# Determining the mechanisms linking cell growth to the cell cycle in the liver

> **NIH NIH R01** · STANFORD UNIVERSITY · 2021 · $352,782

## Abstract

PROJECT SUMMARY
Cell growth triggers human cell division at the G1/S transition before DNA is replicated. This process is important
because it determines the size of proliferating cells, which is important for their physiological functions. Larger
cells, including macrophages and hepatocytes, often have additional copies of their genome in proportion to their
increased cell size. Typically, such large cells maintain their DNA-to-cytoplasm ratio by triggering DNA synthesis,
but not division, at cell sizes in proportion to their ploidy. However, while cell size and ploidy are frequently
correlated, the function of maintaining the DNA-to-cytoplasm ratio is unclear. Moreover, we do not know the
molecular mechanisms controlling the DNA-to-cytoplasm ratio despite having identified many key cell cycle
regulatory proteins. Here, we propose to determine both the function of the DNA-to-cytoplasm ratio and the
regulatory mechanisms linking cell growth to DNA replication in vivo by examining mouse hepatocytes in
developmental and regenerative contexts. The scientific premise of this work is a recent breakthrough that my
laboratory made in understanding how cell growth triggers division. Contrary to expectations that growth would
increase Cyclin D-Cdk4,6 activity, we found instead that cell growth dilutes the cell cycle inhibitor Rb to trigger
division in cultured cells. Our discovery of the Rb dilution mechanism in vitro raises the question if this mechanism
operates in vivo. Here, we propose to definitively test the Rb dilution, and an alternative model in which small
cells activate p38 to inhibit cell division in mouse hepatocytes. We will measure changes in hepatocyte cell size
and how cell growth is coupled to cell cycle progression in a series of mouse lines in which Rb1 has been
conditionally deleted, knocked down or over-expressed. Preliminary data indicate that for hepatocytes of the
same ploidy, the DNA-to-cytoplasm ratio is inversely correlated with Rb1 gene dosage, consistent with the Rb-
dilution model. We will use these genetic models that change the DNA-to-cytoplasm ratio to test its function in
the liver. More specifically, we will use inducible knockdown and over-expression alleles to generate hepatocytes
that are larger and smaller than wild type and have aberrant DNA-to-cytoplasm ratios. We will then perform a
panel of liver function and regeneration tests. This is important because aberrant DNA-to-cytoplasm ratio is
associated with various pathological states, but its function in vivo is still unclear in any cell type. Taken together,
the proposed work is important because determining how cell growth triggers cell division is a fundamental
question in cell and developmental biology. Its understanding will also provide insight into cancers, where this
process is misregulated.

## Key facts

- **NIH application ID:** 10184964
- **Project number:** 1R01DK128578-01
- **Recipient organization:** STANFORD UNIVERSITY
- **Principal Investigator:** Jan M Skotheim
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $352,782
- **Award type:** 1
- **Project period:** 2021-04-01 → 2025-03-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10184964

## Citation

> US National Institutes of Health, RePORTER application 10184964, Determining the mechanisms linking cell growth to the cell cycle in the liver (1R01DK128578-01). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10184964. Licensed CC0.

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