Diet, Muscle Protein Synthesis and Autophagy Relationships in Cancer. An Attempt to Understand Where Are We Going, and Why

Protein-based structures are indispensable to maintain life, so identification and removal of worn out structures achieved through proteostasis, the sum of micro and macro-autophagy (autophagy) plus ubiquitin-proteasome system, must balance renewal by new synthesis. Many of the elements controlling dynamically equilibria between protein synthesis and protein degradation have been identified and modalities of activation actively studied, still we are quite far from mastering how this balance is ruled. Failure to maintain a positive balance between protein synthesis and protein degradation would result in sarcopenia, defined as the loss of skeletal muscle mass and function, a major clinical problem frequently accompanying chronic illnesses, but peculiarly spotted in cancer and in elderly patients. Also, how cancer is fed, and how nutrition in cancer patients may affect evolution and therapy effectiveness is another field of opinions and uncertainty. On the other hand, exercise and nutrition tailored to provide adequate amounts of amino acids are widely considered a necessary strategy for prevention and treatment of protein synthetic deficits in muscles. This paper will synthetically review how different nutritional strategies and energy production may interconnect efficiently synthesis and scavenging of aged and overused protein molecules by autophagy. Finally, since energy availability rules life and death of cells and organisms, an hypothesis predicting how energy may control the ratios among protein synthesis and autophagy is proposed: in normal conditions, protein syntheses have a key role in autophagy activation by consuming large amounts of energy when forming peptidic bonds, that is adenosine tri-phosphate (ATP) is consumed to monophospahate (AMP), thus decreasing ATP to AMP ratios. Conversely, both protein syntheses and autophagy may be scarcely activated when low availability of ATP would result also in lowest concentrations of AMP. In this peculiar setting, reduced rates of both protein syntheses and autophagy would be observed, resulting in worsening of protein balance and functions.