Caveolin–1–mediated regulation of cytoprotective mechanism upon ionizing radiation

Introduction:

The integral membrane protein caveolin–1 (cav1) plays multiple roles in cell physiology and pathology such as endocytosis, signal transduction and tumorigenesis. In cancer, cav1 has a paradoxical function depending on the type of the cancer cells, as an oncogene or as a tumor suppressor. The exact mechanisms by which cav1 controls cancer cell survival are unclear. Our previous studies show that cav1 is overexpressed in human pancreatic ductal adenocarcinoma, contribute to cellular radioresistance and cancer cell survival. In this study, we examined how cav1 co–regulates the repair of radiogenic DNA double strand breaks.

Methods:

As models, we used normal and radioresistant cancer cells (i.e. MiaPaCa 2 pancreatic cancer cells, PC3 human prostate cancer cells, MCF7 breast cancer cells, and MEFs). We tracked fluorescently tagged cav1 upon x–ray irradiation by performing super resolution live–cell 3D imaging in combination with cell biological, biochemical and biophysical methods. DNA repair was measured in stably 53BP1-GFP-transfected cells as well as staining of 53BP1 and γH2AX.

Results:

We found that both cav1 expression and its mobility are correlated with the dynamics of DNA repair upon ionizing radiation in both normal and radioresistant cancer cells. We further confirm that cav1 upregulation is correlated with resistance to x–ray irradiation. Intriguingly, cells irradiated in suspension and subsequently washed prior to co–culturing with unirradiated cells showed significantly less DNA double strand breaks relative to non–co–culture conditions. This cell–to–cell signaling phenomenon was correlated with and conducted through a transfer of cav1 and mitochondria from unirradiated to irradiated cells via intercellular membrane nanotubes.

Conclusion:

Cell stress responses can be modulated via multiple pathways. Our data provide insight into the cytoprotective, cell–to–cell and DNA repair–modulating functions of the integral membrane protein cav1. In addition to the “classical” bystander effect mediated by gap junction-related cell–cell contact and soluble factors, intercellular membrane nanotubes serve as a potent and novel survival nexus. Moreover, this mitochondria–cav1 complex and intercellular membrane nanotubes may serve as potential targets for molecular–targeted therapies to overcome radio- and chemoresistance of cancer cells.