Intramyocellular lipids: effect of age, obesity, and exercise

Leptin receptor defect with diabetes causes skeletal muscle atrophy in female obese Zucker rats where peculiar depots networked with mitochondrial damages

Tibialis anterior muscles of 45-week-old female obese Zucker rats with defective leptin receptor and non-insulin dependent diabetes mellitus (NIDDM) showed a significative atrophy compared to lean muscles, based on histochemical-stained section's measurements in the sequence: oxidative slow twitch (SO, type I) < oxidative fast twitch (FOG, type IIa) < fast glycolytic (FG, type IIb). Both oxidative fiber's outskirts resembled 'ragged' fibers and, in these zones, ultrastructure revealed small clusters of endoplasm-like reticulum filled with unidentified electron contrasted compounds, contiguous and continuous with adjacent mitochondria envelope. The linings appeared crenated stabbed by circular patterns resembling those found of ceramides. The same fibers contained scattered degraded mitochondria that tethered electron contrasted droplets favoring larger depots while mitoptosis were widespread in FG fibers. Based on other interdisciplinary investigations on the lipid depots of diabetes 2 muscles made us to propose these accumulated contrasted contents to be made of peculiar lipids, including acyl-ceramides, as those were only found while diabetes 2 progresses in aging obese rats. These could interfere in NIDDM with mitochondrial oxidative energetic demands and muscle functions.

Perilipin 2 and Age-Related Metabolic Diseases: A New Perspective

Perilipin 2 (Plin2), a protein associated with the metabolism of intracellular lipid droplets (LDs), has long been considered only for its role in lipid storage. However, the manipulation of its expression affects the severity of a variety of metabolic and age-related diseases, such as fatty liver, insulin resistance and type 2 diabetes (T2D), cardiovascular disease, atherosclerosis, sarcopenia, and cancer, suggesting that this protein may play a role in these pathological conditions. In particular, its downregulation in mice prevents or mitigates some of the above mentioned diseases. Conversely, in humans high levels of Plin2 are present in sarcopenia, hepatic steatosis, atherosclerosis, and some types of cancer. We propose that inhibition of Plin2 might be a strategy to counteract several metabolic and age-related diseases.

Lkb1 deletion promotes ectopic lipid accumulation in muscle progenitor cells and mature muscles

Excessive intramyocellular triglycerides (muscle lipids) are associated with reduced contractile function, insulin resistance, and Type 2 diabetes, but what governs lipid accumulation in muscle is unclear. Here we report a role of Lkb1 in regulating lipid metabolism in muscle stem cells and their descendent mature muscles. We used Myod(Cre) and Lkb1(flox/flox) mice to specifically delete Lkb1 in myogenic cells including stem and differentiated cells, and examined the lipid accumulation and gene expression of myoblasts cultured from muscle stem cells (satellite cells). Genetic deletion of Lkb1 in myogenic progenitors led to elevated expression of lipogenic genes and ectopic lipid accumulation in proliferating myoblasts. Interestingly, the Lkb1-deficient myoblasts differentiated into adipocyte-like cells upon adipogenic induction. However, these adipocyte-like cells maintained myogenic gene expression with reduced ability to form myotubes efficiently. Activation of AMPK by AICAR prevented ectopic lipid formation in the Lkb1-null myoblasts. Notably, Lkb1-deficient muscles accumulated excessive lipids in vivo in response to high-fat diet feeding. These results demonstrate that Lkb1 acts through AMPK to limit lipid deposition in muscle stem cells and their derivative mature muscles, and point to the possibility of controlling muscle lipid content using AMPK activating drugs.

Pomegranate and green tea extracts protect against ER stress induced by a high-fat diet in skeletal muscle of mice

PURPOSE

We tested the hypothesis that polyphenol-rich extracts can reduce endoplasmic reticulum (ER) stress induced by a high-fat diet (HFD) in skeletal muscle of mice.

METHODS

Mice were randomly assigned to four groups receiving during 20 weeks either a standard chow control (CTRL), or a HFD supplemented, or not, with pomegranate (HFD + P) or green tea (HFD + GT) extracts. After the nutritional intervention, mice were killed and gastrocnemius muscles were taken. Proteins and mRNA were measured by Western blot and RT-qPCR, respectively.

RESULTS

Body weight gain and visceral fat were higher in HFD, HFD + P and HFD + GT than in CTRL. The markers of the unfolded protein response BiP, XBP1u, XBP1s and ATF4 were higher only in HFD. In HFD + P and HFD + GT, this increase was not observed except for CHOP, which was elevated in all HFD groups. HFD increased also markers of ubiquitin-proteasome pathway, autophagy and oxidative stress, which were kept low in HFD + P and HFD + GT groups.

CONCLUSION

Our data provide evidence for a protective effect of pomegranate and green tea extracts against ER stress, oxidative stress and protein degradation induced by HFD in skeletal muscle. They give arguments for a usefulness of these natural nutritional compounds to fight against cellular dysfunctions related to fat excess.

mTORC1 and the regulation of skeletal muscle anabolism and mass

The mass and integrity of skeletal muscle is vital to whole-body substrate metabolism and health. Indeed, defects in muscle metabolism and functions underlie or exacerbate diseases like diabetes, rheumatoid arthritis, and cancer. Physical activity and nutrition are the 2 most important environmental factors that can affect muscle health. At the molecular level, the mammalian target of rapamycin complex 1 (mTORC1) is a critical signalling complex that regulates muscle mass. In response to nutrition and resistance exercise, increased muscle mass and activation of mTORC1 occur in parallel. In this review, we summarize recent findings on mTORC1 and its regulation in skeletal muscle in response to resistance exercise, alone or in combination with intake of protein or amino acids. Because increased activity of the complex is implicated in the development of muscle insulin resistance, obesity, and some cancers (e.g., ovarian, breast), drugs that target mTORC1 are being developed or are in clinical trials. However, various cancers are associated with extensive muscle wasting, due in part to tumour burden and malnutrition. This muscle wasting may also be a side effect of anticancer drugs. Because loss of muscle mass is associated not only with metabolic abnormalities but also dose limiting toxicity, we review the possible implications for skeletal muscle of long-term inhibition of mTORC1, especially in muscle wasting conditions.

Angiotensin inhibition and longevity: a question of hydration

With the advancement of medical and investigative science, it is somewhat surprising that although it is possible to stabilise medical patients with hypertension and the associated kidney dysfunction, obesity, diabetes and even cancer, there is still no clear method of significantly reducing these chronic disease pathologies, and thus, extending life expectancy. There is one hormone common to these pathologies, the antagonism of which goes some way to clinical improvements, and this is angiotensin, which is released during hypovolaemia. Angiotensin antagonists are used to treat many of these pathologies, and it has been shown in the obesity literature that angiotensin antagonists decrease weight, but also increase the drinking of water. Increased cellular hydration, and hence, improved mitochondrial metabolism could be one of the mechanisms for the reduction in weight seen in these studies, as well as for reducing the other pathologies, all showing metabolic dysfunction. It appears that the application of straightforward physiological regulation might be an appropriate medical approach to the prevention of hypertension, kidney disease, obesity, diabetes and cancer, and thus, to an increased life expectancy.