Abstract SY21-01: Networks of proto-oncogenes and tumor suppressors regulate stem cell self-renewal
Authors: Morrison Sean J. 
Published: June 2012

Abstract
Advances in several areas over the past year have illuminated the critical roles played by networks of proto-oncogenes and tumor suppressors in the regulation of stem cell maintenance. To better understand the mechanisms that regulate stem cell identity and function we sought to identify genes that are preferentially expressed by stem cells and critical for their function in multiple tissues. Prdm16 is a transcription factor that regulates leukemogenesis and brown fate development, but which was not known to be required for stem cell function. We discovered that Prdm16 is preferentially expressed by stem cells throughout the nervous and hematopoietic systems and required for their maintenance [1]. Prdm16 deficiency led to changes in reactive oxygen species (ROS) levels, increased cell death, altered cell cycle distribution, and stem cell depletion in the hematopoietic and nervous systems. In neural stem/progenitor cells, Prdm16 bound the Hgf promoter and in the absence of Prdm16 Hgf expression declined. Addition of recombinant HGF to culture partially rescued the increase in ROS levels and the depletion of Prdm16 deficient neural stem cells. Administration of the antioxidant, N-acetyl-cysteine, to Prdm16 deficient mice partially rescued defects in neural stem/progenitor cell function and neural development. The physiological function of the Prdm16 proto-oncogene is to promote stem cell maintenance in multiple tissues, partly by modulating oxidative stress.
The Pten tumor suppressor is also critically required for hematopoietic stem cell (HSC) maintenance, by virtue of its role in the negative regulation of PI 3-kinase pathway signaling. Pten deficiency depletes HSCs but expands leukemia-initiating cells and the mTOR inhibitor, rapamycin, blocks these effects [2]. Understanding the opposite effects of mTOR activation on HSCs versus leukemia-initiating cells could improve antileukemia therapies. We found that the depletion of Pten-deficient HSCs was not caused by oxidative stress and could not be blocked by N-acetyl-cysteine. Instead, Pten deletion induced, and rapamycin attenuated, the expression of p16Ink4a and p53 in HSCs, and p19Arf and p53 in other hematopoietic cells [3]. p53 suppressed leukemogenesis and promoted HSC depletion after Pten deletion. p16Ink4a also promoted HSC depletion but had a limited role suppressing leukemogenesis. p19Arf strongly suppressed leukemogenesis but did not deplete HSCs. Secondary mutations attenuated this tumor suppressor response in some leukemias that arose after Pten deletion. mTOR activation therefore depletes HSCs by a tumor suppressor response that is attenuated by secondary mutations in leukemogenic clones.
Little is known about metabolic regulation in stem cells and how this modulates tissue regeneration or tumor suppression. We studied the Lkb1 tumor suppressor, and its substrate AMPK, kinases that coordinate metabolism with cell growth [4]. Lkb1 deletion caused increased HSC division, rapid HSC depletion, and pancytopenia. HSCs depended more acutely on Lkb1 for cell cycle regulation and survival than many other haematopoietic cells. HSC depletion did not depend on mTOR activation or oxidative stress. Lkb1-deficient HSCs, but not myeloid progenitors, had reduced mitochondrial membrane potential and ATP. AMPK-deficient HSCs showed similar changes in mitochondrial function but remained able to reconstitute irradiated mice. Lkb1-deficient HSCs, but not AMPK-deficient HSCs, exhibited defects in centrosomes and mitotic spindles, and became aneuploid. Lkb1 is therefore required for HSC maintenance through AMPK-dependent and AMPK-independent mechanisms, revealing differences in metabolic and cell cycle regulation between HSCs and some other hematopoietic progenitors.
References:
1. Chuikov, S., et al., Prdm16 promotes stem cell maintenance in multiple tissues, partly by regulating oxidative stress. Nature Cell Biology, 2010. 12:999-1006.
2. Yilmaz, O.H., et al., Pten dependence distinguishes haematopoietic stem cells from leukaemia-initiating cells. Nature, 2006. 441:475-82.
3. Lee, J.Y., et al., mTOR activation induces tumor suppressors that inhibit leukemogenesis and deplete hematopoietic stem cells after Pten deletion. Cell Stem Cell, 2010. 7:593-605.
4. Nakada, D., T. Saunders, and S.J. Morrison, Lkb1 regulates cell cycle and energy metabolism in hematopoietic stem cells. Nature, 2010. In Press.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr SY21-01. doi:10.1158/1538-7445.AM2011-SY21-01