p16^INK4a Is a Key Downstream Mediator of the Deleterious Effects of FoxO Deficiency on Maintenance of the Hematopoietic Stem Cell Compartment.

Regulation of oxidative stress in the hematopoietic stem cell (HSC) compartment is critical for the maintenance of HSC self-renewal. A number of reports have previously implicated p16 in aging of HSCs, pancreatic β-islet cells and subventricular zone progenitors in the brain [1–3]. In the context of the hematopoietic system, p16^INK4a expression in HSCs increases with age, and correlates with decreased HSC repopulating ability, decreased self-renewal, and increased apoptosis with stress [1]. We and others have recently reported that FoxO play essential roles in the response to physiologic oxidative stress and thereby mediate quiescence and enhanced survival in the HSC compartment [4, 5]. Young mice deficient in FoxO1, FoxO3, and FoxO4 in the adult hematopoietic system, with striking similarity to aging wild-type mice, show a defect in bone marrow repopulating ability, decrease in self-renewal, myeloid skewing in differentiation and increased levels of apoptosis. Furthermore, young FoxO-deficient HSC show increased levels of p16 when compared to their wildtype counterparts. These collective findings suggested the possibility that FoxO loss could result in accelerated aging of HSC due to increased expression of p16 as a consequence of increased ROS. To test the hypothesis that p16 is one of the key mediators of FoxO loss responsible for accelerated aging of HSC, we deleted FoxO1, FoxO3, and FoxO4 in the adult hematopoietic system of mice deficient in p16^INK4a. Young mice deficient in FoxO and p16 shared the same characteristics of their HSC(Lin⁻Sca1⁺c-kit⁺) compartment as mice deficient in FoxO only, including decreased number of HSC, increased percentage of HSC entering S/G2/M and apoptosis, and increased levels of ROS as compared to their wildtype counterparts. However, in a setting of long-term repopulation studies, bone marrow isolated from mice deficient in p16 and FoxO demonstrated a rescue of long-term repopulation for up to 20 weeks, as compared to FoxO deficient bone marrow that showed a severe defect in long-term repopulation. p16 deficiency in the setting of FoxO deficiency did not result in reduction of ROS levels in the HSC compartment. Taken together, these findings indicate that p16 is a critical downstream mediator of FoxO in the maintenance of the HSC compartment, and that it can dissociate the detrimental effects of ROS on HSC self-renewal in a setting of FoxO deficiency.