Synergistic anticancer activity of valproate combined with nicotinamide enhances anti-proliferation response and apoptosis in MIAPaca2 cells

Histone deacetylase is strongly associated with epigenetic regulation and carcinogenesis, and its inhibitors can induce cell cycle arrest and apoptosis of the cancer cells. In this study we aimed to examine the antiproliferative effects a combination of the valproate with nicotinamide in MIAPaca2 cell line. We revealed that valproate acted in a synergistic/additive with nicotinamide to inhibit the proliferation and induction of apoptosis in MIAPaca2 cancer cell line. MIAPaca2 was treated with various concentrations of valproate. The MTT assay and colony formation in soft agar indicated that valproate at 0.5 mM, when used alone weakly, suppressed proliferation of cells (37 ± 3.02 %) whereas the combination treatment of valproate + nicotinamide significantly suppressed cell proliferation (58 ± 3.5 %). The effect of nicotinamide at 25 mM on cell proliferation and cell colonization induced 50 % apoptosis of MIAPaca2 cells. To identify the anti-proliferation and apoptotic effects of valproate and nicotinamide we performed flow cytometric and microscopic analyses. The results indicated significant apoptosis induction and nuclear morphological alterations greater than when valproate was used alone. Furthermore, western blot analyses was performed to study the role of acetyl-histone H3 levels, and quantitative RNA expression analyses were performed on expression of thrombospondin (TSP) and maspin genes in MIAPaca2. We found that the combination treatment of valproate + nicotinamide enhanced the expression of maspin and TSP genes and the biological response of the cell line was correlated with the increase of histone H3 acetylation after nicotinamide and valproate application. Together our findings indicate that valproate which act as inhibitor of cell proliferation and inducer of apoptosis in human cancer MIAPaca2 cells when used in combination with nicotinamide makes it a potentially good candidate for new anticancer drug development.     

Impaired DNA double-strand breaks repair by kinesin family member 4A inhibition renders human H1299 non-small-cell lung cancer cells sensitive to cisplatin

Patients with non-small-cell lung cancer (NSCLC) are routinely treated with the platinum-based chemotherapeutics such as cisplatin. The drug exerts anticancer effects via multiple mechanisms, including DNA double-strand breaks (DSBs). Enhanced DNA DSB repair capacity would be associated with innate or acquired drug resistance. However, despite strong evidence for the role of the chromokinesin kinesin family member 4A (KIF4A) in DSB repair, the relationship between the chromokinesin and cisplatin sensitivity of human NSCLC cells remains unknown. Furthermore, little is known regarding the effect of targeting KIF4A on the function of DSB repair-related proteins in these cells. In the current study, we demonstrated that cisplatin treatment stimulated the expression of KIF4A protein in human NSCLC cells. Depletion of KIF4A by small interfering RNA significantly enhanced cisplatin-induced cell cycle arrest in S and G2/M phases and cytotoxicity in human NSCLC cells. Furthermore, we found that KIF4A inhibition suppressed the ability of cisplatin to induce BRCA2 and Rad51 focus formation and limits the further increase in poly(ADP-ribose) polymerase 1 activity induced by cisplatin treatment in human NSCLC cells. These studies thus identify the chromokinesin KIF4A as a novel modulator of cisplatin sensitivity that is significantly enhanced by the chromokinesin in human NSCLC cells via multiple mechanisms.     

BCI induces apoptosis via generation of reactive oxygen species and activation of intrinsic mitochondrial pathway in H1299 lung cancer cells

The compound(E)-2-benzylidene-3-(cyclohexylamino)-2,3-dihydro-1 H-inden-1-one(BCI) is known as an inhibitor of dual specific phosphatase 1/6 and mitogen-activated protein kinase. However, its precise anti-lung cancer mechanism remains unknown. In this study, the effects of BCI on the viability of non-small cell lung cancer cell lines NCI-H1299, A549, and NCI-H460 were evaluated. We confirmed that BCI significantly inhibited the viability of p53(-) NCI-H1299 cells as compared to NCI-H460 and A549 cells, which express wild-type p53. Furthermore, BCI treatment increased the level of cellular reactive oxygen species and pre-treatment of cells with N-acetylcysteine markedly attenuated BCI-mediated apoptosis of NCI-H1299 cells. BCI induced cellular morphological changes, inhibited viability, and produced reactive oxygen species in NCI-H1299 cells in a dose-dependent manner. BCI induced processing of caspase-9, caspase-3, and poly ADP-ribose polymerase as well as the release of cytochrome c from the mitochondria into the cytosol. In addition, BCI downregulated Bcl-2 expression and enhanced Bax expression in a dose-dependent manner in NCI-H1299 cells. However, BCI failed to modulate the expression of the death receptor and extrinsic factor caspase-8 and Bid, a linker between the intrinsic and extrinsic apoptotic pathways in NCI-H1299 cells. Thus, BCI induces apoptosis via generation of reactive oxygen species and activation of the intrinsic pathway in NCI-H1299 cells.     

Dihydroartemisinin potentiates the anticancer effect of cisplatin via mTOR inhibition in cisplatin-resistant ovarian cancer cells: involvement of apoptosis and autophagy

Dihydroartemisinin (DHA) exhibits anticancer activity in tumor cells but its mechanism of action is unclear. Cisplatin (DDP) is currently the best known chemotherapeutic available for ovarian cancer. However, tumors return de novo with acquired resistance over time. Mammalian target of rapamycin (mTOR) is an important kinase that regulates cell apoptosis and autophagy, and its dysregulation has been observed in chemoresistant human cancers. Here, we show that compared with control ovarian cancer cells (SKOV3), mTOR phosphorylation was abnormally activated in cisplatin-resistant ovarian cancer cells (SKOV3/DDP) following cisplatin monotherapy. Treatment with cisplatin combined with DHA could enhance cisplatin-induced proliferation inhibition in SKOV3/DDP cells. This mechanism is at least partially due to DHA deactivation of mTOR kinase and promotion of apoptosis. Although autophagy was also induced by DHA, the reduced cell death was not found by suppressing autophagic flux by Bafilomycin A1 (BAF). Taken together, we conclude that inhibition of cisplatin-induced mTOR activation is one of the main mechanisms by which DHA dramatically promotes its anticancer effect in cisplatin-resistant ovarian cancer cells.     

Synergistic inhibitory effect of berberine and icotinib on non-small cell lung cancer cells via inducing autophagic cell death and apoptosis

Apoptosis - Resistance to epidermal growth factor receptor-tyrosin kinase inhibitors (TKIs, e.g. icotinib) remains a major clinical challenge. Non-small cell lung cancer patients with wild-type...     

Inhibition of tumor propellant glutathione peroxidase 4 induces ferroptosis in cancer cells and enhances anticancer effect of cisplatin

Glutathione peroxidase 4 (GPX4) has been confirmed to inhibit ferroptosis in cancer cells, however, whether GPX4 serves as an oncogene is not clear. In this study, the expression of GPX4 and its influence to survival of patients with cancer were analyzed via public databases. Furthermore, the epigenetic regulation of GPX4 and the relation between GPX4 and chemoresistance of different anticancer drugs was also detected. Most importantly, cytological assays were performed to investigate the function of GPX4 in cancer cells. The results showed that GPX4 was higher expressed in cancer tissues than normal and was negatively associated with prognosis of patients. Furthermore, at upstream of GPX4 there was low DNA methylation sites and enhanced level of H3K4me3 and H3K27ac, indicating that high level of GPX4 in cancer may resulted from epigenetic regulation. Moreover, GPX4 was positively related to chemoresistance of anticancer drugs L-685458, lapatinib, palbociclib, and topotecan. In addition, GPX4 may potentially be involved in translation of protein, mitochondrial respiratory chain complex I assembly, electron transport oxidative phosphorylation, nonalcoholic fatty liver disease, and metabolic pathways. Finally, we detected that GPX4 inhibited ferroptosis in cancer cells, the inhibition of GPX4 via RSL3 could enhance the anticancer effect of cisplatin in vitro and in vivo. In conclusion, GPX4 acts as an oncogene and inhibits ferroptosis in cancer cells, the anticancer effect of cisplatin can be enhanced by GPX4 inhibition.     

Ibuprofen enhances the anticancer activity of cisplatin in lung cancer cells by inhibiting the heat shock protein 70

Hsp70 is often overexpressed in cancer cells, and the selective cellular survival advantage that it confers may contribute to the process of tumour formation. Thus, the pharmacological manipulation of Hsp70 levels in cancer cells may be an effective means of preventing the progression of tumours. We found that the downregulation of Hsp70 by ibuprofen in vitro enhances the antitumoural activity of cisplatin in lung cancer. Ibuprofen prominently suppressed the expression of Hsp70 in A549 cells derived from lung adenocarcinoma and sensitized them to cisplatin in association with an increase in the mitochondrial apoptotic cascade, whereas ibuprofen alone did not induce cell death. The cisplatin-dependent events occurring up- and downstream of mitochondrial disruption were accelerated by treatment with ibuprofen. The increase in cisplatin-induced apoptosis caused by the depletion of Hsp70 by RNA interference is evidence that the increased apoptosis by ibuprofen is mediated by its effect on Hsp70. Our observations indicate that the suppression of Hsp70 by ibuprofen mediates the sensitivity to cisplatin by enhancing apoptosis at several stages of the mitochondrial cascade. Ibuprofen, therefore, is a potential therapeutic agent that might allow lowering the doses of cisplatin and limiting the many challenge associated with its toxicity and development of drug resistance.     

Synergistic effect of curcumin on epigallocatechin gallate-induced anticancer action in PC3 prostate cancer cells

Epigallocatechin gallate (EGCG) and curcumin are well known to naturally-occurring anticancer agents. The aim of this study was to verify the combined beneficial anticancer effects of curcumin and EGCG on PC3 prostate cancer cells, which are resistant to chemotherapy drugs and apoptosis inducers. EGCG showed weaker inhibitory effect on PC3 cell proliferation than two other prostate cancer cell lines, LNCaP and DU145. Co-treatment of curcumin improved antiproliferative effect of EGCG on PC3 cells. The protein expressions of p21 were significantly increased by the co-treatment of EGCG and curcumin, whereas it was not changed by the treatment with each individual compound. Moreover, treatments of EGCG and curcumin arrested both S and G2/M phases of PC3 cells. These results suggest that the enhanced inhibitory effect of EGCG on PC3 cell proliferation by curcumin was mediated by the synergic up-regulation of p21-induced growth arrest and followed cell growth arrest. [BMB Reports 2015; 48(8): 461-466]     

Cl-IB-MECA enhances TRAIL-induced apoptosis via the modulation of NF-κB signalling pathway in thyroid cancer cells

Apoptosis is an endogenous process that can be a useful anti-cancer tool. This study aimed to investigate the effect of Cl-IB-MECA, adenosine receptor A3 agonist, on TRAIL-induced apoptosis of thyroid carcinoma cells. Cl-IB-MECA enhanced TRAIL-mediated apoptosis in FRO but not in ARO cells. This effect was correlated to higher expression levels of DR5 on FRO than ARO cells, that instead presented higher levels of decoy receptors, DcR1 and DcR2. To understand the cross-talk between the effect of Cl-IB-MECA and TRAIL, we evaluated the nuclear translocation of p65 and c-Rel. Since the dependency by NF-κB, TRAIL promoted the nuclear translocation of both p65 and c-Rel subunits. However, the addition of Cl-IB-MECA led to the predominant translocation of c-Rel after TRAIL addition. Furthermore, Bcl-2, cFLIP and pAkt were lower induced than caspase-3 and -9 in FRO cells. To discriminate a specific effect of TRAIL, we used tumour necrosis factor-alpha (TNF-α) with Cl-IB-MECA. In this case, no synergism was observed. In addition, the effect of Cl-IB-MECA was not A3 receptor-dependent since its antagonists, MRS1191 and FA385, failed to block Cl-IB-MECA activity on TRAIL-treated FRO cells. In conclusion, Cl-IB-MECA enhanced TRAIL-mediated apoptosis via NF-κB/c-Rel activation and DR5-dependent manner. This study may shed light on a potential drug cocktail that may prove useful as anti-cancer in an in vivo animal model. J. Cell. Physiol. 221: 378–386, 2009. © 2009 Wiley-Liss, Inc.     

Ginkgetin derived from Ginkgo biloba leaves enhances the therapeutic effect of cisplatin via ferroptosis-mediated disruption of the Nrf2/HO-1 axis in EGFR wild-type non-small-cell lung cancer

BackgroundCisplatin (DDP) is the first-in-class drug for advanced and non-targetable non-small-cell lung cancer (NSCLC). A recent study indicated that DDP could slightly induce non-apoptotic cell death ferroptosis, and the cytotoxicity was promoted by ferroptosis inducer. The agents enhancing the ferroptosis may therefore increase the anticancer effect of DDP. Several lines of evidence supporting the use of phytochemicals in NSCLC therapy. Ginkgetin, a bioflavonoid derived from Ginkgo biloba leaves, showed anticancer effects on NSCLC by triggering autophagy. Ferroptosis can be triggered by autophagy, which regulates redox homeostasis. Thus, we aimed to elucidate the possible role of ferroptosis involved in the synergistic effect of ginkgetin and DDP in cancer therapy.MethodsThe promotion of DDP-induced anticancer effects by ginkgetin was examined via a cytotoxicity assay and western blot. Ferroptosis triggered by ginkgetin in DDP-treated NSCLC was observed via a lipid peroxidation assay, a labile iron pool assay, western blot, and qPCR. With ferroptosis blocking, the contribution of ferroptosis to ginkgetin + DDP-induced cytotoxicity, the Nrf2/HO-1 axis, and apoptosis were determined via a luciferase assay, immunostaining, chromatin immunoprecipitation (CHIP), and flow cytometry. The role of ferroptosis in ginkgetin + DDP-treated NSCLC cells was illustrated by the application of ferroptosis inhibitors, which was further demonstrated in a xenograft nude mouse model.ResultsGinkgetin synergized with DDP to increase cytotoxicity in NSCLC cells, which was concomitant with increased labile iron pool and lipid peroxidation. Both these processes were key characteristics of ferroptosis. The induction of ferroptosis mediated by ginkgetin was further confirmed by the decreased expression of SLC7A11 and GPX4, and a decreased GSH/GSSG ratio. Simultaneously, ginkgetin disrupted redox hemostasis in DDP-treated cells, as demonstrated by the enhanced ROS formation and inactivation of the Nrf2/HO-1 axis. Ginkgetin also enhanced DDP-induced mitochondrial membrane potential (MMP) loss and apoptosis in cultured NSCLC cells. Furthermore, blocking ferroptosis reversed the ginkgetin-induced inactivation of Nrf2/HO-1 as well as the elevation of ROS formation, MMP loss, and apoptosis in DDP-treated NSCLC cells.ConclusionThis study is the first to report that ginkgetin derived from Ginkgo biloba leaves promotes DDP-induced anticancer effects, which can be due to the induction of ferroptosis.     

Activation of c-Jun NH2-terminal kinase is required for gemcitabine's cytotoxic effect in human lung cancer H1299 cells

Although gemcitabine is a potent therapeutic agent in the treatment of human non-small cell lung cancer (NSCLC), resistance to gemcitabine is common. In this study, we investigated the molecular mechanisms involved in acquired gemcitabine resistance against NSCLC cells. Gemcitabine-resistant NSCLC H1299 cells (H1299/GR) were selected by long-term exposure of parental H1299 cells to gemcitabine. The median inhibitory concentrations of gemcitabine in H1299 and H1299/GR cells were 19.4 and 233.1 nM, respectively. Gemcitabine induced activation of c-Jun NH2-terminal kinase (JNK) in parental H1299 cells but not in H1299/GR cells after 48 h. Blocking JNK activation by pretreatment with SP600125, a specific JNK inhibitor, or by transfection with dominant-negative JNK vectors abrogated gemcitabine-induced apoptosis in parental H1299 cells as evidenced by interruption of caspase activation. Transient transfection with a JNKK2-JNK1 plasmid expressing constitutive JNK1 partially restored the effect of gemcitabine in H1299/GR cells. Our results indicate that gemcitabine-induced apoptosis in human NSCLC H1299 cells requires activation of the JNK signaling pathway. Attenuated JNK activation may contribute to development of acquired gemcitabine resistance in cancer cells.     

Ischemia and reperfusion injury combined with cisplatin induces immunogenic cell death in lung cancer cells

A first-line chemotherapeutic drug for non-small cell lung cancer (NSCLC), cisplatin (CDDP), fails to induce immunogenic cell death (ICD) because it fails to induce calreticulin (CRT) exposure on the cell surface. We investigated the potential of ischemia and reperfusion injury (I/R) combined with CDDP to induce ICD in lung cancer cells. The in vitro model of I/R, oxygen-glucose deprivation and reperfusion (OGD/R), effectively induced CRT exposure, ATP secretion, high mobility group box 1 (HMGB1) release and eIF2α phosphorylation in both Lewis lung carcinoma (LLC) and A549 cells when combined with CDDP. By using a vaccine assay and coculture with bone marrow-derived dendritic cells (BMDCs), we showed that OGD/R restored the immunogenicity of CDDP by phosphorylating eIF2α and demonstrated that OGD/R + CDDP (O + C) is an ICD inducer. Using the inguinal tumor model, we found that I/R significantly enhanced the tumor-killing effect of CDDP and Mitomycin C, and this effect relied on adaptive antitumor immunity. Consistently, I + C altered the ratio of interferon-gamma-secreting T lymphocytes, thus overcoming the immunosuppressive effect induced by CDDP. In conclusion, our research presents a new combination strategy and indicates that I/R is a potential anticancer immunogenic modality when combined with nonimmunogenic chemotherapy.Subject terms: Cancer immunotherapy, Chemotherapy     

E2F1-mediated DNA damage is implicated in 8-Cl-adenosine-induced chromosome missegregation and apoptosis in human lung cancer H1299 cells

Gambogic acid (GA) is the dry resin of Garcinia hanburyi (Guttiferae) with potent anti-tumor activity, various bioactivities, including detoxification, homeostasis, anti-inflammatory, and parasiticide, whereas the effect of this natural compound on cancer cells has not been clearly clarified. Here, we examined cellular cytotoxicity by cell viability assay and DNA fragmentation by DNA-ladder assay. Activation of different protein expressions were detected by western blot analyses. We first demonstrated that GA reduces the human SH-SY5Y neuroblastoma cell viability with IC₅₀ of 1.28 μM at 6 h which has less toxicity in fibroblast cells. However, lower concentration GA significantly downregulated the expression of anti-apoptotic protein including Bcl-2, Bcl-xL, and Mcl-1, which also dramatically activated cleaved caspase-9 and -3 in a dose- and time-dependent manner. Consequently, GA-induced cytotoxicity was not mediated by the Fas/FasL and PI3 K/AKT/GSK-3β signaling pathway. In addition, GA-induced cells showed damage morphology which had become cell rounding, neurite retraction, membrane blebbing and shrunken in a dose- and time-dependent manner that clearly indicates this morphological change might be due to the process of apoptosis which shows fragmented DNA. Therefore, the findings presented in this study demonstrate that apoptotic effects of GA on SH-SY5Y cells are mediated by intrinsic mitochondrion-dependent caspase pathway which suggests this natural compound might be effective as an anti-cancer agent for neuroblastoma malignancies.     

Flavokawain B induces apoptosis of non-small cell lung cancer H460 cells via Bax-initiated mitochondrial and JNK pathway

Flavokawain B (FKB) possesses strong anti-neoplastic activity against many cancer cells. Here we assessed its antitumor activity and molecular mechanisms in lung cancer H460 cells in vitro. FKB significantly inhibited cell proliferation and caused arrest of the cell cycle G2-M of H460 cells in a dose-dependent manner. FKB also inducted apoptosis, which was associated with cytochrome c release, caspase-7 and caspase-9 activation and Bcl-xL/Bax dys-regulation. FKB significantly down-regulated survivin and XIAP, and the inhibitory effect induced by FKB was greatly attenuated by through over-expression of survivin or Bax(-/-) MEFs. Furthermore, FKB activated the mitogen-activated protein kinases and the JNK inhibitor SP600125 significantly decreased the growth-inhibitory and apoptotic effects of FKB. Together, these results suggest the anti-lung cancer potential of flavokawain B for the prevention and treatment of lung cancer.     

Curcumin sensitizes human lung cancer cells to apoptosis and metastasis synergistically combined with carboplatin

Although carboplatin is one of the standard chemotherapeutic agents for non-small cell lung cancer (NSCLC), it has limited therapeutic efficacy due to activation of a survival signaling pathway and the induction of multidrug resistance. Curcumin, a natural compound isolated from the plant Curcuma longa, is known to sensitize tumors to different chemotherapeutic agents. The aim of this study is to evaluate whether curcumin can chemosensitize lung cancer cells to carboplatin and to analyze the signaling pathway underlying this synergism. We investigated the synergistic effect of both agents on cell proliferation, apoptosis, invasion, migration, and expression of related signaling proteins using the human NSCLC cell line, A549. A549 cell was treated with different concentrations of curcumin and carboplatin alone and in combination. Combined treatment with curcumin and carboplatin inhibited tumor cell growth, migration, and invasion compared with either drug alone. Matrix metalloproteinase (MMP)-2 and MMP-9 were more efficiently downregulated by co-treatment than by each treatment alone. mRNA and protein expression of caspase-3 and caspase-9 and proapoptotic genes was increased in cells treated with a combination of curcumin and carboplatin, whereas expression of the antiapoptotic Bcl-2 gene was suppressed. Co-treatment of both agents substantially suppressed NF-κB activation and increased expression of p53. Phosphorylation of Akt, a protein upstream of NF-κB, was reduced, resulting in inhibition of the degradation of inhibitor of κB(IκBα), whereas the activity of extracellular signal-regulated kinase (ERK1/2) was enhanced. Our study demonstrated that the synergistic antitumor activity of curcumin combined with carboplatin is mediated by multiple mechanisms involving suppression of NF-κB via inhibition of the Akt/IKKα pathway and enhanced ERK1/2 activity. Based on this mechanism, curcumin has potential as a chemosensitizer for carboplatin in the treatment of patients with NSCLC.     

Viability inhibition effect of gambogic acid combined with cisplatin on osteosarcoma cells via mitochondria-independent apoptotic pathway

We previously demonstrated that gambogic acid (GA) is a promising chemotherapeutic compound for human osteosarcoma treatment. The aim of this study was to detect whether the combination of lower-dose GA (0.3 mg/L) and cisplatin (CDDP) (1 mg/L) could perform a synergistic effect on inhibiting tumor in four osteosarcoma cell lines. Our results showed that the combination between GA at lower dose and CDDP significantly exerts a synergistic effect on inhibiting the cellular viability in MG63, HOS, and U2OS cells. In contrast, an antagonistic character was detected in SAOS2 cells exposed to the combined use of lower-dose GA (0.3 mg/L) and CDDP (1 mg/L). Then, analysis of cell cycle showed the combination of both drugs significantly induced the G₂/M phase arrest, without any difference relative to GA treatment alone, in MG63 cells. Flow-cytometric analysis of cell apoptosis displayed that the apoptotic rate in the combination group is higher than that in GA treatment alone in MG63, HOS, and U2OS cells. The combined use of both drugs had no effect on mitochondrial membrane potential, but promoted the apoptosis-inducing function through triggering of CDDP in the three cell lines. By measurement of mitochondrial membrane potential, the activity of caspase-3 and the expressions of caspase-8 and caspase-9, it was showed that the apoptosis-promoting effect of the combined use of both drugs could be dependent on the death receptor apoptosis pathway, not dependent on the mitochondria apoptosis mechanism. This research, for the first time, demonstrates that GA could increase the chemotherapeutic effect of CDDP in human osteosarcoma treatment through inducing the cell cycle arrest and promoting cell apoptosis.     

The bystander effect in C3H 10T cells and radon-induced lung cancer.

Little, M. P. and Wakeford, R. The Bystander Effect in C3H 10T(1/2) Cells and Radon-Induced Lung Cancer. Radiat. Res. 156, 695-699 (2001).Bystander effects, whereby cells that are not directly exposed to radiation exhibit adverse biological effects, have been observed in a number of experimental systems, including C3H 10T(1/2) cells exposed to alpha-particle radiation. The bystander effect implies that risks from exposure to low doses of radiation obtained by linear extrapolation from data for high-dose exposures might be substantial underestimates. The best estimate of the ratio of the lung cancer risk among persons exposed to low (residential) doses of radon daughters to that among persons (underground miners) exposed to high doses of radon daughters is in the range of 2.4-4.0, with an upper 95% confidence limit of about 14. Assuming that the bystander effect observed in the in vitro C3H 10T(1/2) cell system applies to human lung cells in vivo, these epidemiological data imply that the central estimate of the number of neighboring cells that can contribute to the bystander effect is between 0 and 1, with an upper 95% confidence limit of about 7. As a consequence, the bystander effect observed in the experimental C3H 10T(1/2) cell system probably does not play a large part in the process of radon-induced lung carcinogenesis in humans.