In the present study, we observed a significant negative association of cytoplasmic caspase-8 with grade of ccRCCs; cytoplasmic caspase-8 was highest in low-grade tumors, in contrast to previous studies in breast and lung carcinomas [22, 23]. This negative relationship implies that caspase-8 exerts an apoptotic action within low-grade ccRCCs, hence enhanced cytoplasmic distribution in grade 1 and 2 tumors. Our results seem to reflect the process in which malignant cells of high-grade specimens simultaneously display enhanced proliferative and decreased apoptotic activity [5, 24].
A significant association was noted between increased caspase-8 cytoplasmic expression and better patients' survival. This implication has never been reported in RCCs and is in agreement to recently published findings in papillary thyroid carcinomas .
p-p38 was detected in the majority of carcinomas localized in the nuclei of the malignant cells, in agreement with a previous study in ccRCCs . Increased p-p38 expression was positively related to high grade ccRCCs, in line with studies performed in a variety of tumors [26, 27]. To our knowledge, this is the first study in ccRCCs demonstrating this kind of association.
In contrast to published data regarding the apoptotic and anti-proliferative action of p-p38 in RCCs, our results suggest that p-p38 seems to exhibit a proliferative or anti-apoptotic action, being increased in high-grade tumors . This finding is also strengthened by the identification of decreased expression of the apoptotic cytoplasmic caspase-8 with increasing the degree of malignancy within our group. Indeed, investigators have showed that bone morphogenetic protein 6 played its anti-apoptotic effect in breast cancer through activation of p38 pathway .
We additionally detected, in harmony with others, a significant inverse relationship between bcl-2 expression and grade as well as pT-stage of RCCs [6, 29]. A possible explanation to this might be based on the suggested antiproliferative role of bcl-2 protein [6, 30]. This function, which seems to be distinct from its well-known antiapoptotic action, has been reported in various malignancies [31, 32]. Therefore, it could be hypothesized that high expression of bcl-2 prevents cell proliferation, suppresses tumor growth and thereby is associated with a lower grade and pT-stage in RCCs, as previously stated .
Provided that a possible but not yet adequately studied mechanism, linking the p38 pathway with caspases activity has been previously implied [14, 15], we attempted to correlate the expression of caspase-8 and p-p38 in our series of RCCs. In this framework, researchers have suggested that caspase-8 is essential for the activation of the p38 pathway through death receptors , while others pointed out that an enhanced activation of p38 kinase pathway may result in caspase-8 activation and enhancement of apoptosis, based on experiments in a human astrocytoma cell line .
However, no statistically significant associations between cytoplasmic or nuclear caspase-8 and p-p38 were revealed in our work, but only a suggestive negative and positive relationship, respectively. These findings backup our aforesaid hypothesis regarding the apoptotic action of cytoplasmic caspase-8 and proliferative or anti-apoptotic role of p-p38 within our ccRCCs [28, 35]. Moreover, we cannot exclude a potential inhibitory effect of p38 signaling upon caspase activity, as it has been previously reported . The reason however, for the increased, though statistically insignificant, expression of nuclear caspase-8 in parallel with high p-p38 immunoreactivity, is still to be determined.
As regards p-p38 and bcl-2 association, no strong linkage was identified in our study. However, p-p38 tended to be inversely related to bcl-2 expression. In this context, scientists have stressed that p38 can induce apoptosis via phosphorylation of bcl-2 [16, 17]. Concerning RCCs, there are no related studies. According to our work, the negative, though loose, correlation found between p-p38 and bcl-2, constitutes an early indication of a probable antagonistic interaction of these two factors within ccRCCs. The major question to be identified is whether this interaction concerns the proliferative, the apoptotic or both potential functions of these factors.
No statistically important correlation between bcl-2 and caspase-8 was found. This was expected, since these substances act through different apoptotic pathways [3, 7]. We also demonstrated that low-grade tumors (grade 1 and 2) depicted greater probability of survival in comparison to high-grade (grade 3 and 4), in accordance with previous data [20, 21]. The absence of statistical significance regarding the association of pT-stage with prognosis could be attributed to the small cohort.