The incidence of fungal infections in humans has increased considerably over the decades. It may be attributed to the rise of the AIDS epidemic, the increasing number of patients at risk, such as those with advanced age and those who have undergone major surgery, immunosuppressive therapy, and solid-organ and hematopoietic stem cell transplantation, among others [1,2]. In addition, despite their wide use, antifungal agents have certain limitations due to their side effects, such as toxicity and the emergence of resistant strains, especially those resistant to fluconazole via the efflux pump mechanism [3,4,5].

Of all the fungi regarded as human pathogens, the members of the genus Candida are the most frequently recovered from human fungal infection. Of the Candida species isolated from humans, C. albicans is the most prevalent under both healthy and disease conditions; however, non-C. albicans species such as C. glabrata, C. tropicalis and C. parapsilosis can be frequently identified as human pathogens [6].

Candida pathogenicity is facilitated by a number of virulence factors, especially adherence to host surfaces including medical devices, biofilm formation and secretion of hydrolytic enzymes. These enzymes can facilitate the spread of this yeast to deeper organs, as well as protect it from the humoral immune response of the host [7].

The term protease is synonymous with peptidase, proteolytic enzyme and peptide hydrolase. The proteases include all enzymes that catalyse the cleavage of the peptide bonds (CO_NH) of proteins, digesting these proteins into peptides or free amino acids. The proteases are initially classified following their mode of action and their active sites: Aspartic, cystein, metallo, serine, and threonine proteases [8].

Candida albicans expresses a vast number of hydrolytic enzymes, playing roles in several phases of yeast–host interactions [9]. Candida spp. metalloproteinase may represent a new pathogenic factor for degrading components of the extracellular matrix of the host and the inhibition of metalloproteinase may have therapeutic implications since it can control the pathological breakdown of collagen, which represents a promising approach for the infectious diseases [10,11,12].

Although it is now clear that proteases may act differently as virulence factors, knowledge on protease substrate specificities remains rather poor and few studies have focused on the research of specific inhibitors [8].

The virulence factors and mechanisms of invasion of Candida spp. are the subject of many research groups; however, many of their interactions with the host are singularly complex and still remains poorly understood.

For development of this review we based in a methodology of Cochrane Handbook for Systematic Reviews of Interventions [13], which a systematic review attempts to collate all empirical evidence that fits pre-specified eligibility criteria in order to answer a specific research question. It uses explicit, systematic methods that are selected with a view to minimizing bias, thus providing more reliable findings from which conclusions can be drawn and decisions made. The key characteristics of a systematic review are:

• a clearly stated set of objectives with pre-defined eligibility criteria for studies;

• an explicit, reproducible methodology;

• a systematic search that attempts to identify all studies that would meet the eligibility criteria;

• an assessment of the validity of the findings of the included studies, for example through the assessment of risk of bias; and

• a systematic presentation, and synthesis, of the characteristics and findings of the included studies [13].

Information sources and search

To identify studies to be included for this review, we searched on the electronic databases MEDLINE via PubMed, Scopus and Web of Science. The last search was carried out in March 2016 with no language or date restrictions. The search strategies defined for the databases described are listed below.

Search strategy

Inclusion Criteria for Included Studies

All studies that describe the metalloproteinases secreted by Candida spp. were included.

Study selection

After the database screening [PubMed (72), Scopus (22) and Web of Science (29)] and removal of duplicates, 112 studies were identified, according to Figures 1 and 2. After title screening, 13 studies remained and this number was reduced to 8 after careful examination of the abstracts. The full texts of these 8 studies were assessed to check if they were eligible.

Figure 1: Flow diagram of the study.

Figure 2: Database screening [PubMed (72), Scopus (22) and Web of Science (29)].

The systematic review method used is scientific and described according to the Cochrane Handbook for Systematic Reviews of Interventions. Two independent reviewers (Juliana L. S. Souza and Rafael G. Lund) initially screened the titles and all identified studies. If the title indicated possible inclusion, the abstracts were evaluated. After the abstracts were carefully appraised, manuscripts considered eligible for the review (or in case of doubt) were selected for full-text reading.

Characteristics of included articles

Three studies detected and purified a metalloproteinase from C. albicans (El Moudni et al., de Brito Costa et al., and dos Santos et al.) [9,14,15], one study used this same metalloproteinase for the development of an immunoassay enzyme (El Moudni et al.) [16], two works investigated the ability of this purified metalloproteinase to degrade proteins of the extracellular matrix (Rodier et al., Klinke et al.) [11,12] and two studies investigated the influence of the agents antifungals for inhibition of the secretion of metalloproteinase purified from C. albicans (Imbert et al., Imbert et al.) [10,17].

Studies involving Candida metalloproteinase are poorly described in the literature. Some metalloproteinases have been purified and characterized, but little is known about the function of these enzymes as potential pathogens.

The first study about metalloproteinase from Candida albicans was by, El Moudni et al. [14] that purified a metalloproteinase from C. albicans by high performance liquid chromatography (HPLC). This enzyme was originally described as a protein of 52 kDa with optimal activity at pH 7.2. The same authors in 1998 [16] used this same metalloproteinase for the development of an immunoassay enzyme (ELISA) to detect antibodies directed against this antigen in the serum of patients with candidiasis. The study by El Moudni et al. [16], used the purified metalloproteinase and showed satisfactory values for diagnostic specificity (97.15%) and sensitivity (83%), demonstrating clearly that the use of a purified antigen of C. albicans, instead a crude extract, provides a more specific test for the diagnosis of systemic candidiasis.

According to Rodier et al. [11] other proteolytic enzymes could play a role in the degradation of extracellular matrix proteins. In his work they purified a metalloproteinase of C. albicans, located in the cell wall, and reported that this enzyme, previously described as a 52 kDa protein was indeed recovered in a native form of 95 kDa. Thus, they investigated the ability of this 95kDa metalloproteinase to degrade four proteins of the extracellular matrix (collagen type I and IV, laminin and fibronectin). They incubated the purified enzyme with each of these components ananalysed its degradation products by an electrophoresis method. It was observed that the enzyme totally degraded collagen type I and fibronectin, but only partially degraded laminin and type IV collagen. These findings showed that C. albicans metalloproteinase may play a role in the breakdown of subendothelial extracellular matrix components of the host and promotes fungus migration after crossing the endothelial layer. And it allows the invasion of the fungus of the target organ and, therefore, can be considered a factor pathogenicity of C. albicans [11].

Inhibition of metalloproteinase may have therapeutic implications, since it can control the pathological collagen degradation, thus representing a promising approach for the treatment infectious diseases.

A study conducted by Imbert et al. [10] tested the inhibitory effect of doxycycline, cephalothin, gentamicin, galardin, and oleic and elaidic acids on C. albicans 95kDa metalloproteinase in relation to the enzyme’s ability to degrade components of the extracellular matrix. Among these agents, only the oleic acid was able, significantly, to inhibit purified metalloproteinase in low concentrations. Oleic acid acted not only on the enzyme directly, but also on its secretion into the culture medium in a dose-dependent way without altering the viability of the yeast.

Later in 2006, the same researchers investigated the influence of three azole antifungals on the secretion of 95kDa metalloproteinase purified from C. albicans. Eight isolates were grown in the presence of voriconazole, fluconazole and itraconazole in MIC, MIC/2 and MIC/4. Voriconazole and fluconazole decreased the secretion of metalloproteinase. However, itraconazole increased the secretion of the enzyme in three isolates [17].

A study by de Brito Costa et al. [15], with the purpose of characterizing the proteinases secreted in oral, clinical isolates of C. albicans showed a profile of extracellular proteolytic activity rather complex and boosted the heterogeneity of this species. In this study, four distinct patterns of extracellular proteolytic activity of serines (30-58 kDa) and metalloproteinases (64-95kDa) were detected, demonstrating that the number of proteinase enzymes produced ranged between strains of the same species. The variability in expression or activity of proteinases may be partly responsible for the immune response after the initial infection.

In addition dos Santos et al. [9] identified two novel extracellular peptidase classes in C. albicans. Using gelatin-sodium dodecyl sulfate polyacrylamide gel electrophoresis two gelatinolytic activities were detected at physiological pH: a 60-kDa metallopeptidase, completely blocked by 1,10-phenanthroline, and a 50-kDa serine peptidase inhibited by phenylmethylsulfonyl fluoride. In an effort to establish a probable functional implication for these novel peptidase classes, they demonstrated that the 50-kDa secretory serine peptidase was active over a broad pH range (5.0-7.2) and was capable to hydrolyze some soluble human serum proteins and extracellular matrix components. Conversely, when this isolate was grown in yeast carbon base supplemented with bovine serum albumin, a secretory aspartyl peptidase activity was measured, instead of metalloand serine peptidases, suggesting that distinct medium composition induces different expression of released peptidases in C. albicans [9].

The study more recently about metalloproteinase from C. albicans have identified the enzyme CaApe2, isolated from the cell wall of C. albicans, and structural and kinetic data support classification as a neutral metalloaminopeptidase arginine/alanine/leucine-specific. However, the authors reported that the enzyme CaApe2 was not able to degrade in vitro collagen type I and IV and that its function is still unknown, but have not ruled out their involvement in the pathogenicity of Candida since CaApe2 can cooperate with proteinases of the host and contribute indirectly in the degradation of collagen. According to the authors, the molecular weight of CaApe2 (107.361 kDa) is significantly higher than the value of 95kDa metalloproteinase [12].

As proposed by Rodier et al. [11] and Klinke et al. [12], C. albicans metalloproteinase may play a role in fungi spread, since some of them are capable of degrading components of the extracellular matrix, facilitating their access to deeper organs after crossing the endothelial barrier. Thus, metalloproteinases represent a potential target for the development of diagnostic strategies and antifungal drugs based on inhibitors of this metalloproteinase as was demonstrated by Imbert et al. [10,17] in his two studies.

Metalloproteinases secreted by Candida is one factor of pathogenicity of this fungi genus and represents a promising approach to the treatment of candidiasis. However, research in this field needs advancement and improvement in terms of rigor and quality of the studies involved with this theme.