Quantitative changes in the lysosomal vacuolar system of rat hepatocytes during short-term starvation

Abstract

Ultrastructural morphometric analysis was used to study time-dependent variations in macro- and microautophagy in rat hepatocytes. Except during periods of short-term starvation for up to 24 h, animals were kept under standardized conditions of food intake. In hepatocytes of meal-fed rats the volume fraction of macroautophagic vacuoles is significantly higher at 23:00 h, i.e., immediately before food intake, compared to 11:00 h, i.e., 12 h following feeding. During fasting, macroautophagy drops to a low level. Microautophagic vacuoles in hepatocytes of meal-fed rats, sacrificed at 11:00 or 23:00 h respectively, do not show any significant quantitative differences. However, during 12 h of starvation, the volume fraction of microautophagic vacuoles rises significantly, whereas the numerical density remains constant. Subsequently, during the second 12-h period of fasting, the volume fraction of microautophagic vacuoles remains unchanged, but the numerical density increases. Over a period of 24 h of starvation the volume fraction of the total lysosomal system does not change significantly, whereas the numerical density rises. The time-dependent changes of the macroautophagic vacuolar system correlate with the circadian, food-related variations in the protein content of individual hepatocytes from meal-fed animals. The increase in volume fraction and thereafter in number of microautophagic vacuoles, as observed during starvation, coincides with a large decrease in protein content of individual hepatocytes.

Microautophagy in mammalian cells: Revisiting a 40-year-old conundrum

“…56 In rat myocardial cells, the appearance of autophagic vacuoles was observed as late as 2 days after the onset of starvation, indicating a delayed lysosomal response in the process of protein degradation. 54 By contrast, in rat hepatocytes microautophagy (as determined by the measurement of [ 14 C]valine release from degradation of radiolabeled long-lived proteins) was a rather prominent feature during short-term starvation (12-24 h or sometimes up to 48 h) (Fig. 2C).…”

“…Several studies highlight a range of findings including: (1) that lysosomes show microautophagy activity in vitro, which may be a mechanism for degradation of soluble cytoplasmic constituents in vivo, 51 (2) a lysosomal wrapping mechanism having features resembling those of microautophagy, 6,52 (3) changes in microautophagy apparently related to intrinsic alterations within cells (e.g., liver protein content, rates of protein synthesis and degradation, volumes of lysosomal-vacuolar components), 53 and (4) characterization of starvation-induced microautophagic vacuoles. 54 Glaumann and colleagues 51 showed that isolated rat liver lysosomes incubated with Percoll particles in vitro contain a number of vesicles, some of which contain (one or more) Percoll particles plus surrounding fluid. Percoll particles can sometimes be seen free in the lysosomal matrix, presumably 'released' from the vesicle by degradation of the vesicle membrane.…”

“…2C). 54,56 It appears unlikely that the initiating event in the onset of cardiac protein degradation would be a change in lysosomal function. The additional work necessary to and subsequent fusion of the membrane folds.…”

“…6,52 Using electron microscopy as the main detection technique, several forms of microautophagy-like sequestration structures have been described in mouse/rat liver hepatocytes. [53][54][55][56] How these structures are formed, however, has not been elucidated. It has been suggested that microautophagy might be responsible for basal protein degradation/turnover of long-lived (resident) proteins by measurement of the release of [ 14 C]valine in liver homogenates.…”

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“…During microautophagy, portions of the cytoplasm are taken up directly by the lysosomes by invagination of their membranes (Ahlberg et al, 1982;Mortimore et al, 1983Mortimore et al, , 1988ade Waal et al, 1986). Microautophagosomes are formed, but they are inside the lysosomes, where they are rapidly degraded.…”