A proliferating cell expends most of its energy in the biogenesis of nascent ribosomes. The increased number of ribosomes provides the translational capacity required for the cell to synthesize the full complement of proteins necessary to enter the mitotic cell cycle and divide. The importance of ribosome biogenesis to cell proliferation is underscored by the fact that a lesion in ribosome biogenesis alters the ratio of ribosomes to mRNA, which eventually modifies the pattern of translation and the genetic program, resulting in aberrant growth. Given these observations, we initially predicted that cells would have evolved a checkpoint to monitor nascent ribosome biogenesis. The importance of this concept to human pathology has been demonstrated in two haemopoietic disorders, 5q- syndrome and Diamond Blackfan Anemia (DBA), which are characterized by hypomorphic mutations or deletions in ribosomal protein (rp) genes. Patients with these diseases first develop myelodysplasias, but have the propensity to progress to a wide range of cancers later in life, particularly acute myeloid leukemia (AML). The concept that rp genes act as haploinsufficient tumor suppressors initially came from studies in model systems, including Drosophila and zebrafish. Recent studies from this laboratory have provided a model to explain both the myelodysplasia and the potential to develop cancer. We showed that upon disruption of either 40S or 60S ribosome biogenesis there is an upregulation of p53 mediated by the binding of the 60S rps, rpL5 and rpL11, to human double minute 2 (MDM2 or Hdm2), leads to G1 cell-cycle arrest. However, in circumstances where 40S, but not 60S, ribosome biogenesis is impaired, this effect requires the translational upregulation of rpL5 and rpL11 to generate sufficient protein to bind to Hdm2 in the face of continued 60S ribosome biogenesis and a decrease in global protein synthesis. The translational upregulation of rpL5 and rpL11 is dependent on the 5'TOP sequence, which acts as translational repressor, and resides at the transcriptional start site of rp mRNAs. Loss of this checkpoint is most likely responsible for the propensity of 5q- syndrome and DBA patients to develop AML. The importance of this checkpoint in p53 positive tumors is underscored by two findings: (i) the extent of tumor progression directly parallels nucleolar size and RNA polymerase 1 (Pol 1) activity and (ii) actinomycin D, which at low concentrations, selectively inhibits RNA Pol 1, arrests p53/MDM2 wild type (WT) tumor cells in G1. Together, these observations have led us to hypothesize that the rpL5/rpL11 complex is upregulated in response to impaired ribosome biogenesis to induce p53 cell-cycle arrest and to prevent tumor progression. These studies are relevant to tumor biology as they are designed to elucidate the mechanism by which impaired ribosome biogenesis mediates cell cycle progression, the identification of components in addition to rp5/rpL11 which mediate this response, and the role of general translation versus the p53 checkpoint in controlling tumor progression beyond 5q- syndrome and DBA, which will aid in the development of novel cancer treatments.

Public Health Relevance

Normal cell division requires the production of new ribosomes, the cellular structures responsible for protein synthesis, so that each new daughter cell will have adequate capacity to produce protein to grow. Cells respond to defects in ribosome production by shutting down cell division and, in this way, prevent abnormal cell growth and tumor formation. The project proposed here will explore a cellular anti-tumor mechanism, which is lost in cancer progression, and exploit information from these studies that will help in designing new cancer treatments.

Agency
National Institute of Health (NIH)
Institute
National Cancer Institute (NCI)
Type
Research Project (R01)
Project #
1R01CA158768-01A1
Application #
8236578
Study Section
Cancer Molecular Pathobiology Study Section (CAMP)
Program Officer
Hildesheim, Jeffrey
Project Start
2012-03-01
Project End
2017-02-28
Budget Start
2012-03-01
Budget End
2013-02-28
Support Year
1
Fiscal Year
2012
Total Cost
$325,775
Indirect Cost
$118,275
Name
University of Cincinnati
Department
Anatomy/Cell Biology
Type
Schools of Medicine
DUNS #
041064767
City
Cincinnati
State
OH
Country
United States
Zip Code
45221