Effects of lectins and tunicamycin on cAMP response to parathyroid hormone

Carbohydrate moieties of cell surface glycoproteins with an external orientation play a role in hormone recognition and/or transmembrane signal transmission. We have examined the effect of various lectins, which interact with specific cell surface glycosyl residues, and of tunicamycin, an antibiotic that inhibits glycosylation of proteins, on the adenosine 3',5'-cyclic monophosphate (cAMP) response to parathyroid hormone (PTH) in confluent cultured osteoblast-like rat osteosarcoma cells (UMR-106) and opossum kidney cells (OK cells). Incubation of both cell lines with wheat germ lectin (WGL), but not with concanavalin A, succinylated wheat germ, ricin, or soybean lectins, markedly reduced the PTH-induced cAMP production, whereas the stimulation obtained with forskolin, a compound that acts directly on the adenylate cyclase enzyme, was not affected. In contrast, tunicamycin did not cause any decrease in the cAMP response to PTH. These results indicate that the masking of sialic acid residue by WGL considerably blunted PTH-stimulated cAMP production in cultured osteoblast-like and kidney cells. An 80% inhibition of glycosylation of cell surface proteins did not appear to affect the response to PTH. Thus the functional role of this carbohydrate moiety in the PTH receptor remains to be determined.