Therapy-naïve and radioresistant 3D pancreatic cancer cell cultures are effectively radiosensitized by β1 integrin targeting

Purpose: Pancreatic ductal adenocarcinoma (PDAC) is one of the cancers with unmet need. The role of highly conformal radiotherapy is still under debate for PDAC. Owing to its desmoplastic nature, integrin-mediated interactions between PDAC cells and extracellular matrix (ECM) profoundly contribute to PDAC resistance. In this study, we investigated the radiochemosensitizing potential of β1 integrin targeting in therapy-naïve and radioresistant PDAC cell cultures grown in three-dimensional (3D) extracellular matrix (ECM).
Materials and Methods: In a panel of 3D, ECM based PDAC cell cultures, β1 integrin was inhibited by antibodies or siRNA-mediated knockdown. Together with X-ray irradiation and specific chemotherapies, we determined 3D colony formation capacity in therapy-naïve and radioresistant PDAC cultures. Kinome profiling, Western blotting and immunofluorescence stainings were employed to characterize these cell lines. Various siRNA screens were conducted to identify novel therapeutic targets.
Results: A significant radiosensitizing potential of β1 integrin inhibition was found both in therapy-naïve and radioresistant PDAC cell cultures. Kinome profiling upon β1 integrin targeting identified a generally declined tyrosine and serine/threonine kinase activity, which presented less prominent in radioresistant than in therapy-naïve PDAC cells. siRNA screens employing the top 34 deregulated kinases in combination with β1 integrin inhibition revealed less efficacy and less radiosensitization in radioresistant relative to therapy-naïve PDAC cell cultures. Triple inhibition of β1 integrin, protein kinase D1 (PDK1) and rearranged during transfection (RET) turned out to be most effective in reducing 3D colony formation of radioresistant PDAC cells.
Conclusion: Our study clearly shows that β1 integrins are robust targets for overcoming radioresistance in PDAC. This seems to apply equally to therapy-sensitive and radioresistant cells. Concerning tumor heterogeneity, this dual therapy-sensitizing potential might be exploitable for a significant improvement of patient survival.