Overall proteolysis in perfused and subfractionated chemically induced malignant hepatoma of rat: effects of amino acids.

Control livers and chemically induced hepatoma-bearing livers of nonstarved rats were perfused cyclically with and without the addition of amino acids (known to suppress proteolysis) to the perfusate. Morphologic analysis of the fractional cytoplasmic volume of the lysosomal apparatus (dense bodies and autophagic vacuoles) demonstrated that the addition of amino acids to the perfusion medium inhibited autophagic sequestration of cytoplasm in both tumor and control hepatocytes, although the inhibition was stronger in control than in tumor hepatocytes. The fractional cytoplasmic volume of autophagic vacuoles (AVs) was larger in hepatoma cells than in control hepatocytes regardless of whether amino acids were added or not. The transition (degradation of sequestered cytoplasm) of AVs into dense bodies seems to be prolonged in malignant hepatoma cells. Assessment of rates of protein degradation both in the perfusion medium and in isolated lysosomes disclosed that proteolysis was much lower in tumor liver than in control liver. This can be explained by lower lysosomal enzyme activities in tumor cells, as was evident from tissue homogenate and isolated lysosomes. The addition of amino acids to the perfusate reduced total proteolysis from 1.73 to 0.78% per hour in control hepatocytes and from 0.49 to 0.33% per hour in tumor hepatocytes, i.e., inhibitions of 55 and 33%, respectively. Proteolysis as estimated from isolated lysosomes was also inhibited by amino acids added to the perfusion medium but the inhibition was more conspicuous in control (from 14 to 7.4%) than in tumor cells (from 5.2 to 3.6%). In conclusion, the results show that the relative cytoplasmic volume of AVs is higher but overall proteolysis lower in malignant hepatoma tissue than in control liver. Amino acids in perfusion medium inhibit overall proteolysis and AVs sequestration in both tumor and control hepatocytes, although the inhibition is stronger in control hepatocytes. Thus, even highly neoplastic cells maintain their ability to respond to physiologic regulators.