Initial inhibition and recovery of protein synthesis in cycloheximide-treated hepatocytes

Previous studies conducted with intact rats had demonstrated that protein synthesis was reversibly inhibited by cycloheximide. Polysome aggregation occurred during inhibition with a return to normal during recovery, suggesting that the block of translational activity involved termination and release of polypeptides. This study involving freshly isolated hepatocytes was undertaken to clarify the mechanism of the biphasic response to cycloheximide. Cycloheximide at 1 μM inhibited [³H]leucine incorporation into both cellular and secreted proteins by at least 86%, without having deleterious effects on membrane integrity as indicated by trypan blue uptake and lactate dehydrogenase (LDH) (EC 1.1.1.27) release. After removal of cycloheximide, incorporation of labeled amino acids into cellular protein and protein secreted into the medium returned to control levels. Kinetically, incorporation into secreted protein exhibited a lag of 30–45 min, indicating that a longer recovery period for restoration of proteosynthetic ability is required for membrane-bound polysomes. During the first 100 min of the recovery period, 30% of the cellular protein, which had been prelabeled during cycloheximide inhibition, was secreted into the medium; treated cells, however, secreted prelabeled protein at a lower initial rate. To elucidate the mechanism of action of cycloheximide, the content of the cytoplasmic ribo-nucleoprotein complexes (RFC), polysome size classes, and the distribution of radioactivity among the various ribosome classes were determined during inhibition and recovery. Larger size class polysomes (7+) were increased by cycloheximide treatment and remained increased during recovery. During inhibition, there was enhanced [³H]leucine labeling with increasing polysome size, implicating termination as the rate-limiting step, whereas during the recovery phase the labeled nascent polypeptides were removed from the ribonucleoprotein complex at a 3- to 4-fold greater rate than control, indicating an accelerated release of completed proteins.