# Molecular basis of tumor suppression by Cdk4/6 inhibition

> **NIH NIH R01** · UNIVERSITY OF CALIFORNIA SANTA CRUZ · 2020 · $361,407

## Abstract

PROJECT SUMMARY
The retinoblastoma protein (Rb) pathway is a critical regulator of cell proliferation and a promising target for
cancer therapeutics. Rb normally inhibits the transcription program for cell division driven by E2F transcription
factors and thus promotes cell cycle arrest in G0/G1. Rb is commonly inactivated by Cyclin-dependent kinase
(Cdk) phosphorylation in cancer cells, including by Cdk4/6-CycD complexes.
Key unanswered questions in this central cellular pathway include how Cdk4/6 activity is regulated, how
specific Rb phosphorylation events mediate E2F activation, why is Rb a more potent tumor suppressive than
its close paralogs p107 and p130, and what are the mechanisms of resistance to chemical Cdk4/6 inhibitors.
These inhibitors have shown promise in the clinic, but we need to better exploit how they act and what factors
influence their response. We will apply our unique combined expertise in biochemical and genetic approaches
to answer previously intractable questions about the Cdk4/6-Rb pathway and tumor suppression.
Our first goal is to uncover the mechanisms of Cdk4/6-CycD activation in cancer cells. We will use structural,
biochemical, and cellular assays to investigate the critical role of the p27 protein in modulating Cdk4/6 activity
and the cellular response to Cdk4/6 small molecule inhibitors. Our second goal is to reveal the key molecular
changes that occur upon inactivating phosphorylation and the key molecular features that confer tumor
suppressor potency to Rb. We will examine how specific Cdk phosphorylation events in Rb lead to its
inactivation in cells. We will also explore a small domain in Rb that we hypothesize confers unique tumor
suppressive ability compared to p107 and p130. Finally, our third goal is to identify new regulators of the
Cdk4/6-Rb pathway using unbiased screening approaches. These new regulators may dictate how we use Cdk
inhibitors as therapeutics and innovate new strategies for targeting cancer cell division.
These experiments will address fundamental issues in the field of cell-cycle regulation and will transform our
understanding of Rb tumor suppressor function, how it is regulated, and how it may be rescued to arrest
cancer growth.

## Key facts

- **NIH application ID:** 9841367
- **Project number:** 5R01CA228413-02
- **Recipient organization:** UNIVERSITY OF CALIFORNIA SANTA CRUZ
- **Principal Investigator:** Seth Michael Rubin
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $361,407
- **Award type:** 5
- **Project period:** 2019-01-01 → 2023-12-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9841367

## Citation

> US National Institutes of Health, RePORTER application 9841367, Molecular basis of tumor suppression by Cdk4/6 inhibition (5R01CA228413-02). Retrieved via AI Analytics 2026-05-22 from https://api.ai-analytics.org/grant/nih/9841367. Licensed CC0.

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