Jan Sap

v-erbA oncogene activation entails the loss of hormone-dependent regulator activity of c-erbA

The v-erbA oncogene, one of the two oncogenes of the avian erythroblastosis virus, efficiently blocks erythroid differentiation and suppresses erythrocyte-specific gene transcription. Here we show that the overexpressed thyroid hormone receptor c-erbA effectively modulates erythroid differentiation and erythrocyte-specific gene expression in a T3-dependent fashion, when introduced into erythroid cells via a retrovirus. In contrast, the endogenous thyroid hormone receptor does not detectably affect erythroid differentiation. The analysis of a series of chimeric v-/c-erbA proteins suggests that the v-erbA oncoprotein has lost one type of thyroid hormone receptor function (regulating erythrocyte gene transcription in response to T3), but constitutively displays another function: it represses transcription in the absence of T3. The region responsible for the loss of hormone-dependent regulator activity of v-erbA has been mapped to the very C-terminus of c-erbA, encompassing a cluster of highly conserved amino acid residues with the potential to form an amphipathic alpha-helix.

DNA Binding Properties of the Thyroid Hormone Receptor/c-erbA Protein and Its Viral Homologue P75gag-v-erbA

The erbA proteins encoded by the cellular proto-oncogene c-erbA and its viral homologue v-erbA differ in their DNA binding region as well as in other domains. We have investigated the possible differences in regulation of transcription and the binding to thyroid hormone responsive elements (TREs) by these proteins. The results show that in chicken erythroblasts P75gag-v-erbA constitutively represses expression of the genes for band 3 and carbonic anhydrase, whereas the normal thyroid hormone receptor represses expression in the absence of ligand and induces transcription upon binding of hormone. In a direct test of binding to TREs in the rat growth hormone gene the c-erbA protein bound with high affinity, whereas the viral ptotein exhibited no binding at all. The experiments demonstrate that the two proteins can recognize the same regulatory elements in erythroblasts but with distinct effects on transcription, and that other TREs bind to only one of the proteins, suggesting that mutations in the primary structure of the viral protein has altered its specificity.

The Thyroid Hormone Receptor/c-erbA Protein and its Viral Homologue P75gag-v-erbA

The erbA proteins encoded by the cellular proto-oncogene c-erbA and its viral homologue verbA differ in their DNA binding region as well as in other domains. We have investigated the possible differences in regulation of transcription and the binding to thyroid hormone responsive elements (TREs) by these proteins. The results show that in chicken erythroblasts P75gag-v-erbA constitutively represses expression of the genes for band 3 and carbonic anhydrase, whereas the normal thyroid hormone receptor represses expression in the absence of ligand and induces transcription upon binding of hormone. In a direct test of binding to TREs in the rat growth hormone gene the c-erbA protein bound with high affinity, whereas the viral ptotein exhibited no binding at all. The experiments demonstrate that the two proteins can recognize the same regulatory elements in erythroblasts but with distinct effects on transcription, and that other TREs bind to only one of the proteins, suggesting that mutations in the primary structure of the viral protein has altered its specificity.

Functions of the erbA and erbB Oncogenes in Avian Erythroblatosis

The avian erythroblastosis virus (AEV) induces an acute erythro id leukemia and sarcomas in virus—infected chickens. AEV strain ES4 was isolated already during the thirties by Engelbreth-Holm by serial passage of a weak leukemia inducing virus (RAV1) in chicks. During the mid—seventies T. Graf demonstrated that a biologically cloned AEV causes both types of diseases, and transforms erythroblasts as well as fibroblasts in vitro.