Mfn1 Deficiency in the Liver Protects Against Diet-Induced Insulin Resistance and Enhances the Hypoglycemic Effect of Metformin

Abstract

Mitochondrial function can be influenced by mitochondrial shape and connectivity with other cellular organelles through fusion and fission processes. Disturbances in mitochondrial architecture and mitochondrial fusion-related genes are observed in situations of type 2 diabetes and obesity, leading to a highly fissioned mitochondrial network. To directly test the effect of reduced mitochondrial fusion on hepatic metabolism, we generated mice with a liver-specific deletion of the Mfn1 gene (Mfn1LKO) and monitored their energy homeostasis, mitochondrial function, and susceptibility to diet-induced insulin resistance. Livers from Mfn1LKO mice displayed a highly fragmented mitochondrial network. This was coupled to an enhanced mitochondrial respiration capacity and a preference for the use of lipids as the main energy source. Although Mfn1LKO mice are similar to control mice fed a low-fat diet, they are protected against insulin resistance induced by a high-fat diet. Importantly, Mfn1 deficiency increased complex I abundance and sensitized animals to the hypoglycemic effect of metformin. Our results suggest that targeting Mfn1 could provide novel avenues to ameliorate glucose homeostasis in obese patients and improve the effectiveness of metformin.

Mfn2 is critical for brown adipose tissue thermogenic function

“…Mfn1 is an essential protein for mitochondria fusion events. Accordingly, decreased Mfn1 function has been shown to trigger a profound shift in the fusion/fission balance, toward a dramatically fragmented mitochondrial network in most, if not all, cells and tissues tested to date (Chen et al , 2003; Park et al , 2008; Papanicolaou et al , 2012; Dietrich et al , 2013; Kulkarni et al , 2016). Further, While Mfn1 mRNA levels did not compensate for Mfn2 loss, we observed that Mfn1 protein slightly decreased in the BAT and WAT form Mfn2‐adKO mice (Fig EV1A).…”

“…The values obtained with Ama were substracted, and the relative increases in O 2 consumption triggered by CL were measured respective to basal respiration for each adipocyte preparation. Respirometry analyses in isolated mitochondria were performed as recently described (Kulkarni et al , 2016). …”

“…MEFs (Kulkarni et al , 2016) and immortalized mouse brown pre‐adipocytes (Boutant et al , 2015) were grown and differentiated as previously described in mycoplasma‐free conditions. Adipocytes were considered fully differentiated at day 6 after the onset of the differentiation protocol, when they exhibited multilocular lipid droplets in their cytoplasm.…”

“…For Western blotting, proteins were separated by SDS–PAGE and transferred onto nitrocellulose membranes. The following antibodies were used: antibodies against mouse Mfn1 (covering the amino acid span from 720 to 741) and Mfn2 (739–757) were generated by YenZym Antibodies LLC (San Francisco, USA) and validated internally using specific Mfn1 or Mfn2 overexpression and gene ablation models (see Figs 1B and EV4B and also Kulkarni et al , 2016); GAPDH (#2118), HK I (#2024), HK II (#2867), P‐HSL (#4139S), HSL (#4107S), ACC (#3676S), and PLIN1 (#9349) were purchased from Cell signaling; P‐ACC was obtained from Millipore (#07‐303); Antibodies against Complex I (NDUFA9, ab14713), Complex II (SDHA, ab14715), Complex III (UQCRC1, ab110252), Complex V (ATP5A, ab14748), GLUT4 (ab654), PFK1 (ab154804), IRE (ab37073), and P‐IRE (ab104157) were from Abcam, while α‐tubulin (T9026) was purchased from Sigma. For protein immunoprecipitation (IP) experiments, antibodies were pre‐incubated with magnetic beads (Invitrogen 10002D) and 300 μg of proteins extracts was pre‐cleared with clean beads during 1 h at 4°C under agitation.…”

Mitochondrial Dysfunction in Age‐Related Metabolic Disorders

“…Targeting liver specific MFN1 has been shown to induce a metabolic switch from glucose to lipid metabolism, enhance respiratory capacity, and offer protection against insulin‐resistance in a high fat diet (HFD) induced obese mice model. [ 70 ] Interestingly, loss of MFN1 in the liver enhances hypoglycemic activity of Metformin by increasing AMPK activity. Hypothalamic Proopiomelanocortin neurons play a major role in regulating food intake, nutrient sensing, energy expenditure, and metabolic adaptations.…”