Jan Skotheim, Postdoctoral Faculty Sponsor
The G1/S transition is promoted by Rb degradation via the E3 ligase UBR5.
bioRxiv : the preprint server for biology
Mammalian cells make the decision to divide at the G1/S transition in response to diverse signals impinging on the retinoblastoma protein Rb, a cell cycle inhibitor and tumor suppressor. Rb is inhibited by two parallel pathways. In the canonical pathway, cyclin D-Cdk4/6 kinase complexes phosphorylate and inactivate Rb. In the second, recently discovered pathway, Rb's concentration decreases during G1 through an unknown mechanism. Here, we found that regulated protein degradation via the E3 ubiquitin ligase UBR5 is responsible for Rb's concentration drop in G1. UBR5 knockout cells have increased Rb concentration in early G1, exhibited a lower G1/S transition rate, and are more sensitive to inhibition of Cdk4/6. This last observation suggests that UBR5 inhibition can strengthen the efficacy of Cdk4/6 inhibitor-based cancer therapies.
View details for DOI 10.1101/2023.10.03.560768
View details for PubMedID 37873473
View details for PubMedCentralID PMC10592979
Increasing cell size remodels the proteome and promotes senescence.
Cell size is tightly controlled in healthy tissues, but it is unclear how deviations in cell size affect cell physiology. To address this, we measured how the cell's proteome changes with increasing cell size. Size-dependent protein concentration changes are widespread and predicted by subcellular localization, size-dependent mRNA concentrations, and protein turnover. As proliferating cells grow larger, concentration changes typically associated with cellular senescence are increasingly pronounced, suggesting that large size may be a cause rather than just a consequence of cell senescence. Consistent with this hypothesis, larger cells are prone to replicative, DNA-damage-induced, and CDK4/6i-induced senescence. Size-dependent changes to the proteome, including those associated with senescence, are not observed when an increase in cell size is accompanied by an increase in ploidy. Together, our findings show how cell size could impact many aspects of cell physiology by remodeling the proteome and provide a rationale for cell size control and polyploidization.
View details for DOI 10.1016/j.molcel.2022.07.017
View details for PubMedID 35987199
The cell cycle inhibitor RB is diluted in G1 and contributes to controlling cell size in the mouse liver.
Frontiers in cell and developmental biology
2022; 10: 965595
Every type of cell in an animal maintains a specific size, which likely contributes to its ability to perform its physiological functions. While some cell size control mechanisms are beginning to be elucidated through studies of cultured cells, it is unclear if and how such mechanisms control cell size in an animal. For example, it was recently shown that RB, the retinoblastoma protein, was diluted by cell growth in G1 to promote size-dependence of the G1/S transition. However, it remains unclear to what extent the RB-dilution mechanism controls cell size in an animal. We therefore examined the contribution of RB-dilution to cell size control in the mouse liver. Consistent with the RB-dilution model, genetic perturbations decreasing RB protein concentrations through inducible shRNA expression or through liver-specific Rb1 knockout reduced hepatocyte size, while perturbations increasing RB protein concentrations in an Fah -/- mouse model increased hepatocyte size. Moreover, RB concentration reflects cell size in G1 as it is lower in larger G1 hepatocytes. In contrast, concentrations of the cell cycle activators Cyclin D1 and E2f1 were relatively constant. Lastly, loss of Rb1 weakened cell size control, i.e., reduced the inverse correlation between how much cells grew in G1 and how large they were at birth. Taken together, our results show that an RB-dilution mechanism contributes to cell size control in the mouse liver by linking cell growth to the G1/S transition.
View details for DOI 10.3389/fcell.2022.965595
View details for PubMedID 36092730
Delineation of proteome changes driven by cell size and growth rate.
Frontiers in cell and developmental biology
2022; 10: 980721
Increasing cell size drives changes to the proteome, which affects cell physiology. As cell size increases, some proteins become more concentrated while others are diluted. As a result, the state of the cell changes continuously with increasing size. In addition to these proteomic changes, large cells have a lower growth rate (protein synthesis rate per unit volume). That both the cell's proteome and growth rate change with cell size suggests they may be interdependent. To test this, we used quantitative mass spectrometry to measure how the proteome changes in response to the mTOR inhibitor rapamycin, which decreases the cellular growth rate and has only a minimal effect on cell size. We found that large cell size and mTOR inhibition, both of which lower the growth rate of a cell, remodel the proteome in similar ways. This suggests that many of the effects of cell size are mediated by the size-dependent slowdown of the cellular growth rate. For example, the previously reported size-dependent expression of some senescence markers could reflect a cell's declining growth rate rather than its size per se. In contrast, histones and other chromatin components are diluted in large cells independently of the growth rate, likely so that they remain in proportion with the genome. Finally, size-dependent changes to the cell's growth rate and proteome composition are still apparent in cells continually exposed to a saturating dose of rapamycin, which indicates that cell size can affect the proteome independently of mTORC1 signaling. Taken together, our results clarify the dependencies between cell size, growth, mTOR activity, and the proteome remodeling that ultimately controls many aspects of cell physiology.
View details for DOI 10.3389/fcell.2022.980721
View details for PubMedID 36133920
RB depletion is required for the continuous growth of tumors initiated by loss of RB.
2021; 17 (12): e1009941
The retinoblastoma (RB) tumor suppressor is functionally inactivated in a wide range of human tumors where this inactivation promotes tumorigenesis in part by allowing uncontrolled proliferation. RB has been extensively studied, but its mechanisms of action in normal and cancer cells remain only partly understood. Here, we describe a new mouse model to investigate the consequences of RB depletion and its re-activation in vivo. In these mice, induction of shRNA molecules targeting RB for knock-down results in the development of phenotypes similar to Rb knock-out mice, including the development of pituitary and thyroid tumors. Re-expression of RB leads to cell cycle arrest in cancer cells and repression of transcriptional programs driven by E2F activity. Thus, continuous RB loss is required for the maintenance of tumor phenotypes initiated by loss of RB, and this new mouse model will provide a new platform to investigate RB function in vivo.
View details for DOI 10.1371/journal.pgen.1009941
View details for PubMedID 34879057
Cell growth dilutes the cell cycle inhibitor Rb to trigger cell division.
Science (New York, N.Y.)
2020; 369 (6502): 466–71
Cell size is fundamental to cell physiology. For example, cell size determines the spatial scale of organelles and intracellular transport and thereby affects biosynthesis. Although some genes that affect mammalian cell size have been identified, the molecular mechanisms through which cell growth drives cell division have remained elusive. We show that cell growth during the G1 phase of the cell division cycle dilutes the cell cycle inhibitor Retinoblastoma protein (Rb) to trigger division in human cells. RB overexpression increased cell size and G1 duration, whereas RB deletion decreased cell size and removed the inverse correlation between cell size at birth and the duration of the G1 phase. Thus, Rb dilution through cell growth in G1 provides one of the long-sought molecular mechanisms that promotes cell size homeostasis.
View details for DOI 10.1126/science.aaz6213
View details for PubMedID 32703881
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