Up-regulation of hTERT expression by low-dose cisplatin contributes to chemotherapy resistance in human hepatocellular cancer cells

Abstract

Abstract. The telomerase is specifically activated in most malignant tumors but is usually inactive in normal somatic cells. It has been reported that telomerase has an anti-apoptotic role and up-regulation of telomerase helps cancer cells to be resistant to chemotherapeutic agent-induced cell death. The effect of cisplatin on telomerase activity is complex, and the exact mechanism remains largely unknown. In this study, we found that cisplatin activated telomerase activity and human telomerase reverse transcriptase (hTERT) expression in SMMC7721 human hepatocellular carcinoma cell line. Lowdose cisplatin up-regulated hTERT and NF-κB p65 expression and increased telomerase and NF-κB activity. Inhibition of NF-κB attenuated the hTERT expression and telomerase activity exposed to cisplatin, suggesting that NF-κB is responsible for the cisplatin-induced activation of the hTERT. Furthermore, preincubation of low-dose cisplatin which induced high expression of hTERT help hepatocellular carcinoma SMMC7721 cells survive under the high concentration of anti-cancer drugs. Inhibition of hTERT increased sensitivity of SMMC7721 cells to chemotherapy. Taken together, these results suggested that up-regulation of hTERT expression by low-dose cisplatin is NF-κB-dependent and contributes to chemotherapy resistance in human hepatocellular cancer cells. IntroductionTelomerase is a cellular reverse transcriptase that catalyses the synthesis and extension of telomeric DNA (1,2). This enzyme is specifically activated in most malignant tumors but is usually inactive in normal somatic cells (3,4). The catalytic core of human telomerase is composed of an RNA subunit known as hTER (human telomerase RNA) and a protein subunit named as hTERT (human telomerase reverse transcriptase). Although the RNA subunit (hTR) of the human telomerase complex is constitutively expressed in both tumor and normal somatic tissues, expression of the catalytic subunit (hTERT) correlates with telomerase activity and is a key step of activation of telomerase (5-7). Substantial experiments demonstrate that the transcriptional regulation of hTERT expression represents the primary and rate-limiting step in the activation of telomerase activity in most cells (8)(9)(10)(11). Several studies have indicated that hTERT has an anti-apoptotic activity in addition to its role in telomere length maintenance (12,13).Chemotherapeutic drugs have complex effects on telomerase activity and these complex effects are cell typedependent (14-19). Cisplatin is a DNA cross-linking agent used in the treatment of various tumors including hepatocellular carcinoma (20,21). Human testicular cancer cells treated with lethal doses of cisplatin exhibit decreased telomerase activity (22). In leukemic cells, telomerase activity was also downregulated after treatment with cisplatin (23). Additionally, long-term exposure of colorectal carcinoma cells to cisplatin results in increased telomerase activity (18). However, the exact mechanism of the complex effect of cisplatin on telomerase activity...