Muscarinic receptors in human SH-SY5Y neuroblastoma cell line: regulation by phorbol ester and retinoic acid-induced differentiation

The specific binding of the muscarinic ligand [³H](-)quinuclidinyl benzilate ([³H]QNB) to cell membranes of human SH-SY5Y neuroblastoma cells was studied. Saturation isotherms yielded a K_{d = 0.28 ± 0.06} nM and a B_max of 337 ± 47 pmol/g protein. Pirenzepine inhibited [³H]QNB binding; inhibition data showed best fit to a 2-site binding model revealing both a high affinity pirenzepine site (34%, K_H = 10 nM) and a low affinity site (66%, K_{L = 1} μM). These results indicate that muscarinic receptors on SH-SY5Y cells may be subclassified as M₁/M₂ subtypes. Morphological and biochemical differentiation of these cells after treatment with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or retinoic acid (RA) resulted in a decrease and an increase in the number of muscarinic binding sites, respectively. Furthermore, TPA- and RA-treated cells showed a significant increase in acetylcholinesterase activity compared with non-treated cells. However, only RA-treated cells showed significant increase in choline acetyltransferase activity compared to nontreated cells. These findings demonstrate that TPA and RA can regulate both the number of muscarinic receptors and the acetylcholinesterase activity in human SH-SY5Y neuroblastoma cells.