Downregulating oncogenic receptor: From bench to clinic

In 1989-1991, George Zhu first postulate oncogenic receptor (Voice of America,1992,12:31;Curr Pharm Biotechnol,2013,14:859-863), as a result of his experiments in which steroid PML-RARa fusion receptor gene rearrangement within it(15;17) translocation in acute promyelocytic leukemia (APL), FGFR-1) receptor induced tumours of the breast glands in two aplastic anemia during the course of testosterone treatment (hormonal tumorigenesis). In the past 2-3 decades, there are at least 30 receptor genes involving in various of oncogenic process. These normal physiologic receptors linked to gene amplification, rearrangement, deletion and activating mutations, which converted receptor to oncogenic (also oncogenic receptor) in development, progressive and pathogenesis of benign and malignant diseases. Receptors included nuclear receptor family members (oncogenic estrogen/estrogen receptor alpha signaling, oncogenic androgen receptor, oncogenic receptor pml/RARa, and GRβ); Oncogenic growth factor receptors (EGF/oncogenic receptor EGFR, Neu oncogenic receptor, insulin receptor substrate 4 IRS4/insulin receptor/c-ros protooncogenic receptor, insulin and IGF-1 and -II/oncogenic IGF-1R, oncogenic PDGFAR, oncogenic TEL/PDGFRB, HGF/HGFR、met oncogenic receptor); Cytokine receptor rc family members (IL-2-BCM fusion,IL-3/oncogenic IL-3Ra,IL-7/oncogenic IL-7R and IL-21R-BCL6 fusion); Cytokine receptors including the βc family (G-CSF/oncogenic CSF3R,oncogenic EPOR), and oncogenic autocrine growth hormone/nuclear GHR and other VEGFR2. Upon ligand binding or external antigen stimuli, mutated growth factor receptors and mutant cytokine receptors including type I cytokine receptor may be activated via ligand binding receptor complex, receptor dimerizes especially receptor homodimerization, and induces transphosphorylation of tyrosine residues in the cytoplasmic domains which serves as docking sites for several adaptor molecular harboring SH2 domain or phosphotyrosine binding domain. These adaptor molecules recruit and activate downstream signaling molecules such as Ras/MAPK, phospholipase C-r,JAK-STAT molecules, NF-KB pathways through tyrosineor serine/threnonine-phosphorylation. Among them, Ras /MAPK/ERK, PI3-K/akt and STAT pathways act as the major oncogenic signaling pathway. Overall, these receptors coupled with their ligands are key importance in human subtle balance in physically regulating multiple cellular processes, for example, in cellular proliferation and differentiation. Oncogenic receptors mutational activation and/or aberrant gene rearrangement are promiscuously interference with normal cell survival, anti-apoptosis and proliferation, and malignant initiation and progression. Others, in a special APL case, oncogenic pml/RARa fusion behave as a potent (constitutive) transcriptional repressor of RAR and retinoic acid signaling, inducing a differentiation blockage at promyelocytic stage which can be overcome with therapeutic doses of 9-cis retinoic acid or ATRA ligand (see in detail model, George Zhu, January 1991. Curr Pharm Biotechnol, 2013, 14: 849-8E). This is the classical model of retinoic acid action. This encourage receptor normal agonist, and oncogenic receptor antagonists (or inhibitors) target therapy. Correspondence to: George Zhu, Institute of Oncology of George Zhu, 422407, Beijing, China, E-mail: sansan4240732@163.com


Introduction
Since introduction of oncogenic receptor concept by George Zhu from oncogenic pml/RARa fusion in etiology of a specific APL and androgen/androgen receptor oncogenic signaling in role of hormonal tumorigenesis in early 1989s [1][2][3][4][5][6][7], de The H and Chomienne C [8,9], at the same peroid, found that in t(15;17) APL the translocated retinoic acid receptor alpha (an RAR mutant) contribute to leukemogenesis. Neil JC [10] is in detail receptor-mediated leukaemogenesis from the oncogenic function of T cell antigen receptor (TCR oncogenic signal) on lymphoid cells not only bind external ligand but are crucial in cellular proliferation. In transgenic mice expressing a mutated TCRbeta lacking the variable chain (Detta V-TCRbeta) developed CD+, CD8+, IL-2Ra positive T-cell lymphoma [11]. Patients with ataxia-telanglectasia are particularly prone to development of T-cell chronic lymphocytic leukemia with chromosomal abnormalities. The breakpoint is composed of a TCR J alpha chain region (from 14q11) fused to sequences derived from 14q32 but on the centromeric side of the IgCmu (IgH), indicating that a 14:14 translocation [t(14;14) (q11;q32); inv(14)(q11;q32)] in the development of T-cell tumors [12][13]. Like mutated oncogenic growth factor receptors [14], antigen-independent B cell receptor (BCR) signaling drives the oncogenic process. The genetic defects in heavy-chain disease(HCD) result in the production of abnormal membrane-associated heavy chain lacking an antigen-binding domain, these aberrant B-cell antigen receptors might engage in ligand-independent signaling, indicating a role in the genesis of HCD neoplasia [14][15].
Yarden Y [16] proposes neu oncogenic receptor from neu oncogenerelated receptor tyrosine kinase (RTK), and this might be useful in neu oncogenic receptor [17][18][19][20][21] antagonists lapatinib and trastuzumab target therapy in metastatic breast cancer with HER2V659E mutation [22]. In a large trials of 48 HER2-positive early breast cancer patients, the adjuvant trastuzumab treatment demonstrates highly favorable outcome. Five-year overall survival rates and disease free survival rates were 95.8% and 93.8% respectively. Al-Nedawi [23] explore the area that microvesicles containing oncogenic receptor EGFRVIII released to cellular surroundings and blood of tumour-bearing mice, and can merge with the plasma membranes of cancer cells lacking EGFRVIII. This intercellular transfer of membrane-derived microvesicles ('oncosomes'-small plasma membrane buds, Robinson, [24]) leads to the transfer of oncogenic activity including activation of MAPK and Akt and autocrine activation of its key signaling receptor (VEGF receptor-2), and increases in anchorage-independent growth capacity. Santos [25] studies oncogenic GRPR (gastrin-releasing peptide receptor) in neoplastic multiple signaling pathway. Somatic mutations in cholecystokinin 2 receptor(CCK2R) alter receptor activity that promote oncogenic phenotype, and the importance of evaluating CCK2R inhibitors to block mutant forms of this receptor [26]. Intriguing, epithelial cell adhesion molecule (EpCAM), as a homophilic adhesion protein, is a novel oncogenic receptor which frequently overexpressed in epithelia, progenitors, embryonic stem cells, carcinoma and cancer-initiating cells, and target its antibodies [27]. The present will in brief focus on recent innovative diagnostic and therapeutic strategies forthcoming in this area.

Steroid hormone receptors oncogenic signaling
The estrogen receptor(ER) is found in a wide variety of species and is involved in the programming and regulation of gene expression in vertebrate female sex-accessory tissue [28]. Estrogen E2/ERa signaling plays an important role in the regulation of mammary gland development and function, and also contributes to the onset and progression of breast cancer. More than 70% of human breast cancers express ERa, and elevated levels of ERa in benign breast epithelium correlate with increased risk of breast cancer [29]. Green S and Chambon P [28] described oncogenic hormone receptor from human oestrogen receptor cDNA sequence. This carcinogenicity of estrogen is attribute to receptor-mediated stimulation of cellular proliferation. Increased proliferation could result in turn in accumulation of genetic damage and stimulation of the synthesis of growth factors that act on the mammary epithelial cells via an autocrine or paracrine loop [30]. There were evidence that estrogen-dependent cell line(MCF-7) cells under estradiol(E2) stimulation release some known polypeptide growth factor activities (EGF-like, IGF-1-like) [31]. Dickson RB and Stancel GM [32] discuss estrogen receptor-mediated processes in normal and cancer cells. ER-mediated regulation of gene expression plays many significant roles in normal and cancer cells and this will improve the understanding of hormonal carcinogenesis. Furthermore, Russo [33], Santen, et al. [34], Yager and Yue [35] discussed estrogen receptor-dependent and independent mechanisms of breast carcinogenesis. ERa mediated stimulation of breast cell proliferation with a concomitant enhanced rate of mutations, and estradiol metabolites to genotoxic DNA mutation cause DNA damage. Thus, ERa function as estrogen activated transcription factor and involved in the stimulation of estrogen target genes in the regulation of cell cycle progression and growth of breast epithelium [36]. As Clarke described [37], some ERa/PR-positive epithelial cells are quiescent breast stem cells that acts as 'steroid hormone sensors'. Such hormone sensor cells are likely to secrete positive or negative paracrine/juxtacrine factors dependent on the prevailing estrogen or progesterone concentration to influence the proliferative activity of adjacent ERa/PR-epithelial cells. This might represent one step toward the development of neoplasia and malignancy-invasive tumors. . Thus, GR activation is sufficient to provide a potent signal for cell survival in mammary epithelial cells. Induction of serum and glucocorticoidregulated kinase-1 (SGK-1) expression is required for GR-mediated epithelial cell survival signaling. SGK-1 is a direct GR target and encodes a serine-threonine kinase with significant homology to Akt-1. In immunohistochemical analysis, nucleus GR-immunostaining and glucocorticoid receptor (GR)-immunopositive cells were abundantly present in subclinical Cushing's disease due to pituitary adenomas (2 macroadenomas and 8 microadenomas), suggesting impaired glucocorticoid action, at least in part, in tumorigenesis of this disease [92]. RU486(mifepristone), a glucocorticoid receptor antagonist, has been successful treatment of the Cushingoid phenotype with markedly elevated lymphocyte glucocorticoid receptor numbers in a transient Cushing's syndrome [94]. Mifepristone may also be a useful strategy for increasing tumor cell apoptosis in chemotherapy-resistant GR+ triple-negative breast cancer(TNBC) [95]. In addition, the cortisol/ cortisone-responsive AR (AR(ccr)) has two mutations (L701H and T877A) that were detected in the MDA PCa human prostate cancer cell lines established from a castrated patient whose metastatic tumor exhibited androgen-independent growth. GR antagonist RU38486 showed inhibitory activity [90], but its therapeutic application to treat prostate cancer may be limited. Thus, the combination of ARccr receptor antagonists bicalutamide (casodex) together with a ligand suppressor(triamcinolone) represents a new therapeutic strategy for the treatment of the subset of androgen-independent prostate cancers harboring the L701H or ARccr type.
So far, there is a conflict among hormone FSH/FSH receptor in etiology of epithelial ovarian cancer [96,97]. Actions of FSH (folliclestimulating hormone) in reproductive physiology are essential for folliculogenesis and steroidogenesis [98], FSH receptor (FSHR), a transmembrane receptor with a G protein-coupled signaling, is expressed by granulosa cells in developing ovarian follicles. Overexpression of FSHR activates oncogenic pathways through FSHRinduced EGFR amplification, and HER-2/neu and activated ERK1/2 in preneoplastic immortalized ovarian surface epithelial(IOSE) cells [99]. The data by epidemiological studies that an increased occurrence of ovarian cancer has been suggested with exposure to high levels of gonadotropins during postmenopause or infertility therapy [100-102].
Retinoids can regulate proliferation and differentiation of normal myeloid cells, and that supraphysiological levels of retinoic acid (9cis RA or all-trans RA) can induce abnormal promyelocytes toward maturation. These leukemic cells harboring steroid PML-RARA translocation play a central role in etiology of APL [4,8,9,103]. The PML-RARa chimera can function as a strong transcriptional repressor of retinoic acid (RA) signaling and RAR function by pertubing normal retinoid signaling, leading to a block in differentiation and in an accumulation of leukemic cells at promyelocytic stage [4,6,[103][104][105][106][107][108][109][110][111][112][113][114][115]. This oncogenic receptor is locked in its "off" regulated mode thereby constitutively repressing transcription of genes or key enzymes that are critical for differentiation of hematopoietic cells [4,116]. Moreover,the PML-RARa apparently involved in part the defects in ability to release corepressor under physiological hormone concentration. The wild-type RARa readily form heterodimer with the retinoid X receptors(RXRs). In contrast, chimeric pml/RARa oncoprotein exhibited an enhanced ability to bind to DNA as homodimer. This enhanced homodimerization by RAR chimeras implicates in aberrant corepressor interaction properties of these oncogenic receptor derivatives, influences their DNA recognization, and appears to prevail in neoplastic cells of these oncoproteins [103]. These differences in target gene recognition by the normal and oncogenic RARa protein may contribute to the leukemogenic phenotype [103].

Targeting oncogenic growth factor receptors
The EGF receptor(EGFR) [117] has a key role in normal embryonic development, adult tissue hemeostasis and many pathological processes, particular tumor formation. Aberrant EGFR activation becomes oncogenic due to overexpression and/or amplification of the EGFR gene or by autocrine/paracrine growth factor loops, whereas activating dimerized mutations promote EGFR signaling, which lead to ligand-independent [118,119]. Phosphorylation of this oncogenic receptor at residues Tyr845, Tyr1045 and Tyr1173 leads to receptor activation and downstream signaling [120,121]. Oncogenic receptor EGFR [2,5,7,119,[122][123][124] was found to involved in A431 human carcinoma cells [125][126][127][128], squamous cell carcinoma (SSC) [129], epithelial cell lines from mammary carcinoma [130], glioblastoma stem cells [5,131,132,EGFR+++ positive in one glioma, George Zhu], and colorectal carcinoma tissue [133]. Oncogenic EGFR mutations are found in 10% to 35% of lung adenocarcinomas, with predominant in a subset of patients with non-small cell lung cancer (NSCLC) [134][135][136]. These mutations, which commonly occur as either small inframe deletions in exon19 or point mutations T790M and L858R in exon21 within the EGFR tyrosine kinase domain, confer constitutive activity and sensitivity to EGFR tyrosine kinase inhibitors(TKI) [137,138]. Recent, Gallant [137] identified a novel EGFR alteration in lung cancer: EGFR exon18-25 kinase domain duplication(EGFR-KDD). EGFR-KDD is oncogenic and oncogenic EGFR-KDDtransformed cells are sensitive to the EGFR TKI afatinib. Konduri and colleagues [138] reported five patients with metastatic lung cancer whose tumors harbored EGFR fusion, most commonly RAD51, are recurrent in lung cancer. Four of whom were treated with EGFR TKI erlotinib with documented antitumor response for 5,6,8, and 20 months respectively. These patients whose tumors harbored EGFR fusions are oncogenic in preclinal studies. In mouse model, transgenic mice expressing EGFR L858R in type II pneumocytes developed atypical adenomatous hyperplasia and multifocal adenocarcinoma, and gefitinib inhibited tumorigenesis completely [139]. Blesa [140] present a durable complete remission of a relapsed glioblastoma with a complete radiologic response and the combination of cetuximab and bevacizumab in a third-line setting, that has offered a progression-free survival of 20 months. In Cuba, CimaVax-EGF, promising, an active vaccine targeting EGF as the major ligand of EGFR, it is in use as a cancer therapy against non-small-cell lung cancer (NSCLC) [ PDGFRa mutations are oncogenic(tumorigenic) and developed 100% of mice with brain tumors whereas only one of 19 mice implanted with cells overexpressing wild-type PDGFRa developed into brain tumor in vivo [143]. This D842V mutant was effectively inhibited by crenolanib, a specific inhibitor of PDGFRa and PDGFRβ. Golub [144] identified that PDGF receptor β is oncogenic in pathogenesis of chronic myelomonocytic leukemia with t(5;12) chromosomal translocation, targetable oncogenic receptor tyrosine kinases PDGFRa/ PDGFRβ, and FGFR fusions [145][146][147]  In vitro HGF can transform immortalized mouse liver epithelial cells [165]. Serum HGF levels are elevated in patients suffering from chronic hepatitis and liver cirrhosis. Moreover, hepatocytes from transgenic mice expressing HGF grew more rapidly than did those from normal siblings. In vivo, Fao HGF cells produced tumors when transplanted into nude mice [166]. These conditions cause persistent hepatocellular damage and regeneration; consequently, there are associated with the subsequent of hepatocellular carcinoma(HCC). Thus, HGF-HGF receptor signaling might play an important role in carcinogenesis [163,167]. PRS-110 (starting at 0.8 mg/kg and going up to 30 mg/kg) specifically binds to Met receptor with high affinity and blocks HGF interaction on ligand-dependent (U87-MG) and ligand-independent (Caki-1) xenograft model [162]. Animals were randomized for the treatment with a novel met inhibitor EMD1214063 (50 mg/kg/d), which resulted in a complete regression of the sensitive H1112L met variant-derived tumors [168]. Foretinib, the first multitarget c-met TKI to under clinical investigation, produced a promising benefit in HCC patients [163]. Recently, the chemically-modified monovalent antibody DN30 was found to inhibit ligand-independent activation of the met oncogenic receptor, providing an another target therapy [169-171].  [189][190][191]. Interesting, the oncogenic mechanism in Ewing sarcoma harboring oncogenic EWS/NR4A3 fusion involves a novel pro-oncogenic IGF/IGF-1 receptor signaling pathway including post-transcriptional depression of IGF signaling by the EWS/Fli1 fusion oncoprotein via miRs [192], and this provide therapeutic targeting anti-IGF-1 receptor antibody in Ewing Sarcoma [193][194][195].

Cytokine receptors had oncogenic mutant variants in cancer
G-CSF has been used in clinic for more than 2 decades to treat congenital and acquired neutropenias [196][197][198][199][200]. It is highlight the clinical application of G-CSF to children with severe congenital neutropenia(SCN) and especially in patients with neutropenia harboring in the G-CSF receptor(CSF3R) gene, which is correlated to an increased risk for development of MDS and acute myeloid leukemia(AML) [201]. There are two classes of CSF3R mutant variants: truncations of the cytoplasmic domain [202,203] and membrane proximal point mutations including T618I [204]. Truncated CSF3R mutations are the mutant type nearly almost observed in SCN and abnormal signaling of this oncogenic receptor variants in malignant transformation, whereas membrane proximal mutations (particularly T618I) are the predominant mutation type observed in chronic neutrophilic leukemia(CML) and atypical (BCR-ABL negative) chronic myeloid leukemia (aCML), and confer ligand-independent growth [204]. Maxson and colleagues [205] identified activating mutations for CSF3R in 59% (16/27) of patients with CNL or atypical CML. In vivo, the activating mutation in the CSF3R gene induces hereditary chronic neutrophilia [206]. Mice transplantanted with CSF3R T618Iexpressing hematopoietic cells developed a myeloproliferative disorder [204]. Treatment with the JAK inhibitor ruxolitinib lowered the white blood cells and reduced spleen weight. These results indicate that activating mutant CSF3R is oncogenic [205] and sufficient to drive a myeloproliferayive disorder resembling CML and CNL that is sensitive to pharmacologic JAK inhibition [204].
Another more example is that a translocation t(14;19) (q32;P13) was involved IGH@ and the cytokine receptor EPOR at 19P13 in two patients with B-cell precursor acute lymphoblastic leukemia (BCP-ALL) [207,208]. An over 230-and 241-fold increase in the expression of EPOR were observed in 2 patients at diagnostic and relapse samples respectively. The EPOR-IGH/IGK chain fusions result in truncation of the cytoplasmic tail of EGFR at residues similar to the mutant EPOR in PFCP, with preservation of the proximal tyrosine essential for receptor activation and loss of distal regulatory residues. Expression of truncated EPOR in mouse B cell progenitors induced ALL in vivo. The data implicate oncogenic erythropoietin receptor in role of both benign erythrocytosis and malignancy [209,210]. Moreover, mutation of erythropoitin receptor (EPOR) gene was associated with primary familial and congenital polycythemia(PFCP).Truncated mutations of 59 to 84 amino acids of EPOR at c-terminus lead to loss of the intracellular cytoplasmic tail of the receptor, and prolonged proliferative signal resulting in hypersensitivity (5-10 fold increased sensitivity) to erythropoitin, and prolonged activity of JAK2 kinase and STAT5 activity [211][212][213][214].The leukemic cells with oncogenic EPOR fusions were sensitive to JAK-STAT inhibition, suggesting a therapeutic option in high-risk ALL.
In vitro nude mice, fibroblasts transfected with a chimera molecule containing the extracellular IL-2 binding domain of the IL-2R cDNA and the transmembrane and intracellular kinase domain of the EGF receptor cDNA, were morphologically transformed and produced rapidly growing tumor [223]. Moreover, retroviral expression of IL-2RG restore signaling by IL-7 receptor to X-SCID precursor cells in T-cell progression to the pro-leukemic effects of ectopic LMO2 [224]. Recently, the chromosomal translocation t(5;9) (q13;q22) in peripheral T cell lymphoma generating the interleukin 2 inducible T cell kinase(ITK)-spleen tyrosine kinase(SYK) fusion kinase mimics a T-cell receptor signal and drives oncogenesis in mouse models.
Activation of the interleukin-3(IL-3) gene by the enhancer of the IgH fusion leads to the overexpression of IL-3 gene product in 2 cases of acute lymphocytic leukemia with t(5;14)(q31;q21) translocation [225]. Serum IL-3 correlated with the clinical cause. When the patient's leukemic cell burden was highest (WBC 116,500/ul, lymphoblasts 33,785/ul), the serum IL-3 level was highest (799,5 pg/ml);whereas complete remission(WBC 123,00/ul, lymphoblasts 0/ul), serum IL-3 levels decreased to 105,1 pg/ml. Therefore, overexpression of IL-3 gene coupled with the presence of aberrant IL-3 receptor in these cells could account for oncogenic effects for proliferatative advantage and may play a central role in the pathogenesis of leukemia [225,226]. Aberrant presence of cysteine residues was shown to induce homodimerization of mutant interleukin-7 receptor(IL-7R), which drive constitutive signaling via JAK1 and independently of IL-7, rc or JAK3 [227], promoting cell transformation. This abnormality is involved in -10% human pediatric T-cell leukemogenesis, paving tthe way for therapeutic targeting oncogenic IL-7R-mediated signaling in T-ALL [228][229][230]. Interleukin-21 receptor (IL-21R) is capable of signal transduction through homodimerization or potentially heterodimerization with IL-2R gamma. IL-21 and IL-21R not only regulates proliferation of mature B cells and T cells in response to activating stimuli but also mediate expansion of NK population from bone marrow. The gene for IL-21R is found the partner of BCL6 in t(3;16)(q27;p11),which is recurrently observed in diffuse large B-cell lymphoma and a lymphoma cell line YM [231].This IL-21R/BCL6 fusion gene is clearly associated with lymphoid cell origin.

Conclusion
To date, antibodies that target oncogenic receptors are often targeted toward lysosome [232] or blockade of translocation from the endoplasmic reticulum to the cell surface of specific antigens such as vascular endothelial growth factor receptors(VEGFR2) or Tie2 [233]. For instance, targeting cells with biotinylated ligands and addition of streptavidin efficiently targets trastuzumab to lysosome and this crosslinking of trastuzumab increased lysosomal degradation of its cognate oncogenic receptor Her2 in breast cancer cell lines [232]. Another burgeoning class of targeted chemotherapies called antibodydrug conjugates(ADCs).This ADCs that have demonstrated sufficient efficacy to gain and retain clinical approved are ado-tratuzumab emtansine (brand name Kadcyla) and brentuximab vedotin (brand name Adcetris) [234]. The auristatin-based antibody-drug conjugate BAY1187982 is for the treatment of FGFR2-positive solid tumors [195]. Another, the nicotinic acetylcholine receptor a7-nAchR (toxicology) is the oncogenic receptor, which mediated nicotine(NNK and NNN) oncogenic signaling in an important role in the initiation and progression of cancer including lung cancer and this oncogenic response was in parallel with the mutagenic and cytotoxic effects of tobacco smoke to promote the growth and angiogenesis of the tobacco related cancers [235]. Thus nAchRs yield new targets for the prevention and treatment of tobacco related cancers. Therefore, downregulating oncogenic receptors may be useful paradigm and perspective in our better understanding of clinical cancer biology .