Control of Muscle Mitochondria by Insulin Entails Activation of Akt2-mtNOS Pathway: Implications for the Metabolic Syndrome

Abstract

BackgroundIn the metabolic syndrome with hyperinsulinemia, mitochondrial inhibition facilitates muscle fat and glycogen accumulation and accelerates its progression. In the last decade, nitric oxide (NO) emerged as a typical mitochondrial modulator by reversibly inhibiting citochrome oxidase and oxygen utilization. We wondered whether insulin-operated signaling pathways modulate mitochondrial respiration via NO, to alternatively release complete glucose oxidation to CO2 and H2O or to drive glucose storage to glycogen.Methodology/Principal FindingsWe illustrate here that NO produced by translocated nNOS (mtNOS) is the insulin-signaling molecule that controls mitochondrial oxygen utilization. We evoke a hyperinsulinemic-normoglycemic non-invasive clamp by subcutaneously injecting adult male rats with long-lasting human insulin glargine that remains stable in plasma by several hours. At a precise concentration, insulin increased phospho-Akt2 that translocates to mitochondria and determines in situ phosphorylation and substantial cooperative mtNOS activation (+4–8 fold, P<.05), high NO, and a lowering of mitochondrial oxygen uptake and resting metabolic rate (−25 to −60%, P<.05). Comparing in vivo insulin metabolic effects on gastrocnemius muscles by direct electroporation of siRNA nNOS or empty vector in the two legs of the same animal, confirmed that in the silenced muscles disrupted mtNOS allows higher oxygen uptake and complete (U-14C)-glucose utilization respect to normal mtNOS in the vector-treated ones (respectively 37±3 vs 10±1 µmolO2/h.g tissue and 13±1 vs 7.2±1 µmol 3H2O/h.g tissue, P<.05), which reciprocally restricted glycogen-synthesis by a half.Conclusions/SignificanceThese evidences show that after energy replenishment, insulin depresses mitochondrial respiration in skeletal muscle via NO which permits substrates to be deposited as macromolecules; at discrete hyperinsulinemia, persistent mtNOS activation could contribute to mitochondrial dysfunction with insulin resistance and obesity and therefore, to the progression of the metabolic syndrome.

The role of nNOS and PGC-1α in skeletal muscle cells

“…In fact, reduced mitochondrial energy production and decreased energy expenditure contribute largely to metabolic dysfunctions that are typical of aging, including insulin resistance and diabetes, which ultimately lead to lipid and glycogen accumulation, in turn further increasing insulin resistance (Derave et al, 2000;Finocchietto et al, 2008).…”

“…2v) (Elfering et al, 2002). mtNOS localises to mitochondria of several tissues, including skeletal muscle, where it regulates oxygen consumption (Aguirre et al, 2012;Finocchietto et al, 2008). At least in skeletal muscle, mtNOS appears to be a mitochondrial form of nNOS and can regulate mitochondrial respiration.…”

“…upon hyperinsulinemia), mtNOS can be activated in situ by Akt, leading to decreased mitochondrial respiration in the muscle (Finocchietto et al, 2008). Hence, persistent mtNOS activation could contribute to mitochondrial dysfunction upon insulin resistance and, therefore, to the progression of the metabolic syndromes (Finocchietto et al, 2008).…”

Mitochondrial nitric oxide metabolism during rat heart adaptation to high altitude: effect of sildenafil,l-NAME, andl-arginine treatments

“…mtNOS activity has been found to be regulated by important physiological effectors; it is downregulated by ANG II (6) and thyroid hormones (15) and upregulated by insulin (16), the autonomic system (15), and during cold acclimation (37). Environmental O 2 pressure has been recognized as a physiological regulator of heart mtNOS activity.…”

The Akt isoforms, their unique functions and potential as anticancer therapeutic targets

“…In light of the data presented by Santi and Lee who used isoform-specific antibodies and siRNA in their experiment (9), the Akt associated with the mitochondria described by Bijur and Jope is probably Akt2, not a selective uptake of (any) Akt. Consistent with this interpretation, Finocchietto et al (112) found that phosphorylated Akt2 translocated to the inner membrane of the mitochondria in response to insulin and phosphorylated nNOS.…”