In yeast, loss of Hog1 leads to osmosensitivity of autophagy

In mammalian liver, proteolysis is regulated by the cellular hydration state in a microtubule- and p38^MAPK (p38 mitogen-activated protein kinase)-dependent fashion. Osmosensing in liver cells towards proteolysis is achieved by activation of integrin receptors. The yeast orthologue of p38^MAPK is Hog1 (high-osmolarity glycerol 1), which is involved in the hyperosmotic-response pathway. Since it is not known whether starvation-induced autophagy in yeast is osmosensitive and whether Hog1 is involved in this process, we performed fluorescence microscopy experiments. The hog1Δ cells exhibited a visible decrease of autophagy in hypo-osmotic and hyperosmotic nitrogen-starvation medium as compared with normo-osmolarity, as determined by GFP (green fluorescent protein)–Atg8 (autophagy-related 8) fluorescence. Western blot analysis of GFP–Atg8 degradation showed that WT (wild-type) cells maintained a stable autophagic activity over a broad osmolarity range, whereas hog1Δ cells showed an impaired autophagic actitivity during hypo- and hyper-osmotic stress. In [³H]leucine-pre-labelled yeast cells, the proteolysis rate was osmodependent only in hog1Δ cells. Neither maturation of pro-aminopeptidase I nor vitality was affected by osmotic stress in either yeast strain. In contrast, rapamycin-dependent autophagy, as measured by degradation of GFP–Atg8, did not significantly respond to hypo-osmotic or hyperosmotic stress in hog1Δ or WT cells. We conclude that Hog1 plays a role in the stabilization machinery of nitrogen-deprivation-induced autophagy in yeast cells during ambient osmolarity changes. This could be an analogy to the p38^MAPK pathway in mammalian liver, where osmosensing towards p38^MAPK is required for autophagy regulation by hypo-osmotic or amino-acid-induced cell swelling. A phenotypic difference is observed in rapamycin-induced autophagy, which does not seem to respond to extracellular osmolarity changes in hog1Δ cells.