The sialomucin complex of the ascites sublines of rat 13762 mammary adenocarcinoma: Molecular cloning and analysis of its expression in tumor cells and normal tissues

The 9.2 kb cDNA of pSMC-1, the precursor for both ASGP-1 and ASGP-2 in the ascites sublines of the rat 13762 adenocarcinoma, was cloned and sequenced. The mucin ASGP-1 portion of the pSMC-1 gene encodes a protein of 2,202 amino acids with a calculated Mr of 224,190 Da. pSMC-1 has characteristic features of a typical mucin with 11 complete repeated sequences of about 120 amino acid residues and three partial repeats, a cysteine-containing carboxyl terminus and a high content of serines and threonines (40%), which can be O-glycosylated. ASGP-1 can be divided into four structural domains: a 30 amino acid signal sequence at the amino terminus, a 50 amino acid N-terminal unique sequence, a repeat region of 1,513 amino acids and a 609 amino acid C-terminal unique sequence.The cDNA sequence for ASGP-2 is the 3$\sp\prime$ portion of pSMC-1 and encodes a transmembrane protein of 728 amino acid residues. ASGP-2 is a glycoprotein with 24 potential N-glycosylation sites and can be divided into seven domains including two EGF-like domains and a short cytoplasmic tail at the C-terminus. Cleavage of pSMC-1 into ASGP-1 and ASGP-2 occurs during its transit to the cell surface. ASGP-1 is strongly associated with the transmembrane ASGP-2.pSMC-1 is a heterodimeric complex which might have arisen by a gene-rearrangement in the tumor cells. However, restriction enzyme mapping of the pSMC-1 gene showed the gene to be present in normal rat genome. A 5-fold amplification of the gene was observed in 13762 ascites tumor cells compared to that of normal rat liver DNA.pSMC-1 mRNA was found in normal rat brain and lung by RT-PCR, and Northern blot analysis showed a 500-fold more expression in the ascites cells than brain. This expression pattern was confirmed by immunohistochemical localization of ASGP-2, which was present on the cell surfaces of bronchus of lung, ependymal epithelia of newborn brain and endoderm of rat embryo, and in the cell body of neurons of newborn rat brain.Other studies suggested that ASGP-2 is a ligand for protooncogene c-erbB-2. Immunostaining with ASGP-2 and c-erbB-2 antibodies showed the co-expression of ASGP-2 and c-erbB-2 in some human breast cancer samples