BACKGROUNDAmeloblastoma is the most common benign odontogenic tumor. The neoplasm shows a locally aggressive behaviour and wide surgical excision is the treatment of choice. Recent studies have focused on the potential usefullness of novel molecules implicated in osteoclastogenesis and apoptosis of ameloblastic cells in treating ameloblastomas.Activation of Receptor Activator of Nuclear Factor kappa Β (RANK) by its ligand RANKL on the surface of pre-osteoclasts stimulates the maturation and activation of osteoclasts whereas, direct binding of Osteoprotegerin (OPG) with RANKL interrupts this interaction, thus inhibiting bone resorption. On the other hand, Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand (TRAIL) is an apoptosis-inducing ligand in ameloblastomas. Although OPG preferentially binds with RANKL than TRAIL, at low levels of RANKL, binding of OPG with TRAIL suppresses its function in inducing apoptosis.OBJECTIVEThe aim of this retrospective study was to evaluate the immunohistochemical expression of RANKL, OPG, TRAIL and Ki-67 in ameloblastoma in order to investigate any possible differences in the expression of the above molecules among the various histological subtypes of ameloblastoma.MATERIALS AND METHODSParaffin-embedded tissue sections from 40 cases of ameloblastoma (solid n=29, unicystic n=11) were immunohistochemically stained using a standard streptavidine-biotin peroxidase method, and polyclonal antibodies against RANKL, OPG, TRAIL and monoclonal Ki-67. The immunohistochemical expression for each of the above bone remodelling related molecules was evaluated by using a semi-quantitative method in a scale of 0-3 according to the staining intensity, and the extent of the immunoreactivity in the tumour. In particular, 0= absence of staining or weak staining in <5% positively stained cells, 1= intense staining in 1-5% or weak staining in 5-60% positively stained cells, 2= intense staining in 6-50% or weak staining in > 50% positively stained cells, 3= intense staining in > 50% positively stained cells. For the Ki67, the percentage of positively stained nuclei was assessed in at least 500 cells for each section. RESULTSThirty two cases were located in the mandible (80%). The male-to-female ratio was 1,5:1 while the mean age of the patients was 40,8 years (Min-Max: 6-78 years , SD 21,63). OPG was expressed as a cytoplasmic or/and cell mebrane staining in ameloblastic epithelium in all cases, whereas no immunoreactivity for RANKL and TRAIL was observed in 16 (40%) and 5 (12,5%) cases respectively. Twenty one cases (52,5 %), 18 of which solid, showed intense OPG immunoreactivity, in contrast to only 5% and 10% of the RANKL and TRAIL intensely stained cases. There were no differences between the central and peripheral areas in the ameloblastic nests or between ameloblastoma types. Twenty-nine out of 40 (72,5%) and 60% (24/40) of cases showed greater expression of OPG over RANKL and TRAIL respectively, while 50% of cases (20/40) exhibited a lower expression of RANKL than TRAIL.No statistically significant differences were found in the expression of TRAIL and RANKL with regards to age, sex, clinical type or histological variant. There was no statistical significant difference in the expression of OPG with regards to sex. OPG expression was found significantly increased in the solid type ameloblastoma samples (p=0,004) and in older patients (p=0,037). The OPG over-expression in solid ameloblastomas reflects the possible role of OPG in inactivating TRAIL-induced apoptosis and the subsequent survival of ameloblastic cells. OPG over-expression in solid ameloblastomas may also be related to the aggressive behaviour of this clinical type compared to the unicystic ameloblastoma. In cases that OPG was overexpressed RANKL and TRAIL were positively associated (mainly low RANKL and low TRAIL, p=0,005). Low RANKL concentrations may favor the binding of OPG to TRAIL.Low proliferative activity (< 5%) was observed in all, but two (5-10%) cases. This finding may reflect the slow growing, as well as the overall benign nature of this lesion.CONCLUSIONSThe intense OPG expression in ameloblastic tissue indicates a possible role of this molecule in ameloblastoma pathogenesis. Prevalence of the OPG/TRAIL over the OPG/RANKL signalling pathway with inhibition of TRAIL-induced apoptosis of the ameloblastic cells and subsequent infiltration of the surrounding bone, rather than an increased osteoclastic function seems to be the pathogenetic mechanism that underlies the locally aggressive behaviour of this tumour.