Sorafenib resistance and autophagy in hepatocellular carcinoma: A concealed threat

K Ashokachakkaravarthy; Biju Pottakkat

doi:10.4103/JCRP.JCRP_6_19

REVIEW ARTICLE

Year : 2019 | Volume : 6 | Issue : 3 | Page : 107-116

Sorafenib resistance and autophagy in hepatocellular carcinoma: A concealed threat

K Ashokachakkaravarthy, Biju Pottakkat
Department of Surgical Gastroenterology, JIPMER, Puducherry, India

Date of Submission	19-Nov-2018
Date of Decision	13-Mar-2019
Date of Acceptance	19-Mar-2019
Date of Web Publication	20-Aug-2019

Correspondence Address:
Dr. Biju Pottakkat
Department of Surgical Gastroenterology, JIPMER, Puducherry - 605 006
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/JCRP.JCRP_6_19

Abstract

Objective: To investigate the relationship between sorafenib resistance and autophagy in hepatocellular carcinoma (HCC). Data Sources: Literature from PubMed (ncbi) database relevant to autophagy and sorafenib resistance in HCC. Study Selection: Studies were selected based on their experimental and observational nature with regards to autophagy and sorafenib resistance in HCC. Observational human studies and sorafenib clinical trials were selected to analyze the epidemiology of HCC, pharmacological properties of sorafenib, autophagy in human HCC, sorafenib treatment in humans, and sorafenib resistance in HCC. In-vivo and In-vitro preclinical studies were selected to analyze the effect of sorafenib on autophagy in HCC and the effect of sorafenib-induced autophagy in HCC. Results: Sorafenib blocked the Akt/mTOR and MEK/ERK pathways which are downstream of ras/raf signaling. By blocking these pathways, sorafenib altered autophagic regulatory signaling pathways, thereby initiating autophagy as a collateral effect. In addition, sorafenib paradoxically activated AMPK, thereby initiating autophagy in human HCC cells. Sorafenib also increased autophagy by upregulating pro-autophagic proteins such as beclin-1, Atg5, LC3II and Vps34. Sorafenib resistance developed in HCC as a consequence of autophagy. Conclusion: Autophagy induced by sorafenib could be a mechanism for the development of sorafenib resistance in HCC.

Keywords: Autophagy, hepatocellular carcinoma, hypoxia, sorafenib resistance

How to cite this article:
Ashokachakkaravarthy K, Pottakkat B. Sorafenib resistance and autophagy in hepatocellular carcinoma: A concealed threat. J Cancer Res Pract 2019;6:107-16

How to cite this URL:
Ashokachakkaravarthy K, Pottakkat B. Sorafenib resistance and autophagy in hepatocellular carcinoma: A concealed threat. J Cancer Res Pract [serial online] 2019 [cited 2021 Jan 15];6:107-16. Available from: https://www.ejcrp.org/text.asp?2019/6/3/107/264844

Introduction

Hepatocellular carcinoma (HCC) is a major contributor to cancer-related mortality and morbidity, and it is the second leading cause of cancer mortality worldwide.^[1] Around 80% of cases of HCC are caused by chronic infection with either hepatitis B virus or hepatitis C virus. Other risk factors include chronic and excessive alcohol consumption, aflatoxin, hemochromatosis, and tyrosinemia.^[2] HCC has a poor prognosis and is usually diagnosed during advanced stages. Recurrence after curative treatment is a major obstacle in the management of HCC and contributes to the overall reduction in the survival rate of HCC after treatment. Hepatic resection remains the first line of treatment for HCC, even though many treatment options are available such as radiofrequency ablation, transarterial chemoembolization, and percutaneous ethanol injection. Liver transplantation and hepatic resection are limited to treating only early-stage HCC but not for advanced stages with metastasis, which requires systemic therapy.^[3],[4]

Advanced HCC with metastasis requires palliative treatment such as systemic therapy with drugs. A multikinase inhibitor, sorafenib is one such systemic drug that has been approved by the Food and Drug Administration to treat advanced HCC.^{[5],[6],[7],[8]} Sorafenib acts as an anti-angiogenic drug by inhibiting receptor tyrosine kinases such as vascular endothelial growth factor receptor (VEGFR), FMS-like tyrosine kinase 3, platelet-derived growth factor receptor (PDGFR), and fibroblast growth factor receptor-1 (FGFR-1).^[7],[9] It also acts as an anti-proliferative drug by inhibiting serine/threonine kinases which have a remarkable impact on the p38/MAPK and RAF/MEK/ERK pathways. Most patients are refractory to sorafenib and only about 30% of patients benefit from sorafenib treatment. Most patients develop sorafenib resistance, which suggests primary and acquired sorafenib resistance in HCC cells.^[7],[9],[10] It has been reported that sorafenib resistance is mediated through autophagy in cancer cells, which is called “sorafenib-mediated autophagy.”^[7]

Molecular Mechanism of Autophagy

Autophagy is a highly conserved and well-regulated biological process to increase cell survival against stressful conditions such as hypoxia, starvation, chemotherapy, and radiotherapy [Figure 1]. Autophagy provides nutrition to cells during stressful conditions by forming and degrading autophagolysosomes. The activation of autophagy induces autophagosome formation by engulfing damaged organelles, which in turn forms autophagolysosomes by fusion with lysosomes.^[11] Autophagy provides nutrition to cells by degrading the organelles inside the autophagolysosomes through lysosomal lytic enzymes. By this means, autophagy provides amino acids, fatty acids, and carbohydrates for cellular metabolism during stress.^[12] In response to cellular stress, tumor cells utilize this inherent autophagy mechanism to overcome hypoxia and nutrient deprivation. Tumor cells use this survival advantage to combat stressful conditions to survive and proliferate.^[13] Autophagy occurs in a consecutive sequential manner by the following steps: initiation of phagophore formation, nucleation of phagophores, elongation of phagophores, formation of autophagosomes, fusion of autophagosomes to lysosomes, and the formation of autophagolysosomes.^[14]

Figure 1: Various cellular conditions acts as autophagic stimulus. Nutrient deprivation due to low blood circulation as well as starvation acts as autophagic stimulus. Hypoxia due to low oxygen tension and low blood circulation acts as autophagic stimulus. ↑ AMP: ATP ratio due to increased catabolism or low nutrient supply acts as autophagic stimulus. Metabolic stress due to high catabolism acts as autophagic stimulus. Chemotherapy and radiation raises cytotoxic effects in cell which gives autophagic stimulus. AMP: Adenosine monophosphate, ATP: Adenosine triphosphate

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Molecular machinery of autophagy sequence is delineated in [Figure 2].

Figure 2: Molecular mechanism of autophagy. (1) Initiation: Initiation complex activates ULK1 phosphorylation. (2) Nucleation: Nucleation complex assembles by the help of phosphorylated ULK1. (3) Elongation: Nucleation complex recruits Atg9 and Atg2 to form elongation complex for elongation. Phagophore forms as a consequence of elongation with the help of Atg12. Maturation of phagophore occurs to form autophagosome. (4) Fusion: Autophagosome gets fused with lysosome by LC3II activation which forms the autophagolysosome where destruction of organelles and proteins occurs with the help of lysosomal enzymes. ULK: UNC51-like kinase

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The initiation of stimuli can be driven by any cellular stress such as nutrient deprivation or hypoxia. The molecular control of autophagy initiation depends on a complex consisting of adenosine monophosphate-activated kinase (AMPK), mammalian target of rapamycin complex 1 (mTORC1) and UNC51-like kinase 1 (ULK1). mTORC inhibits ULK1-mediated autophagosome formation.^[15],[16] During nutrient deprivation or any cellular stress, energy stores in cells are used up in the form of adenosine triphosphate (ATP) which increases intracellular levels of AMP. AMP activates AMPK, which initiates the process by inhibiting mTORC, which in turn permits ULK1 phosphorylation, thereby activating the nucleation complex beclin-1-class-III-PI3K^[14],[16]
Nucleation of phagophores: Unlike other membrane trafficking processes, phagophore nucleation begins de novo from the sequestering of vesicles evoked by the beclin-1-class-III-PI3K complex, which consists of beclin-1, Vps15, Vps34, Ambra1, and Atg14 (L)/Barkor. This regulatory complex is promoted by an exocyst complex of RalB and Exo84^[14],[17]
Elongation: The nucleated phagophores start to elongate to form a complete autophagosome through two ubiquitin-like protein conjugated regulatory complexes called LC3-II complex and Atg5-Atg12-Atg16 L1. These complexes function along with E1-like protein, E2-like protein, Atg7 and Atg10^{[15],[16],[18]}
Docking and fusion: The docking complex consists of the proteins Mon1 and Ccz1 to execute docking/Ypt-7-dependent tethering. The next complex of SNARE proteins including Vtil, Ykt6, NSF sec18, Vam3, Vam7, Rab GTPase Ytp7, α-SNAP sec17, Class C Vps/HOPS, Ccz1, and Mon1 constitutes the molecular machinery for autophagosome vesicular fusion.^[19] The formed vesicular autophagosome fuses with vacuoles/lysosomes with the help of the SNARE fusion protein complex inside the cell for degradation. This fused complex of autophagosome and lysosome is called an autophagolysosome. The autophagosome vesicular contents are composed of macromolecules and are degraded by lysosomal enzymes to release monomeric units such as amino acids and glucose.^{[14],[18],[19]}

Interplay between Autophagy and Angiogenesis in Hepatocellular Carcinoma

Cancer cells rely on autophagy to sustain various stress conditions such as metabolic, hypoxic, and therapeutic stress to yield the required energy to survive and propagate. Autophagy has a potent pro-survival effect during the development of HCC.^{[20],[21],[22]} Autophagy response has been found to be a promoting factor for HCC invasion through epithelial-to-mesenchymal activation, and autophagy has also been reported to have a strong correlation with poor prognosis and malignant progression of HCC.^[23],[24] Experimental studies on animal models of HCC have shown that autophagy is required to inhibit tumor suppressor genes to promote hepatocarcinogenesis, and it has also been demonstrated that the tumor suppressor gene p53 is activated and expressed in the absence of autophagy.^[20],[25]

A recent study reported that deletion of the autophagy gene Atg7 induced the expression of the tumor suppressor p53.^[26] A similar study also demonstrated the functions of autophagy genes Atg5 and Atg7 with regards to their effects on p53-mediated suppression of tumorigenesis, and also a partial restoration of tumorigenesis upon p53 deletion.^[25],[27] Tumor angiogenesis plays an important role in tumor cell survival by supplying nutrients to tumor cells. Autophagy also plays a role in promoting tumor angiogenesis. The high mobility group box 1 (HMGB1) pathway influences tumor angiogenesis and supports the survival of cancer cells by crosstalk between cancer cells and endothelial cells (ECs) in the tumor vasculature.^[28]

The extracellular HMGB1 released from damaged or dead cancer cells functions as a damage-related protein which evokes autophagy by binding to beclin-1, an essential protein in the phagophore nucleation complex. Accordingly, HMGB1 relieves dependency on the phagophore nucleation complex beclin-1-class-III-PI3K on ULK1 to activate the phagophore nucleation complex.^[16],[29] Hypoxic stress damages the cancer cells and releases HMGB1, which binds to beclin-1 to initiate autophagy. The released HMGB1 also binds to beclin-1 inside the ECs of the tumor vasculature, which activates the autophagy mechanism and then modulates autophagy-related gene Atg5 function through the HMGB1 pathway leading to hypoxia-induced angiogenesis. This interplay of HMGB1 between autophagy and angiogenesis makes it an effector molecule for crosstalk between cancer cells and ECs in tumors, which is an essential factor for cancer cell survival and angiogenesis in a hypoxic tumor microenvironment.^{[28],[30],[31]}

Sorafenib as Anti-Angiogenic and Anti-Proliferative Drug for Hepatocellular Carcinoma

Chemically, sorafenib is a biaryl urea which has the ability to inhibit Raf serine/threonine kinases as well as receptor tyrosine kinase downstream pathways, thereby interfering with Ras/Raf/MEK/ERK and Ras/Raf/MAPK pathway-mediated cancer cell proliferation and angiogenesis.^{[32],[33],[34]} Tumor growth primarily depends on the Ras/Raf/MEK/ERK pathway for cell proliferation and angiogenesis. In addition, the VEGFR-mediated angiogenic signal transduction system transduces its signal through the Ras/Raf/MEK/ERK pathway to increase the cellular expressions of angiogenic proteins.^[35] Tyrosine kinase receptors such as VEGFR2 and 3 have Raf/MEK/ERK as a downstream signaling pathway which is controlled by Raf subtypes of serine/threonine kinases, and sorafenib is a well-known inhibitor of Raf proteins, and thus it can inhibit angiogenesis.^[36],[37] Sorafenib directly targets the tyrosine kinases VEGFR2/3, PDGFR beta (PDGFR-β) and serine/threonine kinases Raf-1 and B-Raf. Through these mechanisms, sorafenib directly inhibits cancer cell proliferation and angiogenesis to disrupt the tumor microvasculature and inhibit HCC tumor growth and progression.^[38],[39]

Micrornas Involved in Autophagy and Sorafenib Resistance

Several microRNAs are involved in the regulation of autophagy as well as cancer-related autophagy dysregulation, and also in the posttranscriptional and translational regulation of gene expressions. Many microRNAs have been reported to be downregulated, and a few have been reported to be upregulated in HCC and other cancers, which has implications with regards to autophagic regulation and dysregulation. A few microRNAs have also been reported to be involved in sorafenib resistance in HCC.^{[40],[41],[42],[43]} MiR7, miR21, miR181a, and miR224 have been reported to be upregulated in HCC,^{[40],[44],[45],[46]} and miR26a, miR26b, miR100, miR101, miR142-3p, miR199A-5p, and miR375 have been reported to be downregulated in HCC.^{[42],[43],[47],[48],[49],[50]} [Table 1] shows the microRNAs involved in autophagy dysregulation and sorafenib resistance in HCC.

Table 1: MicroRNAs involved in autophagy and sorafenib resistance in hepatocellular carcinoma

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Autophagy Signalling Pathways in Hepatocellular Carcinoma

The PI3K/AKT/mTOR, AMPK and MEK/ERK pathways are the major signaling pathways that control autophagy. The signaling pathways involved in autophagy regulation in HCC are described below. [Figure 3] depicts the cellular signaling pathways involved in autophagy in HCC.

Figure 3: Autophagy signaling pathways. P13K/AKT activates mTORC1 which inhibits ULK1 leads to decrease in beclin1 levels. Autophagic stimulus activates AMPK which further inhibits mTOR and increases the beclin1 levels. AMPK upregulates MEK/ERK level inhibits autophagy by preventing the disassembly of mTORC1 and mTORC2. Moderate MEK/ERK activity disassembles mTORC1 and mTORC2 which increases beclin1 levels moderately which causes cytoprotective autophagy. High MEK/ERK activity disassembles mTORC1 and mTORC2 which further elevates beclin1 levels which causes cytodestructive autophagy. mTORC: Mammalian target of rapamycin complex, ULK: UNC51-like kinase, AMPK: Adenosine monophosphate-activated kinase

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PI3K/AKT/mTOR pathway

The PI3K/AKT/mTOR pathway is a well-known regulatory pathway for many physiological cellular processes such as metabolism, survival, cell growth, and apoptosis. Cancer cells also exploit these pathways for survival, growth, and proliferation.^[53] AKT is a central regulator of cellular processes and plays a crucial role in the regulation of autophagy. AKT activates mTOR which in turn inhibits the ULK1 complex leading to inhibition of autophagy.^[54] In addition, arenobufagin has been shown to inhibit the PI3K/AKT/mTOR pathway, thereby leading to autophagy in HCC cells. These studies reveal the role of the PI3K/AKT/mTOR pathway in the initiation of autophagy.^[55] PTEN is a regulatory protein closely associated with the PI3K/AKT/mTOR pathway that dephosphorylates PIP3 to form PIP2, thereby leading to a reduction in PIP3 and hampering autophagosome formation and autophagy initiation in HCC.^[56],[57] Sorafenib has been shown to decrease the expressions of PI3K, AKT, and mTOR, and also to downregulate the PI3K/AKT/mTOR pathway. Although it leads to inhibition of cell proliferation and hampers tumor growth, it may positively contribute to autophagy, since high mTOR levels are needed to inhibit autophagy.^[58]

AMPK pathway

AMPK is a sensor of cellular energy status which is activated in conditions of decreased cellular ATP levels and increased AMP levels as it is activated by AMP.^[59] During starvation, cellular ATP levels are depleted and AMP levels increase which activates AMPK and involves cell survival pathways including autophagy. Autophagy acts as a counter-regulatory process for cellular stress such as nutrient starvation.^[60] AMPK inhibits mTORC but not its upstream PI3K/AKT, thereby positively regulating autophagy. AMPK-mediated mTORC inhibition has been shown to further relieve mTORC-mediated ULK1 inhibition and initiate autophagy in HCC.^[61] Experimental evidence has shown that sorafenib can paradoxically activate AMPK by increasing the AMP: ATP ratio by altering mitochondrial metabolism.^[60] This paradoxical activation of AMPK by sorafenib positively contributes to autophagy in HCC. Since sorafenib acts as anti-angiogenic and anti-proliferative agent, it may also cause decreased blood flow and nutrient deprivation, which may activate AMPK and thereby positively contribute to autophagy in HCC.

MEK/ERK pathway

The MEK/ERK pathway is a cell surface receptor signal transducer system involved in cell survival, growth, proliferation, and differentiation.^[62] The MEK/ERK pathway positively contributes to autophagy in cancers by upregulating beclin-1. This MEK/ERK signaling is controlled by AMPK. Although basal MEK/ERK signaling is involved in the regulation of autophagy, the autophagic signal transduced by AMPK amplifies the MEK/ERK pathway which upregulates beclin-1 leading to autophagy initiation. This MEK/ERK-mediated upregulation of beclin-1 is controlled by an intermediate called the tuberous sclerosis complex (TSC). mTORC1 and mTORC2 are composed of mTOR, Raptor, and GβL, and mTOR, Rictor, and GβL, respectively. Activated AMPK phosphorylates and activates TSC, which further causes mTORC1 to disassemble and increase beclin-1 levels. The disassembly of mTORC2 depends on ERK itself rather than TSC which also increases beclin-1, and the overall increase in beclin-1 initiates autophagy.^[63] Thus, a moderate and transitory activation of MEK/ERK by AMPK and a moderate increase in beclin-1 leads to cytoprotective autophagy. In contrast, a strong and persistent activation of MEK/ERK by AMPK results in cytodestructive autophagy. MEK/ERK signaling has also been shown to occur in response to both starvation and nonstarvation stimuli.^[63],[64] In addition, basal MEK/ERK activity can inhibit autophagy by protecting mTORC1 and mTORC2 from disassembly. However, during autophagic stimulus, high levels of AMPK further activate the MEK/ERK pathway which can then disassemble mTORC1 and mTORC2 leading to the accumulation of beclin-1 and initiation of autophagy.^[63] Sorafenib has been shown to activate AMPK,^[10] which in turn may activate its downstream cascade MEK/ERK signaling to initiate autophagy in HCC. In addition to this, sorafenib also blocks MEK/ERK signaling and angiogenesis.^[10] This molecular conflict may contribute to inefficient blockade of MEK/ERK that mimics the moderate activation of the MEK/ERK pathway which could potentially contribute to cytoprotective autophagy and survival in HCC cells. This might be a possible mechanism of autophagy initiation in sorafenib treatment.

Autophagy in Hepatocellular Carcinoma: Friend or Foe?

Autophagy is essentially a cytoprotective process that leads to cell survival, but it may also be destructive to cells if the autophagic signal is persistent and stronger than the threshold.^[63] In HCC, the cellular stress created in the tumor microenvironment may provide a Pro-autophagic stimulus for cell survival. A previous study reported correlations between the expression levels of autophagic proteins and lymph node metastasis, vascular invasion, and inferior 5-year overall survival rate, suggesting that autophagy is positively associated with a poor prognosis and the development of HCC.^[23] Another study showed that experimental hepatocarcinogenesis is prevented by autophagy and that autophagy is necessary to reduce oxidative stress. Once cancer is initiated, it also participates in the suppression of tumor suppressor genes, thereby promoting the development of HCC.^[25] Hence, autophagy in normal cells may be beneficial as it supports cell survival; however, cancer cells can also exploit the same mechanism to survive and proliferate during metabolic stress, nutrient deprivation, and therapeutic stress caused by drugs.

Sorafenib-Induced Autophagy in Hepatocellular Carcinoma

Sorafenib has been shown to induce autophagy in HCC through releasing beclin-1.^[7] Autophagy and apoptosis are concomitantly controlled by a key regulator called myeloid cell leukemia-1 (MCL-1), an anti-apoptotic protein which coordinates both processes depending on the cellular conditions. MCL-1 has also been shown to have an apoptosis-independent effect, such as initiating autophagy during cellular stress. Since MCL-1 is an antiapoptotic protein, the release of MCL-1 also inhibits the apoptosis and degradation of MCL-1 in response to nutrient deprivation and inhibition of mTOR. Therefore, the fate of cells may be decided by the cellular levels of MCL-1.^[65],[66] A study investigated the role of sorafenib on autophagy in human HCC cells, and the results revealed that autophagy is initiated as a consequence of endoplasmic reticulum (ER)-stress induced by sorafenib independently of the MEK/ERK pathway. This sorafenib-induced ER-stress is essential to induce autophagy in HCC cells.^[67] A number of studies have also reported that sorafenib is directly involved in the release of beclin-1 from the MCL-1-beclin-1 complex and induction of autophagy, which in turn induces sorafenib resistance in HCC cells. Sorafenib has also been shown to induce autophagy via an alternative mechanism by inhibiting mTOR and thereby converting LC3I to LC3II.^{[67],[68],[69],[70]}

Sustained exposure of sorafenib to sorafenib-resistant and sensitive human HCC cell lines has been shown to result in the upregulation of the autophagy-related proteins beclin-1, Atg5, LC3II, and Vsp34.^[68],[71] Transitory phosphorylation and activation of AMPK is another mechanism by which sorafenib can influence autophagic signaling through altering both MEK/ERK and PI3K/Akt/mTOR pathways.^[70],[72] [Figure 4] shows the action of sorafenib on the signaling pathways of autophagy. These studies suggest that autophagy is activated during treatment with sorafenib and acts as a compensatory mechanism to withstand sorafenib-mediated apoptosis.^[73] [Table 2] shows the effects of sorafenib on autophagy in various HCC models.

Figure 4: Effect of sorafenib on autophagy. Sorafenib inhibits P13K/AKT/mTOR pathway leads to ULK1 activation and autophagy. Sorafenib blocks MEK/ERK/mTORC2 axis leads to inhibition of autophagy. Sorafenib blocks MEK/TSC/mTORC1 axis leads to activation of autophagy. Sorafenib blocks MEK/ERK/TSC/mTORC1 axis leads to activation of autophagy. Sorafenib activates AMPK which further blocks mTORC1 and activates autophagy. Sorafenib enhances autophagy by increases the expression of autophagy related proteins beclin1, LC3II, Atg5 and Vsp34. mTORC: Mammalian target of rapamycin complex, ULK: UNC51-like kinase

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Table 2: Effect of sorafenib on autophagy in hepatocellular carcinoma

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Autophagy Induces Sorafenib Resistance in Hepatocellular Carcinoma

Sorafenib induces autophagy in HCC directly via stimulation of ER stress, and conversely, inhibition of autophagy in HCC induces ER stress-related apoptosis.^[67] One study investigated the role of autophagy in sorafenib lethality in mediating ER stress and apoptosis, and the results revealed that ER stress mediated by sorafenib increased the formation of autophagosomes in addition to apoptosis. The crucial molecular signaling involved in sorafenib-induced autophagy is mediated through inositol-requiring enzyme 1 (IRE1, an ER stress-mediated protective pro-autophagic factor).^[76] Several studies have conclusively suggested that the ability of liver cancer to exhibit drug resistance is largely caused by sorafenib-induced autophagy. Thus, the fate of the cell is decided by the severity of the damage caused by sorafenib-induced ER stress, in which the cell will either induce pro-apoptotic signaling if the damage is intense and lasting or it may induce pro-autophagic signaling if the damage and ER stress is moderate. This molecular switching of autophagy to apoptotic signaling is regulated by CHOP (an ER-stress mediated proapoptotic factor).^[76],[77] Moreover, the direct involvement of IRE1 by sorafenib-induced ER stress, which acts as a pro-survival mechanism in altering the effect of sorafenib, has been suggested to be a mechanism for acquired sorafenib resistance in HCC. This suggests that pro-survival effects mediated by sorafenib-induced autophagy may facilitate sorafenib resistance in HCC cells.^{[67],[77],[78]}

Shi et al. reported that sorafenib-induced autophagy via ER-stress-mediated mechanisms and the inhibition of autophagy increased cell death in HCC cells treated with sorafenib. Additionally,in vivo experiments revealed that the tumor size was significantly reduced by sorafenib treatment in combination with the autophagy inhibitor chloroquine, but there was no change in the tumor size when treated with sorafenib alone, suggesting that autophagy is involved in sorafenib resistance.^[67] Another study explored the role of autophagy in acquired sorafenib resistance, and the results showed that sustained exposure to sorafenib increased the expressions of beclin-1, LC3II, Atg5, and vps34 in human HCC HepG2 and Huh7 cells.^[68] Collectively, these observations indicate that sorafenib not only obstructs cell proliferation but also initiates autophagy as a consequence of inhibiting signaling kinases involved in both cell proliferation and autophagy. Based on the strength of the stimulus and its persistence, molecular mechanisms switch from proliferation to survival against sorafenib by inducing autophagy, which in turn leads to the acquisition of resistance to sorafenib.^[67],[77] This might be one of the driving forces for drug resistance, tumor recurrence, and metastasis in HCC.

Conclusion

Sorafenib continues to be the recommended drug for HCC. However, it has limitations since it is a nonspecific multikinase inhibitor that exerts a general antiproliferative and antiangiogenic effect on HCC by blocking several kinase signaling pathways which are also involved in the regulation of autophagy. Thus, the sorafenib-mediated inhibition of kinases involved in proliferation and angiogenesis leads to dysregulation of autophagy. Eventually, HCC becomes resistant to sorafenib treatment and exhibits a simultaneous autophagic response. The concurrence of autophagy and sorafenib resistance raises the suspicion that sorafenib-induced autophagy may play a role in acquired sorafenib resistance. Experiments on the effect of sorafenib on autophagy have revealed that sustained exposure can increase autophagic proteins and autophagy, thereby leading to sorafenib resistance in HCC. Sorafenib can also induce the hypoxia-induced release of HMGB1 and autophagy, which contradicts its actual therapeutic effect. Therefore, we conclude that sorafenib-induced autophagy may be the underlying mechanism for acquired sorafenib resistance in HCC.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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