mTOR inactivation governs adaptive survival to ribosome biogenesis deficiency.

Ribosome biogenesis is a resource-consuming process that facilitates rapid growth and feeds uncontrolled, cancerous traits. Constraining ribosome biogenesis and protein translation has become a tenable therapeutic strategy for cancer. Yet, we do not know how cells that rely on high metabolic activity adapt and sustain their growth when deprived of their translational capacity. Conversely, stem cells and treatment-resistant cells persist under low metabolic states challenging their eradication. These are critical questions in cancer therapies. To delineate survival mechanisms that allow cancer cells to adapt to ribosome biogenesis defects, we conducted functional genomics screens during inhibition of RNA polymerase I. We identified that inactivation of mTOR enabled cell survival despite severe translational suppression. This was paradoxical as activation of mTOR is considered oncogenic by boosting ribosome biogenesis and cellular translational programs. We show that mTORC1 inhibition does neither restore rRNA synthesis nor ribosome biogenesis, but redistributes limited ribosomes from highly translated 5'TOP mRNAs to survival-essential transcripts. This mTOR inactivation-mediated prioritization of translational resources represents a minimal requirement for cell survival when translational capacity is compromised, which we term "translational fitness." Our findings redefine the role of mTOR in cell survival and highlight the need for strategic targeting of translation regulation in cancer therapy.