The continuous turnover of intracellular protein and other macromolecules is a basic cellular process that serves, among other functions, to regulate cytoplasmic content and provide amino acids for ongoing oxidative and biosynthetic reactions during nutrient deprivation. The intensity of breakdown and pattern of regulation, though, vary widely among cells. Rat hepatocytes, for example, exhibit high absolute rates of proteolysis and regulatory effects that diminish during starvation, while corresponding responses in skeletal and cardiac muscle move in the opposite direction. It is also becoming apparent that effects of insulin and other acute regulatory agents on muscle breakdown are limited to nonmyofibrillar components. The latter may be sequestered and degraded within autophagic vacuoles, whereas myofibrillar proteins require an initial attack by calcium-dependent proteases in the cytosol. By contrast, most if not all of the breakdown of resident (long-lived) proteins as well as RNA in the hepatocyte can be explained by lysosomal mechanisms. The uptake of cytoplasmic components by lysosomes can be divided into two major categories, macroautophagy and micro- or basal autophagy. The first is induced by amino acid or insulin/serum deprivation. In the hepatocyte, amino acids alone can regulate this process almost instantaneously over two thirds of the full range of proteolysis, 4.5% to 1.5% per hour. Glucagon, cyclic AMP, and beta-agonists also stimulate macroautophagy in hepatocytes but have opposite effects in skeletal and cardiac myocytes. Basal autophagy differs from the macro type in that the cytoplasmic "bite" is smaller and sequestration is not acutely regulated. It is, however, adaptively decreased during starvation in parallel with absolute rates of basal turnover. Since endoplasmic reticulum comprises an appreciable fraction of the vacuolar content, volume sequestration would be compatible with the known heterogeneity of individual protein turnover if some proteins (or altered proteins) selectively bind to membranes. The amino acid control of macroautophagy in the hepatocyte is accomplished by a small group of direct inhibitors (Leu, Tyr/Phe, Gln, Pro, Met, Trp, and His) and the permissive effect of alanine whereas only leucine is involved in myocytes and adipocytes. Of unusual interest is the fact that the inhibitory amino acid group alone evokes responses in perfused livers that are identical to those of a complete plasma mixture at 0.5 and 4 times normal plasma levels but loses effectiveness almost completely at normal concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)

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Amino acid deprivation and glucagon are both potent inducers of autophagy and proteolysis in liver. Because glucagon enhanced the metabolic utilization of some amino acids, the catabolic response to both of these stimuli could be achieved by a lowering of intracellular amino acid pools. Al-ternatively, glucagon could act independently of amino acids.To clarify the mode of hormonal action and also the relationship between the two cellular responses, livers from fed rats were perfused, with and without glucagon, with plasma amino acids over a concentration range of 0 to 10 times normal. Individual amino acids constancy at each level was ensured by perfusion in the single-pass mode. Amino acids alone strongly regulated autophagy and proteolysis in a coordinated fashion; maximal suppression was achieved at twice normal concentration; both effects increased rapidly to maximum at less than normal concentration. Corresponding effects of glucagon, however, could be elicited only at intermediate amino acid levels. None was noted at 4 and 10 times normal; at 0, hormonal stimulation was minimal. The amino acid inhibition was selective because it did not block cyclic AMP production or glycogenolysis. Intracellular pool measurements and systematic alteration of perfusate amino acid composition indicated that the autophagic and proteolytic effects of glucagon are mediated by a hormonally induced depletion of glycine, alanine, glutamate, and glutamine; of these, glutamine alone is the most effective. We conclude that the stinfulation of intracellular protein degradation in liver is a manifestation of deprivation-induced autophagy which results from a decrease in certain intracellular glucogenic amino acids, notably glutamine. The enhancement of intracellular protein breakdown in response to nutritional deprivation is a basic process that is manifested to varying degrees in most cells (1). In rat liver the response is of sufficient sensitivity and magnitude that it could play an important role in the homeostasis of extracellular amino acids during the postabsorptive period (2, 3). Although the mechanism of this deprivation-induced proteolysis is not completely known, it does seem clear that in cells of isolated, normal rat livers there is a dramatic internalization of cytoplasmic constituents within autophagic vacuoles when extracellular amino acid levels are decreased or insulin is lacking (4, 5). The quantity of cytoplasmic protein that is continuously taken up and digested by lysosomes under these conditions appears to explain adequately the associated increase in total protein breakdown (6).Cellular catabolism also appears to be enhanced by certain hormones. It is well known that glucagon accelerates glycogen breakdown in liver. This hormone also brings about a net loss of liver protein (7-9). In addition, it is a potent inducer of hepatic autophagy (10-12) and has been reported to decrease some intracellular amino acid pools, an effect attributable largely to the enhanced utilization of amino acids by gluconeogene...

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