CDK4/6 initiates Rb inactivation and CDK2 activity coordinates cell-cycle commitment and G1/S transition

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Cell-cycle entry is externally controlled until irreversible commitment ensures DNA replication.
• Cyclin-dependent kinases (CDKs) and the retinoblastoma protein (Rb) tightly regulate this process.

Purpose of the Study:

• To investigate the roles of CDK4/6 and CDK2 activities in cell-cycle commitment and G1/S transition.
• To elucidate the mechanism of retinoblastoma protein (Rb) inactivation and its link to cell-cycle progression.

Main Methods:

• Utilized live-cell sensors to monitor CDK4/6 and CDK2 activities.
• Performed experiments involving acute CDK4/6 inhibition and cyclin E1 overexpression to manipulate CDK activity.

Main Results:

• CDK4/6 activation initiates Rb inactivation, leading to E2F activation and a gradual increase in CDK2 activity.
• High CDK2 activity is necessary for CDK2-Rb positive feedback and CDK4/6-independent cell-cycle progression.
• Cells commit to the cell cycle before DNA replication, with high CDK2 activity creating a bistable switch for Rb phosphorylation.

Conclusions:

• CDK4/6 inactivates Rb, initiating E2F and CDK2 activation.
• High CDK2 activity is essential for cell-cycle commitment prior to the G1/S transition and DNA replication.
• The findings reveal a critical regulatory mechanism for cell-cycle initiation and commitment.