HCC cell lines, Hep-G2 and Huh7, were purchased from ECACC. Cells were cultured in DMEM (Invitrogen, Karlsruhe, Germany), supplemented with 10% fetal calf serum (FCS, Biochrom, Berlin, Germany), 1% Pen/Strep (PAA laboratories, Pasching, Austria), 1% HEPES and 1% L-Glutamine (Cambrex, Verviers, Belgium). Cells were cultivated at 37°C with a concentration of 5% CO₂. Transfection experiments were performed in OPTIMEM (Invitrogen).

Reagents were purchased from the following suppliers: recombinant TRAIL (with Enhancer applied in a concentration of 1 μg/mL) and SuperKillerTRAIL (SkTRAIL) from Alexis Biochemicals (SanDiego, CA, USA), goat anti-human IgG F(ab)’2 from Meridian Life Science (Cincinnati, USA), 5-fluorouracil (5-FU), doxorubicin (Doxo) from Sigma (Deisenhofen, Germany), SP600125, AG1478, PD98059, LY294002 and rapamycin (RAPA) from Calbiochem (Schwalbach, Germany). LBY135 was supplied from Novartis (Basel, Switzerland), Sorafenib (BAY 43-9006) from Bayer (Leverkusen, Germany).

Cell viability was determined by a colorimetric 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. HCC cell lines were seeded onto 96-well plates. On day one after seeding, cells were treated as indicated. We added 10 μL MTT (5 mg/mL) 48 h after treatment and incubated cells for a further 3 h at 37°C. Next, supernatant was discarded and cells were lysed by adding 100 μL 1-propanol to each well followed by shaking plates till complete lysis. Absorbance was measured at 550 nm in a microtiter plate reader. A viability of 1 was defined as the absorbance of untreated cells.

To ease ligand-receptor interaction with the crosslinking supplement IgG F(ab)’2, 96-well plates were coated with IgG F(ab)’2 before seeding cells. One hundred microliters of sterile filtered 100 nmol/L sodium bicarbonate buffer (pH 9.2) containing 5 μg/mL IgG F(ab)’2 was added to each well and incubated for 2 h at room temperature (RT). After replacement of F(ab)’2 buffer by cell culture media, plates were stored at 4°C. Coated plates were stable for at least 1 wk.

To knock-down protein expression, we administered specific small interfering RNA (siRNA) against MCL-1 or BCL-x_L. As a control we used siRNA specific for green fluorescent protein (GFP). The following siRNA sequences were applied (MWG Biotech, Ebersberg, Germany): BCL-x_L, 5'-gcuuggauaaagaugcaaTT-3' (sense) and 5'-uugcaucuuuaucccaagcAG-3' (antisense), MCL-1, 5'-aaguaucacagacguucucTT-3' (sense) and 5'-gagaacgucugugauacuuTT-3' (antisense), GFP, 5'-ggcuacguccaggagcgcaccTT-3' (sense) and 5'-ggugcgcuccuggacguagccTT-3' (antisense). Here, capitals represent deoxyribonucleotides and lower case letters represent ribonucleotides. Huh7 cells were seeded onto 12-well plates and after 24 h transiently transfected in OPTIMEM with Lipofectamine RNAiMax (Invitrogen) according to the manufacturer’s protocol. Expression levels were analyzed 24, 48 and 72 h after transfection via Western blotting analysis.

HCC cells were seeded onto 12-well plates and treated as indicated 1 d after transfection. Forty eight hours after treatment, cells were washed with cold PBS, collected and resuspended in a hypotonic buffer containing 0.1% (w/v) sodium citrate, 0.1% (v/v) Triton X-100, and 50 μg/mL Propidium iodide (PI, Sigma). After 3 h incubation at 4°C, nuclei from apoptotic cells were quantified by fluorometric absorbance cell sorting according to the protocol of Nicoletti et al[24].

Cell lysis, SDS-PAGE and Western blotting were performed as described previously[13]. Immunodetection of proteins was performed using the following antibodies: anti-BCL-x_L (Labvison/NeoMarkers, Warm Springs Blvd. Fremont, Canada), anti-MCL-1 (Santa Cruz Biotechnology, Heidelberg, Germany) and anti-α-tubulin (Sigma) as loading control.

HCC cells were cultured as described and collected. Five hundred thousand cells for each receptor analysis were transferred to polystyrene tubes, washed twice with PBS and resuspended in PBS containing 0.5% BSA (Sigma). A specific monoclonal antibody to either TRAIL-R1, -R2, -R3, -R4 or unspecific mouse IgG1 as isotype control was applied at 5 μg/mL. Cells were incubated for 20 min with gentle rocking at RT. Cells were washed twice in PBS and secondary fluorescein isothiocyanate-conjugated polyclonal goat antibody to mouse IgG1 (1:200 in PBS containing 0.5% BSA) was added, followed by incubation protected from light for 30 min with gentle rocking at RT. Cells were then washed and resuspended in PBS containing 0.5% BSA. Analysis of receptor expression was performed via flow cytometry. All antibodies were purchased from Alexis.

All results are expressed as mean ± SD. Data were analyzed by students t-test (paired, two-sided) based on normal data distribution. P < 0.05 was considered significant.

It is known that TRAIL resistance can be mediated at the receptor level, either by low expression of TRAIL-R1 and -R2 or by a comparably high expression of TRAIL-R3 and -R4[25]. Firstly, we analyzed surface receptor expression of the HCC cell lines Huh7 and Hep-G2. Except for TRAIL-R3, all receptors were found to be expressed: we detected high expression levels of TRAIL-R1, -R2 and -R4 in both cell lines (Figure 1A). Next, we analyzed the expression levels after treatment with TRAIL and consequently the possibility of TRAIL-induced regulation in a feedback manner. After 12 h-treatment with TRAIL, we observed downregulation of TRAIL-R1 and a moderate upregulation of TRAIL-R4 in Hep-G2 cells. In contrast, no changes in receptor expression were detected in Huh7 cells (Figure 1B).

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Figure 1 Surface expression of tumor necrosis factor-related apoptosis inducing ligand (TRAIL) receptors on Huh7 and Hep-G2 cells. Flow cytometric analysis of TRAIL receptors was performed using monoclonal mouse IgG1, anti-TRAIL-R1, -R2, -R3, -R4 antibodies and secondary FITC-conjugated polyclonal goat anti mouse-IgG1 antibodies. Unspecific mouse IgG1 antibodies were used as isotype control. Receptor surface expression was analyzed in untreated Huh7 and Hep-G2 cells (A) and 12 h after treatment with 100 ng/mL rec. TRAIL + 1 μg/mL Enhancer (B), 50 μg/mL 5-fluorouracil (5-FU) (C) and 0.5 μmol/L doxorubicin (D). Diagrams are representative of at least two independent experiments. Doxo: Doxorubicin.

In order to study the effect of chemotherapeutics on TRAIL receptor expression, we treated HCC cells with 5-FU and Doxo, both applied for transarterial chemoembolization in patients with HCC[26]. 12 h-treatment with 5-FU resulted in upregulation of TRAIL-R1 and -R2 in both cell lines. In contrast, TRAIL-R3 was downregulated in Huh7 and unaffected in Hep-G2 cells. For TRAIL-R4, we observed a significant downregulation in both Hep-G2 and Huh7 cells (Figure 1C). 12 h-treatment with Doxo resulted in a slight upregulation of TRAIL-R1 in both cell lines. Remarkably, TRAIL-R2 was considerably upregulated. TRAIL-R3 surface expression was detectable in both cell lines after Doxo treatment. TRAIL-R4 was upregulated in Hep-G2 and unaffected in Huh7 cells (Figure 1D).

Next, we determined sensitivity of HCC cells towards TRAIL-induced apoptosis. Firstly, we analyzed the effect of recombinant TRAIL in concentrations from 20 up to 200 ng/mL (combined with Enhancer, 1 μg/mL). Hep-G2 cells were sensitive towards recombinant TRAIL in a dose-dependent manner, whereas Huh7 were resistant (Figure 2A). Next, we tested LBY135, a chimeric monoclonal antibody targeting TRAIL-R2, in concentrations from 50 to 2000 ng/mL, together with the cross linker F(ab)’2 (2 μg/mL). To optimize the interaction between LBY135 and F(ab)’2, we coated the plates with the F(ab)’2-fragment before seeding the cells. Hep-G2 showed moderate sensitivity towards LBY135-induced apoptosis in a dose-dependent manner, whereas Huh7 cells were resistant (Figure 2B). To discover whether resistance was due to impaired interactions between enhancer or F(ab)’2, the ligand and the receptor, we included SkTRAIL in our study. SkTRAIL interacts effectively with TRAIL receptors without additional supplements. Again, Hep-G2 cells revealed a dose-dependent sensitivity to SkTRAIL in concentrations from 10 to 200 ng/mL. In contrast, Huh7 cells were resistant to SkTRAIL (Figure 2C).

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Figure 2 TRAIL-induced apoptosis in hepatocellular carcinoma (HCC) cells. Huh7 and Hep-G2 cells were seeded onto 96-well plates and treated on day one after seeding with different TRAIL compounds. A: Cells were treated for 48 h with rec. TRAIL + 1 μg/mL Enhancer as indicated; B: Plates were coated with crosslinker IgG F(ab)’2 for 24 h before seeding of cells. Cells were then treated for 48 h with LBY135 + 1 μg/mL F(ab)’2 as indicated; C: Cells were treated for 48 h with SkTRAIL as indicated. Cell viability was analyzed by MTT assay. Viability is shown relative to untreated controls. Assays were performed in six-fold values and are representative of three independent experiments. Values are expressed as mean ± SD. Enh: Enhancer.

Next, we analyzed whether HCC cells were sensitized to TRAIL-induced apoptosis by co-treatment with the chemotherapeutic drugs 5-FU and Doxo. As a first step, we analyzed whether the chemotherapeutics induced loss of viability if applied alone: after 48 h treatment of Huh7 and Hep-G2 cells with 5-FU (50, 100 and 200 μg/mL) and Doxo (0.1, 0.5, 1 and 2 μmol/L), we observed a dose-dependent decrease of cell viability (Figure 3A). Next, we applied these agents in concentrations which exhibited less significant cytotoxic effects, in combination with recombinant TRAIL (+ Enhancer 1 μg/mL). 5-FU (50 μg/mL) or TRAIL (100 ng/mL) did not induce apoptosis in Huh7 cells when administered alone. However, combination of 5-FU and TRAIL induced apoptosis in 62% of Huh7 cells. In Hep-G2 cells, TRAIL (100 ng/mL) induced apoptosis in 15% of cells. 5-FU treatment alone triggered apoptosis of 12% of Hep-G2 cells. 5-FU and TRAIL co-treatment of Hep-G2 resulted in 93% apoptotic cells (Figure 3B, upper panel). Next, we tested the combination of Doxo (0.5 μmol/L) and TRAIL (100 ng/mL). Doxo induced apoptosis in less than 5% of Huh7 cells, whereas the combination of Doxo and TRAIL resulted in 17% apoptotic cells. Treatment of Hep-G2 with Doxo alone induced apoptosis in 24% of cells, whereas Doxo and TRAIL in combination led to apoptosis rates of 43% (Figure 3B, lower panel).

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Figure 3 Treatment of HCC cells with TRAIL in combination with 5-FU and doxorubicin. Values are expressed as mean ± SD. A: Huh7 and Hep-G2 cells were analyzed for cell viability after treatment with the chemotherapeutic agents 5-FU and doxorubicin alone. Cells were seeded onto 96-well plates and treated on day one after seeding. Cells were treated for 48 h with 5-FU (upper left panel) and doxorubicin (lower left panel) as indicated. Cell viability was analyzed by MTT assay. Viability is shown relative to untreated controls. Assays were performed in six-fold values; B: Apoptosis induction in Huh7 and Hep-G2 cells treated with 50 μg/mL 5-FU (upper right panel) and 0.5 μmol/L doxorubicin (lower right panel) either alone or in combination with 100 ng/mL TRAIL + 1 μg/mL Enhancer. Cells were seeded onto 12-well plates, harvested 48 h after treatment and analyzed for apoptosis induction by flow cytometry. Assays were performed in triplicate and are representative of at least two independent experiments.

Antiapoptotic pathways such as PI3K/Akt, epidermal growth factor receptor (EGFR), [mitogen-activated protein kinase (MAPK)/extracellular signal regulated kinase (ERK) kinase] (MEK)/ERK are well known to be activated in malignant cells, thus contributing to cell cycle progression and tumor growth. Therefore, we analyzed whether inhibition of kinases involved in these pathways could overcome resistance towards TRAIL-mediated apoptosis. Firstly, we applied the multi-kinase inhibitor Sorafenib to inhibit RAF/MEK/ERK signaling, in escalating concentrations (2.5, 5 and 10 μmol/L). A dose-dependent decrease of cell viability in Huh7 and Hep-G2 was observed (Figure 4A, upper panel). In a second step, we analyzed the impact of Sorafenib on TRAIL treatment. In Huh7 cells, Sorafenib (10 μmol/L) induced apoptosis rates of 50%. Strikingly, the combination of SkTRAIL (50 ng/mL) and Sorafenib (10 μmol/L) induced apoptosis in 80% of the cells. In Hep-G2 cells Sorafenib caused only minor apoptosis rates (33%). However, combination of TRAIL and Sorafenib led to 98% apoptotic cells (Figure 4A, lower panel).

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Figure 4 Treatment of HCC cells with TRAIL in combination with specific kinase inhibitors. Viability of HCC cells treated with kinase inhibitors alone (upper panels). On day one after seeding of Huh7 and Hep-G2 cells onto 96-well plates, cells were treated with multi-kinase inhibitor Sorafenib (A), PI3 kinase inhibitor LY294002 (B), EGFR kinase inhibitor AG1478 (C), JNK inhibitor SP600125 (+ 0.2% DMSO as vehicle) (D), MEK inhibitor PD98059 (E) and mTOR inhibitor rapamycin (RAPA) (F) at the indicated concentrations for 48 h. Cell viability was analyzed by MTT assay. Viability is shown relative to untreated or 0.2% DMSO treated controls, respectively. Assays were performed in six-fold values. Values are expressed as mean ± SD. Apoptosis induction in Huh7 and Hep-G2 cells treated with 10 μmol/L Sorafenib (A), 10 μmol/L LY294002 (B), 20 μmol/L AG1478(C), 20 μmol/L SP600125 (+ 0.2% DMSO as vehicle) (D), 50 μmol/L PD98059 (E) and 20 ng/mL rapamycin (F) in combination with 50 ng/mL SkTRAIL (lower panels). Cells were seeded 1 d before treatment onto 12-well plates, harvested 48 h after treatment and analyzed for apoptosis induction by flow cytometry. Assays were performed in triplicate and are representative of at least two independent experiments. Values are expressed as mean ± SD.

Next, we inhibited the PI3K/Akt pathway by application of the PI3K inhibitor LY294002. A slight decrease of cell viability was observed in Huh7 and Hep-G2 after 48 h treatment in concentrations lower than 50 μmol/L (Figure 4B, upper panel). However, combination of LY294002 (10 μmol/L) and SkTRAIL (50 ng/mL) doubled apoptosis rates in Hep-G2 cells to 59% compared to SkTRAIL treatment alone. In Huh7, we observed an increased rate of apoptosis after treatment with the combination of LY294002 and SkTRAIL compared to SkTRAIL alone (23% vs 11%, respectively. Figure 4B, lower panel). Furthermore, we used AG1478 to inhibit EGFR kinase. Interestingly, inhibition of EGFR kinase increased cell viability in Hep-G2 cells in concentrations up to 5 μmol/L. In Huh7 cells, AG1478 caused no significant changes in cell viability when applied in low concentrations (Figure 4C, upper panel). AG1478 (20 μmol/L) and SkTRAIL (50 ng/mL) co-treatment increased the rate of apoptotic cells to 27% in Huh7 and 74% in Hep-G2 cells (Figure 4C, lower panel). Next, we inhibited the c-Jun N-terminal kinases 1 and 2 (JNK1 and JNK2) with the anthrapyrazolone inhibitor SP600125. We observed increased cell viability in Huh7 cells and a slight, dose-dependent decrease of cell viability in Hep-G2 cells after 48 h of SP600125 treatment (Figure 4D, upper panel). Notably, a high percentage of cells were arrested in the G2 phase 48 h after treatment with the JNK inhibitor (data not shown). Combined treatment with SP600125 (20 μmol/L) and SkTRAIL (50 ng/mL) led to 28% apoptosis of Huh7 and to 80% apoptosis of Hep-G2 cells (Figure 4D, lower panel). Next, we included a specific inhibitor of MAP kinase kinase (MEK), PD98059, in our study. Again, a death inducing effect of MEK inhibition alone was only observed when applied in high concentrations of more than 50 μmol/L (Figure 4E, upper panel). However, in combination (50 μmol/L PD98059 and 50 ng/mL SkTRAIL), a two-fold increase of apoptosis, compared to monotherapy with SkTRAIL, was detectable in Huh7 and Hep-G2 cells (Figure 4E, lower panel).

Finally, we inhibited mammalian target of rapamycin (mTOR) with rapamycin (Sirolimus). Rapamycin alone only caused a moderate decrease of cell viability (20%) in Huh7 and Hep-G2 cells (Figure 4F, upper panel). Combination of 20 ng/mL rapamycin with 50 ng/mL SkTRAIL resulted in a slight increase of apoptosis rates in Huh7 cells (18% vs 15% SkTRAIL alone) and a profound increase of apoptosis in Hep-G2 cells (43% vs 27%, Figure 4F, lower panel).

The antiapoptotic BCL-2 proteins, MCL-1 and BCL-x_L, are profoundly expressed in tissues of human HCC, thus contributing to apoptosis resistance of HCC cells[13,15,27]. To analyze the role of antiapoptotic BCL-2 proteins in TRAIL-induced apoptosis, we manipulated their expression in Huh7 cells via specific siRNA-mediated knock-down. An effective reduction of MCL-1 and BCL-x_L expression levels was observed 24 h after transfection (Figure 5A).

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Figure 5 TRAIL-induced apoptosis in Huh7 cells after targeted therapy approaches and knock-down of BCL-x_L and MCL-1. A: Huh7 cells were transfected with siRNAs (40 nmol/L) specific for MCL-1 and BCL-x_L either alone or in combination. SiGFP was used as control. Whole cell lysates were prepared 24 h after transfection. MCL-1 and BCL-x_L expression was analyzed by Western blotting. α-Tubulin expression was used to control equal loading; B: 24 h after siRNA transfection, cells were treated for 48 h with 100 ng/mL TRAIL (+ 1 μg/mL Enhancer); C: 20 μmol/L SP600125 (left panel) or 10 μmol/L LY294002 (+ 0.2% DMSO as vehicle, right panel), either alone or in combination with 100 ng/mL TRAIL (+ 1 μg/mL Enhancer). Cells were harvested on day two after treatment and analyzed for apoptosis induction by flow cytometry. Assays were performed in triplicate and are representative of at least two independent experiments. Values are expressed as mean ± SD. NS: Not significant. ^aP < 0.05, ^bP < 0.01, ^cP < 0.001.

A knock-down of BCL-x_L induced significant apoptosis in comparison to mock transfected cells (P < 0.05). Knock-down of MCL-1 did not induce significant apoptosis rates. Additionally, combined knock-down of MCL-1 and BCL-x_L induced spontaneous apoptosis in 8% of Huh7 cells (P < 0.05, Figure 5B). Downregulation of either MCL-1 or BCL-x_L significantly enhanced susceptibility towards TRAIL-induced apoptosis (17% vs 6% and 18% vs 6%, respectively, P < 0.001). Remarkably, we detected 34% apoptotic cells in Huh7 lacking BCL-x_L and MCL-1 expression after treatment with TRAIL (P < 0.001, Figure 5B). Furthermore, we analyzed whether lack of MCL-1 and BCL-x_L expression sensitized cells towards the JNK inhibitor SP600125 and the PI3K inhibitor LY294002. Inhibition of JNK and PI3K showed significantly enhanced anti-tumoral efficacy after knock-down of BCL-x_L and MCL-1. In cells lacking BCL-x_L expression, apoptosis was induced in 27% vs 11% of control cells after treatment with SP600125 (20 μmol/L) (P < 0.05, Figure 5C). In contrast, cells lacking MCL-1 did not show increased susceptibility to JNK inhibition (14% vs 11%, not significant, Figure 5C). Knock-down of MCL-1 and BCL-x_L increased SP600125-induced apoptosis rates to 57% (P < 0.005, Figure 5C, left panel). Additionally, single knock-down of BCL-x_L (P < 0.001) and double knock-down of MCL-1 and BCL-x_L (P < 0.001) significantly increased apoptosis after combined treatment of SP600125 with recombinant TRAIL (100 ng/mL). Single knock-down of MCL-1 did not exhibit sensitizing effects (differences not significant, Figure 5C).

Next, we analyzed the effects of MCL-1 and BCL-x_L knock-down in combination with the PI3K inhibitor LY294002. We observed a significant sensitizing effect of BCL-x_L knock-down on LY294002-induced apoptosis in Huh7 cells (P < 0.005, Figure 5C, right panel). Knock-down of MCL-1 did not increase LY294002-induced apoptosis. However, in Huh7 cells lacking both MCL-1 and BCL-x_L, apoptosis rates increased to 35% after LY294002 treatment (P < 0.001). Finally, we found an increased rate of apoptosis after combined treatment of LY294002 (10 μmol/L) with recombinant TRAIL (100 ng/mL) in cells lacking MCL-1 (48% vs 27% of mock transfected Huh7, P < 0.05). A moderate sensitizing effect in cells lacking BCL-x_L was observed (not significant). Importantly, the combined knock-down of MCL-1 and BCL-x_L caused apoptosis rates of 83%, if cells were treated with a combination of LY294002 and recombinant TRAIL (P < 0.05, Figure 5C, right panel).

Hepatocellular carcinoma (HCC) is the fifth most common malignancy worldwide. Numerous clinical trials have failed to establish an effective therapy regimen in patients with advanced or metastasized HCC. Thus, new strategies for these patients are mandatory. Defects in apoptosis signaling contribute to resistance of HCC cells towards the death receptor ligand tumor necrosis factor-related apoptosis inducing ligand (TRAIL), which is a promising anti-tumor agent since it is capable of killing tumor cells via receptor-mediated apoptosis. New combined treatment regimens have the aim of overcoming resistance towards TRAIL and to make TRAIL an effective treatment option in patients suffering from HCC.

Hyperactivation of the PI3K/Akt, EGFR and [Mitogen-activated protein kinase/extracellular signal regulated kinase (ERK) kinase] (MEK)/ERK survival pathways and decreased mitochondrial sensitivity due to overexpression of the BCL-2 proteins MCL-1 and BCL-x_L are key mechanisms of TRAIL resistance in HCC. Furthermore, resistance towards TRAIL can be located at receptor level, contributing to inefficient treatment of HCC cells with TRAIL compounds.

Previous articles have demonstrated that TRAIL is an effective treatment option in HCC in a combined setup with sensitizing agents. In this study the authors demonstrate new treatment options for the sensitization of HCC cells towards TRAIL-induced apoptosis by combination of chemotherapeutic drugs doxorubicin (Doxo) and 5-fluorouracil (5-FU) or kinase inhibitors, such as LY294002, AG1478, PD98059 and SP600125 with TRAIL. In addition, the authors reveal a pivotal role of the antiapoptotic BCL-2 proteins MCL-1 and BCL-x_L for HCC resistance towards TRAIL. The importance of MCL-1 and Bcl-x_L for mitochondrial integrity has been extensively studied in this study.

TRAIL is an effective treatment option in HCC if combined with the chemotherapeutic drugs Doxo and 5-FU or kinase inhibitors, such as LY294002, AG1478, PD98059 and SP600125. These results open the possibility of a treatment regime which includes reduced doses of chemotherapeutic drugs in combination with TRAIL. The authors also revealed a pivotal role of the antiapoptotic BCL-2 proteins MCL-1 and BCL-x_L for HCC resistance towards TRAIL. Thus, downregulation of these anti-apoptotic proteins alone (e.g. by application of so-called “BH3-only mimetics”) is a promising approach for the treatment of HCC patients. “BH3-only mimetics” have already entered clinical trials in cancer patients.

Apoptosis, also described as programmed cell death, is a genetically determined process of controlled cellular suicide characterized by typical morphological changes, e.g. fragmentation of DNA. TRAIL ligates two different types of receptors: (1) death receptors triggering TRAIL-induced apoptosis and (2) decoy receptors possibly inhibiting the TRAIL death-signaling pathway. MCL-1 and BCL-x_L are antiapoptotic members of the BCL-2 family serving as protective factors against several death stimuli. Antiapoptotic pathways such as PI3K/Akt, EGFR, MEK/ERK are well known as being activated in malignant cells, thus contributing to cell cycle progression and tumor growth

This is an interesting work, elegantly performed and with possible future clinical applications. Although the article contains a wealth of experimental details, making it a difficult reading for the uninitiated, I believe it will be of interest for both clinicians and basic science readers of the World Journal of Gastroenterology because of its possible clinical implications