Franny Jongbloed

67 3 PROTECTION OF FASTING ON IRINOTECAN TOXICITY irinotecan in healthy tissue due to fasting. This may be an indication that fasting results in a state that favors maintenance and repair and in which normal cells are protected against toxic stressors during periods of nutrient deprivation 8,10,12 . Fasting is able to amend and dampen the genotoxic response in the liver; in AL fed mice treated with irinotecan growth signaling induction and general activation of signaling pathways are prominent adaptations, whereas these pathways are blunted by fasting. Particular of interest is the reduction in cellular injury and cytokine signaling. Cytokines are crucial players in the activation and adaptation to oxidative stress, and therefore the reduction is strong evidence of increased stress resistance induced by fasting 13 . Another marker of cellular stress is RhoA, which is activated by reactive oxygen species (ROS) in fibroblasts and may lead to apoptosis when activated by caspase 3 14,15 . RhoA signaling was activated in the fasting vehicle group of the liver as indication of a preconditioned state of stress. In the fasting irinotecan group, RhoA activation did not come forward whereas a known inhibitor of RhoA ( i.e. RhoGDI) was highly activated. An inhibition of RhoA results in a dampening of TGF-beta 1, a transcription factor able to activate ROS and stress- induced injury 16,17 . These findings are consistent with reduction of cellular stress by fasting, possibly via blunting of cytokine and RhoA signaling, which may play a key role in the protection of DR against adverse side effects of irinotecan. The mTOR signaling was downregulated as well via inhibition of the p7056 pathway, thereby initiating autophagy 18,19 . Autophagy is a process initiated for cell survival in healthy tissue, however autophagy in tumor cells also promotes tumorigenesis and tumor progression and therefore has an outcome dependent on the nature of the cells 13 .The effect of mTOR on stress resistance 20 also refers to the anti-inflammatory response due to fasting, and the beneficial role for promoting stress resistance after DR therefore holds also true for the side effects of chemotherapy 9,10 . The activation of the NRF2 stress response might also provide protection against the damage-induced response after irinotecan 21-23 . Posttranscriptional validation is needed to further understand the exact role of cytokine signaling, RhoA, mTOR and NRF2 on stress resistance and prevention of side effects of chemotherapy. The changes induced by fasting in healthy liver tissue did not come forward in the tumor transcriptome analysis. The PCA plots of the AL and fasting groups revealed a high heterogeneity within all the groups of tumor tissue, which is in contrast with the orchestrated response of the liver. Especially both AL groups (with and without irinotecan treatment) revealed a small number of DEPS. This number was about a 5-fold higher after fasting, however these genes showed only little overlap with the response in liver tissue of fasted irinotecan treated mice. On pathway level, the AL and fasting comparisons with and without irinotecan did not show a common response either; in the AL group