Involvement of sodium nitrite in the regulation of cancer stem cell properties in human hepatoma cells / 药学学报

ABSTRACT

Increasing evidence suggests that hepatocellular carcinomas (HCCs) are sustained by a distinct subpopulation of self-renewing cells known as cancer stem cells (CSC). However, our understanding of their regulation is limited. Rapid reversible changes of CSC-like cells within tumors may result from the effect of biological mediators found in the tumor microenvironment. This paper aims to explore how nitrite, a key cellular modulator whose level is elevated in many tumors, affects CSC-like phenotypes of human hepatoma cells SMMC-7721 cells. The SMMC-7721 cell line was cultured under serum-free conditions to produce floating spheres. The distribution of cell cycle was analyzed by flow cytometry, the capability of cells self-renew was detected by colony-forming capabilities and spheroid-formation assay, the expression of stemness protein such as CD133, CD90 and EpCAM were determined by flow cytometry and Western blot, cell invasion was analyzed by transwell assay, and viability of SMMC-7721 parental cells and spheroids cancer cells was determined by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) assay. Xenograft tumor models were established by subcutaneously injecting SMMC-7721 spheroids cancer cells, the transplanted tumor tissue ROS levels was detected by reactive oxygen species (ROS) test kits, the expression of HIF-1α was observed by immunofluorescence. Our results showed that the SMMC-7721 spheroid cells were enriched with CSCs properties, indicated by the ability to self-renew, increased expression of CSCs markers, and increased resistance to chemotherapeutic drugs. Additionally, SMMC-7721 parental cells and spheroids cancer cells were treated with 150 μmol·L-1 sodium nitrite for 6 days, compared with control cells, an increased accumulation of G0/G1 phase cells was observable in treatment cells. Indeed, our data demonstrated that in parent cells and spheres cells that were treated with sodium nitrite for different time, the cells' ability to chemoresistance and invasion, clone-forming efficiencies and the spheres forming ability were significantly higher than that of control cells. Exposure of sodium nitrite regulated CSC-like phenotype, indicated by increased expression of known CSC markers, CD133, CD90 and EpCAM in the exposed parental cells, as well as in dormant spheroids cancer cells. Compared with the parent cells, the above effects of nitrite on the spheres cells were significantly enhanced. In vivo data also presented a more significant promotion of tumor xenograft growth from the nitrite treatment than from either of the control. Mechanistic analysis indicated that nitrite induced the upregulation of HIF-1α as well as the downregulation of ROS in the tumor microenvironment. These results suggest that nitrite increases the invasiveness of SMMC-7721 cells through up-regulation of tumor stemness.