Radiation Induces Pericyte-Myofibroblast Transition

Purpose: Radiation not only kills Cancer cells directly through DNA damage but also indirectly by inducing vascular changes. The effects of radiation on tumor vasculature are critical for therapeutic efficacy; however, its impact on pericytes remains largely unknown. In this study, we investigated the effect of radiation on pericytes and sought to elucidate the mechanism by which radiation affects pericytes.

Methods and materials: C3H10T1/2 (10T1/2) mouse embryonic mesenchymal pericyte precursor cells and human pericytes from the placenta (hPC-PL) cells were irradiated to study the effects of radiation on pericyte differentiation, maturation, and transition of pericytes into myofibroblasts by flow cytometry, quantitative reverse transcription-polymerase chain reaction (qRT-PCR), and immunofluorescence. Phospho-kinase array, Western blot, and Reactive Oxygen Species (ROS) detection assay were conducted to elucidate the signaling pathways activated by radiation in pericytes. To investigate the pericyte-myofibroblast transition in vivo, MC38 tumor cells were implanted in NG2DsRedBAC (NG2DsRed) transgenic mice whose pericytes express DsRed. Tumors harvested 3, 5, and 7 days after radiation were assessed for myofibroblast markers in DsRed⁺ cells.

Results: Radiation promoted pericyte differentiation and increased the expression of adhesion molecules in both 10T1/2 and hPC-PL cells. Permeability and leukocyte adhesion were altered by radiation via an effect on endothelial cells, not pericytes. Radiation increased the expression of myofibroblast markers and induced morphologic changes. Phospho-kinase array indicated radiation activates the Akt signaling pathway, and inhibitors of Akt and ROS suppressed the expression of myofibroblast markers increased by radiation. MC38 tumors implanted in NG2DsRed transgenic mice harvested 3 days after radiation exhibited increased expression of myofibroblast markers in DsRed⁺ cells, indicating radiation-induced pericyte-myofibroblast transition in vivo. Administration of an Akt Inhibitor in combination with radiation in tumor-bearing NG2DsRed mice reduced vimentin expression in tumors.

Conclusions: Our data suggest radiation promotes pericyte maturation and transition into myofibroblasts via Akt signaling.