The rate of internalization of the mannose 6-phosphate/insulin-like growth factor II receptor is enhanced by multivalent ligand binding

The cation-independent mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) undergoes constitutive endocytosis, mediating the internalization of two unrelated classes of ligands, mannose 6-phosphate (Man-6-P)-containing acid hydrolases and insulin-like growth factor II (IGF-II). To determine the role of ligand valency in M6P/IGF-II receptor-mediated endocytosis, we measured the internalization rates of two ligands, beta-glucuronidase (a homotetramer bearing multiple Man-6-P moieties) and IGF-II. We found that beta-glucuronidase entered the cell approximately 3-4-fold faster than IGF-II. Unlabeled beta-glucuronidase stimulated the rate of internalization of 125I-IGF-II to equal that of 125I-beta-glucuronidase, but a bivalent synthetic tripeptide capable of occupying both Man-6-P-binding sites on the M6P/IGF-II receptor simultaneously did not. A mutant receptor with one of the two Man-6-P-binding sites inactivated retained the ability to internalize beta-glucuronidase faster than IGF-II. Thus, the increased rate of internalization required a multivalent ligand and a single Man-6-P-binding site on the receptor. M6P/IGF-II receptor solubilized and purified in Triton X-100 was present as a monomer, but association with beta-glucuronidase generated a complex composed of two receptors and one beta-glucuronidase. Neither IGF-II nor the synthetic peptide induced receptor dimerization. These results indicate that intermolecular cross-linking of the M6P/IGF-II receptor occurs upon binding of a multivalent ligand, resulting in an increased rate of internalization.     

Lack of association between the polyadenylation polymorphism in the NAT1 (acetyltransferase 1) gene and colorectal adenomas

Smoking and a high intake of red meat are risk factors for colorectal tumors. These effects could be due to aromatic amine carcinogens. Individual susceptibility to aromatic amines has been related to acetylation phenotype, which plays a role in the bioactivation of arylamines. Polymorphisms in both N-acetyltransferase genes, NAT1 and NAT2, have been associated with an increased risk of colorectal tumors. We studied the NAT1*10 fast acetylator allele (1088 T-->A mutation) and distal adenomas in a sigmoidoscopy-based case-control study (441 cases, 484 controls). We found neither an increased adenoma prevalence in subjects homozygous or heterozygous for the NAT1*10 fast acetylator allele (odds ratio 1.04; 95% confidence interval 0.79-1.36), nor a gene-gene interaction between NA1 and NAT2 (P(interaction) = 0.59). Further NAT1 alleles must be considered for more conclusive results regarding the relevance of NAT1 activity to colorectal tumorigenesis.