Chaperone-mediated autophagy (CMA) is a selective lysosomalprotein degradative process that is activated in higher organismsunder conditions of prolonged starvation and in cell cultureby the removal of serum. Ketone bodies are comprised of threecompounds (-hydroxybutyrate, acetoacetate, and acetone) thatcirculate during starvation, especially during prolonged starvation.Here we have investigated the hypothesis that ketone bodiesinduce CMA. We found that physiological concentrations of -hydroxybutyrate (BOH)induced proteolysis in cells maintained in media with serumand without serum; however, acetoacetate only induced proteolysisin cells maintained in media with serum. Lysosomes isolatedfrom BOH-treated cells displayed an increased ability to degradeboth glyceraldehyde-3-phosphate dehydrogenase and ribonucleaseA, substrates for CMA. Isolated lysosomes from cells maintained inmedia without serum also demonstrated an increased ability todegrade glyceraldehyde-3-phosphate dehydrogenase and ribonucleaseA when the reaction was supplemented with BOH. Such treatmentdid not affect the levels of lysosome-associated membrane protein2a or lysosomal heat shock cognate protein of 70 kDa, two rate-limitingproteins in CMA. However, pretreatment of glyceraldehyde-3-phosphateand ribonuclease A with BOH increased their rate of degradationby isolated lysosomes. Lysosomes pretreated with BOH showedno increase in proteolysis, suggesting that BOH acts on thesubstrates to increase their rates of proteolysis. Using OxyBlotTManalysis to detect carbonyl formation on proteins, one commonmarker of protein oxidation, we showed that treatment of substrateswith BOH increased their oxidation. Neither glycerol, anothercompound that increases in circulation during prolonged starvation,nor butanol or butanone, compounds closely related to BOH, hadan effect on CMA. The induction of CMA by ketone bodies mayprovide an important physiological mechanism for the activationof CMA during prolonged starvation.
Source: Patrick F. Finn and J. Fred Dice. J. Biol. Chem., Vol. 280, Issue 27, 25864-25870, July 8, 2005