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Many metabolic processes occur simultaneously in the liver in different locations along the porto-central axis of the liver units. These processes are often regulated by hormones, one of which is thyroid hormone which for its action depends on the presence of the different isoforms of the thyroid hormone receptor (TR). These are encoded by two genes: c-erbA-alpha encoding TRalpha1 and TRalpha2 and their respective Delta isoforms, and c-erbA-beta which encodes TRbeta1, TRbeta2 and TRbeta3. We recently found a zonal (pericentral) expression of and a diurnal variation in the TRbeta1 isoform in rat liver. We were therefore also interested to see whether TRalpha1 and TRalpha2 expression showed similar characteristics. For this reason we raised both polyclonal and monoclonal antibodies against TRalpha1 and TRalpha2 isoforms and characterised these. Antibody specificity was tested using Western blots and immunohistochemistry in liver of TR isoform-specific knockout animals. Using these antibodies we found that the TRalpha1 and TRalpha2 isoforms are zonally expressed around the central vein in rat liver. The experiments show that the portal to central gradient of TRalpha1 is broader than that of TRbeta1. Moreover, the expression of the TRalpha2 protein showed a diurnal variation with a peak in the afternoon when the animals are least active whereas no such variation was found for the TRalpha1 protein.From our data it appears that both the TRalpha1 and TRalpha2 isoforms show a zonal distribution in liver. This finding, together with the observed diurnal rhythm, has major implications for interpreting and timing experiments concerning the TR and its downstream actions in liver.

Thyroid hormone receptor alpha 1, beta 1 and beta 2-deficient mice (TR alpha 1-/-beta-/- mice) demonstrate growth retardation and defective ossification in the epiphyses associated with an inhibition of the GH/IGF-I axis. There are differences between TR alpha 1-/-beta-/- mice (receptor deficient) and the hypothyroid animal model (ligand deficient). Such differences include possible repressive actions exerted by unliganded receptors in the ligand-deficient (hypothyroid) model but not in the receptor-deficient model. In the present study we have investigated whether or not GH substitution rescues the skeletal phenotype of TR alpha 1-/-beta-/- mice. TR alpha 1-/-beta-/- and wild-type (WT) mice were treated with GH from day 18 until 10 weeks of age. GH substitution of mutant mice resulted in a significant and sustained stimulatory effect on the body weight that was not seen in WT mice. GH-treated mutant mice but not GH-treated WT mice demonstrated increased length and periosteal circumference of the femur. However, GH substitution did not reverse the defective ossification seen in TR alpha 1-/-beta-/- mice. TR alpha 1-/-beta-/- mice displayed increased width of the proximal tibial growth plate, which was caused by increased width of the proliferative but not the hypertrophic layer. GH substitution did not restore the disturbed morphology of the growth plate in TR alpha 1-/-beta-/- mice. In summary, GH substitution reverses the growth phenotype but not the defective ossification in TR alpha 1-/-beta-/- mice. Our data suggest that TRs are of importance both for the regulation of the GH/IGF-I axis and for direct effects on cartilage.

The primary structure of the chicken epidermal growth factor (EGF) receptor was deduced from the sequence of a cDNA clone containing the complete coding sequence and shown to be highly homologous to the human EGF receptor. NIH-3T3 cells devoid of endogenous EGF receptor were transfected with the appropriate cDNA constructs and shown to express either chicken or human EGF receptors. Like the human EGF receptor, the chicken EGF receptor is a glycoprotein with an apparent molecular weight of 170,000. Murine EGF bound to the chicken receptor with approximately 100-fold lower affinity than to the human receptor molecule. Surprisingly, human transforming growth factor alpha (TGF-alpha) bound equally well or even better to the chicken EGF receptor than to the human EGF receptor. Moreover, TGF-alpha stimulated DNA synthesis 100-fold better than did EGF in NIH 3T3 cells that expressed the chicken EGF receptor. The differential binding and potency of mammalian EGF and TGF-alpha by the avian EGF receptor contrasts with the similar affinities of the mammalian receptor for the two growth factors.

Avian erythroblastosis virus (AEV) induces acute erythroleukemia and sarcomas in vivo and it transforms erythroblasts and fibroblasts in vitro. The virus has two host cell‐derived genes, v‐erbA and v‐erbB. The latter encodes the oncogenic capacity of the virus, whereas v‐erbA enhances the erythroblast transforming effects of v‐erbB while being unable to induce neoplasms independently. Recently, human cellular homologues of these viral erb genes have been isolated. The chromosomal locations of two of these genes have been determined using EcoRI‐digested DNA prepared from human‐mouse somatic cell hybrids. The human c‐erbA1 gene has been assigned to chromosome 17 and is located between 17p11 and 17q21. The human c‐erbB sequence has been assigned to chromosome 7 and is located between 7pter and 7q22. Thus, in the human genome these genes are on two separate chromosomes. No evidence for the involvement of the human c‐erb genes in neoplasia has been found.

The chicken genome contains nucleotide sequences homologous to transforming genes (oncogenes) of a number of avian retroviruses. We have isolated chicken DNA (c-myc) that is homologous to the oncogene (v-myc) of the avian myelocytomatosis virus MC29 and have compared the structures of the cellular and viral genes. Results from restriction endonuclease mapping of c-myc and from analysis of heteroduplexes between the DNAs of the cellular and viral genes show that c-myc is homologous to 1,500 nucleotides in v-myc DNA. This homologous region is interrupted in c-myc by an intron-like sequence of 1,100 nucleotides which is absent from v-myc. Nuclear RNA from normal chicken cells contains at least five species of transcripts from c-myc ranging from 2.5 to 6.5 kilobases in length. By contrast, cytoplasm contains only the 2.5-kilobase c-myc RNA. These features of the c-myc gene and its nuclear transcripts are characteristic of normal cellular genes and suggest that the myc gene is of cellular rather than viral origin. The exons in c-myc may define two functional domains in the gene and may therefore facilitate the dissection of the different oncogenic potentials of the MC29 virus.

HBI is a recombinant avian retrovirus with novel pathogenic properties that was derived from the myc-containing virus MC29. In contrast to MC29, which causes endotheliomas in chickens, HBI induces lymphoid tumors. The results of molecular cloning and nucleotide sequencing of HBI reported here show that the virus contains sequences derived from both c-myc and ring-neck pheasant virus, in addition to MC29. The 3' half of the myc gene was largely replaced by c-myc sequences, and most of the long terminal repeat and gag regions were replaced by ring-neck pheasant virus sequences. The long terminal repeat contained a triplicate sequence which was homologous to the core enhancer sequence of the simian virus 40 72-base-pair repeat. The significance of these changes in relation to the unusual biological properties of the virus are discussed.