Loss of MK2 Enhances Radiation-Mediated Apoptosis in Bladder Cancer.

Background: Bladder cancer patients unable to receive cystectomy or who choose to pursue organ-sparing approach are managed with definitive (chemo)radiotherapy. However, this standard of care has not evolved in decades and disease recurrence and survival outcomes remain poor. Identifying novel therapies to combine with radiotherapy (RT) is therefore paramount to improve overall patient outcomes and survival. One approach is to find cellular mechanisms that can be targeted to increase the radiosensitivity of bladder cancer. The stress-activated kinase directly downstream from p38 mitogen-activated protein kinase (MAPK), mitogen-activated protein kinase activated protein kinase 2 (MAPKAPK2 or MK2), has been shown to enhance cancer-mediated inflammation, mesenchymal gene expression, and in vivo tumor growth. Here we examined the impact that MK2 knockdown (KD) has on bladder cancer cell radiosensitivity.

Methods: We utilized short hairpin RNA (shRNA) KD of MK2 using lentiviral transfection in the bladder cancer cell lines, T24 and HTB9. We compared the growth of KD cells to wild type using colony formation assays, proliferation assays and cell counts to determine differences in cell growth. Apoptosis was examined by annexin-based flow cytometry and western blots. Flow cytometry was also used for cell cycle analysis.

Results: KD clones showed a greater than 90% inhibition of MK2 expression as determined by western blot. Clonogenic assays exhibited an increase in radiosensitivity among the MK2 KD bladder cancer cells. These data were supported with proliferation assays that displayed a greater reduction in cell number following RT in MK2 KD bladder cancer cells. Annexin V binding in bladder cancer cells suggested increased apoptosis in MK2 KD cells. This was confirmed by comparing the amount of cleaved caspase products for the caspases 3 and 8 to scrambled control (SCR), and the release of cytochrome C into the cytosol. Both cell types showed disruptions in the cell cycle but at different points in the cycle.

Conclusion: These results show that MK2 controls irradiation-induced apoptosis in bladder cancer cells.