Abstract:
Although cisplatin has been considered as a first-line chemotherapeutic drug for the treatment strategy of lung cancer, its curative efficacy is limited by the occurrence of drug resistance and adverse effects. The application of non-toxic natural products for sensitization of cancer cells to conventional drugs is a recent novel strategy for cancer management to tackle these problems. This study aims to investigate effective chemosensitizer produced by Streptomyces sp. on the growth inhibition of human H460 lung cancer cells. Herein, three of eight strains isolated from peat swamp forest, Yala Province, Thailand which showed good purity analyzed by 16S rRNA sequencing were screened for cytotoxic activity. Among these, crude extract ST1-45 with the highest anticancer activity (IC50 = 9.51 µg/mL) was selected for further investigations. The optimal non-toxic dose of ST1-45 (1.25 µg/mL) was used to examine the sensitization effect on cisplatin-induced apoptosis in H460 cells. The viability assay suggested that the pretreatment of 1.25 ug/ml of ST1-45 significantly reduced IC50 of cisplatin from 37.60 µM to 13.65 µM. Therefore, this study revealed that non-toxic dose of ST1-45 can be used to sensitize H460 cells to cisplatin treatment. The findings from the annexin V/PI costaining by flow cytometry revealed that the induction of cell death by cotreatment with ST1-45 and cisplatin was due to apoptosis. The results of RT-PC analysis and western bot analysis showed that the underlying apoptotic mechanism of sensitization effect did not depend on IRE1 α-mediated ER stress pathway since IRE1 α mediated XBP-1 splicing ratio which indicates mild ER stress condition did not change after treatment with ST1-45. Moreover, protein expression level of the ER stress marker pIRE1 α which is an upstream signaling molecule of XBP-1 splicing and JNK activation did not alter in both treatment groups compared to control. Surprisingly, upregulation of pJNK protein was found in the cells treated only with ST1-45 and a combination of ST1-45 with cisplatin at 10 µM while there was no difference in total JNK protein. So, other upstream signaling pathways for example AMPK pathway could be one of the contributors that induce JNK activation. Therefore, Further studies are required to clarify the exact molecular mechanism of chemosensitization effect of ST1-45 on cisplatin-induced apoptosis. Lastly, strain identification of promising crude extract ST1-45 by whole genome sequence analysis proved that the strain ST1-45 could represent the candidate of novel species. The novel findings in this study highlight further to investigate major constituents in ST1-45 and subsequently evaluate the therapeutic effects of pure compound in cancer management.
