Serine auxotrophy of myoblasts in primary and secondary culture

Targeting Nutrient Dependency in Cancer Treatment

Metabolic reprogramming is one of the hallmarks of tumor. Growing evidence suggests metabolic changes that support oncogenic progression may cause selective vulnerabilities that can be exploited for cancer treatment. Increasing demands for certain nutrients under genetic determination or environmental challenge enhance dependency of tumor cells on specific nutrient, which could be therapeutically developed through targeting such nutrient dependency. Various nutrients including several amino acids and glucose have been found to induce dependency in genetic alteration- or context-dependent manners. In this review, we discuss the extensively studied nutrient dependency and the biological mechanisms behind such vulnerabilities. Besides, existing applications and strategies to target nutrient dependency in different cancer types, accompanied with remaining challenges to further exploit these metabolic vulnerabilities to improve cancer therapies, are reviewed.

Extracellular serine and glycine are required for mouse and human skeletal muscle stem and progenitor cell function

OBJECTIVE

Skeletal muscle regeneration relies on muscle-specific adult stem cells (MuSCs), MuSC progeny, muscle progenitor cells (MPCs), and a coordinated myogenic program that is influenced by the extracellular environment. Following injury, MPCs undergo a transient and rapid period of population expansion, which is necessary to repair damaged myofibers and restore muscle homeostasis. Certain pathologies (e.g., metabolic diseases and muscle dystrophies) and advanced age are associated with dysregulated muscle regeneration. The availability of serine and glycine, two nutritionally non-essential amino acids, is altered in humans with these pathologies, and these amino acids have been shown to influence the proliferative state of non-muscle cells. Our objective was to determine the role of serine/glycine in MuSC/MPC function.

METHODS

Primary human MPCs (hMPCs) were used for in vitro experiments, and young (4-6 mo) and old (>20 mo) mice were used for in vivo experiments. Serine/glycine availability was manipulated using specially formulated media in vitro or dietary restriction in vivo followed by downstream metabolic and cell proliferation analyses.

RESULTS

We identified that serine/glycine are essential for hMPC proliferation. Dietary restriction of serine/glycine in a mouse model of skeletal muscle regeneration lowered the abundance of MuSCs 3 days post-injury. Stable isotope-tracing studies showed that hMPCs rely on extracellular serine/glycine for population expansion because they exhibit a limited capacity for de novo serine/glycine biosynthesis. Restriction of serine/glycine to hMPCs resulted in cell cycle arrest in G0/G1. Extracellular serine/glycine was necessary to support glutathione and global protein synthesis in hMPCs. Using an aged mouse model, we found that reduced serine/glycine availability augmented intermyocellular adipocytes 28 days post-injury.

CONCLUSIONS

These studies demonstrated that despite an absolute serine/glycine requirement for MuSC/MPC proliferation, de novo synthesis was inadequate to support these demands, making extracellular serine and glycine conditionally essential for efficient skeletal muscle regeneration.

Loquat leaf extract enhances myogenic differentiation, improves muscle function and attenuates muscle loss in aged rats

A main characteristic of aging is the debilitating, progressive and generalized impairment of biological functions, resulting in an increased vulnerability to disease and death. Skeletal muscle comprises approximately 40% of the human body; thus, it is the most abundant tissue. At the age of 30 onwards, 0.5‑1% of human muscle mass is lost each year, with a marked acceleration in the rate of decline after the age of 65. Thus, novel strategies that effectively attenuate skeletal muscle loss and enhance muscle function are required to improve the quality of life of older subjects. The aim of the present study was to determine whether loquat (Eriobotrya japonica) leaf extract (LE) can prevent the loss of skeletal muscle function in aged rats. Young (5-month-old) and aged (18‑19-month-old) rats were fed LE (50 mg/kg/day) for 35 days and the changes in muscle mass and strength were evaluated. The age‑associated loss of grip strength was attenuated, and muscle mass and muscle creatine kinase (CK) activity were enhanced following the administration of LE. Histochemical analysis also revealed that LE abrogated the age‑associated decrease in cross‑sectional area (CSA) and decreased the amount of connective tissue in the muscle of aged rats. To investigate the mode of action of LE, C2C12 murine myoblasts were used to evaluate the myogenic potential of LE. The expression levels of myogenic proteins (MyoD and myogenin) and functional myosin heavy chain (MyHC) were measured by western blot analysis. LE enhanced MyoD, myogenin and MyHC expression. The changes in the expression of myogenic genes corresponded with an increase in the activity of CK, a myogenic differentiation marker. Finally, LE activated the Akt/mammalian target of rapamycin (mTOR) signaling pathway, which is involved in muscle protein synthesis during myogenesis. These findings suggest that LE attenuates sarcopenia by promoting myogenic differentiation and subsequently promoting muscle protein synthesis.

Human skeletal muscle cell differentiation is associated with changes in myogenic markers and enhanced insulin-mediated MAPK and PKB phosphorylation

AIM

We hypothesized that myogenic differentiation of HSMC would yield a more insulin responsive phenotype.

METHODS

We assessed expression of several proteins involved in insulin action or myogenesis during differentiation of primary human skeletal muscle cultures (HSMC).

RESULTS

Differentiation increased creatine kinase activity and expression of desmin and myocyte enhancer factor (MEF)2C. No change in expression was observed for big mitogen-activated protein kinase (BMK1/ERK5), MEF2A, insulin receptor (IR), hexokinase II, and IR substrates 1 and 2, while expression of glycogen synthase, extracellular signal-regulated kinase 1 and 2 (ERK1/2 MAP kinase) and the insulin responsive aminopeptidase increased after differentiation. In contrast to protein kinase B (PKB)a, expression of (PKB)b increased, with differentiation. Both basal and insulin-stimulated PI 3-kinase activity increased with differentiation. Insulin-mediated phosphorylation of PKB and ERK1/2 MAP kinase increased after differentiation.

CONCLUSION

Components of the insulin-signalling machinery are expressed in myoblast and myotube HSMC; however, insulin responsiveness to PKB and ERK MAP kinase phosphorylation increases with differentiation.