Mechanism of selective induction of apoptosis of HCT116 tumor cells in circulating blood by riboflavin photochemistry

Abstract

The removal or inactivation of circulating tumor cells (CTCs) can prevent distant metastasis by hematogenous route, but there is still a lack of mature and effective technical means. In the previous research, our team has initially established a method of riboflavin photosensitized treatment (RPT) for continuous treatment of peripheral blood in vitro for the inactivation of CTCs. The core of this technology is that it can selectively induce apoptosis of CTCs (HCT116 cells) without damaging immunocyte (mainly Peripheral Blood Mononuclear Cells，PBMCs) under specific parameters. To clarify the specific mechanism, firstly, the enrichment of riboflavin in HCT116 cells and PBMCs was observed under fluorescence microscope. Secondly, the apoptotic signaling pathways in HCT116 cells and PBMCs in response to RPT treatment were analyzed by transcriptomics. Finally, the mitochondrial damage in HCT116 cells and PBMCs before and after RPT treatment was observed under electron microscope. The results showed that under the same treatment conditions, HCT116 cells were significantly enriched in riboflavin compared with PBMCs. Besides, RPT treatment reduced the expression of long non﹣coding RNA (lncRNA) NEAT1, an effector gene of HCT116 cells, which further down-regulated the expression of target gene PAX2 and promoted the expression of Bax, leading to mitochondrial outer membrane permeabilization (MOMP), and consequently increased the release of pro-apoptotic factors such as cytochrome c(Cyt C), high-temperature requirement protein A2(HTRA2), apoptosis-inducing factor (AIF), endonuclease G(ENDOG), finally leading to apoptosis of HCT116 cells. In contrast, lncRNA NEAT1 remained unchanged in PBMCs before and after RPT treatment, and was unable to stimulate the PBMCs apoptotic signaling pathway. The results of the study indicated that under the specific treatment conditions, RPT technology could selectively induce apoptosis of HCT116 cells by activating the mitochondrial apoptosis pathway, which would further provide a theoretical and technical support for the effective inactivation of CTCs by RPT technology, thereby reducing the risk of recurrence of malignant tumors and improving the cure rate of malignant tumors.

Keywords: Apoptosis; Circulating tumor cells; Mitochondrial outer membrane permeabilization; Nuclear enriched abundant transcript 1(NEAT1); Paired Box 2 (PAX2); Riboflavin photosensitized treatment; Selectively induce.