The Ah receptor nuclear translocator gene (ARNT) is located on q21 of human chromosome 1 and on mouse chromosome 3 near Cf-3

Hypoxic microenvironment in cancer: molecular mechanisms and therapeutic interventions

Having a hypoxic microenvironment is a common and salient feature of most solid tumors. Hypoxia has a profound effect on the biological behavior and malignant phenotype of cancer cells, mediates the effects of cancer chemotherapy, radiotherapy, and immunotherapy through complex mechanisms, and is closely associated with poor prognosis in various cancer patients. Accumulating studies have demonstrated that through normalization of the tumor vasculature, nanoparticle carriers and biocarriers can effectively increase the oxygen concentration in the tumor microenvironment, improve drug delivery and the efficacy of radiotherapy. They also increase infiltration of innate and adaptive anti-tumor immune cells to enhance the efficacy of immunotherapy. Furthermore, drugs targeting key genes associated with hypoxia, including hypoxia tracers, hypoxia-activated prodrugs, and drugs targeting hypoxia-inducible factors and downstream targets, can be used for visualization and quantitative analysis of tumor hypoxia and antitumor activity. However, the relationship between hypoxia and cancer is an area of research that requires further exploration. Here, we investigated the potential factors in the development of hypoxia in cancer, changes in signaling pathways that occur in cancer cells to adapt to hypoxic environments, the mechanisms of hypoxia-induced cancer immune tolerance, chemotherapeutic tolerance, and enhanced radiation tolerance, as well as the insights and applications of hypoxia in cancer therapy.

Existence of Leukemic Clones Resistant to Both Imatinib Mesylate and Rituximab before Drug Therapies in a Patient with Philadelphia Chromosome-Positive Acute Lymphocytic Leukemia

Imatinib mesylate and rituximab are molecularly targeted drugs against the BCR-ABL fusion protein and the CD20 antigen, respectively. Although these drugs have excellent anticancer effects, a major concern is drug resistance. We have investigated the case of a patient with Philadelphia chromosome-positive and CD20+ acute lymphocytic leukemia who acquired resistance to imatinib and rituximab. Imatinib therapy resulted in prompt cytogenetic remission, but resistance developed shortly thereafter. Sequencing of the kinase domain of the ABL gene and allele-specific polymerase chain reaction analysis revealed a point mutation resulting in an E255V substitution that was present before the therapy. After the patient received mild chemotherapy followed by rituximab administration, hematologic and cytogenetic remission was sustained for 5.5 months. The recurrent leukemic cells after the rituximab therapy showed not only the E255V mutation in the ABL gene but also loss of the CD20 antigen due to impaired transcription of the CD20 gene. The results of 2-color flow cytometry analysis showed that a small population of CD20(-) leukemic cells existed before the imatinib therapy. These results suggest that leukemic subclones carrying a genetic perturbation of the targeted molecules for both imatinib and rituximab were present before the therapies. The preexistence of primary resistant clones suggests the inability of combination therapy with 2 molecularly targeted drugs to overcome drug resistance in leukemia.

Aryl hydrocarbon receptor nuclear translocator 2 (ARNT2): structure, gene mapping, polymorphisms, and candidate evaluation for human orofacial clefts

BACKGROUND

Nonsyndromic orofacial clefts have an estimated incidence of 1/1000 live births. Population genetic and embryologic studies suggest that cleft palate only (CPO) may be a distinct clinical entity from cleft lip with or without cleft palate (CL/P). Both CPO and CL/P are thought to be multifactorial in etiology, with evidence indicating that genetic, environmental, and developmental determinants may all play a role. The ARNT2 gene localizes to a conserved linkage group on mouse chromosome 7 that is syntenic with human chromosome 15q23-25. This chromosomal region was previously identified as a teratogen-induced clefting susceptibility locus in a genome-wide scan of AXB and BXA recombinant inbred mice. Arnt2 is expressed in the first branchial arch in mice. The teratogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) acts through the aryl hydrocarbon receptor (Ahr) pathway to produce dose-dependent CPO and thymic wasting in mice exposed in utero. Arnt2 and Ahr proteins dimerize in vitro. TCDD exposure is also associated with orofacial clefting in children of parents involved in agricultural work.

METHODS

To determine whether ARNT2 influences human craniofacial development, we identified the human ARNT2 gene and conducted genomic structural analysis. Mutational screening was performed in infants with nonsyndromic CPO or CL/P who were identified by the Iowa Birth Defects Registry.

RESULTS

A common amino acid polymorphism was detected but, no obvious disease-causing mutations were detected by SSCP analysis. The microsatellite marker, GATA89D04 (D15S823) was identified within intron 11 of the human ARNT2 gene, and linkage disequilibrium of nonsyndromic CPO and CL/P parent-infant trios was conducted.

CONCLUSIONS

No association was demonstrated with CPO (n = 45) and CL/P (n = 37). Teratology 66:85-90, 2002.

The t(1;12)(q21;p13) translocation of human acute myeloblastic leukemia results in a TEL-ARNT fusion

The TEL/ETV6 gene is located at 12p13 and encodes a member of the ETS family of transcription factors. Translocated ETS leukemia (TEL) is frequently involved in chromosomal translocations in human malignancies, usually resulting in the expression of fusion proteins between the amino-terminal part of TEL and either unrelated transcription factors or protein tyrosine kinases. We have characterized a t(1;12)(q21;p13) translocation in an acute myeloblastic leukemia (AML-M2). At the protein level, the untranslocated TEL copy and, as a result of the t(1;12) translocation, a fusion protein between TEL and essentially all of aryl hydrocarbon receptor nuclear translocator (ARNT) are expressed. The involvement of ARNT in human leukemogenesis has not been previously described. The ARNT protein belongs to a subfamily of the "basic region helix-loop-helix" (bHLH) protein that shares an additional region of similarity called the PAS (Per, ARNT, SIM) domain. ARNT is the central partner of several heterodimeric transcription factors, including those containing the aryl hydrocarbon (dioxin) receptor (AhR) and the hypoxia-inducible factor 1alpha (HIF1alpha). Our results show that the TEL-ARNT fusion protein is the crucial product of the translocation and suggest that interference with the activity of AhR or HIF1alpha can contribute to leukemogenesis.

Physical localization of the mouse aryl hydrocarbon receptor nuclear translocator-2 (Arnt2) gene within the c112K deletion

The albino deletions identify at least seven functional intervals essential for pre- and postnatal development in the 6- to 10-cM region surrounding the albino coat color (c = tyrosinase) locus on mouse chromosome 7. The c112K deletion identifies a putative thymus functional region not removed by the overlapping c3H deletion. Cloning the c3H proximal breakpoint provided a starting point for construction of an 840-kb BAC contig spanning the c112K and c3H (D7Ssb3Hp) proximal breakpoints. These breakpoints are separated by 320-350 kb. The aryl hydrocarbon receptor nuclear translocator-2 (Arnt2) is completely removed by the c112K deletion and spans 130-170 kb of the interval. Although Arnt2 is a candidate for the thymus defects in c112K homozygotes, the possibility that other as yet unidentified genes in the c112K deletion are responsible for the abnormalities has not been ruled out. Arnt2 is a member of the bHLH-PAS (Per, Ahr, Arnt, Sim) family of transcription factors and shares the highest similarity with Arnt. The survival of c112K homozygotes markedly contrasts the embryonic lethality observed in Arnt-deficient embryos and suggests distinct roles for these related transcription factors during embryogenesis.

Localization of the gene for familial partial lipodystrophy (Dunnigan variety) to chromosome 1q21-22

Obesity is strongly implicated in the pathophysiology of insulin resistance, diabetes mellitus and dyslipidemia. The mechanisms, however, by which obesity causes these complications are not known. The study of single-gene disorders affecting adipose tissue may elucidate some of the mechanisms involved in these processes. Familial partial lipodystrophy, Dunnigan variety, (FPLD, OMIM 308980) is an autosomal-dominant condition characterized by marked loss of subcutaneous adipose tissue affecting the trunk and extremities but with excess fat deposition in the head and neck areas. Affected individuals show an increased preponderance of insulin resistance, diabetes mellitus, dyslipidemia and acanthosis nigricans. The genetic basis of FPLD is unknown. We carried out a genome-wide scan with a set of highly polymorphic short tandem-repeats (STR) in individuals from five well-characterized pedigrees and mapped the FPLD locus to chromosome 1q21-22. The maximum two-point lod score obtained with a highly polymorphic microsatellite at D1S2624 at theta(max)=0 was 5.84. Multipoint-linkage analysis yielded a peak lod score of 8.25 between D1S305 and D1S1600. There was no evidence for genetic heterogeneity (alpha=1) in the pedigrees.

A point mutation responsible for defective function of the aryl-hydrocarbon-receptor nuclear translocator in mutant Hepa-1c1c7 cells

A 3,4-benzopyrene-resistant mutant clone (c4) of the mouse hepatoma Hepa-1c1c7 cell line was examined for the mutation that causes the defective function of aryl-hydrocarbon receptor (AHR) nuclear translocator (Arnt). Arnt dimerizes with AHR and mediates the induction signal of aryl-hydrocarbon hydroxylase activity. The Arnt cDNAs of c4 cells were cloned by reverse-transcription/PCR to compare the sequences with that of wild-type Arnt cDNA. The Arnt cDNA of c4 cells was found to have a single point mutation, leading to replacement of Gly326 with Asp between two internal repeats in the highly conserved Per-Arnt-Sim (PAS) domain, PAS A and PAS B. The inability of [Asp326]Arnt/AHR heterodimers to enhance reporter gene transcription under the control of the CYP1A1 gene promoter and enhancer confirmed that the G326-->D substitution was a causative mutation. While fluorescence microscopy and coimmunoprecipitation experiments showed that this mutant form of Arnt was not changed from wild-type Arnt in terms of nuclear localization or heterodimer formation with AHR, the binding activity of the [Asp326]Arnt x AHR heterodimer to the xenobiotic-responsive element was reduced markedly. Determination of the turnover rate in COS-7 cells transfected with expression plasmids for mutant Arnt or normal Arnt showed that the mutant protein turned over with an accelerated rate compared with that of the normal. Moreover, the mutant protein displayed increased proteolytic digestibility in vitro with various proteases.

Structural and functional analysis of hypoxia-inducible factor 1

Hypoxia-inducible factor 1 (HIF-1) is a basic helix-loop-helix protein that activates transcription of hypoxia-inducible genes, including those encoding: erythropoietin, vascular endothelial growth factor, heme oxygenase-1, inducible nitric oxide synthase, and the glycolytic enzymes aldolase A, enolase 1, lactate dehydrogenase A, phosphofructokinase I, and phosphoglycerate kinase 1. Hypoxia response elements from these genes consist of a HIF-1 binding site (that contains the core sequence 5'-CGTG-3') as well as additional DNA sequences that are required for function, which in some elements include a second HIF-1 binding site. HIF-1 is a heterodimer. The HIF-1 alpha subunit is unique to HIF-1, whereas HIF-1 beta (ARNT) can dimerize with other bHLH-PAS proteins. Structural analysis of HIF-1 alpha revealed that dimerization with HIF-1 beta (ARNT) requires the HLH and PAS domains, DNA binding is mediated by the basic domain, and that HIF-1 alpha contains a carboxyl-terminal transactivation domain. Co-transfection of HIF-1 alpha and HIF-1 beta (ARNT) expression vectors and a reporter gene containing a wild-type hypoxia response element resulted in increased transcription in non-hypoxic cells and a superinduction of transcription in hypoxic cells, whereas HIF-1 expression vectors had no effect on the transcription of reporter genes containing a mutation in the HIF-1 binding site. HIF-1 alpha and HIF-1 beta (ARNT) protein levels were induced by hypoxia in all primary and transformed cell lines examined. In HeLa cells, the levels of HIF-1 alpha and HIF-1 beta protein and HIF-1 DNA-binding activity increased exponentially as cellular oxygen tension decreased, with maximum values at 0.5% oxygen and half-maximal values at 1.5 to 2% oxygen. HIF-1 alpha and HIF-1 beta (ARNT) mRNAs were detected in all human, mouse, and rat organs assayed and mRNA expression was modestly induced in rodents subjected to hypoxia. HIF-1 alpha protein levels were induced in vivo when animals were subjected to anemia or hypoxia. The HIF1A gene was mapped to human chromosome 14q21-q24 and mouse chromosome 12.

A genetic analysis of processes regulating cytochrome P4501A1 expression

Cytochrome P4501A1 and its associated aryl hydrocarbon hydroxylase activity are highly inducible in the mouse hepatoma cell line, Hepa-1, by substrates of the enzyme and related compounds, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Mutants of this cell line, deficient in P4501A1 inducibility, were isolated. Some of the mutants show a dominant phenotype. Such mutants may have resulted from a genetic alteration leading to the inappropriate activation of a repressor gene that normally functions to restrict high level inducibility to the liver and certain other organs or to certain developmental stages. One dominant mutant was shown to express a protein that prevents binding of the liganded aryl hydrocarbon (Ah) receptor (which mediates induction of P4501A1) to its recognition sequence in DNA (the xenobiotic responsive element, or XRE). The majority of mutants are recessive, and were assigned to four different complementation groups (which probably correspond to four different genes). Gene A corresponds to the structural gene (Cyp1a-1) for P4501A1. Mutations in genes B, C and D all affect functioning of the Ah receptor. A cDNA for gene C was cloned. The encoded protein (ARNT) is required for ligand-dependent translocation of the Ah receptor to the nucleus and its binding to the XRE. ARNT and the Ah receptor form a heterodimeric complex which binds the XRE in a fashion such that both subunits bind the XRE directly. Both ARNT and the Ah receptor contain basic helix-loop-helix motifs. Such motifs have been identified in several transcription factors that bind DNA as heterodimers or homodimers. The roles of the proteins corresponding to the B and D genes are presently under investigation.