCa. This was in order to present study characteristics and results, to derive relevant information from studies, allowing the reader to make an assessment of the literature and final conclusions.

PCa will be diagnosed in one in seven patients in their lifetime [6]. This makes the topic of PCa the subject of intense research to clarify its biology and ensure appropriate treatment [7].

PCa is the fourth most common cancer in the world in terms of the overall population and the second most common cancer in the male population. As for the widespread availability of determination of the concentration of specific prostate antigen (PSA) and prostate biopsy, more than 70% of PCa cases are diagnosed in well-developed regions [8]. In the last 3 decades, the global recognition of PCa has significantly increased, which can be explained by the introduction of PSA concentration in blood serum into diagnostics, an increase in the number of surgical procedures performed for BPH, and the aging of the population [9]. A systematic review of studies on the prevalence of PCa in autopsy studies found that the incidence of PCa increases with each decade of life [10]. The value of the mortality rate per 100,000 inhabitants in 2000 was 16.97, while in 2015 it increased to 26.22 [11]. Despite its high incidence, the etiology of PCa remains unclear. The only established risk factors remain advanced age, family history, and the African American race. It is postulated that the development of PCa may be influenced by additional factors whose role in the development of PCa remains unclear, including physical and sexual activity, androgens, obesity, inflammation, and dietary factors [12]. Nutrients, such as carotenoids and retinoids, are noteworthy, as they can have anti-inflammatory, antioxidant, anti-androgen cytoprotective effects, and thus can counteract the hyperplasia of prostate cells and show a selective pro-apoptotic effect on prostate cells showing the features of neoplastic transformation. The strong potential for lycopene (LYC) in preventing PCa has been suggested, the risk for which appears to be increased in the male population carrying the TMPRSS2:ERG fusion gene, present in approximately half of all PCa cases worldwide. LYC has also been shown to inhibit PCa metastasis by down-regulating of MMP-9 and of the 1 type of intercellular adherence molecule (ICAM-1) expression by LYC on DU145 cells, which are most commonly used in PCa cell culture [13,14].

The prostate gland is divided into 4 histological zones, among which there are the anterior part, which is a fibro-muscular stroma and does not contain glandular tissue, a transitional zone containing 5% of glandular tissue, a central zone containing approximately 20% of glandular tissue, and a peripheral zone containing 70%–80% glandular tissue. About 70% to 75% of PCa cases develop in the peripheral zone, and the remaining 20%–30% originate in the transition zone [15,16].

The final diagnosis of PCa is based on a histopathological examination. The dominant histological type is adenocarcinoma (AC). AC subtypes include acinar type, accounting for 95% of cases, ductal type, accounting for less than 5% of cases, low-differentiated small-cell type, and large-cell neuroendocrine type, accounting for less than 1% of cases in total [17].

The guidelines of the European Society of Urology (EAU), based on the classification proposed by D’Amico, stratify patients with non-metastatic and locally advanced PCa into three risk groups of biochemical recurrence (Table 1) [18]. The classification is based on 3 basic parameters, including serum PSA concentration, Gleason score, and tumor grade (TNM) (Table 2) [19].

PCa is usually confirmed on the basis of the abnormal results of digital rectal examination (DRE) and/or serum PSA concentration. PSA is a serine protease of the kallikrein family, produced by the secretory cells of the prostate. The introduction of PSA determination has revolutionized the diagnosis of PCa. PSA is an organ-specific, but not tumor-specific, marker, therefore its concentration may also be increased in BPH, prostatitis, and other benign prostate pathologies. PSA is a continuous parameter, meaning that the higher the PSA concentration, the higher the probability of PCa [20].

Among the biomarkers assessed in serum, a new diagnostic test for the detection of PCa should be distinguished: the Prostate Health Index (PHI), which includes the assessment of the concentration of free PSA, total PSA, and the 2-proPSA isoform, as well as the 4-kallikrein test (4K), which assesses the concentration of free and total PSA, intact PSA (iPSA), and human kallikrein-2 (hK-2) activity. Both of the above-mentioned tests are characterized by similar sensitivity [21,22].

Urinary biomarkers include the PCa-3 specific gene (PCA-3), which is a long, non-coding mRNA fragment obtained from urine sediment collected after prostate massage, which is determined using the commercial Progensa^® PCA-3 test [23]. Other markers determined in the urine sediment are the mRNA of the homeodomain of the HOXC-6 gene and the mRNA of the transcription factor of the homeodomain of the DLX-1 gene, for detection of which the SelectMDX^TM assay is used [24]. Another test used in the diagnosis of PCa is ConfirmMDX^®, the essence of which is based on the assumption that BPH biopsy tissues obtained in a patient with suspected PCa show epigenetic changes in the form of methylation of three promoter regions in the gene of adenomatous polyposis coli (APC) and mutations in the tumor suppressor gene of RAS-associated factor-1 (RASSF-1) and gene of P-1-glutathione-S-transferase (GSTP-1). Of the above biomarkers, only PHI and Progensa^® have been approved by the Food and Drug Administration (FDA) [25].

MMPs are a family of proteolytic enzymes called endopeptidases, which act mainly extracellularly and contain a zinc ion in their active center that breaks down the elements of the extracellular matrix (ECM) (Table 3). The first enzyme in this group was discovered over 50 years ago in the tadpole's tail, where it was responsible for its proteolytic degradation during the metamorphosis of the organism. These enzymes are similar both functionally and structurally, and the differences that separate them have become the basis for the isolation of several smaller groups, which were described in detail in our previous publication [5]. The production of MMPs takes place in most connective tissue cells, neoplastic cells, leukocytes, vascular endothelial cells, as well as in neurons, glial cells, and macrophages. Like other proteolytic enzymes, MMPs are produced and secreted in the form of inactive proenzymes. Activation of MMPs occurs extracellulary [26]. The solid components of MMPs are propeptide, the task of which is to inhibit catalysis by blocking the active center of the enzyme, and a catalytic domain with a zinc ion, enabling the hydrolysis of the peptide bond of substrates [5]. The hinge region or the hemopexin-like domain, are not present in all MMPs [26]. Detailed structure of MMPs and the process of activating proenzymes of MMPs were also discussed in our previous publication [5].

The activity of MMPs is regulated by TIMPs, which, by binding to the active center of the enzyme, block the attachment of substrates. The main function of TIMPs is to inhibit the activity of MMPs by forming non-covalent complexes with them [3].

The most important role in physiological and pathological processes is assigned to the tissue inhibitor of metalloproteinases-1 (TIMP-1) and the tissue inhibitor of metalloproteinases-2 (TIMP-2) [27].

TIMP-1 has the ability to inhibit most MMPs, but the strongest effect is on MMP-3 and MMP-9. It shows the lowest affinity for membrane metalloproteinases [28]. TIMP-1 was discovered about 40 years ago [29]. It is an alkaline glycoprotein composed of 184 aminoacid residues and its molecular weight is 28 kDa. It includes two domains in its structure. The N-terminal domain binds to the active site of the MMP [30]. The structure of TIMP-1 is encoded by a gene on the X chromosome (Xp11.23-11.4) and the possible variability of its molecular weight depends on the number of carbohydrate residues. TIMP-1 exhibits an expression mechanism of an inducible type [31]. Its synthesis is stimulated by erythropoietin (EPO), epithelial growth factor (EGF), and platelet-derived growth factor (PDGF), and pro-inflammatory cytokines such as tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β) and interleukin 6 (IL-6) [3,32]. Factors that induce TIMP-1 expression also include transforming growth factor β (TGF-β), which undergoes negative feedback inhibition by TIMP-1, thanks to which TIMP-1 reduces the influence of TGF-β on self-expression [28]. TIMP-1 combines with both the latent form and the active form of MMP-9. The biological activity of TIMPs is not limited to inhibiting the activity of MMPs [33]. TIMP-1 has been shown to stimulate erythropoiesis by directly affecting erythropoietic cells [34]. Apart from inhibiting the activity of MMPs, TIMP-1 stimulates somatic karyokinesis in tumor cells, keratinocytes, epithelial cells, fibroblasts, and cartilage cells. The effect of TIMP-1 on the CD63 receptor (CD63-R), located on the cell surface, has an anti-apoptotic effect [28]. The interaction of TIMP-1 with CD63-R is mediated by the C-terminal fragment of TIMP-1, to which pro-MMP-9 also shows the ability to bind. Pro-MMP-9 has the effect of regulating the anti-apoptotic effect of TIMP-1 on the cell. Increasing the concentration of pro-MMP-9 in the studied environment promotes the production of pro-MMP-9/TIMP-1 complexes and enhances the pro-apoptotic effect of TIMP-1 on the cell by reducing the binding of CD63-R to TIMP-1 [33].

TIMP-2 is a 194 amino acid–soluble protein with a molecular weight of 21 kDa. In contrast to TIMP-1, TIMP-2 shows constitutive expression [35]. In its structure, it contains the C-terminal domain, common to all proteins from the TIMPs family, responsible for binding the inhibitor to the hemopexin-like fragment of the MMP, and the N-terminal domain binding to the active center of the MMP. It does not contain carbohydrate residues in its structure [30]. TIMP-2 has the ability to bind to both the active and inactive forms of MMP-2 and has an activating effect on pro-MMP-2. In in vitro studies, TIMP-2 blocks both the activity of MMP-9 as well as MMP-2 [36]. Studies show the effect of TIMP-2 on the induction of growth factor–dependent mitogenesis, probably by affecting membrane receptors and the induction of secondary messengers in the form of cyclic adenosine monophosphate (cAMP) and protein tyrosine kinases (PTKs) [35,37]. Moreover, TIMP-2, like TIMP-1, is involved in stimulating erythropoiesis, and is also involved in the regulation of proliferation of fibroids and sarcomas in bones and fibroblasts in various tissues of the body [37,38,39].

The ratio of enzyme concentration to inhibitor concentration is often used as a measure of the activity of MMPs. This rule most often applies to the inhibitor with the highest affinity for a given MMP [40,41].

MMPs and TIMPs play an important role in numerous physiological and pathological processes, including proliferation, migration, invasion, cell differentiation, and angiogenesis. Many studies indicate a direct relationship between MMPs and tumor invasion, angiogenesis, and the formation of distant metastases [4]. The degradation of basement membranes, the main building component of which is type IV collagen, is crucial in the process of local tumor infiltration and metastasis formation [42]. Degradation of ECM elements due to the proteolytic activity of MMPs promotes primary tumor growth and the formation of distant metastases [43]. Metastases to distant organs remain the leading cause of death among patients with diagnosed neoplastic disease [44]. MMPs promote the migration of neoplastic cells by the degradation of ECM components, such as type IV collagen, laminin, and fibronectin, and also facilitate the invasion of neoplastic cells due to the cleavage of surface particles such as E-cadherin, integrins, or CD-44 particle, which in turn leads to the disturbance of cell-cell interactions [45]. In addition, MMPs can promote cell migration independent of their proteolytic activity. MMP-14 promotes the production of adenosine triphosphate (ATP) in macrophages by activation of the type 1 hypoxia-induced factor (HIF-1) [46]. Neoplastic cells gain invasiveness and increased mobility potential, thanks to the gradual transformation of the phenotype of epithelial cells into mesenchymal cells. This process is called the epithelial-mesenchymal transfer (EMT) [47]. In addition, EMT is triggered by several transcription factors such as SNAI-1, SNAI-2, ZEB-1, and TWIST-1, whichblock the epithelial phenotype and promote the mesenchymal phenotype through processes leading to the disappearance of epithelial cell markers, rearrangement of cytoskeleton actin, and stimulation of mesenchymal cell marker expression. This results in the acquisition of a mesenchymal phenotype by cells and an increase in their biomobility, and thus a greater ability to form distant metastases. Exogenous demethylating agents such as 5-aza-2-deoxycytidine have been shown to induce the expression of the above-mentioned transcription factors responsible for triggering EMT, to an extent differing, depending on the type of PCa cell lines. Overexpresion of SNAI-1 was observed in all the studied cell lines of PCa. Overexpression of TWIST-1 was observed only in Du145 and PC3 cell lines of PCa. Overexpression of SNAI-2 was observed exclusively in Du145 cell lines of PCa and overexpression of ZEB-1 was apparent only in LNCaP cell lines of PCa [48]. In addition, it was confirmed that the increased expression of the transcription factors discussed above positively correlates with the increased expression of CD146 in PCa cells subjected to the demethylational action of 5-aza-2-deoxycytidine and is an additional stimulant of ETM. Epigenetic studies have shown that methylation and demethylation processes in the CpG island of the promoter of the CD146 gene may inhibit or stimulate its expression in the progression of PCa. The CpG island encompassing exon 1 of the promoter DNA, which is closely related to transcription silencing of the CD146 gene, has been shown to be a constitutively non-methylated region. Therefore, it is assumed that the processes of methylation and demethylation of DNA fragments outside the CpG island are responsible for the transcription control of the CD146 gene, respectively. Methylation at the upper and lower edges of the CpG islands of the promoter of the CD146 gene inhibit its transcription in cancer cells. Hence, the use of 5-aza-2-deoxycytidine in the treatment of PCa, which has strong demethylation properties, may be counterproductive as it triggers the expression of CD146, which is a strong EMT inducer. MMPs may play a regulatory role in CD146 induction of EMT because they have the ability to remove an extracellular fragment of CD146 from the surface of the cell membrane, but this mechanism in the PCa progression is unexplored [48,49,50].

The role of MMPs in the progression of PCa is closely related to the intercellular signaling processes involving intracellular SMAD proteins, E2F proteins and p38 protein kinases [51,52,53,54]. The uncontrolled proliferation of tumor cells in PCa is also associated with aberrant regulation of the E2F5/p38/SMAD3 axis. The change in E2F5 expression significantly influenced the activity of MMP-2 and MMP-9 and the recruitment of E2F5 on the promoters of type 2 tissue factor pathway inhibitor (TFPI-2), MMP-2, and MMP-9 was confirmed by the real-time reverse transcription polymerase chain reaction(RT-PCR), WesternBlot, and microarray tests. Importantly, the combined blocking of E2F5 and TFPI-2 expression significantly influenced the gelatinolytic activity of MMP-9 and the active form of MMP-2 in PC3 cells of PCa. In prostate biopsy tissues from PCa patients, the concentration of E2F5 and the activity of MMP-2 and MMP-9 were significantly increased, and TFPI-2 expression was significantly decreased compared to prostate biopsy tissues in BPH patients. In PC3 cells of PCa treated with artemisinin, a decrease in E2F5 concentration and a decrease in the activity of MMP-2 and MMP-9 as well as an increase in TFPI-2 expression were observed, which led to a decrease in the aggressiveness of PC3 cells of PCa, most likely by inhibiting the activity of MMPs. This proves the oncogenic role of E2F5, which, acting as a transcription factor, induces invasiveness of PCa cells [54,55].

It has also been shown that MMPs are involved in the initiation and termination of immunological processes by modifying membrane-bound proteins and ECM proteins. Particular importance is attached to the anti-tumor immune response, especially with regard to immune checkpoints [56]. MMPs, through their proteolytic activity, participate in the control of the type 1 programmed death ligand (PDL-1) expression on fibroblasts [57].

Increased expression of MMPs is confirmed in solid tumors of various origins and correlates with a low overall survival rate [58]. High MMP-9 expression is associated with poor prognosis in breast, lung, colorectal, and prostate cancers [59]. A similar correlation has been observed between MMP-13 and PCa [60].

MMPs play a vital role in the development of tumor blood vessels. This process depends on the mutual interaction of many factors, including, importantly, fibroblast growth factor (FGF) and vascular endothelial growth factor (VEGF) and its receptors [61,62,63,64]. Release of growth factors from ECM takes place mainly in the mechanism of ECM degradation by MMPs, while VEGF and FGF have the ability to reverse MMP expression, whose aftermath stimulates the angiogenesis.

The interrelationships between VEGF secretion, MMP-9 expression, and activation of tyrosine kinases in neoplastic cells are particularly emphasized. Focal-adhesion kinase (FAK) activation plays a very important role in controlling cellular functions such as proliferation, adhesion, invasion, and migration. The strong expression of MMP-9 and FAK have been shown to play a pivotal role in the progression of PCa. FAK is a non-receptor cytoplasmic protein tyrosine kinase and consists of an N-terminal FERM domain, a central kinase domain, and a C-terminal domain. The N-terminal domain and the C-terminal domain are responsible for the biological activity of FAK, including the regulation of the activity of integrins and transmembrane receptors. Activation of FAK through signal transduction from the ECM to the cell nucleus promotes the signaling of integrins and proteins such as interleukin 8 (IL-8), bombesin, and urokinase-type plasminogen activator (uPA). FAK, by activating phosphoinositide-3-kinase (PI3K) with subsequent activation of protein kinase C (PKC), stimulates the expression of MMP-9, which in turn influences the secretion of VEGF. In addition, FAK promotes the activation of extracellular signal-regulated kinase (ERK) by p130CAS multiadaptor, GRB2 protein adaptor, and PI3K, which may potentially lead to the induction of VEGF transcription and intensification of neoplastic angiogenesis processes, but the main role in stimulating VEGF secretion is attributed to the interaction of MMP-9, whose strong expression is associated with shorter disease-free survival (DFS) [65,66,67,68,69].

The activity of MMPs may also inhibit the maturation of tumor microcirculation. The degradation of several ECM components by MMPs, which include collagen type IV, type XV, type XVII, and thrombospondin, fibronectin, and plasminogen, leads to the production of bioactive fragments, which are designed to interrupt the differentiation process of vessels. This is evidence of the regulatory role of MMPs in the angiogenesis process [63,70]. In the process of maturation of the tumor blood vessel network, the role of MMP-14 is also emphasized, whose experimental inhibition of activity leads to maturation disorders of the capillary vessels [64,71]. The role in the regulation of MMP-14 expression is attributed to epidermal growth factor (EGF) and specific ligands for its receptor, showing both autocrine and paracrine effects on differentiating cells of blood vessels [72]. The role of EGF in the process of angiogenesis is inherently associated with the intensification of MMP-3 expression, the main function of which is the degradation of ECM elements, such as fibronectin, laminin, thrombospondin, and various types of collagen, which results in the production of molecules capable of inhibiting angiogenesis (Figure 1) [31,73,74].

Figure 1

In addition to the mechanisms described above, the role of MMPs in the process of tumor angiogenesis was described in detail in our previous publication [5].

A number of MMPs play an important role in the pathology of PCa. In a study involving 154 men with PCa, the expression of MMP-1, MMP-9, and TIMP-2 in tumor tissues, stromal tissue, and normal gland tissue was assessed using an immunohistochemical method. It was shown that higher MMP-1 expression in PCa cells was associated with lower Gleason score, lower PSA concentration before treatment, and lower frequency of vascular and perineural invasion, and extraprostatic extension (EPE). In addition, higher expression of MMP-1 and MMP-9 in tumor cells and undetectable expression of TIMP-2 in normal prostate tissue were associated with a longer DFS. The results of this work suggest that evaluation of MMP and TIMP expression may provide useful prognostic information for PCa patients [75].

The conducted studies also showed a correlation between the expression of MMP-11 and the occurrence of PCa. The analysis of the relationship between the presence of PCa and the expression of MMP-11 was the subject of a study conducted on PCa tissue collected from 103 men, in which the expression of MMP-11 was assessed by immunohistochemistry. None of the patients received anticancer treatment prior to the collection of the material for the study. The results of this study showed a strong correlation between high MMP-11 expression and a higher Gleason score, a higher clinical tumor grade and the presence of bone metastases. There was no correlation with the patient's age or serum PSA concentration. Moreover, patients with high MMP-11 expression showed significantly shorter overall survival [76].

In an immunohistochemical study on tissue samples obtained from 40 PCa patients who underwent radical prostatectomy, the expression of MMP-2, MMP-9, MMP-14, as well as TIMP-1 and TIMP-2 was assessed in a microarray of localized PCa. The obtained results were correlated with the Gleason scale, pre-operative serum PSA concentration, and biochemical recurrence (BCR) during the observation period up to 92.5 months. It has been shown that the loss of TIMP-1 expression is associated with a significantly higher percentage of BCR. BCR affected 56.3% of patients who did not express TIMP-1, compared to 22.2% who did express TIMP-1. TIMP-2 expression was not found in any of the above cases. Thus, loss of TIMP-1 expression is associated with a higher risk of BCR, possibly due to the mechanism of loss of control over MMP-9 expression, which plays an important role in PCa progression [77].

MMPs also play an important role in PCa-related EPE processes. In a study by Baspinar et al., the expression of nerve growth factor (NGF), glial neurotrophic factor (GDNF), and MMP-9 was assessed in three tissue compartments, including BPH, high-risk prostatic intraepithelial neoplasia (HG-PIN), and PCa, and analyzed correlation with clinical and pathological parameters. Tissues were assessed by immunohistochemistry in 30 BPH patients, 40 patients with HG-PIN, and 121 patients with PCa. MMP-9 expression was significantly different in all groups. High expression of NGF, GDFN, and MMP-9 was found in PCa associated with high Gleason scores and higher clinical advancement. All markers were associated with perineural invasion, lymphatic invasion, and EPE. This indicates the participation of the abovementioned factors in the process of carcinogenic transformation of prostate tissue cells covered by BPH and HG-PIN, as well as in the process of PCa progression [78].

The evaluation of MMP activity as a potential biochemical marker of treatment response is also currently under investigation. The greatest hopes are related to the assessment of the activity of MMP-7 as a potential marker of response to treatment with docetaxel, enzalutamide and abiraterone. The study of MMP-7 activity in the course of treatment with docetaxel, enzalutamide, and abiraterone in hormone-refractory PCa was carried out on a group of 320 patients with metastatic hormone-refractory PCa. Of 320 patients, 95 received docetaxel, 140 abiraterone and 85 enzalutamide. A total of 836 blood samples were collected from patients before treatment and during treatment to assess MMP-7 activity. Patients with high MMP-7 activity treated with abiraterone had a significantly higher overall survival rate compared to patients who also had high MMP-7 activity but received docetaxel for treatment. Such a correlation was not observed in the group of patients with low MMP-7 activity. This clearly suggests that patients who have high MMP-7 activity will benefit more from treatment with abiraterone than with docetaxel [79].

As mentioned in the introduction, there are also s-MMP-Is that have been synthesized as drugs. The first of them was batimastat – a molecule mimicking the most common substrate for MMPs. Batimastat was inhibitory against most enzymes in the MMP family, and preclinical data indicated a potentially beneficial anti-tumor effect. However, due to the lack of water solubility, batimastat was characterized by low bioavailability [80]. Marimastat was developed as a new-generation drug and showed similar effects to batimastat, and it entered phase II and III clinical trials in metastatic PCa. Despite promising reports, no survival benefit has been identified. In addition, patients suffered from increased symptoms of the musculoskeletal syndrome that led to treatment discontinuation [81]. More selective s-MMP-Is were also tested, such as tanomastat, which is an inhibitor of MMP-2, MMP-3, MMP-8, MMP-9, and MMP-13;prinomastat, which is an inhibitor of MMP-2, MMP-3, MMP-9, MMP-13, and MMP-14; and rebimastat, which is an inhibitor of MMP-1, MMP-2, MMP-3, MMP-8, MMP-13, and MMP-14. The above-mentioned molecules have been studied in disseminated tumors originating in the ovary, breast, and prostate gland; however, due to the lack of survival benefit, clinical trials were discontinued [4,82].

In 2016 alone, 1.5 million new cases of PCa and 595,000 deaths from the disease were found in the United States alone, therefore new biomarkers are needed that can be used in making clinical decisions, as well as potential targets for new cancer therapies [4,6].

Despite hundreds of observational studies that clearly confirm the association of increased MMP activity with relapse or spread of neoplastic disease, only MMP-11 has become part of the OncotypeDXTM prognostic test, which is used to predict recurrence of and conduct therapy for hormone-dependent breast cancer lacking the human type 2 epidermal growth factor receptor (HEGF-2-R) [83].

The concentrations of TIMPs also change as the cancer becomes more aggressive. It might be assumed that lowering TIMP concentrations would result in tumor progression, but evidence suggests that these processes are more complex. Indeed, some TIMPs are upregulated while others are silenced, and thus regulating their expression contributes to invasion and metastasis [84].

Moreover, MMPs can be a very important diagnostic marker for many cancers. Diagnosing cancer at an early stage improves the survival rate. MMPs offer theoretical potential for highly sensitive, cost-effective, and non-invasive diagnostic tests [85].

It is worth emphasizing that influence on MMP activity is the target of some anticancer drugs. As described above, the therapeutic potential of artemisinin, a natural compound whose anti-tumor activity is based on correcting the dysfunctional E2F5/p38/SMAD3 axis in PCa, is noteworthy [54].

On the other hand, demethylating drugs used in anticancer therapy, such as 5-aza-2-deoxycytidine, used under the name of decitabine, should be thoroughly reviewed. Apart from its low effectiveness in preliminary clinical trials, decitabine is still considered in the treatment of solid tumors and has recently been proposed as an epigenetic therapy in the treatment of PCa. Decitabine triggers the expression of CD146, a very potent EMT inducer in PCa cells, and its use could potentially be counterproductive [48].

Moreover, the goal of more extensive research should be to create s-MMP-Is characterized by high affinity and selectivity. Perhaps this will reduce the blocking of the activity of MMPs showing physiological protective effects and reduce the risk of developing the musculoskeletal syndrome, which is a serious side effect of previous generations of s-MMP-Is [86].