Reciprocal regulation of autophagy and dNTP pools in human cancer cells

Abstract

Ribonucleotide reductase (RNR) plays a critical role in catalyzing the biosynthesis and maintaining the intracellular concentration of 4 deoxyribonucleoside triphosphates (dNTPs). Unbalanced or deficient dNTP pools cause serious genotoxic consequences. Autophagy is the process by which cytoplasmic constituents are degraded in lysosomes to maintain cellular homeostasis and bioenergetics. However, the role of autophagy in regulating dNTP pools is not well understood. Herein, we reported that starvation- or rapamycin-induced autophagy was accompanied by a decrease in RNR activity and dNTP pools in human cancer cells. Furthermore, downregulation of the small subunit of RNR (RRM2) by siRNA or treatment with the RNR inhibitor hydroxyurea substantially induced autophagy. Conversely, cancer cells with abundant endogenous intracellular dNTPs or treated with dNTP precursors were less responsive to autophagy induction by rapamycin, suggesting that autophagy and dNTP pool levels are regulated through a negative feedback loop. Lastly, treatment with si-RRM2 caused an increase in MAP1LC3B, ATG5, BECN1, and ATG12 transcript abundance in xenografted Tu212 tumors in vivo. Together, our results revealed a previously unrecognized reciprocal regulation between dNTP pools and autophagy in cancer cells.

Keywords: :

• autophagy
• dNTP pools
• rapamycin
• ribonucleotide reductase
• RRM2

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

We thank Sofia Loera and her colleagues from her core facility for assisting with the immunohistochemical analysis, Dr Brian Armstrong, Mariko Lee, and Tina Patel for helping with the digital image capturing in the Microscope Core Lab, Dr Zhuo Li for helping with the digital image capturing in the EM Core Lab, and Dr Frank Hong for his critical review of the manuscript. This work was supported by the National Natural Science Foundation of China grant No. 31000775 (WC), Scientific Research Foundation for Returned Overseas Chinese Scholars grant (Human Resources and Social Security Department, 2012) (WC), National Key Technology R&D Program of China grant No. 2012BAD33B08 (WC), Research Fund for the Doctoral Program of Higher Education of China grant No. 20103326120006 (WC), City of Hope’s Women’s Cancers Program Award (Y-RC) and NIH grant 5R01DE10742, R01DE14183 (DKA) and 5R01CA127541 (LZ, KZ, BZ, M-LK, SH, and YY).