Rare codon translation regulates growth factor-dependent cell proliferation

In animal cells, growth factor signaling promotes both cell cycle progression and cell growth, but the connection between these two processes is not well understood. Here, we investigated whether cell cycling and cell growth are coupled through protein translation. Using ribosome profiling and mRNA sequencing we examined changes in translational activities in human Retinal Pigment Epithelial (RPE-1) cells as they entered the cell cycle in response to serum growth factor stimulation. We found that, in addition to mRNAs encoding factors in ribosome biogenesis and translational initiation, mRNAs encoding many DNA replication factors were translationally upregulated by serum. We also noted increases of snoRNAs and tRNAs, which facilitate translation. By analyzing the distribution of 21nt mRNA ribosome footprints, produced by stalled ribosomes that lack amino-acyl-tRNAs, we found that growth factor withdrawal promoted ribosome stalling preferentially at specific rare codons. DNA replication factor genes that were translationally upregulated by serum and essential for cell cycle progression were enriched in many of these rare codons, and the cognate tRNAs that read these codons were induced by growth signaling. The serum-dependent induction of translation was more mTOR-dependent than MEK-dependent. Our results support a novel regulatory mechanism wherein growth factor signaling promotes cell proliferation by inducing tRNAs that decode rare codons, which in turn promote translational elongation of mRNAs encoding DNA replication factors to accelerate G1/S progression. Significance StatementWe present a mechanistic explanation for the longstanding question of how cell cycle progression is coupled to cell growth. Our findings emphasize the role of translation machinery and reveal a "growth checkpoint" that ensures that DNA replication gene translation occurs only when growth signaling has activated the protein translation machinery. Our work complements the textbook model of the Growth Factor-Cyclin D-Rb-E2F transcriptional mechanism for cell cycle entry and is a fundamental advance for the fields of cell proliferation and growth factor signaling.