Background: In a previously published in vitro study based on top-down proteomics we found that the calcium-binding proteins S100A6 and S100A4 were affected by exposure to ionizing radiation in a p53-dependent fashion. Both proteins showed post-translational modification changes, and S100A6 also showed increased expression and translocation in response to irradiation. Aim of the present study was to evaluate the expression of S100A6 and S100A4 in non-small-cell lung cancer (NSCLC).
Methods: S100A6 expression on archival tumor cell lysates from 39 patients with radically resected NSCLC was assessed with SELDI-TOF-MS. S100A6 identity was confirmed using a SELDI-based antibody-capture method on lysates from the A549 lung cancer cell line, cell lysates from two freshly prepared NSCLC samples, four plasma samples and one pleural effusion sample. Immunostainings for S100A6, S100A4 and p53 were performed on tissue microarrays containing 103 stage I surgically resected NSCLC cases and 14 normal lung parenchyma specimens.
Results: The presence of post-translationally modified S100A6 forms was confirmed with SELDI-MS on enriched tumor cell lysates, as well as in plasma and pleural effusion samples. In addition, high S100A6 peak intensity was associated with longer median survival (35 months vs. 18 months for high and low peak intensity, respectively; p=n.s.). The immunohistochemical analysis showed that 25% of tumors were S100A6 positive. S100A6 expression correlated directly with non-squamous histology (p<0.0001) and S100A4 expression (p=0.005), and inversely with p53 expression (p=0.01). S100A6-positive cases showed a trend of longer survival compared with S100A6-negative cases (p=0.07). This difference became significant when the analysis was restricted to p53-negative cases (n=72). In this subgroup of patients, whose tumors likely exhibit a functional p53, S100A6 was an independent prognostic factor of improved survival at multivariate analysis (HR 0.49, 95% CI 0.27-0.81, p=0.017).
Conclusions: In this study we have validated on clinical material our previous findings on cell lines in terms of S100A6 expression and post-translational modifications pattern in NSCLC. Moreover, the survival results obtained in p53-negative stage I NSCLC cases support the proposed pro-apoptotic function of S100A6 and suggest the hypothesis of a cross regulation between these two proteins.