Cell Cycle Phase-Specific Drug Resistance as an Escape Mechanism of Melanoma Cells

Kimberley A. Beaumont, David S. Hill, Sheena M. Daignault, Goldie Y.L. Lui, Danae M. Sharp, Brian Gabrielli, Wolfgang Weninger, Nikolas K. Haass

The tumor microenvironment is characterized by cancer cell subpopulations with heterogeneous cell cycle profiles. For example, hypoxic tumor zones contain clusters of cancer cells that arrest in G1-phase. It is conceivable that neoplastic cells exhibit differential drug sensitivity based on their residence in specific cell cycle phases. Here, we have used two-dimensional and organotypic melanoma culture models in combination with fluorescent cell cycle indicators to investigate the effects of cell cycle phases on clinically used drugs. We demonstrate that G1-arrested melanoma cells, irrespective of the underlying cause mediating G1-arrest, are resistant to apoptosis induced by the proteasome inhibitor bortezomib or the alkylating agent temozolomide. In contrast, G1-arrested cells were more sensitive to MAPK pathway inhibitor-induced cell death. Of clinical relevance, pre-treatment of melanoma cells with a MAPK pathway inhibitor, which induced G1-arrest, resulted in resistance to temozolomide or bortezomib. On the other hand, pre-treatment with temozolomide, which induced G2-arrest, did not result in resistance to MAPK pathway inhibitors. In summary, we have established a model to study the effects of the cell cycle on drug sensitivity. Cell cycle phase-specific drug resistance is an escape mechanism of melanoma cells that has implications on the choice and timing of drug combination therapies.