RPL5 deficiency-induced ribosomal stress targets a select subset of proteins and inhibits the PI3K-Akt-mTOR signaling pathway to eradicate leukemia stem cells

Ribosomal protein L5 (RPL5) is considered a haplo-insufficient tumor suppressor by upregulating p53 expression or promoting the inactivation of c-Myc in solid tumors upon tumor initiation. However, its detailed effect and mechanism in tumor maintenance were more complicated. Particularly, the specific role of RPL5 in acute myeloid leukemia (AML) remains unclear. In this study, we found that RPL5 expression was increased in primary AML blasts compared with normal controls, regardless of TP53 mutation. RPL5 knockdown reduced the survival and colony-forming ability of AML cells in vitro, as well as inhibited the engraftment of leukemia stem cells (LSCs) in vivo. It indicated that RPL5 was required for the survival of AML cells, especially for maintaining the stemness of LSCs. The data analysis of RNA-seq and proteomics revealed that RPL5 deficiency eradicated LSC by inducing a cellular stress response, i.e., ribosomal stress, rather than a specific function relating to RPL5. The inhibited PI3K-Akt-mTOR signaling pathway played a central role in the ribosomal stress induced by RPL5 deficiency. To investigate the selectivity of RPL5 depletion in eradicating LSCs, we found that RPL5 expression was highest in LSCs compared to AML cells and healthy controls. Moreover, ribosomal stress specifically affected transcripts with longer exon lengths and proteins with a lower isoleucine (Ile) to valine (Val) ratio. Ile and Val are glycogenic branched-chain amino acids (BCAAs) that regulate fundamental cell processes by affecting mTOR activation through BCAA metabolism. In conclusion, RPL5 depletion-induced ribosomal stress disrupted stemness maintenance by affecting BCAA metabolism in AML, specifically inhibiting the PI3K-Akt-mTOR signaling pathway, which resulted in LSC eradication.