Proliferation, But Not Growth, Blocked by Conditional Deletion of 40 S Ribosomal Protein S6

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Abstract
Because ribosome biogenesis plays an essential role in cell proliferation, control mechanisms may have evolved to recognize lesions in this critical anabolic process. To test this possibility, we conditionally deleted the gene encoding 40 S ribosomal protein S6 in the liver of adult mice. Unexpectedly, livers from fasted animals deficient in S6 grew in response to nutrients even though biogenesis of 40 S ribosomes was abolished. However, liver cells failed to proliferate or induce cyclin E expression after partial hepatectomy, despite formation of active cyclin D–CDK4 complexes. These results imply that abrogation of 40 S ribosome biogenesis may induce a checkpoint control that prevents cell cycle progression.