Long-lived mice with reduced growth hormone signaling have a constitutive upregulation of hepatic chaperone-mediated autophagy

Chaperone-mediated autophagy (CMA) is the most selective form of lysosomal proteolysis. CMA modulates proteomic organization through selective protein degradation, with targets including metabolic enzymes, cell growth regulators, and neurodegeneration-related proteins. CMA activity is low in ad libitum-fed rodents but is increased by prolonged fasting. AKT negatively regulates CMA at the lysosomal membrane by phosphorylating and inhibiting the CMA regulator GFAP. We have previously reported that long-lived Pou1f1/Pit1 mutant (Snell) mice and ghr (growth hormone receptor) knockout mice (ghr KO) have lower AKT activity when fed compared to littermate controls, suggesting the hypothesis that these mice have increased baseline CMA activity. Here, we report that liver lysosomes from fed Snell dwarf mice and ghr KO mice have decreased GFAP phosphorylation and increased CMA substrate uptake activity. Liver lysosomes isolated from fed Snell dwarf mice and ghr KO mice injected with the protease inhibitor leupeptin had increased accumulation of endogenous CMA substrates, compared to littermate controls, suggesting an increase in CMA in vivo. Mice with liver-specific ablation of GH (growth hormone) signaling did not have increased liver CMA, suggesting that a signaling effect resulting from a loss of growth hormone in another tissue causes enhanced CMA in Snell dwarf and ghr KO mice. Finally, we find Snell dwarf mice have decreased protein levels (in liver and kidney) of CIP2A, a well-characterized CMA target protein, without an associated change in Cip2a mRNA. Collectively, these data suggest that CMA is enhanced downstream of an endocrine change resulting from whole-body ablation of GH signaling. Abbreviations: CMA: chaperone-mediated autophagy; GH: growth hormone; ghr KO: growth hormone receptor knockout; LAMP2A: splice variant 1 of Lamp2 transcript; LC3-I: non-lipidated MAP1LC3; LC3-II: lipidated MAP1LC3; Li-ghr KO: liver-specific ghr knockout; MA: macroautophagy; MTORC1: mechanistic target of rapamycin kinase complex 1; MTORC2: mechanistic target of rapamycin kinase complex 2; PBS: phosphate-buffered saline.