Low-density lipoprotein receptor expression is involved in the beneficial effect of photodynamic therapy using talaporfin sodium on gastric cancer cells

Photodynamic therapy (PDT) is a therapeutic method used to destroy tumor tissue via reactive oxygen. Notably, reactive oxygen is induced by a combination of photosensitizers, including talaporfin sodium (TS) and laser light. Gastric cancer cell lines, MKN45 and MKN74, were used to evaluate the effect of TS-PDT in vitro. The antitumor effect of TS-PDT, which was evaluated via cellular viability assay, on MKN74 was weaker than that on MKN45 cells, suggesting that MKN74 cell could be resistant to TS-PDT. However, using a higher TS concentration or setting a longer treatment time (24 h) resulted in effective TS-PDT treatment on MKN74 cells. In addition, when irradiation power of LED was raised up to 5.06 J/cm(2), TS-PDT was able to induce an antitumor effect on MKN74 cells. This suggested that the difference in TS-PDT efficacy between MKN45 and MKN74 cells is based on the difference in cellular uptake of TS. As expected, uptake of TS by MKN74 cells was lower than that by MKN45 cells. The expression levels of low-density lipoprotein (LDL) receptor in MKN74 cells were lower than those in MKN45 cells. With GW3965 treatment, an agonist/activator of Liver X Receptor, LDL receptor expression was reduced, weakening the TS-PDT effect. Furthermore, as a hydroxymethylglutaryl-Coenzyme A reductase inhibitor, treatment using simvastatin increased LDL receptor expression, leading to enhancement of the TS-PDT effect on MKN74 cells. In conclusion, the difference in LDL receptor expression between the two gastric cell lines could influence TS-PDT efficacy; simvastatin may enhance the antitumor effect of TS-PDT through upregulating the LDL receptor even on PDT-resistant gastric cancer cells.