Fasting regimens can increase lifespan, improve health or both in diverse species including mammals. Fasting also has an emerging role in inhibiting tumor growth, yet little is known about how it impacts tumor initiation or how fasting-imposed metabolism can be therapeutically exploited to treat established tumors. Given that adult stem cells coordinate tissue adaptation and drive tumorigenesis, understanding the mechanism(s) that mediate their response to fasting has important implications for enhancing tissue repair after injury or aging where stem cell function declines, and may provide new therapeutic inroads for cancer. In the mouse intestine, where LGR5+ intestinal stem cells (ISCs) drive the rapid renewal of the intestinal lining, we showed that fasting augments ISC function by inducing a peroxisome proliferator-activated receptor delta (PPARd) driven fatty acid oxidation (FAO) program, which breaks down free fatty acids into acetyl-CoA units. This work raises the critical question of how fasting functions through the FAO pathway to regulate intestinal stemness. We hypothesize that beta-hydroxybutyrate (?OHB), a ketone body and biosynthetic product of FAO generated acetyl- CoA, functions as a signaling metabolite and energetic substrate that mediates the ISC fasting response. In support of this idea, we recently found that the LGR5+ ISCs strongly express enzymes of the ketogenic pathway that produce ?OHB, including its rate-limiting enzyme HMGCS2 (3-hydroxy-3- methylglutaryl-CoA synthetase 2), compared to non-stem cell populations and that fasting strongly elevates HMGCS2 and ?OHB levels in ISCs. HMGCS2 loss in the small intestine reduces ?OHB levels in LGR5+ ISCs and skews their differentiation towards secretory cell fates, which we showed can be rescued by exogenous ?OHB and class I histone deacetylases (HDACs) inhibitor treatment. Mechanistically, ?OHB acts as a signaling metabolite to reinforce the NOTCH program in ISCs by inhibiting HDAC-mediated transcriptional repression. Dynamic control of ?OHB levels in ISCs, therefore, could enable the rapid adaptation of the intestine to diverse physiological states like fasting. Many important questions that form the basis of our aims remain regarding the role ketone bodies as effectors of the fasting response in ISCs such as understanding the in vivo signaling (Aim 1) and energetic (Aim 2) roles of ?OHB in this process. Another critical question is to decipher how the fasting-induced FAO program in ISCs influences tumor initiation and progression (Aim 3). !
In the mammalian intestine, Lgr5+ stem cells (ISCs) drive the rapid renewal of the intestinal lining and its adaptation to diet including to fasting, which enhances the ability of ISCs to repair damage after injury. Although fasting extends lifespan and promotes health in mammals, little is known about how it stimulates the function of ISCs to mediate this repair process in the intestine or how fasting-induced changes in ISCs impact intestinal tumorigenesis?a process often initiated by stem cells. In this proposal, we will investigate the mechanisms by which fasting mediates intestinal stemness and how fasting contributes to intestinal tumor initiation, progression and treatment.
