Table 3.

Important roles of CXCL chemokines and CXCR1/2 in cancer

Cancer type	Summary of findings
Lung Cancer (NSCLC)	IL-1β stimulated more CXC chemokine secretion in A549 cells than in human tracheobronchial epithelium cells via CREB and NF-κB activation [229]. Lewis lung carcinoma (LLC) cells transduced with human IL-1β exhibited increased tumor growth, which was inhibited by CXCR2 antibodies [270]. CXCL8 stimulated epithelial cell proliferation (A549 and NCI-H292) via EGFR trans-activation involving the MAPK pathways [271]. CXCL8 stimulated H460 and MOR/P (NSCLC cell lines) cell proliferation via CXCR1 but not CXCR2 [272]. CXCR2-/- mice implanted with LLC primary tumors in heterotopic and orthotopic models showed reduced tumor growth and vascular density as well as reduced spontaneous metastases [273]. Inhibition of CXCR2 with antibodies impeded the progression of premalignant alveolar lesions in mice with KRAS mutations known to develop lung adenocarcinoma [274]. Inhibition of CXCR2 with AZ10397767 reduced neutrophil infiltration in A549 tumor spheroids and primary tumors in mice [275]. CXCR2 antibodies inhibited SNAIL-mediated tumor burden in orthotopic and heterotopic lung cancer mouse models [276]. Depletion of CXCR2 via shRNA knockdown in a highly metastatic murine adenocarcinoma cell line with Kras/p53 mutant reduced tumor invasion and metastasis in in vitro and in vivo orthotopic syngeneic mouse models [277]. In lung adenocarcinoma, CXCR2 was a poor prognostic marker and promoted invasion and metastasis of tumors [277]. CXCL5 was one of the main drivers of the CXCL8-CXCR2 ligand axis in adenocarcinomas as it was the most upregulated gene in that cluster [277]. Single nucleotide polymorphisms (SNPs) in CXCR2 were associated with CXCR2 expression, signaling and susceptibility to lung cancer [278].
Colorectal Cancer	CXCL8 and CXCR2 were upregulated in colorectal tumor samples (n=8) [230]. CXCL8, CXCR1, and CXCR2 expression were higher in metastatic colon cancer cell lines (KM12C and KM12L4) than in Caco-2 cells. CXCL8 also induced cell proliferation which was attenuated by neutralizing antibodies to CXCR1/2 or CXCL8 [279]. CXCL1 expression was higher in primary colon adenocarcinoma than in normal colon epithelium. Inhibition of CXCL1 by siRNA reduced proliferation and increased apoptosis [231]. Primary colorectal cancer samples expressed CXCL1 and its expression was associated with tumor size and stage, metastasis, and patient survival; colon cancer cell lines also express CXCR2, and CXCL1 stimulation increased their invasiveness [280]. Over-expression of CXCL8 via stable transfection in human colon cancer cells (HCT116 and Caco-2) enhanced cell proliferation, migration, invasion, and resistance to oxaliplatin. CXCL8 over-expressing cells also formed larger tumors with increased microvessel density in xenograft models [281]. Single nucleotide polymorphisms (SNPs) in CXCL8, CXCR1, and CXCR2 were associated with colon and rectal cancer risk [282]. Immunodeficient mice expressing human CXCL8 on the skin had enhanced human and mouse colon cancer tumor growth, angiogenesis, and metastases to the lung and liver. Conversely, CXCR2 knockout mice exhibited reduced tumor growth and angiogenesis, and increased necrosis [283]. CXCL8 was an independent prognostic marker for colon cancer. CXCL8 expression was upregulated in colon cancer and its level was increased with disease progression and metastasis [284]. CXCL8 expression significantly correlated with expression of αvβ6 integrin. The CXCL8-CXCR1/2 axis enhanced migration of colorectal carcinoma cells by increasing αvβ6 integrin expression [285]. SCH527123 inhibited human colon cancer liver metastases in a mouse xenograft model; however, it had no effect on tumor growth [248]. SCH527123 inhibited colon cancer cell (HCT116 and Caco-2) proliferation, migration, and invasion, and increased apoptosis. It also reduced tumor growth and angiogenesis as well as improved oxaliplatin treatment in mice xenograft studies [247]. The CXCL8-CXCR2 axis played an important role in chemoresistance of HCT116 cells [286]. CXCL2-CXCR2 axis helps in the recruitment of tumor-associated neutrophils and thus, regulated colitis-associated colon cancer in mice [287].
Breast Cancer	Increased copy numbers of CXCL1/2 genes contributed to higher expression of CXCL1/2 in invasive breast tumors. CXCL1/2 participated in a paracrine loop involving the tumor microenvironment and cancer cells to enhance chemoresistance and metastasis in breast tumors [232]. Thrombin stimulated CXCL1 expression and secretion in tumor and endothelial cells. Antibodies against CXCL1 inhibited thrombin-induced angiogenesis (endothelial tube formation). Depletion of CXCL1 via shRNA in 4T1 cells reduced tumor growth, angiogenesis, and metastasis [288]. CXCL7 and CXCR2 expression were higher in malignant (MCF10CA1a.c11) than in premalignant (MCF10AT) cells. Premalignant cells transfected with CXCL7 showed increased invasiveness, which was attenuated by a CXCL7 antibody [233]. Activation of the fibroblast growth factor receptor (FGFR) in epithelial breast cancer cells led to downregulation of the TGFβ/SMAD3 pathways in tumor-associated macrophages, which is associated with increased expression of CXCL chemokines. These chemokines also stimulated breast epithelial cancer cell invasiveness which was inhibited by SB225002 (CXCR2 inhibitor) [289]. Mesenchymal stem cells produced CXCL1 and CXCL5 and recruited mammary cancer cells, facilitating bone metastasis. This process was inhibited by antibodies against CXCL1, CXCL5, and CXCR2 as well as by SB265610 [290]. CXCR2 knockdown via shRNA in metastatic murine mammary tumor cell lines (C166, 4T1) reduced cell invasion, but did not alter cell proliferation. Implantation of these cells into an orthotopic mouse model showed that CXCR2 knockdown reduced spontaneous lung metastasis by 40% compared to control. These shRNA knockdown cells also enhanced cytotoxicity of doxorubicin and paclitaxel in in vitro and in vivo mice models [256, 291]. CXCR1 blockade with CXCR1 antibodies or reparixin depleted breast cancer stem cells in HCC1954, MDA-MB-453 and MDA-MB-231 cell lines. Reparaxin also retarded tumor growth and metastasis in xenograft studies [292, 293]. CXCL8 induced activation of EGFR/HER2 signaling pathways mediated by SRC, PI3K, and MEK in breast cancer stem cells from metastatic and invasive breast cancers derived from human patients. CXCL8 also enhanced colony formation ability of these cells. Inhibition of CXCR1/2 with SCH563705 inhibited colony formation and improved the efficacy of lapatinib (tyrosine kinase inhibitor) [255]. CXCL8 levels were increased in breast cancer patients compared to healthy volunteers and the level was associated with the stage of the disease [294]. CXCL8 levels were significantly upregulated in breast cancer patients having bone metastasis compared with patients lacking bone metastasis. There was also a signicant correlation between plasma CXCL8 levels and bone resorption in breast cancer patients [295]. Higher expressions of CXCR2 ligands CXCL1, CXCL3, CXCL5 and CXCL7 were observered in drug resistance breast cancer cells which exhibited delayed tumor growth, but higher metastatic potential in mouse xenograft model [296].
Prostate Cancer	Oxaliplatin increased NF-κB activity and the transcription of CXCL1, CXCL8, and CXCR2. CXCR2 antagonist AZ10397767 inhibited oxaliplatin-induced NF-κB activity and increased oxaliplatin-induced apoptosis in androgen-independent prostate cancer resistant to chemotherapy [249]. 5-FU increased CXCL8 secretion and CXCR1 and CXCR2 gene expression in PC3 cells. AZ10397767 increased 5-FU cytotoxicity and apoptosis [250]. TRAMP (tumor adenocarcinoma of the mouse prostate)/CXCR2-/- mice were smaller than CXCR2 wild-type mice and had reduced angiogenesis [129]. CXCL1 and CXCL8 increased PC3 invasion and adhesion to laminin, while CXCR2 antibodies inhibited CXCL8-induced cell invasion [297]. High-producing and low-producing CXCL8 clones of PC3 cells were isolated and injected into the prostate of nude mice. Tumors with high-producing CXCL8 showed increased growth, vascularization, and lymph node metastasis compared to low-producing CXCL8 tumors [234]. Hypoxia induced CXCL8, CXCR1, and CXCR2 expression in PC3 cells via HIF-1 and NF-κB transcriptional activity. CXCR1/2 siRNA enhanced etoposide-induced cell death in hypoxic PC3 cells [252]. CXCR2 inhibition with AZ10397767 and NF-κB inhibition with BAY11-7082 enhanced ansamycin cytotoxicity in PC3 cells but not DU145 cells [253]. CXCL8 upregulated cFLIP (caspase 8 inhibitor) expression and pretreatment with AZ10397767 inhibited CXCL8-induced cFLIP expression in LnCAP and PC3 cells. It also sensitized PC3 cells to TRAIL treatment (TRAIL induce CXCL8 expression in PC3 and LnCAP cells) [254]. PTEN repression via siRNA and shRNA increased CXCL8, CXCR1 and CXCR2 expression in PCa cells. CXCL8 depletion via siRNA decreased cell viability in PTEN deficient cells through G1 cell cycle arrest and apoptosis [298]. Tumor-derived CXCL8 enhanced secretion of cytokines CCL2 and CXCL12 from stromal cells and augmented proliferation and invasion of PTEN deficient prostate cancer cells [142]. CXCL8 serum level was correlated with increasing grade of metastatic prostate cancer compared to healthy volunteers [237]. CXCL8 induced cyclin D1 translation via Akt and activation of translational components in PC3 and DU145 cells [299]. The highly metastatic PC-3M-LN4 cells overexpress CXCL8 compared to PC-3P cells. Knockdown of PC-3M-LN4 cells with antisense CXCL8 cDNA reduced MMP-9 expression, collagenase activity, and invasion in vitro and in an in vivo orthotopic model. Conversely, upregulation of CXCL8 in PC-3P cells had the opposite effect [300]. CXCL1 and CXCL2, derived from bone marrow adipocytes, accelarated osteolysis and promoted metastasis of prostate cancer to bone [301].
Ovarian Cancer	CXCR2 shRNA knockdown in ovarian cancer cells (T29Gro-1, T29H, and SKOV3) inhibited tumor growth and arrested cells in G0/G1 phase by regulating cell cycle modulators. CXCR2 also induced apoptosis and angiogenesis. CXCR2 expression was correlated with poor overall survival for ovarian cancer [302]. Matrix metalloprotease-1 (MMP-1) activation of protease-activated receptor-1 (PAR1) induced CXCL8, CXCL1, and CCL1 secretion and stimulated endothelial cell proliferation, tube formation, and migration. These activities were attenuated by CXCR1/2 inhibition with X1/2pal-i3 (cell-penetrating pepducin that targets third intracellular loop of CXCR1/2), which also reduced tumor growth in mice and MMP-1-mediated angiogenesis [303]. CXCL8 suppressed TRAIL-medaited OVCAR3 apoptosis via downregulation of death receptors [304]. CXCL1 enhanced epithelial ovarian cancer cell (SKOV3 and OVCAR-3) growth and trans-activated EGFR via EGF release which involved the ERK1/2 signaling pathway [305]. Ovarian cancer patients with the A/A or A/T genotype for the CXCL8 T-251A gene polymorphism were less responsive to cyclophosphamide and bevacizumab treatment than patients with the T/T genotype. The A/A genotype was associated with increased CXCL8 production [306]. Paclitaxel induced CXCL8 promoter activation in ovarian cancer through the activation of both AP-1 and NF-κB. CXCL8 inhibition by antibodies stimulated tumor growth via recruitment of neutrophils to tumor site [307-309]. Stably CXCR2 transfected SKOV3 cells had a faster proliferation rate compared to cells transfected with empty vector. CXCR2 positive cells potentiated EGFR trans-activation, which led to AKT signaling regulated by NF-κB activation. As a result CXCR2 ligands CXCL1/2 were upregulated [310]. CXC chemokines and CXCR2 expression was elevated in sorafenib resistant ovarian tumors comapred to responsive tumors. CXCR2 inhibitor in combination with sorafenib exhibited synergistic inhibition of tumor cell growth [311].
Melanoma	Low tumorigenecity melanoma cell line A375P overexpressing CXCR1 or CXCR2 had enhanced in vivo tumor growth which was associated with increased microvessel density and reduced apoptosis [312]. CXCL8 serum level was associated with patient response to dacarbazine, cisplatin, and vindesine with or without DVP/IFN-2/IL-2 chemotherapy/immunochemotherapy [313]. Expression of CXCR2 and CXCL8 were correlated with melanoma tumor grade [314]. Melanoma cell lines secrete CXCL8 and express CXCR1 and CXCR2. CXCL8 stimulation of melanoma cells enhanced cell proliferation, migration, and invasion, which was reversed with inhibition of CXCR2 with neutralizing antibodies [315]. CXCL8 overexpression in A375P cells or CXCL8 knockdown in A375SM cells showed that CXCL8 regulated cell proliferation, migration, invasion, and colony formation. CXCL8 overexpression was associated with enhanced tumor growth and lung metastasis in vivo [235]. CXCR1 and/or CXCR2 knockdown in A375-SM cells via shRNA inhibited cell proliferation, migration, and invasion in vitro, reduced tumor growth and mircovessel density, and increased apoptosis in nude mice compared to control cells. Similar in vitro and in vivo results were obtained with CXCR1/2 inhibition with small molecule antagonists, SCH-479833 and SCH-527123 [316, 317].
Pancreatic cancer	Capan-1 cells expressed CXCL1, CXCL8, and CXCR2. CXCL1 and CXCL8 antibodies inhibited Capan-1 growth [318]. BxPC3 cells secreted CXCL3, CXCL5 and CXCL8. The supernatant from BxPC3 cells induced neovascularization in a corneal micropocket assay, which was impaired in the presence of CXCR2 antibodies. CXCL5 and CXCL8 were overexpressed in pancreatic cancer tissue samples [236]. ELR+ chemokines were elevated in exocrine pancreatic secretions from pancreatic cancer patients. Pancreatic cancer cell lines (BxPC3, Colo-357, and Panc-28) also expressed more ELR+ chemokines than normal pancreatic ductal epithelial cell line. Supernatants from pancreatic cancer cell lines stimulated HUVEC tube formation, which was attenuated by CXCR2 antibodies. These results were replicated in an orthotopic mouse model [319]. CXCR2 knockout mice with orthotopic and heterotopic pancreatic cancer tumors had impaired mobilization of bone marrow derived endothelial progenitor cells associated with reduced tumor angiogenesis and tumor growth [246]. K-Ras4BG12V transformed human pancreatic duct epithelial cells show enhanced secretion of CXC chemokines and VEGF via MEK1/2 and c-Jun pathways. When these cells were co-cultured with HUVECs, they enhanced HUVEC tube formation and invasiveness which was inhibited by CXCR2 antagonist, SB225002, or VEGF antibody [320]. CXCL5 expression correlated with clinical stage and shorter patient survival in pancreatic cancer. CXCL5 siRNA knockdown inhibited tumor growth in pancreatic cancer xenograft mouse model [238]. CXCR1 expression was positively correlated with lymph node metastasis and poor survival rate in patients with pancreatic ductal adenocarcinoma (PDAC) [321]. CXCR2 formed a macromolecular complex with NHERF1 and PLCβ3 in pancreatic cancer cells. The CXCR2-NHERF1-PLCβ3 complex regulated CXCR2 signaling activity and played important role in tumor progression and invasion [322]. CXCR2 expression is upregulated in human pancreactic ductal adenocarinoma, particularly in neutrophils/myeloid derived suppressor cells and assocoiated with tumorigenesis, metastasis as well as poor prognosis and survival. CXCR2 inhibition significantly enhanced sensitivity to anti-PD1 therapy and improved survival time in mice bearing pancreatic tumors [323-325]. Increased expression of CXCR2 and its ligand was observed in KRAS(G12D) mutation-bearing PDAC cells. Knocking down of CXCR2 or CXCR2 antagonists selectively inhibited in vitro and in vivo tumor cell proliferation as well as altered KRAS protein levels [326].
Liver cancer	CXCL8 along with CXCR1/2 receptors played important role in invasion, angiogenesis and metastasis of different solid tumors including liver cancer [327]. CXCL5 and CXCL8, both bind CXCR2, were significantly overexpressed in liver cancer cells, HCCLM3, with high movement capacity as compared with HepG2 cells with low movement capacity or normal liver L02 cells [328]. Treatment with siRNAs against CXCL5 suppressed tumor growth, proliferation, migration and invasion in liver cancer [329].
Bladder cancer	CXCL5 along with CXCR2 promoted migration and invasion of tumor cells in bladder cancer [330]. CXCL8 expression was increased significantly in monomethylarsenous acid [MMA(III)]-induced malignant transformation of urothelial (UROtsa) cells. Internalization of CXCR1 was increased in those malignant cells [331].
Other cancers	CXCR2 expression is significantly correlated with high grade, advanced stage metastasis as well as shorter overall survival in patients with renal cell carcinoma. Immunohistochemical analysis using CXCR2 could be a positive prognostic marker for renal cell carcinoma [332]. CXCR2-CXCL2 interaction in the tumor microenvironment plays an important role in tumor progression and metastasis in hepatocellular carcinoma (HCC) [333]. CXCR2-CXCL1-axis regulated infiltration of neutrophils, which were enriched in the peripheral stromal cells and associated with reduced recurrence free as well as overall survival of HCC patients [334]. CXCR2 signaling is strongly activated in glioblastoma microenvironment and responsible for tumor neovascularization and metastasis as well as tumor recurrence. Blocking the CXCR2 signaling by anti-CXCR2 antibody or CXCR2 inhibitor suppressed glioma growth and cell migration [335, 336]. CXCR2 is a poor prognostic marker in gastric cancer patients [337]. IL-1β transactivated EGFR via the CXCL1-CXR2 axis by increasing CXCL1 expression in oral squamous cell carcinoma [338]. CXCR2 expression along with postoperative complications affect recurrence free as well as overall survival of patients with esophageal cancer [339]. CXCL8-CXCR1/2 axis plays important role in the head and neck squamous cell carcinoma (HNSCC). IL-8 siRNA inhibited proliferation of HNSCC cells [340].