Reversal of radiation-induced cisplatin resistance in murine fibrosarcoma cells by selective modulation of the cyclic GMP-dependent transduction pathway

Radioresistant Nasopharyngeal Carcinoma Cells Exhibited Decreased Cisplatin Sensitivity by Inducing SLC1A6 Expression

Cisplatin-based regimens are commonly used for the treatment of nasopharyngeal carcinoma (NPC) in patients who receive concurrent chemoradiotherapy. The sensitivity of NPC cells to cisplatin is closely associated with the efficacy of radiation therapy. In this study, we established two radioresistant NPC cell lines, HONE1-IR and CNE2-IR, and found that both cell lines showed reduced sensitivity to cisplatin. RNA-sequence analysis showed that SLC1A6 was upregulated in both HONE1-IR and CNE2-IR cell lines. Downregulation of SLC1A6 enhanced cisplatin sensitivity in these two radioresistant NPC cell lines. It was also found that the expression of SLC1A6 was induced during radiation treatment and correlated with poor prognosis of NPC patients. Notably, we observed that upregulation of SLC1A6 led to elevating level of glutamate and the expression of drug-resistant genes, resulted in reduced cisplatin sensitivity. Our findings provide a rationale for developing a novel therapeutic target for NPC patients with cisplatin resistance.

Soluble guanylate cyclase stimulators increase sensitivity to cisplatin in head and neck squamous cell carcinoma cells

Head and neck squamous cell carcinoma (HNSCC) is an aggressive and often fatal disease. Cisplatin is the most common chemotherapeutic drug in the treatment of HNSCC, but intrinsic and acquired resistance are frequent, and severe side effects occur at high doses. The second messenger cyclic GMP (cGMP) is produced by soluble guanylate cyclase (sGC). We previously reported that activation of the cGMP signaling cascade caused apoptosis in HNSCC cells, while others found that this pathway enhances cisplatin efficacy in some cell types. Here we found that sGC stimulators reduced HNSCC cell viability synergistically with cisplatin, and enhanced apoptosis by cisplatin. Moreover, the sGC stimulators effectively reduced viability in cells with acquired cisplatin resistance, and were synergistic with cisplatin. The sGC stimulator BAY 41-2272 reduced expression of the survival proteins EGFR and β-catenin, and increased pro-apoptotic Bax, suggesting a potential mechanism for the anti-tumorigenic effects of these drugs. The sGC stimulator Riociguat is FDA-approved to treat pulmonary hypertension, and others are being studied for therapeutic use in several diseases. These drugs could provide valuable addition or alternative to cisplatin in the treatment of HNSCC.

Targeting phosphodiesterase 3B enhances cisplatin sensitivity in human cancer cells

We previously reported that human squamous cell carcinoma (SCC) cell lines refractory to cis-diaminedichloro-platinum II (cisplatin [CDDP]) had significant upregulation of the phosphodiesterase 3B gene (PDE3B), suggesting that inhibiting PDE3B suppresses CDDP resistance. shRNA-mediated PDE3B depletion in CDDP-resistant cells derived from SCC cells and Hela cells and induced CDDP sensitivity and inhibited tumor growth with elevated cyclic GMP induction resulting in upregulation of the multidrug-resistant molecule, but this did not occur in the 5-fluorouracil-resistant hepatocellular carcinoma cell lines. Furthermore, the antitumor growth effect of the combination of a PDE3B inhibitor (cilostazol) and CDDP in vivo was also greater than with either cilostazol or CDDP alone, with a significant increase in the number of apoptotic and cell growth-suppressive cancer cells in CDDP-resistance cell lines. Our results provided novel information on which to base further mechanistic studies of CDDP sensitization by inhibiting PDE3B in human cancer cells and for developing strategies to improve outcomes with concurrent chemotherapy.

Role of cAMP-dependent protein kinase in the regulation of DNA repair

Enhanced DNA repair is an important factor in drug resistance in cancer. Using cell-free extracts derived from the fission yeast, Schizosaccharomyces pombe, we demonstrate in an in vitro system DNA repair system that increased cAMP levels, which activates cAMP-dependent protein kinase (PKA), inhibits repair of ultraviolet (UV)-damaged DNA. Supplementing the cell-free system with the catalytic kinase subunit of PKA also inhibits DNA repair. In contrast, addition of the PKA inhibitor H-89 enhances repair activity. These results show that PKA regulates DNA repair synthesis, thus implicating the cAMP signaling pathway in DNA damage response and repair of UV-damaged DNA lesions.

In vitro establishment of cis-diammine-dichloroplatinum(II) resistant lung cancer cell line and modulation of apoptotic gene expression as a mechanism of resistant phenotype

After exposure of H460 cells to an increasing concentrations of cis-diammine-dichloroplatinum(II) (cisplatin, CDDP) for 6 months, cisplatin resistant cells were isolated (H460/CIS). The biologic behaviors of H460 and H460/CIS cells were tested using animal experiments. Only the resistant cells developed lung metastases despite cisplatin treatment. The characteristics of H460/CIS cells are as follows, MTT analyses revealed that H460/CIS cells were markedly resistant to cisplatin compared with their parental cells. Also, H460/CIS cells exhibited cross-resistance to DNA damaging agents such as doxorubicin (DXR) and etoposide. Cisplatin treatment dramatically increased p53 expression in parental cells but not in H460/CIS cells which expressed basal levels of p53. Without cisplatin treatment, Bcl-2 and Bax were expressed in H460/CIS cells, but not in parental cell. Our data suggested that p53, Bax and Bcl-2 were up-regulated in H460/CIS cells. These changes could explain some of the mechanisms of cisplatin resistance. Thus, H460/CIS could be useful to investigate the mechanisms of drug resistance to cisplatin including apoptotic gene expressions conferring drug resistance, thereby making progress in the treatment of cisplatin-resistant tumor cells.

Reduced expression of the ICE-related protease CPP32 is associated with radiation-induced cisplatin resistance in HeLa cells

Low-dose fractionated gamma-irradiation (three cycles of 5 x 2 Gy) induced cisplatin resistance in HeLa cells. The drug resistance was modest (Rf of about 2) and stable, similar to that found previously in murine cells after irradiation. In the drug-resistant HeLa-C3 cells, flow cytometric analysis revealed a decreased number of apoptotic cells compared with the parental cells. Drug resistance was associated with considerably enhanced expression of the p53 suppressor protein in HeLa-C3 cells after cisplatin exposure but seemed not to be regulated by the bcl-2-dependent pathway. Cisplatin resistance correlated with reduced expression of ICE-related proteases (interleukin-1beta-converting enzyme). Basal levels of the 45-kDa precursor ICE protein were reduced in HeLa-C3 cells, while those of the mature 60-kDa heterotetramer were similar. The CPP32 protease, a member of the ICE family with structural homology but different substrate specificity, was expressed at a lowered level. After drug exposure, there was a slight increase of CPP32 in HeLa-C3 cells, equivalent to about 45% of the level attained in the parental cells. This is in contrast to the CPP32 levels measured after irradiation, which were similar in sensitive and in resistant cells. As the radiosensitivity is unchanged in both cell lines, these results suggest that cisplatin resistance in HeLa-C3 cells is associated with alterations of a CPP32-linked apoptotic pathway, which is affected by the damage caused by cisplatin but not by irradiation. Whether these changes are dependent on the observed p53 modifications is now being studied in resistant clones.