A NOVEL JUXTAMEMBRANE DOMAIN ISOFORM OF HER4 ERBB4 - ISOFORM-SPECIFICTISSUE DISTRIBUTION AND DIFFERENTIAL PROCESSING IN RESPONSE TO PHORBOL ESTER/

Human epidermal growth factor receptor 4 (HER4) is a member of the epi dermal growth factor (EGF) receptor subfamily of receptor tyrosine kin ases that is activated by neuregulins (NRG), betacellulin (ETC), and h eparin-binding EGF-like growth factor, Sequencing of full-length human HER4 cDNAs revealed the existence of two HER4 isoforms that differed by insertion of either 23 or 13 alternative amino acids in the extrace llular juxtamembrane (JM) region, The 23-amino acid form (HERC JM-a) a nd the 13-amino acid form (HER4 JM-b) were expressed in a tissue-speci fic manner, as demonstrated by reverse transcriptase-polymerase chain reaction analysis of mouse and human tissues. Both isoforms were expre ssed in neural tissues such as cerebellum, whereas kidney expressed HE R4 JM-a only and heart HER4 JM-b only, In situ hybridization using spe cific oligonucleotides demonstrated transcription of both JM-a and JM- b isoforms in the mouse cerebellum, Tyro sine phosphorylation analysis indicated that both receptor isoforms were activated to the same exte nt by NRG-beta 1 and ETC, and to a lesser extent by NRG-alpha 1 and he parin-binding EGF-like growth factor. A functional difference was foun d, however, in response 60 phorbol ester treatment, Stimulation of cel ls with phorbol ester resulted in a loss of I-125-NRG-beta 1 binding a nd in a reduction of total cell-associated HER4 protein in HER4 JM-a t ransfectants but not in HER4 JM-b transfectants. It was concluded that novel alternatively spliced isoforms of HER4 exist, that they are dis tributed differentially in vivo in mouse and human tissues, that they are both activated by HER4 ligands, and that they may represent cleava ble and noncleavable forms of HER4.