Emerging role of autophagy in mediating widespread actions of ADIPOQ/adiponectin

Abstract

Autophagy can dictate changes in cell metabolism via numerous mechanisms. ADIPOQ/adiponectin has been extensively characterized to have beneficial metabolic effects, both via INS/insulin-sensitizing and INS-independent actions. Our recent work examined the regulation of skeletal muscle autophagy by ADIPOQ and the functional significance. We showed that ADIPOQ directly stimulates autophagic flux in cultured skeletal muscle cells via an AMPK-dependent signaling pathway leading to phosphorylation of ULK1 (Ser555). Pharmacological inhibition of autophagy or overexpressing an inactive mutant of ATG5 to create an autophagy-deficient cell model reduces INS sensitivity. A high-fat diet (HFD) does not induce skeletal muscle autophagy in Adipoq knockout (Ad-KO) mice, whereas it does in wild-type (WT) mice, although ADIPOQ replenishment in Ad-KO mice stimulates autophagy. Changes in skeletal muscle autophagy correlate well with peripheral INS sensitivity and glucose metabolism. Thus, ADIPOQ stimulates autophagic flux in skeletal muscle, which likely represents one important mechanism mediating multiple favorable metabolic effects.

The data in our recent study fit into a broader research context, including rapidly emerging literature on the importance of autophagy in skeletal muscle metabolism. For example, in stimulus-induced autophagy-deficient mice, the beneficial metabolic effects of short- and long-term exercise are attenuated. Activation of autophagy in muscle has also been reported in response to caloric restriction, which is associated with improved INS sensitivity and metabolic profile. Furthermore, fiber-type specificity may be important because starvation-induced autophagic flux is greater in glycolytic versus highly oxidative muscle, and this correlates with AMPK and MTOR activities.

Numerous studies have characterized the antidiabetic effects of ADIPOQ, yet the precise cellular mechanisms in skeletal muscle, in particular the ability of ADIPOQ to stimulate autophagy and the consequent functional significance, was uncharacterized until our recent study. The take-home message from this work is that ADIPOQ directly stimulates autophagic flux in muscle via AMPK activation. Next, it will be important to determine the precise mechanisms via which ADIPOQ regulates autophagy and here we outline several potential avenues of investigation for future research. For example, do ADIPOR1- and/or ADIPOR2-mediated signaling pathways elicit similar outcomes? Our data indicated that AMPK plays a central role. But is it a prerequisite or do alternative pathways exist? We have demonstrated that ADIPOQ induces remodeling of the actin cytoskeleton, which is also a significant player in alterations in autophagy. Does increased autophagic flux, and the consequent alteration in scaffolding of intracellular signaling complexes, directly influence action of ADIPOQ itself or hormones such as INS? Bidirectional crosstalk exists between autophagy and oxidative stress or endoplasmic reticulum stress and we think that ADIPOQ may influence autophagy secondary to attenuation of these cellular stresses.

In our recent study, we overexpressed an inactive mutant of ATG5 to create an autophagy-deficient cell model and, together with pharmacological inhibition of autophagy, demonstrated reduced INS sensitivity under these conditions. It will now be important to further investigate the functional significance of ADIPOQ-induced autophagic flux using additional gain- or loss-of-function approaches both in vivo and in vitro. We have now developed autophagy-deficient skeletal and cardiac muscle cell lines using CRISPR to delete Atg5, Atg7, or Becn1. Furthermore, expanding this work by capitalizing on the rapidly emerging armory of new reagents and approaches will allow us to examine whether ADIPOQ targets specific forms of autophagy, with mitophagy, lipophagy, and ferritinophagy likely the most attractive to consider. Various tissue-specific autophagy-deficient mouse models, in particular on genetic backgrounds with or without ADIPOQ expression, will also be invaluable in further examining the physiological significance of our initial observations.

The phenomenon of increased autophagic flux as an important mechanism of ADIPOQ actions may have widespread importance. For example, ADIPOQ prevents acute liver failure via promoting autophagy-mediated clearance of damaged mitochondria, and we observed that stimulation of autophagy by ADIPOQ in cardiomyocytes contributes to beneficial metabolic and anti-apoptotic effects. ADIPOQ stimulates autophagy in macrophages protected from angiotensin II-induced inflammation and cardiac fibrosis. It is very likely that regulation of autophagy by ADIPOQ, and subsequently altered cellular effects such as apoptosis and metabolism, may also have significant implications in cancer cells. Enhanced autophagic flux may contribute to anticancer effects of ADIPOQ, again likely via mechanisms including clearance of scaffold proteins and reduced oxidative stress. Nevertheless, it is conceivable that enhanced survival in the face of a lack of nutrients may promote cancer under these conditions. It will be particularly interesting to test this in breast cancer, where ADIPOQ has predominantly beneficial effects. Thinking beyond ADIPOQ action only, we hope that multidisciplinary future studies will bear in mind the general concept emanating from our work since it is conceivable that numerous hormones may mediate physiological effects such as changes in INS sensitivity and metabolism via regulating autophagic flux.

In summary, we propose that ADIPOQ stimulates autophagic flux in muscle and other tissues, particularly under pathological conditions, and that this is an underappreciated contributor to advantageous metabolic outcomes. Nevertheless, it is conceivable that excessive activation of muscle autophagy may have adverse consequences. We think it is now time to determine the temporal nature and scale of changes in autophagy, or specific types of autophagy, in response to ADIPOQ in various models of metabolic disease. The ensuing comprehensive understanding of this notion may allow efficient and safe translation toward therapeutic targeting in disease states spanning diabetes, cardiovascular disease, and cancer.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest declared.

Funding

Related work in the authors' laboratories is funded by Canadian Institutes for Health Research, Canadian Diabetes Association, Heart and Stroke Foundation of Canada (GS), and Hong Kong Research Grant Council (XA).