BIOSYNTHETIC PROCESSING OF NEU DIFFERENTIATION FACTOR - GLYCOSYLATION, TRAFFICKING, AND REGULATED CLEAVAGE FROM THE CELL-SURFACE

neu differentiation factor (NDF), also known as heregulin, is structur ally related to the epidermal growth factor family of growth factors; it stimulates tyrosine phosphorylation of the neu/HER-2 oncogene and c auses differentiation of certain human breast cancer cell lines, Alter native splicing of a single gene gives rise to multiple isoforms of ND F/heregulin, as well as the neuronal homologues, designated ARIA (acet ylcholine receptor inducing activity) and GGF (glial growth factor); a t least 15 structural variants are known. All but two of the NDF/hereg ulin cDNAs are predicted to encode transmembrane, glycosylated precurs ors of soluble NDF. In this report we characterized the biosynthetic p rocessing of different NDF isoforms in stably transfected Chinese hams ter ovary cells expressing individual NDF isoforms, and in the native cell line Rat 1-EJ, which expresses at least six different NDF isoform s. We found that the precursors for NDF undergo typical glycosyIation and trafficking. A portion of the molecules are proteolyticafly cleave d intracellularly leading to the constitutive secretion of soluble, ma ture NDF into the culture media, However, a significant portion of the newly synthesized NDF precursor molecules escape intracellufar cleava ge and are transported to the cell surface of both transfected and nat ive cells, where they reside as fulllength, transmembrane proteins. Fi nally we show that these full-length, transmembrane NDF molecules can undergo phorbol ester regulated cleavage from the membrane, releasing the soluble growth factor into the medium.