Interestingly, we detected an application time and con centration dependent loss of Sirt1 protein upon cambinol treatment. The underlying cause for this effect, which abrogates Sirt1 function, remains to be elucidated and may be due more to protein degradation. Consistent with the results by Zhao et al. obtained by immunhistochemistry, qPCR and western blotting, we observed a variable expression of Sirt1 in PDACs but did not see a positive correlation of Sirt1 expression with age, tumor size, and lymphatic spread. The different findings may be explained by distinct cohort characteristics includ ing cohort size, age, and sex. However and in contrast to Zhao et al, we observed a strong correlation with higher tumor grades, i. e. the less differentiated the cancer cells are the more Sirt1 expression they exhibit.

This finding is of interest since there are reports that implicate Sirt1 in the regulation of cellular differentiation and dedifferenti ation processes. Dedifferentiation and the associ ated phenomenon of epithelial to mesenchymal transition play an essential role in the development of early local and distant tumor spread. Observations that link high Sirt1 ex pression to poorly differentiated cancers were also made by other investigators for hepatocellular carcinoma, prostate cancer and glioblastoma. The association between high Sirt1 expression and poor histological grade may also explain why in our cohort Sirt1 expression is associated with poor outcome regardless of the tumor stage as shown by its prognostic indepen dency in multivariate survival analysis.

A Sirt1 positive and poorly differentiated tumor may have acquired a biological profile that allows for e. g. early systemic spread of clinically undetectable micrometastases in lymph nodes and distant organs leading to impaired survival regardless of the tumor size and metastases detected at the point of initial tumor diagnosis. A re cent study by Nalls and colleagues showed that SAHA induced micro RNA 34a expression in human pancreatic cancer cells putatively directly inhibited Sirt1 expression by binding within the 3UTR of Sirt1. On cellular level, restoration of miR34a ex pression led to growth inhibition as well as decreased epithelial to mesenchymal transition and inva sion. Although miR34a does not exclusively target Sirt1, this recent study further argues for an oncogenic role of Sirt1 in PDAC development. Recent results obtained by Pramanik et al. corroborate this view. Functional GSK-3 studies indicate that the subcellular localization of Sirt1 might have functional implica tions in carcinogenesis. Wauters et al.