The human receptor tyrosine kinase Axl gene--promoter characterization and regulation of constitutive expression by Sp1, Sp3 and CpG methylation

AXL signaling in cancer: from molecular insights to targeted therapies

AXL, a member of the TAM receptor family, has emerged as a potential target for advanced-stage human malignancies. It is frequently overexpressed in different cancers and plays a significant role in various tumor-promoting pathways, including cancer cell proliferation, invasion, metastasis, epithelial-mesenchymal transition (EMT), angiogenesis, stemness, DNA damage response, acquired therapeutic resistance, immunosuppression, and inflammatory responses. Beyond oncology, AXL also facilitates viral infections, including SARS-CoV-2 and Zika highlighting its importance in both cancer and virology. In preclinical models, small-molecule kinase inhibitors targeting AXL have shown promising anti-tumorigenic potential. This review primarily focuses on the induction, regulation and biological functions of AXL in mediating these tumor-promoting pathways. We discuss a range of therapeutic strategies, including recently developed small-molecule tyrosine kinase inhibitors (TKIs), monoclonal antibodies, and antibody-drug conjugates (ADCs), anti-AXL-CAR, and combination therapies. These interventions are being examined in both preclinical and clinical studies, offering the potential for improved drug sensitivity and therapeutic efficacy. We further discuss the mechanisms of acquired therapeutic resistance, particularly the crosstalk between AXL and other critical receptor tyrosine kinases (RTKs) such as c-MET, EGFR, HER2/HER3, VEGFR, PDGFR, and FLT3. Finally, we highlight key research areas that require further exploration to enhance AXL-mediated therapeutic approaches for improved clinical outcomes.

Research progress on the role of bypass activation mechanisms in resistance to tyrosine kinase inhibitors in non-small cell lung cancer

The clinical application of small molecule tyrosine kinase inhibitors (TKIs) has significantly improved the quality of life and prognosis of patients with non-small cell lung cancer (NSCLC) carrying driver genes. However, resistance to TKI treatment is inevitable. Bypass signal activation is one of the important reasons for TKI resistance. Although TKI drugs inhibit downstream signaling pathways of driver genes, key signaling pathways within tumor cells can still be persistently activated through bypass routes such as MET gene amplification, EGFR gene amplification, and AXL activation. This continuous activation maintains tumor cell growth and proliferation, leading to TKI resistance. The fundamental strategy to treat TKI resistance mediated by bypass activation involves simultaneously inhibiting the activated bypass signals and the original driver gene signaling pathways. Some clinical trials based on this combined treatment approach have yielded promising preliminary results, offering more treatment options for NSCLC patients with TKI resistance. Additionally, early identification of resistance mechanisms through liquid biopsy, personalized targeted therapy against these mechanisms, and preemptive targeting of drug-tolerant persistent cells may provide NSCLC patients with more sustained and effective treatment.

The transcription factors NR5A1 and JUNB cooperate to activate the Axl promoter in mouse Sertoli cell lines

BACKGROUND

The Axl gene is a receptor tyrosine kinase essential for male fertility. With other Tyro3 family members, it regulates cell apoptosis and preserves the organization of seminiferous tubules. However, the regulation of the expression of Axl in testicular Sertoli cells is not entirely understood. The transcription factors NR5A1 and JUNB are involved in several male fertility mechanisms such as sex development and steroidogenesis. We hypothesize that Axl promoter activity is regulated by cooperation between JUNB and NR5A1 in Sertoli cells.

METHODS AND RESULTS

Following transfections of TM4 Sertoli cells with DsiRNA interference against Axl, our results show that cell morphology may be regulated by AXL. Using transfections of expression plasmids and reporter plasmids containing the Axl promoter, we report that Axl expression is highly activated by cooperation between NR5A1 and JUNB in TM4 and 15P-1 Sertoli cells. Chromatin immunoprecipitation and luciferase reporter assays with 5' promoter deletions demonstrate that JUNB and NR5A1 are being recruited to DNA regulatory elements in the proximal region of the Axl promoter. The fourth intronic region of Axl also participates in the recruitment of JUNB.

CONCLUSION

Thus, Axl expression is regulated by a cooperation between the transcription factors JUNB and NR5A1 and influences the morphology of TM4 Sertoli cells.

Therapeutic advances of targeting receptor tyrosine kinases in cancer

Receptor tyrosine kinases (RTKs), a category of transmembrane receptors, have gained significant clinical attention in oncology due to their central role in cancer pathogenesis. Genetic alterations, including mutations, amplifications, and overexpression of certain RTKs, are critical in creating environments conducive to tumor development. Following their discovery, extensive research has revealed how RTK dysregulation contributes to oncogenesis, with many cancer subtypes showing dependency on aberrant RTK signaling for their proliferation, survival and progression. These findings paved the way for targeted therapies that aim to inhibit crucial biological pathways in cancer. As a result, RTKs have emerged as primary targets in anticancer therapeutic development. Over the past two decades, this has led to the synthesis and clinical validation of numerous small molecule tyrosine kinase inhibitors (TKIs), now effectively utilized in treating various cancer types. In this manuscript we aim to provide a comprehensive understanding of the RTKs in the context of cancer. We explored the various alterations and overexpression of specific receptors across different malignancies, with special attention dedicated to the examination of current RTK inhibitors, highlighting their role as potential targeted therapies. By integrating the latest research findings and clinical evidence, we seek to elucidate the pivotal role of RTKs in cancer biology and the therapeutic efficacy of RTK inhibition with promising treatment outcomes.

DNMT1 determines osteosarcoma cell resistance to apoptosis by associatively modulating DNA and mRNA cytosine-5 methylation

Cellular apoptosis is a central mechanism leveraged by chemotherapy to treat human cancers. 5-Methylcytosine (m5C) modifications installed on both DNA and mRNA are documented to regulate apoptosis independently. However, the interplay or crosstalk between them in cellular apoptosis has not yet been explored. Here, we reported that promoter methylation by DNMT1 coordinated with mRNA methylation by NSun2 to regulate osteosarcoma cell apoptosis. DNMT1 was induced during osteosarcoma cell apoptosis triggered by chemotherapeutic drugs, whereas NSun2 expression was suppressed. DNMT1 was found to repress NSun2 expression by methylating the NSun2 promoter. Moreover, DNMT1 and NSun2 regulate the anti-apoptotic genes AXL, NOTCH2, and YAP1 through DNA and mRNA methylation, respectively. Upon exposure to cisplatin or doxorubicin, DNMT1 elevation drastically reduced the expression of these anti-apoptotic genes via enhanced promoter methylation coupled with NSun2 ablation-mediated attenuation of mRNA methylation, thus rendering osteosarcoma cells to apoptosis. Collectively, our findings establish crosstalk of importance between DNA and RNA cytosine methylations in determining osteosarcoma resistance to apoptosis during chemotherapy, shedding new light on future treatment of osteosarcoma, and adding additional layers to the control of gene expression at different epigenetic levels.

AXL - a new player in resistance to HER2 blockade

HER2 is a driver in solid tumors, mainly breast, oesophageal and gastric cancer, through activation of oncogenic signaling pathways such as PI3K or MAPK. HER2 overexpression associates with aggressive disease and poor prognosis. Despite targeted anti-HER2 therapy has improved outcomes and is the current standard of care, resistance emerge in some patients, requiring additional therapeutic strategies. Several mechanisms, including the upregulation of receptors tyrosine kinases such as AXL, are involved in resistance. AXL signaling leads to cancer cell proliferation, survival, migration, invasion and angiogenesis and correlates with poor prognosis. In addition, AXL overexpression accompanied by a mesenchymal phenotype result in resistance to chemotherapy and targeted therapies. Preclinical studies show that AXL drives anti-HER2 resistance and metastasis through dimerization with HER2 and activation of downstream pathways in breast cancer. Moreover, AXL inhibition restores response to HER2 blockade in vitro and in vivo. Limited data in gastric and oesophageal cancer also support these evidences. Furthermore, AXL shows a strong value as a prognostic and predictive biomarker in HER2+ breast cancer patients, adding a remarkable translational relevance. Therefore, current studies enforce the potential of co-targeting AXL and HER2 to overcome resistance and supports the use of AXL inhibitors in the clinic.

AXL activates YAP through the EGFR-LATS1/2 axis and confers resistance to EGFR-targeted drugs in head and neck squamous cell carcinoma

The Hippo signaling pathway and its downstream effector YAP play a central role in cell proliferation. Dysregulation of the Hippo pathway triggers YAP hyperactivation, thereby inducing head and neck squamous cell carcinoma (HNSCC). Recently, we reported that EGFR promotes tyrosine phosphorylation of MOB1 and subsequent LATS1/2 inactivation, which are core components of the Hippo pathway, resulting in YAP activation. However, EGFR-targeted monotherapy has shown a low response rate in HNSCC patients. Given that YAP is activated in patient samples refractory to EGFR-targeted therapy, EGFR inhibitors may temporarily inactivate YAP, but intrinsic hyperactivation or acquired reactivation of YAP may confer resistance to EGFR inhibitors in HNSCC cells. The mechanism by which YAP is activated in HNSCC resistant to EGFR inhibitors remains unclear. Comprehensive transcriptional analysis revealed that AXL activates YAP through a novel mechanism: AXL heterodimerizes with EGFR, thereby activating YAP via the EGFR-LATS1/2 axis. The combination of AXL and EGFR inhibitors synergistically inactivates YAP and suppresses the viability of HNSCC and lung adenocarcinoma cells. In turn, LATS1/2 knockout and YAP hyperactivation confer resistance to the synergistic effects of these inhibitors. Our findings suggest that co-targeting both AXL and EGFR represent a promising therapeutic approach in patients with EGFR-altered cancers.

AXL in cancer: a modulator of drug resistance and therapeutic target

AXL is a member of the TAM (TYRO3, AXL, and MERTK) receptor tyrosine kinases family (RTKs), and its abnormal expression has been linked to clinicopathological features and poor prognosis of cancer patients. There is mounting evidence supporting AXL's role in the occurrence and progression of cancer, as well as drug resistance and treatment tolerance. Recent studies revealed that reducing AXL expression can weaken cancer cells' drug resistance, indicating that AXL may be a promising target for anti-cancer drug treatment. This review aims to summarize the AXL's structure, the mechanisms regulating and activating it, and its expression pattern, especially in drug-resistant cancers. Additionally, we will discuss the diverse functions of AXL in mediating cancer drug resistance and the potential of AXL inhibitors in cancer treatment.

AXL/Gas6 signaling mechanisms in the hypothalamic-pituitary-gonadal axis

AXL is a receptor tyrosine kinase commonly associated with a variety of human cancers. Along with its ligand Gas6 (growth arrest-specific protein 6), AXL is emerging as an important regulator of neuroendocrine development and function. AXL signaling in response to Gas6 binding impacts neuroendocrine structure and function at the level of the brain, pituitary, and gonads. During development, AXL has been identified as an upstream inhibitor of gonadotropin receptor hormone (GnRH) production and also plays a key role in the migration of GnRH neurons from the olfactory placode to the forebrain. AXL is implicated in reproductive diseases including some forms of idiopathic hypogonadotropic hypogonadism and evidence suggests that AXL is required for normal spermatogenesis. Here, we highlight research describing AXL/Gas6 signaling mechanisms with a focus on the molecular pathways related to neuroendocrine function in health and disease. In doing so, we aim to present a concise account of known AXL/Gas6 signaling mechanisms to identify current knowledge gaps and inspire future research.

Targeting AXL in Mesothelioma: from functional characterization to clinical implication

Malignant pleural mesothelioma (MM) is a highly aggressive and lethal cancer with a poor survival rate. Current treatment approaches primarily rely on chemotherapy and radiation, but their effectiveness is limited. Consequently, there is an urgent need for alternative treatment strategies, a comprehensive understanding of the molecular mechanisms underlying MM, and the identification of potential therapeutic targets. Extensive studies over the past decade have emphasized the role of Axl in driving tumor development and metastasis, while high levels of Axl expression have been associated with immune evasion, drug resistance, and reduced patient survival in various cancer types. Ongoing clinical trials are investigating the efficacy of Axl inhibitors for different cancers. However, the precise role of Axl in MM progression, development, and metastasis, as well as its regulatory mechanisms within MM, remain inadequately understood. This review aims to comprehensively investigate the involvement of Axl in MM. We discuss Axl role in MM progression, development, and metastasis, along with its specific regulatory mechanisms. Additionally, we examined the Axl associated signaling pathways, the relationship between Axl and immune evasion, and the clinical implications of Axl for MM treatment. Furthermore, we discussed the potential utility of liquid biopsy as a non-invasive diagnostic technique for early detection of Axl in MM. Lastly, we evaluated the potential of a microRNA signature that targets Axl. By consolidating existing knowledge and identifying research gaps, this review contributes to a better understanding of Axl's role in MM and sets the stage for future investigations and the development of effective therapeutic interventions.

AXL kinase inhibitors- A prospective model for medicinal chemistry strategies in anticancer drug discovery

Deviant expressions of the tyrosine kinase AXL receptor are strongly correlated with a plethora of malignancies. Henceforth, the topic of targeting AXL is beginning to gain prominence due to mounting evidence of the protein's substantial connection to poor prognosis and treatment resistance. This year marked a milestone in clinical testing for AXL as an anti-carcinogenic target, with the start of the first AXL-branded inhibitor study. It is critical to emphasize that AXL is a primary and secondary target in various kinase inhibitors that have been approved or are on the verge of being approved while interpreting the present benefits and future potential effects of AXL suppression in the clinical setting. Several research arenas across the globe resolutely affirm the crucial significance of AXL receptors in the case study of several pathophysiologies including AML, prostate cancer, and breast cancer. This review endeavors to delve deeply into the biological, chemical, and structural features of AXL kinase; primary AXL inhibitors that target the enzyme (either purposefully or unintentionally); and the prospects and barriers for turning AXL inhibitors into a feasible treatment alternative. Furthermore, we analyse the co-crystal structure of AXL, which remains extensively unexplored, as well as the mutations of AXL that may be valuable in the development of novel inhibitors in the upcoming future and take a comprehensive look at the medicinal chemistry of AXL inhibitors of recent years.

Axl Expression in Renal Mesangial Cells Is Regulated by Sp1, Ap1, MZF1, and Ep300, and the IL-6/miR-34a Pathway

Axl receptor tyrosine kinase expression in the kidney contributes to a variety of inflammatory renal disease by promoting glomerular proliferation. Axl expression in the kidney is negligible in healthy individuals but upregulated under inflammatory conditions. Little is known about Axl transcriptional regulation. We analyzed the 4.4 kb mouse Axl promoter region and found that many transcription factor (TF)-binding sites and regulatory elements are located within a 600 bp fragment proximal to the translation start site. Among four TFs (Sp1, Ap1, MZF1, and Ep300) identified, Sp1 was the most potent TF that promotes Axl expression. Luciferase assays confirmed the siRNA results and revealed additional mechanisms that regulate Axl expression, including sequences encoding a 5'-UTR mini-intron and potential G-quadruplex forming regions. Deletion of the Axl 5'-UTR mini-intron resulted in a 3.2-fold increases in luciferase activity over the full-length UTR (4.4 kb Axl construct). The addition of TMPyP4, a G-quadruplex stabilizer, resulted in a significantly decreased luciferase activity. Further analysis of the mouse Axl 3'-UTR revealed a miRNA-34a binding site, which inversely regulates Axl expression. The inhibitory role of miRNA-34a in Axl expression was demonstrated in mesangial cells using miRNA-34a mimicry and in primary kidney cells with IL-6 stimulated STAT3 activation. Taken together, Axl expression in mouse kidney is synergistically regulated by multiple factors, including TFs and secondary structures, such as mini-intron and G-quadruplex. A unique IL6/STAT3/miRNA-34a pathway was revealed to be critical in inflammatory renal Axl expression.

Epstein-Barr virus miR-BART4-3p regulates cell proliferation, apoptosis, and migration by targeting AXL in gastric carcinoma

To investigate the role of miR-BART4-3p in EBV-associated gastric cancer (EBVaGC) and its regulation of cell proliferation, apoptosis, and migration by targeting AXL in GC. Quantitative real-time PCR and western blot were used to detect the expression of AXL. The methylation status of AXL gene promoter region was determined by bisulfite sequencing PCR. Luciferase reporter assay was used to detect whether miR-BART4-3p targets AXL. The key molecules of EMT and PI3K/AKT pathway were used to examine by western blot. CCK8, Transwell, and flow cytometry were used to detect the phenotypic gastric cancer cells after interference with AXL and miR-BART4-3p. EBV infection inhibited the expression of AXL in GC cells and the inhibition was not caused by the change of promoter methylation status. MiR-BART4-3p directly targeted AXL. Moreover, both inhibition of miR-BART4-3p and AXL inhibited cell proliferation and migration and promoted cell apoptosis. In addition, E-cadherin, Vimentin, ZEB1, and p-AKT were found to be the downstream molecules of the miR-BART4-3p/AXL pathway. The change of promoter methylation status was not the reason for the downregulation of AXL expression in EBV-positive cells. MiR-BART4-3p may inhibit the proliferation and migration and promote apoptosis of GC cells by directly targeting AXL.

Intrinsic and Extrinsic Control of Hepatocellular Carcinoma by TAM Receptors

Simple Summary

Tyro3, Axl, and MerTK are receptor tyrosine kinases of the TAM family, which are activated by their ligands Gas6 and Protein S. TAM receptors have large physiological implications, including the removal of dead cells, activation of immune cells, and prevention of bleeding. In the last decade, TAM receptors have been suggested to play a relevant role in liver fibrogenesis and the development of hepatocellular carcinoma. The understanding of TAM receptor functions in tumor cells and their cellular microenvironment is of utmost importance to advances in novel therapeutic strategies that conquer chronic liver disease including hepatocellular carcinoma.

Abstract

Hepatocellular carcinoma (HCC) is the major subtype of liver cancer, showing high mortality of patients due to limited therapeutic options at advanced stages of disease. The receptor tyrosine kinases Tyro3, Axl and MerTK—belonging to the TAM family—exert a large impact on various aspects of cancer biology. Binding of the ligands Gas6 or Protein S activates TAM receptors causing homophilic dimerization and heterophilic interactions with other receptors to modulate effector functions. In this context, TAM receptors are major regulators of anti-inflammatory responses and vessel integrity, including platelet aggregation as well as resistance to chemotherapy. In this review, we discuss the relevance of TAM receptors in the intrinsic control of HCC progression by modulating epithelial cell plasticity and by promoting metastatic traits of neoplastic hepatocytes. Depending on different etiologies of HCC, we further describe the overt role of TAM receptors in the extrinsic control of HCC progression by focusing on immune cell infiltration and fibrogenesis. Additionally, we assess TAM receptor functions in the chemoresistance against clinically used tyrosine kinase inhibitors and immune checkpoint blockade in HCC progression. We finally address the question of whether inhibition of TAM receptors can be envisaged for novel therapeutic strategies in HCC.

The Antiviral Roles of Hydrogen Sulfide by Blocking the Interaction between SARS-CoV-2 and Its Potential Cell Surface Receptors

The ongoing coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is posing a great threat to the global economy and public health security. Together with the acknowledged angiotensin-converting enzyme 2, glucose-regulated protein 78, transferrin receptor, AXL, kidney injury molecule-1, and neuropilin 1 are also identified as potential receptors to mediate SARS-CoV-2 infection. Therefore, how to inhibit or delay the binding of SARS-CoV-2 with the abovementioned receptors is a key step for the prevention and treatment of COVID-19. As the third gasotransmitter, hydrogen sulfide (H2S) plays an important role in many physiological and pathophysiological processes. Recently, survivors were reported to have significantly higher H2S levels in COVID-19 patients, and mortality was significantly greater among patients with decreased H2S levels. Considering that the beneficial role of H2S against COVID-19 and COVID-19-induced comorbidities and multiorgan damage has been well-examined and reported in some excellent reviews, this review will discuss the recent findings on the potential receptors of SARS-CoV-2 and how H2S modulates the above receptors, in turn blocking SARS-CoV-2 entry into host cells.

Identification and Characterisation of cis-Regulatory Elements Upstream of the Human Receptor Tyrosine Kinase Gene MERTK

BACKGROUND:

MERTK encodes a receptor tyrosine kinase that regulates immune homeostasis via phagocytosis of apoptotic cells and cytokine-mediated immunosuppression. MERTK is highly expressed in the central nervous system (CNS), specifically in myeloid derived innate immune cells and its dysregulation is implicated in CNS pathologies including the autoimmune disease multiple sclerosis (MS).

OBJECTIVE:

While the cell types and tissues that express MERTK have been well described, the genetic elements that define the gene’s promoter and regulate specific transcription domains remain unknown. The primary objective of this study was to define and characterise the human MERTK promoter region.

METHODS:

We cloned and characterized the 5’ upstream region of MERTK to identify cis-acting DNA elements that promote gene transcription in luciferase reporter assays. In addition, promoter regions were tested for sensitivity to the anti-inflammatory glucocorticoid dexamethasone.

RESULTS:

This study identified identified both proximal and distal-acting DNA elements that promote transcription. The strongest promoter activity was identified in an ∼850 bp region situated 3 kb upstream of the MERTK transcription start site. Serial deletions of this putative enhancer revealed that the entire region is essential for expression activity. Using in silico analysis, we identified several candidate transcription factor binding sites. Despite a well-established upregulation of MERTK in response to anti-inflammatory glucocorticoids, no DNA region within the 5 kb putative promoter was found to directly respond to dexamethasone treatment.

CONCLUSIONS:

Elucidating the genetic mechanisms that regulate MERTK expression gives insights into gene regulation during homeostasis and disease, providing potential targets for therapeutic modulation of MERTK transcription.

β-Catenin Restricts Zika Virus Internalization by Downregulating Axl

The latest outbreak of Zika virus (ZIKV) in the Americas was associated with significant neurologic complications, including microcephaly of newborns. We evaluated mechanisms that regulate ZIKV entry into human fetal astrocytes (HFAs). Astrocytes are key players in maintaining brain homeostasis. We show that the central mediator of canonical Wnt signaling, β-catenin, regulates Axl, a receptor for ZIKV infection of HFAs, at the transcriptional level. In turn, ZIKV inhibited β-catenin, potentially as a mechanism to overcome its restriction of ZIKV internalization through regulation of Axl. This was evident with three ZIKV strains tested but not with a laboratory-adapted strain which has a large deletion in its envelope gene. Finally, we show that β-catenin-mediated Axl-dependent internalization of ZIKV may be of increased importance for brain cells, as it regulated ZIKV infection of astrocytes and human brain microvascular cells but not kidney epithelial (Vero) cells. Collectively, our studies reveal a role for β-catenin in ZIKV infection and highlight a dynamic interplay between ZIKV and β-catenin to modulate ZIKV entry into susceptible target cells. IMPORTANCE ZIKV is an emerging pathogen with sporadic outbreaks throughout the world. The most recent outbreak in North America was associated with small brains (microcephaly) in newborns. We studied the mechanism(s) that may regulate ZIKV entry into astrocytes. Astrocytes are a critical resident brain cell population with diverse functions that maintain brain homeostasis, including neurogenesis and neuronal survival. We show that three ZIKV strains (and not a heavily laboratory-adapted strain with a large deletion in its envelope gene) require Axl for internalization. Most importantly, we show that β-catenin, the central mediator of canonical Wnt signaling, negatively regulates Axl at the transcriptional level to prevent ZIKV internalization into human fetal astrocytes. To overcome this restriction, ZIKV downregulates β-catenin to facilitate Axl expression. This highlights a dynamic host-virus interaction whereby ZIKV inhibits β-catenin to promote its internalization into human fetal astrocytes through the induction of Axl.

AXL Is a Key Factor for Cell Plasticity and Promotes Metastasis in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDA), a leading cause of cancer-related death in the United States, has a high metastatic rate, and is associated with persistent immune suppression. AXL, a member of the TAM (TYRO3, AXL, MERTK) receptor tyrosine kinase family, is a driver of metastasis and immune suppression in multiple cancer types. Here we use single-cell RNA-sequencing to reveal that AXL is expressed highly in tumor cells that have a mesenchymal-like phenotype and that AXL expression correlates with classic markers of epithelial-to-mesenchymal transition. We demonstrate that AXL deficiency extends survival, reduces primary and metastatic burden, and enhances sensitivity to gemcitabine in an autochthonous model of PDA. PDA in AXL-deficient mice displayed a more differentiated histology, higher nucleoside transporter expression, and a more active immune microenvironment compared with PDA in wild-type mice. Finally, we demonstrate that AXL-positive poorly differentiated tumor cells are critical for PDA progression and metastasis, emphasizing the potential of AXL as a therapeutic target in PDA. IMPLICATIONS: These studies implicate AXL as a marker of undifferentiated PDA cells and a target for therapy.

AXL Receptor in Breast Cancer: Molecular Involvement and Therapeutic Limitations

Breast cancer was one of the first malignancies to benefit from targeted therapy, i.e., treatments directed against specific markers. Inhibitors against HER2 are a significant example and they improved the life expectancy of a large cohort of patients. Research on new biomarkers, therefore, is always current and important. AXL, a member of the TYRO-3, AXL and MER (TAM) subfamily, is, today, considered a predictive and prognostic biomarker in many tumor contexts, primarily breast cancer. Its oncogenic implications make it an ideal target for the development of new pharmacological agents; moreover, its recent role as immune-modulator makes AXL particularly attractive to researchers involved in the study of interactions between cancer and the tumor microenvironment (TME). All these peculiarities characterize AXL as compared to other members of the TAM family. In this review, we will illustrate the biological role played by AXL in breast tumor cells, highlighting its molecular and biological features, its involvement in tumor progression and its implication as a target in ongoing clinical trials.

AXL as a Target in Breast Cancer Therapy

AXL is a receptor tyrosine kinase (RTK) that has been implicated in diverse tumor-promoting processes such as proliferation, migration, invasion, survival, and apoptosis. AXL therefore plays a role in cancer progression, and AXL has been implicated in a wide variety of malignancies from solid tumors to hematopoietic cancers where it is often associated with poor prognosis. In cancer, AXL has been shown to promote epithelial to mesenchymal transition (EMT), metastasis formation, drug resistance, and a role for AXL in modulation of the tumor microenvironment and immune response has been identified. In light of these activities multiple AXL inhibitors have been developed, and several of these have entered clinical trials in the U.S. In breast cancer, high levels of AXL expression have been observed. The role of AXL in cancer with a focus on therapeutic implications for breast cancer is discussed.

AXL receptor tyrosine kinase as a promising anti-cancer approach: functions, molecular mechanisms and clinical applications

Molecular targeted therapy for cancer has been a research hotspot for decades. AXL is a member of the TAM family with the high-affinity ligand growth arrest-specific protein 6 (GAS6). The Gas6/AXL signalling pathway is associated with tumour cell growth, metastasis, invasion, epithelial-mesenchymal transition (EMT), angiogenesis, drug resistance, immune regulation and stem cell maintenance. Different therapeutic agents targeting AXL have been developed, typically including small molecule inhibitors, monoclonal antibodies (mAbs), nucleotide aptamers, soluble receptors, and several natural compounds. In this review, we first provide a comprehensive discussion of the structure, function, regulation, and signalling pathways of AXL. Then, we highlight recent strategies for targeting AXL in the treatment of cancer.AXL-targeted drugs, either as single agents or in combination with conventional chemotherapy or other small molecule inhibitors, are likely to improve the survival of many patients. However, future investigations into AXL molecular signalling networks and robust predictive biomarkers are warranted to select patients who could receive clinical benefit and to avoid potential toxicities.

Targeting cyclin-dependent kinase 9 by a novel inhibitor enhances radiosensitization and identifies Axl as a novel downstream target in esophageal adenocarcinoma

Cyclin-dependent kinase 9 (CDK9) transcriptionally regulates several proteins and cellular pathways central to radiation induced tissue injury. We investigated a role of BAY1143572, a new highly specific CDK9 inhibitor, as a sensitizer to radiation in esophageal adenocarcinoma. In vitro synergy between the CDK9 inhibitor and radiation was evaluated by clonogenic assay. In vivo synergy between the CDK9 inhibitor and radiation was assessed in multiple xenograft models including a patient’s tumor derived xenograft (PDX). Reverse phase protein array (RPPA), western blotting, immunohistochemistry, and qPCR were utilized to identify and validate targets of the CDK9 inhibitor. The CDK9 inhibitor plus radiation significantly reduced growth of FLO-1, SKGT4, OE33, and radiation resistant OE33R xenografts and PDXs as compared to the cohorts treated with either single agent CDK9 inhibitor or radiation alone. RPPA identified Axl as a candidate target of CDK9 inhibition. Western blot and qPCR demonstrated reduced Axl mRNA (p = 0.02) and protein levels after treatment with CDK9 inhibitor with or without radiation in FLO-1 and SKGT4 cells. Axl protein expression in FLO-1 xenografts treated with combination of CDK9 inhibitor and radiation was significantly lower than the xenografts treated with radiation alone (p = 0.003). Clonogenic assay performed after overexpression of Axl in FLO-1 and SKGT4 cells enhanced radiosensitization by the CDK9 inhibitor, suggesting dependency of radiosensitization effects of the CDK9 inhibitor on Axl. In conclusion, these findings indicate that targeting CDK9 by BAY1143572 significantly enhances the effects of radiation and Axl is a novel downstream target of CDK9 in esophageal adenocarcinoma.

AXL Is a Novel Predictive Factor and Therapeutic Target for Radioactive Iodine Refractory Thyroid Cancer

Papillary thyroid carcinomas (PTCs) have an excellent prognosis, but a fraction of them show aggressive behavior, becoming radioiodine (RAI)-resistant and/or metastatic. AXL (Anexelekto) is a tyrosine kinase receptor regulating viability, invasiveness and chemoresistance in various human cancers, including PTCs. Here, we analyze the role of AXL in PTC prognosis and as a marker of RAI refractoriness. Immunohistochemistry was used to assess AXL positivity in a cohort of human PTC samples. Normal and cancerous thyroid cell lines were used in vitro for signaling, survival and RAI uptake evaluations. 38.2% of human PTCs displayed high expression of AXL that positively correlated with RAI-refractoriness and disease persistence or recurrence, especially when combined with v-raf murine sarcoma viral oncogene homolog B(BRAF) V600E mutation. In human PTC samples, AXL expression correlated with V-akt murine thymoma viral oncogene homolog 1 (AKT1) and p65 nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) activation levels. Consistently, AXL stimulation with its ligand growth arrest-specific gene 6 (GAS6) increased AKT1- and p65 NF-kB-phosphorylation and promoted survival of thyroid cancer cell lines in culture. Enforced expression or activation of AXL in normal rat thyroid cells significantly reduced the expression of the sodium/iodide symporter (NIS) and the radioiodine uptake. These data indicate that AXL expression levels could be used as predictor of RAI refractoriness and as a possible novel therapeutic target of RAI resistant PTCs.

Repression of AXL expression by AP-1/JNK blockage overcomes resistance to PI3Ka therapy

AXL overexpression is a common resistance mechanism to anti-cancer therapies, including the resistance to BYL719 (Alpelisib) - the p110α isoform specific inhibitor of phosphoinositide 3-kinase (PI3K) - in esophagus and head and neck squamous cell carcinoma (ESCC, HNSCC respectively). However, the mechanisms underlying AXL overexpression in resistance to BYL719 remain elusive. Here we demonstrated that the AP-1 transcription factors, c-JUN and c-FOS, regulate AXL overexpression in HNSCC and ESCC. The expression of AXL was correlated with that of c-JUN both in HNSCC patients and in HNSCC and ESCC cell lines. Silencing of c-JUN and c-FOS expression in tumor cells downregulated AXL expression and enhanced the sensitivity of human papilloma virus positive (HPVPos) and negative (HPVNeg) tumor cells to BYL719 in vitro. Blocking of the c-JUN N-terminal kinase (JNK) using SP600125 in combination with BYL719 showed a synergistic anti-proliferative effect in vitro, which was accompanied by AXL downregulation and potent inhibition of the mTOR pathway. In vivo, the BYL719-SP600125 drug combination led to the arrest of tumor growth in cell line-derived and patient-derived xenograft models, and in syngeneic head and neck murine cancer models. Collectively, our data suggests that JNK inhibition in combination with anti-PI3K therapy is a new therapeutic strategy that should be tested in HPVPos and HPVNeg HNSCC and ESCC patients.

An Integrated Genome-wide CRISPRa Approach to Functionalize lncRNAs in Drug Resistance

Resistance to chemotherapy plays a significant role in cancer mortality. To identify genetic units affecting sensitivity to cytarabine, the mainstay of treatment for acute myeloid leukemia (AML), we developed a comprehensive and integrated genome-wide platform based on a dual protein-coding and non-coding integrated CRISPRa screening (DICaS). Putative resistance genes were initially identified using pharmacogenetic data from 760 human pan-cancer cell lines. Subsequently, genome scale functional characterization of both coding and long non-coding RNA (lncRNA) genes by CRISPR activation was performed. For lncRNA functional assessment, we developed a CRISPR activation of lncRNA (CaLR) strategy, targeting 14,701 lncRNA genes. Computational and functional analysis identified novel cell-cycle, survival/apoptosis, and cancer signaling genes. Furthermore, transcriptional activation of the GAS6-AS2 lncRNA, identified in our analysis, leads to hyperactivation of the GAS6/TAM pathway, a resistance mechanism in multiple cancers including AML. Thus, DICaS represents a novel and powerful approach to identify integrated coding and non-coding pathways of therapeutic relevance.

CX3CR1-deficient microglia shows impaired signalling of the transcription factor NRF2: Implications in tauopathies

TAU protein aggregation is the main characteristic of neurodegenerative diseases known as tauopathies. Low-grade chronic inflammation is also another hallmark that indicates crosstalk between damaged neurons and glial cells. Previously, we have demonstrated that neurons overexpressing TAU^P301L release CX3CL1, which activates the transcription factor NRF2 signalling to limit over-activation in microglial cells in vitro and in vivo. However, the connection between CX3CL1/CX3CR1 and NRF2 system and its functional implications in microglia are poorly described. We evaluated CX3CR1/NRF2 axis in the context of tauopathies and its implication in neuroinflammation. Regarding the molecular mechanisms that connect CX3CL1/CX3CR1 and NRF2 systems, we observed that in primary microglia from Cx3cr1^-/- mice the mRNA levels of Nrf2 and its related genes were significantly decreased, establishing a direct linking between both systems. To determine functional relevance of CX3CR1, migration and phagocytosis assays were evaluated. CX3CR1-deficient microglia showed impaired cell migration and deficiency of phagocytosis, as previously described for NRF2-deficient microglia, reinforcing the idea of the relevance of the CX3CL1/CX3CR1 axis in these events. The importance of these findings was evident in a tauopathy mouse model where the effects of sulforaphane (SFN), an NRF2 inducer, were examined on neuroinflammation in Cx3cr1^+/+ and Cx3cr1^-/- mice. Interestingly, the treatment with SFN was able to modulate astrogliosis but failed to reduce microgliosis in Cx3cr1^-/- mice. These findings suggest an essential role of the CX3CR1/NRF2 axis in microglial function and in tauopathies. Therefore, polymorphisms with loss of function in CX3CR1 or NRF2 have to be taken into account for the development of therapeutic strategies.

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CX3CR1-deficient primary microglial cells present impaired expression of the transcription factor NRF2 signature.

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TAM receptors expression is decreased in CX3CR1-deficient microglia.

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AXL receptor is a NRF2-dependent gene.

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Loss of CX3CR1 expression led to impaired phagocytosis and migration of microglia.

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Sulforaphane treatment did not reverse rAAV-TAU^P301L induced microgliosis in CX3CR1-deficient mice.

Does Axl have potential as a therapeutic target in pancreatic cancer?

INTRODUCTION

Pancreatic cancer is a leading cause of cancer-related death. Metastasis, therapy resistance, and immunosuppression are dominant characteristics of pancreatic tumors. Strategies that enhance the efficacy of standard of care and/or immune therapy are likely the most efficient route to improve overall survival in this disease. Areas covered: Axl, a member of the TAM (Tyro3, Axl, MerTK) family of receptor tyrosine kinases, is involved in cell plasticity, chemoresistance, immune suppression, and metastasis in various cancers, including pancreatic cancer. This review provides an overview of Axl and its function in normal conditions, summarizes the regulation and function of Axl in cancer, and highlights the contribution of Axl to pancreatic cancer as well as its potential as a therapeutic target. Expert opinion: Axl is an attractive therapeutic target in pancreatic cancer because it contributes to many of the roadblocks that hamper therapeutic efficacy. Clinical evidence supporting Axl inhibition in pancreatic cancer is currently limited; however, multiple clinical trials have been initiated or are in the planning phase to test the effect of inhibiting Axl in conjunction with standard therapy in pancreatic cancer patients. We anticipate that these studies will provide robust validation of Axl as a therapeutic target in pancreatic cancer.

ZNF224 is a transcriptional repressor of AXL in chronic myeloid leukemia cells

ZNF224 is a KRAB-zinc finger transcription factor that exerts a key tumor suppressive role in chronic myelogenous leukemia. In this study, we identify the receptor tyrosine kinase Axl as a novel target of ZNF224 transcriptional repression activity. Axl overexpression is found in many types of cancer and is frequently associated with drug resistance. Interestingly, we also found that sensitivity to imatinib can be partly restored in imatinib-resistant chronic myelogenous leukemia cells by ZNF224 overexpression and the resulting suppression of Axl expression. These results, in accordance with our previous findings, support the role of ZNF224 in imatinib responsiveness and shed new insights into potential therapeutic use of ZNF224 in imatinib-resistant chronic myelogenous leukemia.

Site specific hypermethylation of CpGs in Connexin genes 30, 26 and 43 in different grades of glioma and attenuated levels of their mRNAs

AIM

Gliomas, the intracranial tumours are considered the deadliest malignancies. The gap junctional Connexins (Cxs) that maintain cellular homeostasis perform a unique function in glial tumour suppression. However, the differential methylation patterns of Cxs were not revealed in glioma so far. The current study attempts to categorise promoter methylation of Cx30 and Cx26 and intron methylation of Cx43 in different grades of human glioma.

MATERIALS AND METHODS

About 85 glioma patients with pathologically confirmed grades and 15 control brain tissues were recruited in the study. Bisulphite-PCR-Single Stranded Conformation analysis(SSCA), Bisulphite sequencing and MeDIP-qPCR were carried out to assess methylation status and Cx mRNA levels were also analysed to evaluate the effect of methylation.

RESULTS

We found that promoter CpG islands(CpGs) reside in Sp1 and Ap2 sites of Cx30 and 26 were hypermethylated in high grades (HG) of glioma rather than low grades. The input % of both was significantly increased (p < 0.03) in progressive grades. Interestingly, Cx43 could exhibit a significant increase (p < 0.05) in input % only in grade IV. While, Cx30 and 26 mRNAs were downregulated according to their methylation status in progressive fashion with grades, Cx43 was downregulated irrespective of intron methylation.

CONCLUSION

Thus, we suggest that the sites and extent of methylation of Cxs (30 and 26 but not in 43) are found to be altered. In different grades of glioma can provide better appreciation of the grade of the patient and might help in strategies based on epigenetic approaches.

Association between AXL promoter methylation and lung function growth during adolescence

AXL is one of the TAM (TYRO3, AXL and MERTK) receptor tyrosine kinases and may be involved in airway inflammation. Little is known about how epigenetic changes in AXL may affect lung development during adolescence. We investigated the association between AXL DNA methylation at birth and lung function growth from 10 to 18 years of age in 923 subjects from the Children's Health Study (CHS). DNA methylation from newborn bloodspots was measured at multiple CpG loci across the regulatory regions of AXL using Pyrosequencing. Linear spline mixed-effects models were fitted to assess the association between DNA methylation and 8-year lung function growth. Findings were evaluated for replication in a separate population of 237 CHS subjects using methylation data from the Illumina HumanMethylation450 (HM450) array when possible. A 5% higher average methylation level of the AXL promoter region at birth was associated with a 48.4 ml decrease in mean FEV1 growth from 10 to 18 years of age in the primary study population (95% CI: -100.2, 3.4), and a 53.9 ml decrease in mean FEV1 growth from 11 to 15 years of age in the replication population (95% CI: -104.3, -3.5). One CpG locus in the promoter region, cg10564498, was significantly associated with decreased growth in FEV1, FVC and MMEF from 10 to 18 years of age and the negative associations were observed in a similar age range in the replication population. These findings suggest a potential association between AXL promoter methylation at birth and lower lung function growth during adolescence.

Self-reported prenatal tobacco smoke exposure, AXL gene-body methylation, and childhood asthma phenotypes

BACKGROUND

Epigenetic modifications, including DNA methylation, act as one potential mechanism underlying the detrimental effects associated with prenatal tobacco smoke (PTS) exposure. Methylation in a gene called AXL was previously reported to differ in response to PTS.

METHODS

We investigated the association between PTS and epigenetic changes in AXL and how this was related to childhood asthma phenotypes. We tested the association between PTS and DNA methylation at multiple CpG loci of AXL at birth using Pyrosequencing in two separate study populations, the Children's Health Study (CHS, n = 799) and the Newborn Epigenetic Study (NEST, n = 592). Plasma cotinine concentration was used to validate findings with self-reported smoking status. The inter-relationships among AXL mRNA and miR-199a1 expression, PTS, and AXL methylation were examined. Lastly, we evaluated the joint effects of AXL methylation and PTS on the risk of asthma and related symptoms at age 10 years old.

RESULTS

PTS was associated with higher methylation level in the AXL gene body in both CHS and NEST subjects. In the pooled analysis, exposed subjects had a 0.51% higher methylation level in this region compared to unexposed subjects (95% CI 0.29, 0.74; p < 0.0001). PTS was also associated with 21.2% lower expression of miR-199a1 (95% CI - 37.9, - 0.1; p = 0.05), a microRNA known to regulate AXL expression. Furthermore, the combination of higher AXL methylation and PTS exposure at birth increased the risk of recent episodes of bronchitic symptoms in childhood.

CONCLUSIONS

PTS was associated with methylation level of AXL and the combination altered the risk of childhood bronchitic symptoms.

Targeted inhibition of Axl receptor tyrosine kinase ameliorates anti-GBM-induced lupus-like nephritis

Glomerulonephritis (GN) is a typical lesion in autoantibody and immune complex disorders, including SLE. Because the Gas6/Axl pathway has been implicated in the pathogenesis of many types of GN, targeting this pathway might ameliorate GN. Consequently, we have studied the efficacy and mechanism of R428, a potent selective Axl inhibitor, in the prevention of experimental anti-GBM nephritis. Axl upregulation was investigated with Sp1/3 siRNA in the SV40-transformed mesangial cells. For Axl inhibition, a daily dose of R428 (125 mg/kg) or vehicle was administered orally. GN was induced with anti-GBM sera. Renal disease development was followed by serial blood urine nitrogen (BUN) determinations and by evaluation of kidney histology at the time of sacrifice. Axl-associated signaling proteins were analyzed by Western blotting and inflammatory cytokine secretion was analyzed by Proteome array. SiRNA data revealed the transcription factor Sp1 to be an important regulator of mesangial Axl expression. Anti-GBM serum induced severe nephritis with azotemia, protein casts and necrotic cell death. R428 treatment diminished renal Axl expression and improved kidney function, with significantly decreased BUN and glomerular proliferation. R428 treatment inhibited Axl and significantly decreased Akt phosphorylation and renal inflammatory cytokine and chemokine expression; similar effects were observed in anti-GBM antiserum-treated Axl-KO mice. These studies support a role for Axl inhibition in glomerulonephritis.

Baicalein inhibits growth of Epstein-Barr virus-positive nasopharyngeal carcinoma by repressing the activity of EBNA1 Q-promoter

Epstein-Barr virus (EBV) can establish a life-long latent infection in the host and is associated with various human malignancies, including nasopharyngeal carcinoma (NPC), the most common cancer originated from nasopharynx. EBV nuclear antigen 1 (EBNA1) is the only viral protein absolutely demanded for segregation, replication, transcription and maintenance of EBV viral genome in host cells. Baicalein, a bioactive flavonoid compound purified from the root of Scutellariae baicaleinsis, displays anti-inflammatory, immunosuppressive, and anti-tumor properties. In this study, the therapeutic effects and functional mechanism of baicalein on EBV-positive human NPC were determined. Cell Counting Kit-8 assays and cell formation colony were performed to investigate that baicalein can suppress proliferation of EBV-infected human NPC cells. Flow cytometric and hoechst 33258 staining results indicated that baicalein induced cell cycle arrest and apoptosis. Western blotting results demonstrated that baicalein down-regulates EBNA1 expression but not reduces the stability and half-life of EBNA1 in EBV-infected NPC cells. Additionally, the mRNA level of EBNA1 was examined by real time-PCR, the activity of EBNA1 Q promoter (Qp) was determined by dual luciferase reporter assay. Considering that transcription factor specificity protein 1 (Sp1) can maintain EBNA1 Qp active. Further analyses also elucidated that baicalein inhibits the expression of Sp1 while knock-down Sp1 by specific shRNAs decreases the expression and transcription levels of EBNA1. Therefore, the results suggested that baicalein may decrease EBNA1 expression level in EBV-positive NPC cells via inhibiting the activity of EBNA1 Q-promoter while over-expression of EBNA1 attenuate the inhibitory effect of baicalein. Finally, it was found that baicalein may strongly reduce growth of tumor in the mouse xenograft model of EBV-positive NPC. These results indicated that baicalein inhibits growth of EBV-positive NPC by repressing the activity of EBNA1 Q-promoter. Baicalein may be used as a therapeutic agent to treat EBV-positive NPC.

Discovery and characterization of Isofistularin-3, a marine brominated alkaloid, as a new DNA demethylating agent inducing cell cycle arrest and sensitization to TRAIL in cancer cells

We characterized the brominated alkaloid Isofistularin-3 (Iso-3), from the marine sponge Aplysina aerophoba, as a new DNA methyltransferase (DNMT)1 inhibitor. Docking analysis confirmed our in vitro DNMT inhibition data and revealed binding of Iso-3 within the DNA binding site of DNMT1. Subsequent increased expression of tumor suppressor gene aryl hydrocarbon receptor (AHR) could be correlated to decreased methylation of CpG sites within the essential Sp1 regulatory region of its promoter. Iso-3 induced growth arrest of cancer cells in G0/G1 concomitant with increased p21 and p27 expression and reduced cyclin E1, PCNA and c-myc levels. Reduced proliferation was accompanied by morphological changes typical of autophagy revealed by fluorescent and transmission electron microscopy and validated by LC3I-II conversion. Furthermore, Iso-3 strongly synergized with tumor-necrosis-factor related apoptosis inducing ligand (TRAIL) in RAJI [combination index (CI) = 0.22] and U-937 cells (CI = 0.21) and increased TRAIL-induced apoptosis via a mechanism involving reduction of survivin expression but not of Bcl-2 family proteins nor X-linked inhibitor of apoptosis protein (XIAP). Iso-3 treatment decreased FLIP_L expression and triggered activation of endoplasmatic reticulum (ER) stress with increased GRP78 expression, eventually inducing TRAIL receptor death receptor (DR)5 surface expression. Importantly, as a potential candidate for further anticancer drug development, Iso-3 reduced the viability, colony and in vivo tumor forming potential without affecting the viability of PBMCs from healthy donors or zebrafish development.

MACC1 is post-transcriptionally regulated by miR-218 in colorectal cancer

Metastasis is a multistep molecular network process, which is lethal for more than 90% of the cancer patients. Understanding the regulatory functions of metastasis-inducing molecules is in high demand for improved therapeutic cancer approaches. Thus, we studied the post-transcriptional regulation of the crucial carcinogenic and metastasis-mediating molecule metastasis associated in colon cancer 1 (MACC1). In silico analysis revealed MACC1 as a potential target of miR-218, a tumor suppressor miRNA. Expression of these two molecules inversely correlated in colorectal cancer (CRC) cell lines. In a cohort of CRC patient tissues (n = 59), miR-218 is significantly downregulated and MACC1 is upregulated compared with normal mucosa. Luciferase reporter assays with a construct of the MACC1-3′-UTR harboring either the wild type or the mutated miR-218 seed sequence confirmed the specificity of the targeting. miR-218 inhibited significantly MACC1 protein expression, and consistently, MACC1-mediated migration, invasion and colony formation in CRC cells. Anti-miR-218 enhanced the MACC1-mediated migration, invasion and colony formation. Similar findings were observed in the gastric cancer cell line MKN-45. Further, we performed methylation-specific PCR of the SLIT2 and SLIT3 promoter, where miR-218 is encoded in intronic regions. The SLIT2 and SLIT3 promoters are hypermethylated in CRC cell lines. miR-218 and SLIT2 expressions correlated positively. Methyltransferase inhibitor 5-Azacytidine induced miR-218 expression and inhibited the expression of its target MACC1. We also determined that MACC1 has alternative polyadenylation (APA) sites, which results in different lengths of 3′-UTR variants in a CRC cell line. Taken together, miR-218 is post-transcriptionally inhibiting the MACC1 expression and its metastasis-inducing abilities.

Key Roles of AXL and MER Receptor Tyrosine Kinases in Resistance to Multiple Anticancer Therapies

A major challenge in anticancer treatment is the pre-existence or emergence of resistance to therapy. AXL and MER are two members of the TAM (TYRO3-AXL-MER) family of receptor tyrosine kinases, which, when activated, can regulate tumor cell survival, proliferation, migration and invasion, angiogenesis, and tumor-host interactions. An increasing body of evidence strongly suggests that these receptors play major roles in resistance to targeted therapies and conventional cytotoxic agents. Multiple resistance mechanisms exist, including the direct and indirect crosstalk of AXL and MER with other receptors and the activation of feedback loops regulating AXL and MER expression and activity. These mechanisms may be innate, adaptive, or acquired. A principal role of AXL appears to be in sustaining a mesenchymal phenotype, itself a major mechanism of resistance to diverse anticancer therapies. Both AXL and MER play a role in the repression of the innate immune response which may also limit response to treatment. Small molecule and antibody inhibitors of AXL and MER have recently been described, and some of these have already entered clinical trials. The optimal design of treatment strategies to maximize the clinical benefit of these AXL and MER targeting agents are discussed in relation to the different cancer types and the types of resistance encountered. One of the major challenges to successful development of these therapies will be the application of robust predictive biomarkers for clear-cut patient stratification.

Decitabine-Vorinostat combination treatment in acute myeloid leukemia activates pathways with potential for novel triple therapy

Despite advancements in cancer therapeutics, acute myeloid leukemia patients over 60 years old have a 5-year survival rate of less than 8%. In an attempt to improve this, epigenetic modifying agents have been combined as therapies in clinical studies. In particular combinations with Decitabine and Vorinostat have had varying degrees of efficacy. This study therefore aimed to understand the underlying molecular mechanisms of these agents to identify potential rational epi-sensitized combinations. Combined Decitabine-Vorinostat treatment synergistically decreased cell proliferation, induced apoptosis, enhanced acetylation of histones and further decreased DNMT1 protein with HL-60 cells showing a greater sensitivity to the combined treatment than OCI-AML3. Combination therapy led to reprogramming of unique target genes including AXL, a receptor tyrosine kinase associated with cell survival and a poor prognosis in AML, which was significantly upregulated following treatment. Therefore targeting AXL following epi-sensitization with Decitabine and Vorinostat may be a suitable triple combination. To test this, cells were treated with a novel triple combination therapy including BGB324, an AXL specific inhibitor. Triple combination increased the sensitivity of OCI-AML3 cells to Decitabine and Vorinostat as shown through viability assays and significantly extended the survival of mice transplanted with pretreated OCI-AML3 cells, while bioluminescence imaging showed the decrease in disease burden following triple combination treatment. Further investigation is required to optimize this triple combination, however, these results suggest that AXL is a potential marker of response to Decitabine-Vorinostat combination treatment and offers a new avenue of epigenetic combination therapies for acute myeloid leukemia.

A Scalable Epitope Tagging Approach for High Throughput ChIP-Seq Analysis

Eukaryotic transcriptional factors (TFs) typically recognize short genomic sequences alone or together with other proteins to modulate gene expression. Mapping of TF-DNA interactions in the genome is crucial for understanding the gene regulatory programs in cells. While chromatin immunoprecipitation followed by sequencing (ChIP-Seq) is commonly used for this purpose, its application is severely limited by the availability of suitable antibodies for TFs. To overcome this limitation, we developed an efficient and scalable strategy named cmChIP-Seq that combines the clustered regularly interspaced short palindromic repeats (CRISPR) technology with microhomology mediated end joining (MMEJ) to genetically engineer a TF with an epitope tag. We demonstrated the utility of this tool by applying it to four TFs in a human colorectal cancer cell line. The highly scalable procedure makes this strategy ideal for ChIP-Seq analysis of TFs in diverse species and cell types.

Identification of potential target genes associated with the effect of propranolol on angiosarcoma via microarray analysis

The purpose of the present study was to explore the effect of propranolol on angiosarcoma, and the potential target genes involved in the processes of proliferation and differentiation of angiosarcoma tumor cells. The mRNA expression profile (GSE42534) was downloaded from the Gene Expressed Omnibus database, including three samples without propranolol treatment (control), three samples with propranolol treatment for 4 h and three samples with propranolol treatment for 24 h. The differentially expressed genes (DEGs) in angiosarcoma tumor cells with or without propranolol treatment were obtained via the limma package of R and designated DEGs-4 h and DEGs-24 h. The DEGs-24 h group was divided into two sets. Set 1 contained the DEGs also contained in the DEGs-4 h group. Set 2 contained the remainder of the DEGs. Functional and pathway enrichment analysis of sets 1 and 2 was performed. The protein-protein interaction (PPI) networks of sets 1 and 2 were constructed, termed PPI 1 and PPI 2, and visualized using Cytoscape software. Modules of the two PPI networks were analyzed, and their topological structures were simulated using the tYNA platform. A total of 543 and 2,025 DEGs were identified in angiosarcoma tumor cells treated with propranolol for 4 and 24 h, respectively, compared with the control group. A total of 401 DEGs were involved in DEGs-4 h and DEGs-24 h, including metallothionein 1, heme oxygenase 1, WW domain-binding protein 2 and sequestosome 1. Certain significantly enriched gene ontology (GO) terms and pathways of sets 1 and 2 were identified, containing 28 overlapping GO terms. Furthermore, 121 nodes and 700 associated pairs were involved in PPI 1, whereas 1,324 nodes and 11,839 associated pairs were involved in PPI 2. A total of 45 and 593 potential target genes were obtained according to the node degrees of PPI 1 and PPI 2. The results of the present study indicated that a number of potential target genes, including AXL receptor tyrosine kinase, coatomer subunit α, DR1-associated protein 1 and ERBB receptor feedback inhibitor 1 may be involved in the effect of propranolol on angiosarcoma.

G protein-coupled KISS1 receptor is overexpressed in triple negative breast cancer and promotes drug resistance

Triple-negative breast cancer (TNBC) lacks the expression of estrogen receptor α, progesterone receptor and human epidermal growth factor receptor 2 (HER2). TNBC patients lack targeted therapies, as they fail to respond to endocrine and anti-HER2 therapy. Prognosis for this aggressive cancer subtype is poor and survival is limited due to the development of resistance to available chemotherapies and resultant metastases. The mechanisms regulating tumor resistance are poorly understood. Here we demonstrate that the G protein-coupled kisspeptin receptor (KISS1R) promotes drug resistance in TNBC cells. KISS1R binds kisspeptins, peptide products of the KISS1 gene and in numerous cancers, this signaling pathway plays anti-metastatic roles. However, in TNBC, KISS1R promotes tumor invasion. We show that KISS1 and KISS1R mRNA and KISS1R protein are upregulated in TNBC tumors, compared to normal breast tissue. KISS1R signaling promotes drug resistance by increasing the expression of efflux drug transporter, breast cancer resistance protein (BCRP) and by inducing the activity and transcription of the receptor tyrosine kinase, AXL. BCRP and AXL transcripts are elevated in TNBC tumors, compared to normal breast, and TNBC tumors expressing KISS1R also express AXL and BCRP. Thus, KISS1R represents a potentially novel therapeutic target to restore drug sensitivity in TNBC patients.

Giving AXL the axe: targeting AXL in human malignancy

The receptor tyrosine kinase AXL, activated by a complex interaction between its ligand growth arrest-specific protein 6 and phosphatidylserine, regulates various vital cellular processes, including proliferation, survival, motility, and immunologic response. Although not implicated as an oncogenic driver itself, AXL, a member of the TYRO3, AXL, and MERTK family of receptor tyrosine kinases, is overexpressed in several haematologic and solid malignancies, including acute myeloid leukaemia, non-small cell lung cancer, gastric and colorectal adenocarcinomas, and breast and prostate cancers. In the context of malignancy, evidence suggests that AXL overexpression drives wide-ranging processes, including epithelial to mesenchymal transition, tumour angiogenesis, resistance to chemotherapeutic and targeted agents, and decreased antitumor immune response. As a result, AXL is an attractive candidate not only as a prognostic biomarker in malignancy but also as a target for anticancer therapies. Several AXL inhibitors are currently in preclinical and clinical development. This article reviews the structure, regulation, and function of AXL; the role of AXL in the tumour microenvironment; the development of AXL as a therapeutic target; and areas of ongoing and future investigation.

The Receptor Tyrosine Kinase AXL in Cancer Progression

The AXL receptor tyrosine kinase (AXL) has emerged as a promising therapeutic target for cancer therapy. Recent studies have revealed a central role of AXL signaling in tumor proliferation, survival, stem cell phenotype, metastasis, and resistance to cancer therapy. Moreover, AXL is expressed within cellular components of the tumor microenvironment where AXL signaling contributes to the immunosuppressive and protumorigenic phenotypes. A variety of AXL inhibitors have been developed and are efficacious in preclinical studies. These agents offer new opportunities for therapeutic intervention in the prevention and treatment of advanced disease. Here we review the literature that has illuminated the cellular and molecular mechanisms by which AXL signaling promotes tumor progression and we will discuss the therapeutic potential of AXL inhibition for cancer therapy.

Significance of microRNA 21 in gastric cancer

Despite promising developments of treatment, the mortality due to gastric cancer remains high and the mechanisms of gastric cancer initiation and the development also remains elusive. It has been reported that patients with positive serologic tests for H. pylori have a higher risk of the development of gastric cancer. microRNAs (miRNAs) are short non-coding RNA molecules consisting of 21-25 nucleotides (nt) in length. The miRNAs silence their cognate target genes by inhibiting mRNA translation or degrading the mRNA molecules by binding to their 3'-untranslated (UTR) regions and plays a very important role in cancer biology. Recent evidences indicate that miR-21 is overexpressed in tumour tissue, including gastric cancer and plays a vital role in tumour cell proliferation, apoptosis, invasion and angiogenesis. Elevated levels of miR-21 is associated with downregulation of tumour suppressor genes, such as programmed cell death 4 (PDCD4), tissue inhibitor of metalloproteinase 3, phosphatase and tensin homolog (PTEN), tropomyosin 1, ras homolog gene family member B, and maspin. Silencing of miR-21 through the use of a miR-21 inhibitor affected cancer cell viability, induced cell cycle arrest and increased chemosensitivity to anticancer agents indicating that miR-21 functions as an oncogene. Although an increased expression level of miR-21 has been observed in gastric cancer, studies related to the role of miR-21 in gastric cancer progression is very limited. The main thrust of this mini review is to explain the potency of miR-21 as a prognostic and/or diagnostic biomarker and as a new target for clinical therapeutic for interventions of gastric cancer progression.

ALK inhibitor resistance in ALK(F1174L)-driven neuroblastoma is associated with AXL activation and induction of EMT

The crizotinib-resistant ALK(F1174L) mutation arises de novo in neuroblastoma (NB) and is acquired in ALK translocation-driven cancers, lending impetus to the development of novel anaplastic lymphoma kinase (ALK) inhibitors with different modes of action. The diaminopyrimidine TAE684 and its derivative ceritinib (LDK378), which are structurally distinct from crizotinib, are active against NB cells expressing ALK(F1174L). Here we demonstrate acquired resistance to TAE684 and LDK378 in ALK(F1174L)-driven human NB cells that is linked to overexpression and activation of the AXL tyrosine kinase and epithelial-to-mesenchymal transition (EMT). AXL phosphorylation conferred TAE684 resistance to NB cells through upregulated extracellular signal-regulated kinase (ERK) signaling. Inhibition of AXL partly rescued TAE684 resistance, resensitizing these cells to this compound. AXL activation in resistant cells was mediated through increased expression of the active form of its ligand, GAS6, that also served to stabilize the AXL protein. Although ectopic expression of AXL and TWIST2 individually in TAE684-sensitive parental cells led to the elevated expression of mesenchymal markers and invasive capacity, only AXL overexpression induced resistance to TAE684 as well. TAE684-resistant cells showed greater sensitivity to HSP90 inhibition than did their parental counterparts, with downregulation of AXL and AXL-mediated ERK signaling. Our studies indicate that aberrant AXL signaling and development of an EMT phenotype underlie resistance of ALK(F1174L)-driven NB cells to TAE684 and its derivatives. We suggest that the combination of ALK and AXL or HSP90 inhibitors be considered to delay the emergence of such resistance.

Gene of the month: Axl

The interaction between Axl receptor tyrosine kinase and its main ligand Gas6 has been implicated in the progression of a wide number of malignancies. More recently, overexpression of Axl has emerged as a key molecular determinant underlying the development of acquired resistance to targeted anticancer agents. The activation of Axl is overexpression-dependent and controls a number of hallmarks of cancer progression including proliferation, migration, resistance to apoptosis and survival through a complex network of intracellular second messengers. Axl has been noted to influence clinically meaningful end points including metastatic recurrence and survival in the vast majority of tumour types. With Axl inhibitors having gained momentum as novel anticancer therapies, we provide an overview of the biological and clinical relevance of this molecular pathway, outlining the main directions of research.

Molecular Pathways: AXL, a Membrane Receptor Mediator of Resistance to Therapy

AXL is a tyrosine kinase membrane receptor that signals via PI3K, MAPK, and protein kinase C (PKC), among other pathways. AXL has oncogenic potential and interacts with other membrane receptors, depending on their relative abundance and availability. The increased expression of AXL in cancer is often the result of pharmacologic selective pressure to a number of chemotherapies and targeted therapies and acts as a mechanism of acquired drug resistance. This resistance phenotype, frequently accompanied by epithelial-to-mesenchymal transition, can be reversed by AXL inhibition. In tumors with high levels of EGFR, including lung, head and neck, and triple-negative breast cancer, AXL dimerizes with this receptor and initiates signaling that circumvents the antitumor effects of anti-EGFR therapies. Likewise, AXL overexpression and dimerization with EGFR can overcome PI3K inhibition by activating the phospholipase C-γ-PKC cascade that, in turn, sustains mTORC1 activity. The causative role of AXL in inducing drug resistance is underscored by the fact that the suppression of AXL restores sensitivity to these agents. Hence, these observations indicate that AXL is selectively expressed in tumor cells refractory to therapy and that cotargeting AXL in this setting would potentially overcome drug resistance. The use of AXL inhibitors should be considered in the clinic.

AXL kinase as a novel target for cancer therapy

The AXL receptor tyrosine kinase and its major ligand, GAS6 have been demonstrated to be overexpressed and activated in many human cancers (such as lung, breast, and pancreatic cancer) and have been correlated with poor prognosis, promotion of increased invasiveness/metastasis, the EMT phenotype and drug resistance. Targeting AXL in different model systems with specific small molecule kinase inhibitors or antibodies alone or in combination with other drugs can lead to inactivation of AXL-mediated signaling pathways and can lead to regained drug sensitivity and improved therapeutic efficacy, defining AXL as a promising novel target for cancer therapeutics. This review highlights the data supporting AXL as a novel treatment candidate in a variety of cancers as well as the current status of drug development targeting the AXL/GAS6 axis and future perspectives in this emerging field.

Racial variation in breast tumor promoter methylation in the Carolina Breast Cancer Study

BACKGROUND

African American (AA) women are diagnosed with more advanced breast cancers and have worse survival than white women, but a comprehensive understanding of the basis for this disparity remains unclear. Analysis of DNA methylation, an epigenetic mechanism that can regulate gene expression, could help to explain racial differences in breast tumor clinical biology and outcomes.

METHODS

DNA methylation was evaluated at 1,287 CpGs in the promoters of cancer-related genes in 517 breast tumors of AA (n = 216) or non-AA (n = 301) cases in the Carolina Breast Cancer Study (CBCS).

RESULTS

Multivariable linear regression analysis of all tumors, controlling for age, menopausal status, stage, intrinsic subtype, and multiple comparisons [false discovery rate (FDR)], identified seven CpG probes that showed significant (adjusted P < 0.05) differential methylation between AAs and non-AAs. Stratified analyses detected an additional four CpG probes differing by race within hormone receptor-negative (HR(-)) tumors. Genes differentially methylated by race included DSC2, KCNK4, GSTM1, AXL, DNAJC15, HBII-52, TUSC3, and TES; the methylation state of several of these genes may be associated with worse survival in AAs. TCGA breast tumor data confirmed the differential methylation by race and negative correlations with expression for most of these genes. Several loci also showed racial differences in methylation in peripheral blood leukocytes (PBL) from CBCS cases, indicating that these variations were not necessarily tumor-specific.

CONCLUSIONS

Racial differences in the methylation of cancer-related genes are detectable in both tumors and PBLs from breast cancer cases.

IMPACT

Epigenetic variation could contribute to differences in breast tumor development and outcomes between AAs and non-AAs.