PARTICIPATION OF LYSOSOMES IN CELLULAR AUTOPHAGY INDUCED IN RAT LIVER BY GLUCAGON

Numerous observations made over the last five years have established the widespread occurrence of cellular autophagy as well as its importance in the processes whereby living cells achieve the degradation of their own constituents (For a review, see reference 5). In all the cases that have been investigated in this respect, autophagic vacuoles have been found to stain positively for acid phosphatase, suggesting strongly that lysosomal enzymes are the main agents of the digestive processes taking place within them. However, the mechanism of the segregation phenomenon and the manner in which hydrolases become associated with the sequestered area are still far from being understood. One theory, proposed by Novikoff et al. (9), considers areas of endoplasmic reticulum, assumed to be involved in the intracellular transport of newly synthesized hydrolases, as providing both the surrounding membrane of autophagic vacuoles and their enzymic complement. According to this theory, cellular autophagy represents a mechanism, though not necessarily the only one, whereby new lysosomes may arise in a living cell. Other workers have expressed similar ideas concerning the origin of lysosomes (1, 8). An alternative hypothesis, put forward by de Duve and Wattiaux (5), considers preexisting lysosomes as the source of the enzymes that are found in autophagic vacuoles, on the assumption, either that the particles actually participate in the sequestration phenomenon, or that they fuse with initially hydrolase-less "autophagosomes" as they are known to do with the "heterophagosomes" formed by endocytosis. In a recent investigation, Deter and de Duve (7) have found that the injection of a large dose of glucagon, a powerful inducer of autophagy in liver, causes a transient, but manifest increase in the mechanical and osmotic fragility and in the median sedimentation coefficient of rat-liver lysosomes. These changes were tentatively attributed to an enlargement of the lysosomes, possibly reflecting extensive participation of these particles in the autophagic process. But in the absence of morphological data, no firm conclusion could be reached. The present communication reports the first results of a quantitative morphological study correlating the appearance of autophagic vacuoles with a marked decrease in the number of recognizable dense bodies present in particulate fractions separated from hepatic homogenates 45 min after glucagon injection.