Heat shock protein 72 regulates hepatic lipid accumulation

Abstract

Induction of the chaperone heat shock protein 72 (HSP72) through heat treatment (HT), exercise, or overexpression improves glucose tolerance and mitochondrial function in skeletal muscle. Less is known about HSP72 function in the liver where lipid accumulation can result in insulin resistance and nonalcoholic fatty liver disease (NAFLD). The purpose of this study was 1) to determine whether weekly in vivo HT induces hepatic HSP72 and improves glucose tolerance in rats fed a high-fat diet (HFD) and 2) to determine the ability of HSP72 to protect against lipid accumulation and mitochondrial dysfunction in primary hepatocytes. Male Wistar rats were fed an HFD for 15 wk and were given weekly HT (41°C, 20 min) or sham treatments (37°C, 20 min) for the final 7 wk. Glucose tolerance and insulin sensitivity were assessed, along with HSP72 induction and triglyceride storage, in the skeletal muscle and liver. The effect of an acute loss of HSP72 in primary hepatocytes was examined via siRNA. Weekly in vivo HT improved glucose tolerance, elevated muscle and hepatic HSP72 protein content, and reduced muscle triglyceride storage. In primary hepatocytes, mitochondrial morphology was changed, and fatty acid oxidation was reduced in small interfering HSP72 (siHSP72)-treated hepatocytes. Lipid accumulation following palmitate treatment was increased in siHSP72-treated hepatocytes. These data suggest that HT may improve systemic metabolism via induction of hepatic HSP72. Additionally, acute loss of HSP72 in primary hepatocytes impacts mitochondrial health as well as fat oxidation and storage. These findings suggest therapies targeting HSP72 in the liver may prevent NAFLD.

Heat Treatment Improves Hepatic Mitochondrial Respiratory Efficiency via Mitochondrial Remodeling

“…Briefly, we inserted synthetic introns (via guide RNA) into mouse embryonic stem cells, where loxP sites were then inserted flanking the functional domain of Hspa1a to create a premature stop codon ( Figure S1 ). C57Bl/6J mice with the floxed Hspa1a gene were bred together and subsequently, 10-week-old male floxed mice were exposed to AAV8-Alb-Cre (107787-AAV8) or AAV8-Alb-GFP (105535-AAV8) obtained from Vector Biolabs (Malvern, PA, USA) via tail vein injection (1 × 10 11 CG/mL diluted in 200 µL of saline) to generate L-HSP72KO and WT littermates, respectively ( Figure S1 ). All WT and L-HSP72KO mice were left for 14 days postviral injection to ensure viral delivery and thus they were 12 weeks of age at the beginning of experimentation ( n = 6/group for acute and chronic studies; SHM vs HT, WT vs l-HSP72KO).…”

“…10 One potential strategy that our group has explored to great effect in animals and primary hepatocytes is heat treatment (HT). 11 , 12 …”

“…First established by Philip Hooper in 1999, 13 we and others have consistently shown that chronic whole-body HT positively impacts glucose homeostasis in both obese humans and animals. 11 , 12 , 14–17 Importantly, the current hypothesis surrounding the positive metabolic effects of HT is tied to increased Heat Shock Protein 72 (HSP72) levels. 18 This hypothesis has been propagated by findings that HSP72 content is reduced in both liver and muscle tissue during obese conditions 15 , 19 and that restoration of HSP72 levels via HT is associated with positive metabolic benefits including lowered hepatic lipid storage.…”

“… 18 This hypothesis has been propagated by findings that HSP72 content is reduced in both liver and muscle tissue during obese conditions 15 , 19 and that restoration of HSP72 levels via HT is associated with positive metabolic benefits including lowered hepatic lipid storage. 11 , 12 , 14 , 15 , 20 Moreover, increased HSP72 content via genetic overexpression or pharmacologic activation in animals fed a high-fat diet (HFD) is shown to benefit insulin sensitivity and skeletal muscle mitochondrial content. 15 , 21–24 We have also recently shown that acute knockdown of HSP72 in primary hepatocytes causes increased lipid uptake, reduced fatty acid oxidation (FAO), and depolarization of mitochondria.…”

“… 15 , 21–24 We have also recently shown that acute knockdown of HSP72 in primary hepatocytes causes increased lipid uptake, reduced fatty acid oxidation (FAO), and depolarization of mitochondria. 11 However, difficulties in generating a tissue-specific knockout model (floxed alleles) have kept the field from definitively showing a role for HSP72 in muscle and/or liver metabolic adaptations with HT.…”