Regulation of surface expression of acetylcholine receptors in response to serum and cell growth in the BC3H1 muscle cell line.

The regulation of acetylcholine receptor synthesis, assembly, and cell surface expression in response to serum and cell growth in the BC3H1 muscle-like cell line have been examined. In the presence of media containing low concentrations of serum, BC3H1 cells withdraw from the cell cycle and express high levels of cell surface acetylcholine receptors. Exposure of quiescent, differentiated BC3H1 cells to high concentrations of serum results in re-entry of cells into the cell cycle and a loss of surface acetylcholine receptors by a first order process (t1/2 = 8.6 h) which appears to be initiated within 1 h following serum addition. Under these conditions, the rate of synthesis of acetylcholine receptor alpha-subunits is not significantly reduced; however, the efficiency of assembly of alpha-subunits into functional acetylcholine receptors is significantly less than that observed in quiescent cells. The receptors which are assembled following serum stimulation of quiescent cells are not transported to the cell surface, nor do they accumulate within the cells in a functional form (defined by the ability to bind alpha-bungarotoxin) in the cells. We conclude that reinitiation of cell division in quiescent cultures of BC3H1 cells results in alterations in several post-translational processing steps required for the surface expression of acetylcholine receptors.