Abstract
Preclinical studies suggest that fasting prior to chemotherapy may be an effective strategy to protect patients against the adverse effects of chemo-toxicity. Fasting may also sensitize cancer cells to chemotherapy. It is further suggested that fasting may similarly augment the efficacy of oncolytic viral therapy. The primary mechanism mediating these beneficial effects is thought to relate to the fact that fasting results in a decrease of circulating growth factors. In turn, such fasting cues would prompt normal cells to redirect energy toward cell maintenance and repair processes, rather than growth and proliferation. However, fasting is also known to upregulate autophagy, an evolutionarily conserved catabolic process that is upregulated in response to various cell stressors. Here, we review a number of mechanisms by which fasting-induced autophagy may have an impact on both chemo-tolerance and chemo-sensitization. First, fasting may exert a protective effect by mobilizing autophagic components prior to chemo-induction. In turn, the autophagic apparatus can be repurposed for removing cellular components damaged by chemotherapy. Autophagy also plays a key role in epitope expression as well as in modulating inflammation. Chemo-sensitization resulting from fasting may in fact be an effect of enhanced immune surveillance as a result of better autophagy-dependent epitope processing. Finally, autophagy is involved in host defense against viruses, and aspects of the autophagic process are also often targets for viral subversion. Consequently, altering autophagic flux by fasting may alter viral infectivity. These observations suggest that fasting-induced autophagy may have an impact on therapeutic efficacy in various oncological contexts.
Highlights
A major hurdle in the fight against cancer is the narrow therapeutic window associated with cytotoxic chemotherapeutic agents
Frontiers in Oncology | www.frontiersin.org van Niekerk et al Fasting, Chemotherapy and Autophagy demonstrated that mice fasted for 48–60 h were protected against a lethal dose of etoposide, a chemotherapeutic with a notoriously narrow therapeutic window [1]
We review emerging evidence supporting a key role played by macroautophagy in attenuating the toxic effects of chemotherapy and in enhancing the efficacy of antineoplastic therapies
Summary
A major hurdle in the fight against cancer is the narrow therapeutic window associated with cytotoxic chemotherapeutic agents. Fasting-induced upregulation of the autophagic process may render cells prepared to deal with a systemic challenge faced during chemotherapeutic induction Importantly, this raises the question whether fasting-induced autophagy may not be protective to cancer cells. Oncogenes that are often upregulated in cancer (e.g., Akt, PI3K, and Bcl-2 proteins) inhibit autophagy, whereas tumor suppressor genes often mutated or lost (e.g., DAPK1 and PTEN) are usually involved in the upregulation of autophagy [16, 17] This would suggest that, at least in some cancers, the differential stress response may manifest from the inability of cancer cells to sustain autophagic processes as effectively as normal cells. There is reason to suspect that host cells, which are more responsive to fasting cues, may upregulate autophagy to a greater extent than cancer cells
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