Odontogenic lesions arise from the activation of epithelial (cysts and tumors) and/or ectomesenchymal (tumors) remnants of odontogenesis that persist in the bone. These remnants can be stimulated by proteins that activate biological events essential for the growth and progression of the lesion. One important event is osteoclastogenesis, which is directly linked to the establishment of intraosseous lesions. The molecular events involved in the bone resorption and growth of odontogenic lesions are not fully understood, but studies have shown that some proteins seem to influence the osteolytic activity and expansion of these lesions [1, 4, 11, 15]. The present study suggests the contribution of RANKL, CatK, MMP8 and OPG during the development of DC, GOC, OKC, and AB.
Analysis of the immunoexpression of osteoclastogenic factors in DC demonstrated high expression of OPG. However, significant expression of MMP8 was also observed in this cyst. Marked expression of OPG in DC is expected because of the indolent nature of the lesion. OPG inhibits the proliferation and differentiation of osteoclasts and competes with RANKL by preventing its binding to RANK, thus inhibiting osteolytic activity [7, 15]. Suojanen et al. (2016) found higher expression of OPG in DC when compared to OKC and AB, in agreement with our results .
Studies investigating the expression of MMP8 in odontogenic lesions are scarce in the literature and there is only one study that involved DC . Our findings are consistent with the results of Suojanen et al. (2016) who also found high immunoexpression of MMP8 in DC. MMP8 is one of the most important endopeptidases that mainly degrades collagen I [16, 17]. By acting together with other factors that stimulate osteolytic activity, this protease would contribute to the growth and progression of intraosseous lesions. It is believe that the presence of MMP8 in CD evaluated in the present study was important for the lesion to begin. However the predominance of OPG is responsible for its indolent characteristic. With respect to CatK, low expression of this protein was detected in the DC cases. CatK participates in the degradation of bone matrix protein components, especially collagen I, the most abundant collagen in bone. However, the low expression of CatK in DC suggests a minor role of this protein in the development of this lesion. There are no studies evaluating the immunoexpression of CatK in DC.
In GOC, MMP8, RANKL, and CatK were expressed at higher levels than OPG. This finding may explain in part the aggressive nature of this cyst since it exhibits higher expression of molecules that favor bone resorption.
The highest immunoexpression of RANKL was observed in OKC. This result is consistent with the literature since OKC is considered a more aggressive lesion compared to other odontogenic cysts [18, 19] and is associated with higher bone resorption [20, 21]. Tekkesin, Mutlu and Oglac (2011), Matos et al.(2013) and Siar et al. (2015) also obtained high immunoexpression of RANKL in OKC [1, 11, 15]. Furthermore, ours results show significant expression of OPG in the capsule of OKC. This result raises the possibility that, although considered an aggressive lesion, OKC normally possesses slower growth than a tumor lesion such as AB. Similar results have been reported by Tekkesin, Mutlu and Oglac (2011) who evaluated the levels of RANKL/OPG in OKC and found higher expression of OPG in 62.4% of the cases studied . In contrast, in the capsule of OKC of our sample, OPG was less expressed than RANKL, CatK and MMP8. It is believed that the different proportions of stimulating and inhibiting factors favor greater or lower bone resorption.
Significant staining for CatK was observed in OKC, suggesting that this protein is important for the development of this lesion. The immunohistochemical expression of MMP8 in OKC was more significant in the capsule. There are no studies that evaluated the expression of CatK and MMP8 in OKC. However, some authors have demonstrated a relationship of CatK with higher bone resorption in the progression of rheumatoid arthritis and periodontitis , and of MMP8 with reduced bone repair and an increased risk of developing malignant tumors [2, 17].
Ameloblastoma parenchyma and stroma studied revealed high levels of RANKL, compatible with an aggressive lesion. Low expression of OPG was also observed in AB, which is consistent with the fact that this protein is an inhibitor of osteoclastogenesis. These results corroborate several studies in the literature [1, 11, 23, 24]. Kumamoto and Ooya (2004) found higher expression of RANKL compared to OPG in the parenchyma of AB . Tekkesin, Mutlu and Oglac (2011) detected higher expression of RANKL in the stroma of AB when compared to OPG .
In the present study, significant CatK staining was observed in the parenchyma and in the stroma of AB. Cathepsin K is a favorable biological marker of osteoclastogenesis which, together with RANKL, could explain the bone resorption and expansion potential of this lesion. Kim et al. (2014) observed important expression of CatK in peripheral ameloblastoma . There are no previous studies evaluating the expression of CatK in intraosseous AB. The immunohistochemical expression of MMP8 in AB was not significant. The studies published so far did not investigate the participation of MMP8 in the pathogenesis of AB. However, other MMPs have been explored and an important role in the establishment, bone resorption and growth of this tumor has been demonstrated [3, 26].
Comparison of the immunohistochemical expression of the proteins between the lesions studied revealed an apparently greater participation of RANKL and CatK in the pathogenesis of OKC, AB and GOC than in DC. This result might be important to explain the greater bone resorption, growth and aggressiveness associated with AB, OKC and GOC [3, 7, 11]. Tekkesin, Mutlu and Oglac (2011) evaluated the immunohistochemical expression of RANK, RANKL and OPG in OKC, AB and RC . In that study, the expression of RANKL was high and similar in AB and OKC and low in RC. RANK expression was higher in OKC and OPG expression was low, without differences between lesions. Brito et al. (2018) evaluated the expression of the RANK/RANKL/OPG triad in unicystic ameloblastoma, OKC, and DC . Higher expression of RANKL was observed in unicystic ameloblastoma, followed by OKC. The latter exhibited higher expression of OPG when compared to the other lesions studied. Higher expression of RANK and lower expression of RANKL were found in DC. The present study also compared the immunohistochemical expression of RANKL and OPG in DC, AB and OKC. Higher expression of RANKL was observed in OKC, followed by AB. Taken together, these results suggest considerable osteolytic activity in AB and OKC, as indicated by the high expression of factors that stimulate osteoclastogenesis.
Higher immunoexpression of OPG was observed in DC and DF. This finding might explain the lower osteolytic activity associated with these processes. Likewise, Tekkesin, Mutlu and Oglac (2011) found high immunopositivity for OPG in 70% of DF cases when compared to AB, OKC and DC . According to Moraes et al. (2011) and Siar et al. (2015), higher expression of OPG is expected in more indolent lesions because of the inhibitory activity of this protein on osteoclastogenesis [4, 15].
Our results indicate higher expression of RANKL and CatK in lesions with a more aggressive behavior such as OKC, AB and GOC. OPG was more expressed in DF and DC and seems to be one of the molecules responsible for the slower growth of DC. MMP8 appears to play an important role in the growth of the GOC and OKC. Further studies involving odontogenic lesions and these markers, especially CatK and MMP8, are necessary to increase our understanding of the process of bone resorption in these lesions.