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. 2017 Oct 5;41(1):85–91. doi: 10.1007/s13402-017-0354-4

Implications of FGF19 on sorafenib-mediated nitric oxide production in hepatocellular carcinoma cells - a short report

Lixia Gao 1, Chloe Shay 2, Fenglin Lv 3, Xuli Wang 4, Yong Teng 1,5,✉
PMCID: PMC12995209  PMID: 28983785

Abstract

Background

Hepatocellular carcinoma (HCC), a primary neoplasm derived from hepatocytes, is the second leading cause of cancer mortality worldwide. Previous work has shown that fibroblast growth factor 19 (FGF19), an oncogenic driver, acts as a negative regulator of the therapeutic efficacy of the tyrosine kinase inhibitor sorafenib in HCC cells. The FGF19-mediated mechanism affecting sorafenib treatment, however, still remains to be resolved. Here, we hypothesize that the FGF19-FGFR4 axis may affect the effectiveness of sorafenib in the treatment of HCC.

Methods

FGF19 and FGFR4 cDNAs were cloned into a pcDNA3.1 vector and subsequently used for exogenous over-expression analyses. FGF19 knockdown cells were generated using a lentiviral-mediated short hairpin RNA (shRNA) methodology and FGFR4 knockout cells were generated using a CRISPR-Cas9 methodology. FGFR4 activation in HCC cells was inhibited by BLU9931. The effects of exogenous gene over-expression, expression knockdown and knockout, as well as drug efficacies in HCC cells, were validated using Western blotting. HCC cell proliferation was assessed using a CellTiter 96® AQueous One Solution Cell Proliferation Assay, whereas NO levels were assessed using DAF-FM DA staining in conjunction with electrochemical biosensors.

Results

We found that FGF19, when exogenously overexpressed, results in a reduced sorafenib-induced NO generation and a decreased proliferation of HCC cells. In contrast, we found that either FGF19 silencing or knockout of its receptor FGFR4 sensitized HCC cells to sorafenib through the induction of NO generation. Concordantly, we found that inactivation of FGFR4 by BLU9931 enhanced the sensitivity of HCC cells to sorafenib.

Conclusion

From our data we conclude that the FGF19-FGFR4 axis may play a critical role in the effects elicited by sorafenib in HCC cells. Blocking the FGF19-FGFR4 axis may provide novel opportunities to improve the efficacy of sorafenib in the treatment of patients with HCC.

Keywords: FGF19, FGFR4, hepatocellular carcinoma, sorafenib, BLU9931

Introduction

Sorafenib is a tyrosine kinase inhibitor that is designed to provide survival benefits for patients with hepatocellular carcinoma (HCC) [1–4]. However, the observed median survival of only 3 months of patients for which sorafenib has been subscribed as first-line therapy questions its therapeutic efficacy [5] and emphasizes the need for obtaining further insights into the key molecular mechanisms underlying sorafenib action and resistance. In addition, it is clear that the design of novel molecular targeted therapies that can be combined with sorafenib may have a major impact on improving the quality of life of HCC patients.

Fibroblast growth factor 19 (FGF19) is an ileal-derived hormone that is produced by cells of the small intestine and to some extend mimics the effect of insulin [6, 7]. FGF19 has also been reported to play a pivotal role in mediating cancer initiation and progression through activation of its corresponding receptor, FGFR4 [7, 8]. A combined amplification of the FGF19 encoding gene and hyperactivation of FGFR4 has been found to control various oncogenic pathways in HCC cells [7–9] including the FGF19-FGFR4 axis, which induces GSK3β/β-catenin/E-cadherin signaling and promotes epithelial-mesenchymal transition (EMT) and invasion in epithelial-like HCC cells [8]. Recent next generation sequencing-based DNA copy number analyses of sorafenib responder cases revealed an association between the therapeutic effects of sorafenib and FGF19 alterations, suggesting that FGF19 amplification may serve as a response predictor [10]. We found that FGF19 plays an essential role in sorafenib resistance through suppression of drug-induced reactive oxygen species (ROS)-associated apoptosis [11]. Together, these findings suggest that FGF19 may play a major role in the efficacy of sorafenib treatment and its resistance in HCC.

Nitric oxide (NO) is a pleiotropic molecule influencing normal physiological and pathological processes [12]. NO has been shown to have dichotomous effects on various biological processes, and its levels have been associated with cancer development and progression [13, 14]. Low NO levels have been found to promote cellular proliferation, invasion and other cancer-associated phenotypes, whereas high NO levels may elicit anticancer effects that are mediated by cytotoxicity, oxidative/nitrosative stress or DNA damage. The exact role of NO in cancer development, however, remains to be established. Here, we addressed the question whether FGF19 may affect the efficacy of sorafenib treatment in HCC by regulating NO levels. We show that FGF19 may indeed play a regulatory role in NO production, thereby contributing to the effectiveness of sorafenib. We found that FGF19 overexpression inhibits sorafenib-induced NO generation and, concomitantly, anti-proliferative effects in HCC cells, whereas FGF19 silencing or FGFR4 receptor deletion results in the opposite effects. Accordingly, we found that FGFR4 inactivation by BLU9931 overcomes the resistance of HCC cells to sorafenib. Our findings indicate that FGF19 may serve as an attractive therapeutic target, and that disruption of the FGF19-FGFR4 axis may augment the efficacy of sorafenib on HCC cells.

Materials and methods

Cell lines and standard assays

MHCC97L, MHCC97H, SMCC7721 and HepG2 cells were purchased from the Cell Bank of Type Culture Collection of the Chinese Academy of Sciences (Shanghai, China) and maintained according to the supplier’s instructions. Cell proliferation and Western blotting assays were carried out as previously reported [8, 11, 15, 16].

Constructs, reagents and antibodies

All the chemicals used for electrochemical analyses were purchased from Sigma-Aldrich (St Louis, MO, USA), whereas 4-Amino-5-Methylamino-2′,7′-Difluorofluorescein Diacetate (DAF-FM DA) was purchased from Thermo Fisher Scientific (San Jose, CA, USA). A CellTiter 96® AQueous One Solution Cell Proliferation Assay kit was purchased from Promega (Madison, MI) and lentiviral vectors harboring shRNAs targeting FGF19 were obtained from GeneCopoeia (Rockville, MD, USA). A LentiCRISPR v2 vector used for generating a CRISPR-Cas9 targeted deletion of FGFR4 was obtained from Feng Zhang (Addgene plasmid #52961) [17]. Full-length human FGF19 and FGFR4 cDNAs were cloned into a pcDNA3.1 vector (Life Technologies, Carlsbad, CA, USA). All the plasmids used in this study were sequence verified. Antibodies directed against FGF19, FGFR4 and β-Actin were purchased from Abcam (Cambridge, MA, USA).

Electrochemical assessment of NO levels

A classic three-electrode system of NO sensors comprising a nanomaterial-functionalized glassy carbon working electrode, a Hg/HgCl2/KCl reference electrode and a platinum wire counter electrode was used. Graphite oxide was synthesized from natural graphite using a modified Hummers’ method, and reduced graphene oxide-ceria (rGO/CeO2) nanocomposites were synthesized using a hydrothermal method. To establish an electrochemical biosensor for NO detection, 900 mg poly-vinylpyrrolidone, 400 mg Ce(NO3)3 .6H2O and 7.5 mg graphene oxide were dissolved in 30 ml deionized water for 0.5 h. The mixture was transferred to a Teflon-lined autoclave and heated at 180 °C for 24 h after which the rGO/CeO2 nanocomposites were dried at 70 °C for 3 h. Next, formulated rGO/CeO2 nanocomposites were diluted at a concentration of 10 mg/ml after which 5 μl rGO/CeO2 nanocomposites were modified on a polished glassy carbon electrode. For the electrochemical detection of intracellular NO, 5 × 105 cells were seeded in a 6-well plate in the presence or absence of sorafenib. Cyclic voltammetry (CV) was used to monitor NO levels on a CHI760E electrochemical station (ChenHua Instruments, Wuhan, China). NG-monomethyl L-arginine (L-NMMA) was used to verify that current changes were caused by NO. The electrochemical sensors were calibrated at different NO concentrations in a fluidic chamber, and the peak percentages (potential = 0.7 V; current enhancement) were compared and calculated against the control curve and used to evaluate the release.

Fluorescence assessment of NO levels

The amount of intracellular NO was also determined using DAF-FM DA at 10 μM for 30 min. Images were captured using an Axio Observer microscope (Carl Zeiss MicroImaging) and the mean fluorescence intensity from at least six different fields was quantified using NIH ImageJ software (https://imagej.nih.gov/ij/).

Statistic analysis

The data are presented as mean ± SD from three or more independent experiments and analyzed using the Student’s t-test at a significance level of p < 0.05.

Results and discussion

To determine the sensitivity of HCC cells to sorafenib, MHCC97L, MHCC97H, SMCC7721 and HepG2 cells were treated with various concentrations of sorafenib for 72 h. We found that sorafenib inhibited the proliferation of all examined cell lines with a half maximal inhibitory concentration (C50) of 4 μM (Fig. 1a). This concentration was subsequently used in the following experiments. Given the fact that NO may play a role in regulating cancer cell proliferation, we tested whether sorafenib suppresses HCC cell proliferation through the induction of NO. To measure intracellular NO release, we established an electrochemical biosensor for NO detection (Fig. 1b). By using this biosensor, we found that the NO production was induced in sorafenib-treated HCC cells (Fig. 1c). This increase reached a peak at 8 h of sorafenib treatment in all cell lines examined, and at this time point the NO production was >50% increased in the sorafenib-treated cells compared to untreated control cells (Fig. 1c). To confirm this observation, we stained the respective HCC cells with DAF-FM DA to quantify the NO production in the presence or absence of sorafenib. We found a similar tendency as in the electrochemical analysis, i.e., higher NO levels in sorafenib-treated cells than in control cells (Fig. 1d). These observations suggest that sorafenib-induced NO release may contribute to its anti-proliferative effects.

Fig. 1.

Fig. 1

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Sorafenib induces NO-associated repression of HCC cell proliferation. (a) MHCC97L, MHCC97H, SMCC7721 and HepG2 cells were treated with the indicated concentrations of sorafenib for 48 h after which cell proliferation was determined using a CellTiter 96® AQueous One Solution Cell Proliferation Assay Kit. (b) Working model of an electrochemical biosensor for NO detection with a three-electrode system. RE: reference electrode; WE: working electrode; CE: counter electrode. (c, d) MHCC97L, MHCC97H, SMCC7721 and HepG2 cells were treated with 4 μM sorafenib for the indicated time points after which NO generation was determined by electrochemical biosensor (c) and DAF-FM DA staining (d). *p < 0.05; **p < 0.01

Previously, we have shown that FGF19 is expressed at a lower level in MHCC97L cells compared to MHCC97H, SMCC-7721 and HepG2 cells [8]. Interestingly, we here found that MHCC97L cells are more sensitive to high doses of sorafenib (6 μM and 10 μM) than the other three cell lines (Fig. 1a), which prompted us to assess whether FGF19 is involved in sorafenib-induced NO release. To this end, we exogenously overexpressed FGF19 in MHCC97L cells (Fig. 2a) and found that, as a result, the increased NO levels induced by sorafenib were significantly attenuated (Fig. 2b and c). Subsequent cell proliferation assays revealed that exogenous FGF19 overexpression abolished the inhibitory effects of sorafenib on MHCC97L cells (Fig. 2d). Conversely, we found that FGF19 expression knockdown in MHCC97H cells (expressing high levels of FGF19) augmented the sorafenib-induced NO levels and anti-proliferative effects (Fig. 2e–h). Collectively, these observations indicate that FGF19 affects the efficacy of sorafenib treatment in HCC cells through inhibition of its anti-proliferative activity via downregulation of intercellular NO levels.

Fig. 2.

Fig. 2

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FGF19 is involved in sorafenib-induced NO release and anti-proliferative effects in HCC cells. (a) The effect of FGF19 overexpression in MHCC97L cells was determined by Western blotting with an antibody directed against FGF19. (b–d) FGF19 overexpressing MHCC97L cells and control cells were treated with 4 μM sorafinib for the indicated time points. NO generation was determined using an electrochemical biosensor (b) and DAF-FM DA staining (c), and cell proliferation was determined using a CellTiter 96® AQueous One Solution Cell Proliferation Assay Kit (d). (e) The effect of FGF19 knockdown in MHCC97H cells was determined by Western blotting with an antibody directed against FGF19. (f–h) FGF19 knockdown MHCC97H cells and control cells were treated with 4 μM sorafinib for the indicated time points. NO generation was determined using an electrochemical biosensor (f) and DAF-FM DA staining (g), and cell proliferation was determined using a CellTiter 96® AQueous One Solution Cell Proliferation Assay (h). EV: empty vector; FGF19 O/E: FGF19 overexpression; shNC: non-target shRNA control; shFGF19: shRNA against the FGF19 gene; *p < 0.05; **p < 0.01

Considering the fact that FGF19 exerts it function through activation of its receptor, FGFR4, we next set out to assess the role of FGFR4 in the NO-associated anti-proliferative activity of sorafenib. To this end, FGFR4 knockout MHCC97L cells were generated using a CRISPR-Cas9 editing system [8], after which no FGFR4 expression could be detected (Fig. 3a). Using the electrochemical biosensor (see above), we found that the FGFR4 knockout cells produced more NO than control cells following sorafenib exposure (Fig. 3b), which was subsequently confirmed using DAF-FM DA assays (Fig. 3c). Subsequent cell proliferation assays revealed that loss of FGFR4 expression enhanced the sorafenib-mediated inhibition of proliferation (Fig. 3d). Similar effects were observed in FGF19 silenced HCC cells (Fig. 2), indicating that the FGF19-FGFR4 axis may represent a key pathway involved in sorafenib-induced proliferation inhibition. Interestingly, we found that FGFR4 overexpression in MHCC97L cells did not result in effects opposite from those observed in FGFR4 knockout cells (Fig. 3), indicating that FGF19-induced FGFR4 activation, rather than FGFR4 expression, triggers these changes.

Fig. 3.

Fig. 3

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FGFR4 activation regulates sorafenib-induced NO release and anti-proliferative effects in HCC cells. (a) The effects of FGFR4 overexpression or knockdown in MHCC97H cells were determined by Western blotting with an antibody directed against FGFR4. (b–d) FGFR4 modulated MHCC97H cells and control cells were treated with 4 μM sorafinib for the indicated time points. NO generation was determined using an electrochemical biosensor (b) and DAF-FM DA staining (c), and cell proliferation was determined using a CellTiter 96® AQueous One Solution Cell Proliferation Assay Kit (d). WT: wild-type; FGFR4 O/E: FGFR4 overexpression; FGFR4 KO: FGFR4 knockout; *p < 0.05; **p < 0.01

Previously, we generated sorafenib-resistant MHCC97H cells [11]. Compared to wild-type cells, these cells exhibited a higher proliferation rate and a lower NO level when exposed to a high dose of sorafenib (20 μM) (Fig. 4a–c), suggesting that an enhancement of NO levels may overcome sorafenib-induced resistance. Since BLU9931 has a high selectivity for FGFR4 [18], we decided to investigate the efficacy of BLU9931 in generating sorafenib resistance. We found that BLU9931 treatment led to a dose-dependent decrease in FGFR4 activation (Fig. 4d) and a concomitant inhibition of proliferation in sorafenib-resistant cells (Fig. 4e). Most importantly, we found that a combination of BLU9931 and 20 μM sorafenib resulted in a significantly decreased proliferation rate in resistant cells compared to untreated cells, and that the synergistic effect was stronger than that of each drug alone (Fig. 4e). In addition, we found that the BLU9931-mediated effects in sorafenib-resistant cells were associated with increased NO levels (Fig. 4f–g), supporting an additive anti-proliferative activity of BLU9931 to sorafenib treatment.

Fig. 4.

Fig. 4

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BLU9931 improves the sensitivity of HCC cells to sorafenib. (a–c) Sorafenib-resistant MHCC97H cells were cultured in the presence of 1 μM sorafenib before treatment after which a high dose of sorafenib (20 μM) was used to test the sensitivity of the resistant cells. NO generation was determined using an electrochemical biosensor (a) and DAF-FM DA staining (b), and cell proliferation was determined using a CellTiter 96® AQueous One Solution Cell Proliferation Assay Kit (c). (d) Sorafenib-resistant MHCC97H cells were treated with the indicated concentrations of BLU9931 for 24 h after which the phosphorylation levels of FGFR4 were determined by Western blotting. (e–g) Sorafenib-resistant MHCC97H cells were treated with 500 nM BLU9931 for 48 h in the presence or absence of 20 μM sorafenib after which cell proliferation was determined using a CellTiter 96® AQueous One Solution Cell Proliferation Assay Kit (e), and NO generation was determined using an electrochemical biosensor (f) and DAF-FM DA staining (g). WT: wild-type; SR: sorafenib resistance; *p < 0.05; **p < 0.01

Although sorafenib has been shown to improve the survival of advanced HCC patients [19, 20], the overall outcomes are far from satisfactory due to the development of drug resistance [21–23]. As a multi-tyrosine kinase inhibitor, sorafenib inhibits a myriad of signaling pathways while addiction switches and compensatory pathways are activated [1–4]. This opens up novel avenues for the design of therapeutic strategies such as combining sorafenib with other anticancer agents in order to enhance its efficacy and to overcome resistance. Given the fact that high levels of FGF19 amplification are positively correlated with resistance to sorafenib [10], we decided to assess the impact of FGF19 on sorafenib efficacy and resistance. Although sorafenib has been found to neutralize proliferative signals in HCC cells, we identified a novel mechanism with a key role of FGF19 in sorafenib-induced NO-associated anti-proliferative effects, which may be of translational relevance.

NO operates in a bimodal fashion and its dichotomous effects on cancer depend on its concentration and the tumor microenvironment in question [13, 14]. Also, polymorphisms in genes that code for NO synthases (NOS) have been linked to the occurrence of various cancer types, adding weight to its association with cancer [24, 25]. Consistent with this concept, sorafenib appears to increase NO levels in HCC cells and, by doing so, to enhance its anti-proliferative effects. This notion is consistent with lower levels of NO observed in sorafenib-resistant HCC cells compared to those in wild-type cells. NO may be generated by three NOS isoforms, i.e., neuronal (nNOS/NOS1), inducible (iNOS/NOS2) and endothelial (eNOS/NOS3) NOS [13, 24]. Future studies should be aimed at identifying the isoform that is regulated by sorafenib.

It has been reported that high FGF19 expression levels lead to aberrant FGF19-FGFR4 signaling, thereby driving various tumorigenic processes ranging from proliferation to metastasis [8, 9]. We have previously shown that FGF19 excreted by HCC cells may act in both autocrine and paracrine ways through FGFR4 activation, thereby representing a prototypical oncogenic loop [8]. In the present study, we identified an additional mechanism by which the FGF19-FGFR4 axis may repress NO in HCC cells after sorafenib treatment, which provides a molecular framework to explore potential FGF19-targeted approaches in combination with sorafenib for the treatment of HCC.

FGF19-targeted strategies, including the use of monoclonal antibodies and small molecules, have recently been developed. It has been found for example that 1A6, a neutralizing antibody directed against FGF19, may be employed to block FGF19-mediated tumorigenic effects [26, 27]. Since FGF19 acts as a multifunctional hormone implicated in regulating bile acid, carbohydrate and liver regeneration [28], a nontumorigenic variant (M70) has been developed to overcome undesired 1A6 effects on preventing normal bile acid homeostasis associated with FGF19 [27, 29]. An in vivo study has shown that M70 can suppress tumorigenic effects mediated by FGF19 without compromising its role in bile acid homeostasis [29]. Disrupting FGF19 action can also be achieved by specifically targeting FGFR4 activation. Here, we applied the FGFR4 inhibitor BLU9931 to treat sorafenib-resistant HCC cells, and found that it elicits an effective suppressive effect on cell proliferation and on improving the sensitivity of HCC cells to sorafenib. Additional in vivo (animal) studies are required to follow up on these promising results and to explore the FGF19-FGFR4 axis as an adjunct therapeutic target next to sorafenib-based treatment of advanced HCC.

Our work shows that hyperactivation of the FGF19-FGFR4 axis represents one of the main mechanisms underlying sorafenib resistance in HCC cells, and that its inactivation can improve the sensitivity of HCC cells to sorafenib through inhibiting NO-associated proliferation induction. Future studies should be aimed at developing methods to successfully identify HCC patients who will benefit from sorafenib and at pre-clinical and clinical evaluation of the efficacy of sorafenib in combination with FGF19-targeted agents in sorafenib-resistant HCC patients.

Acknowledgements

This work was supported in part by Dental College of Georgia Special Funding Initiative.

Compliance with ethical standards

Conflicts of interest

None of the authors has any conflicts of interest related to this study.

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