MCAT Bio/Biochem Question of the Day

Cyclin-dependent kinase (CDK) activity controls progression through cell cycle checkpoints in mammary epithelial cells. Rodriguez et al. (2022) examined the effects of ErbB2 overexpression on cell cycle regulation in MCF-10A cells.

MCF-10A cells were transfected with either pcDNA3.1-ErbB2 or empty vector control using Lipofectamine 3000. After 48 hours, cells were synchronized in G0/G1 by serum starvation for 16 hours, then released into complete medium containing 10% FBS. Cell cycle distribution was analyzed by flow cytometry at 0, 6, 12, and 24 hours post-release. Cells were fixed in 70% ethanol, treated with RNase A (100 μg/mL), and stained with propidium iodide (50 μg/mL). Data were acquired on a BD FACSCalibur instrument, collecting 10,000 events per sample.

At 12 hours post-release, ErbB2-overexpressing cells showed 68.3% in S phase compared to 41.2% in control cells (p < 0.001). The percentage of cells in G2/M phase reached 24.7% in ErbB2-transfected cells versus 12.1% in controls at 24 hours (Figure 1). Western blot analysis was performed on whole cell lysates collected at the same time points. Primary antibodies against cyclin D1 (1:1000), cyclin E (1:2000), CDK4 (1:1500), CDK2 (1:1000), p53 (1:500), and phospho-Rb (Ser780, 1:1000) were incubated overnight at 4°C. β-actin served as loading control.

ErbB2 overexpression induced a 3.4-fold increase in cyclin D1 protein levels at 6 hours compared to vector control. Cyclin E expression peaked at 12 hours with a 2.8-fold elevation. Phosphorylated Rb levels increased progressively, reaching 4.2-fold over baseline by 24 hours. Total p53 protein decreased to 0.31-fold of control levels by 12 hours post-release (Figure 2). CDK4 and CDK2 levels remained relatively constant, though CDK2-cyclin E complex formation increased as measured by co-immunoprecipitation (data not shown).

To assess checkpoint integrity, cells were treated with nocodazole (100 ng/mL) for 16 hours to arrest cells in prometaphase. Following nocodazole washout, 73.4% of control cells arrested in G2/M, while only 42.6% of ErbB2-overexpressing cells maintained arrest (p < 0.01). BrdU incorporation assays confirmed that ErbB2-transfected cells bypassed the spindle checkpoint, with 31.2% incorporating BrdU within 4 hours of nocodazole removal versus 8.7% in controls.

These findings suggest that ErbB2 overexpression disrupts normal cell cycle checkpoint control through sustained cyclin expression and Rb hyperphosphorylation, while simultaneously suppressing p53-mediated growth arrest mechanisms.

Adapted from Eran R Andrechek, William J Muller, Tyrosine kinase signalling in breast cancer: Tyrosine kinase-mediated signal transduction in transgenic mouse models of human breast cancer.