Obesity is a major health problem, that is escalating to epidemic proportions and the importance of this disorder is emphasized by the alarming increase in the prevalence of overweight or obese children over the last 25 years. Obesity can lead to heart disease and stroke and is one component of a series of metabolic disorders that may contribute to metabolic syndrome. Although complex, the underlying cause of obesity is influenced by environmental and social behaviours, in addition to genetic interactions controlling the balance between energy consumption and expenditure. Diet, exercise, metabolism and genetics are all important interacting factors in determining body composition, and while much is known about diet and exercise, the genetic and metabolic mechanisms that control weight gain are incompletely understood.
Skeletal muscle and adipose tissue are the most metabolically active tissues in the body and both tissues play an important role in insulin sensitivity, metabolism and obesity. The molecular mechanisms involved in the interplay between adipose tissue and skeletal muscle are unclear. Current gene targets include the Nuclear Hormone Receptor (NR) superfamily of transcription factors, and other signalling pathways such as Transforming Growth Factor-β (TGFβ). Although, these factors are linked to the obese phenotype and are implicated in lipid and glucose homeostasis and energy expenditure, the molecular mechanisms of their action are incompletely understood. Thus, we hypothesised that the Ski proto-oncogene, a major regulator of NR and TGF-β signalling, plays a significant role in myogenesis and adipogenesis and therefore body composition and metabolism.
The role of Ski in body composition development and metabolism was examined in vivo using the ‘gain of function’ MSV c-Ski transgenic mouse model (skeletal muscle c-Ski overexpression). MSV c-Ski mice have increased muscle mass due to hypertrophy and a leaner phenotype than WT mice. Earlier studies have shown that in accordance with the lean phenotype, skeletal muscle of MSV c-Ski transgenic mice, have an increase in fatty acid oxidation concomitant with gene expression suppression of the master lipogenic regulator, SREBP1c and its target genes, SCD-1 and FAS. In this study when challenged on a high fat diet MSV c-Ski mice were partially resistant to diet-induced obesity despite possessing a similar metabolic rate, food intake and lower activity level than WT mice. Additionally chow fed MSV c-Ski mice exhibited mild glucose intolerance (which was not worsened by a high fat diet), and had impaired insulin-stimulated Akt phosphorylation and skeletal muscle glucose uptake. This phenotype was investigated through gene expression studies in skeletal muscle which revealed the suppression of genes involved in TGFβ signalling such as the Smads, HDACs, activin receptors and myostatin, offering in part an explanation for the increased muscle mass in these mice. Additionally high-fat fed MSV c-Ski mice displayed aberrant skeletal muscle mRNA expression of several key NRs and metabolic pathways involved in glucose transport, insulin signalling, cholesterol metabolism and lipogenesis. Of specific interest, we observed a 10-fold suppression of PPARγ, the master regulator of adipogenesis and metabolism, and a 23-fold suppression of Tbc1d1, a Rab-GAP protein and a 2.3-fold suppression of Glut4 that regulates both insulin and contraction-stimulated glucose transport and uptake in skeletal muscle.
As MSV c-Ski mice possessed a lean phenotype we examined Ski’s role in adipocyte differentiation in vitro. Ski mRNA expression increased as 3T3-L1 pre-adipocyte cells become confluent and was dramatically suppressed (6-fold) at the onset of differentiation. A similar observation was made in MSCs however the Ski repression was not as dramatic as that seen in the 3T3-L1 cells, suggesting an early regulated role for Ski in adipocyte differentiation. Furthermore we revealed that Ski was not modulated by the adipocyte differentiation inducers insulin, dexamethasone or IBMX, triggers for the IGF-1 receptor/insulin pathway, glucocorticoid signalling or the cAMP-dependent protein kinase pathway respectively.
Ski’s role in the mature adipocyte was investigated in vivo by the generation and utilisation of an adipose-specific c-Ski overexpression mouse model, AP2 c-Ski. Overexpression of c-Ski in adipose did not result in any significant body composition, metabolic rate, activity level or food intake difference when compared to WT mice on chow or high fat diet. However high-fat fed AP2 c-Ski mice possessed elevated serum NEFA, leptin and insulin levels and liver triglyceride levels. Additionally in contrast to MSV c-Ski mice, high-fat fed AP2 c-Ski mice possessed significantly improved glucose tolerance and insulin sensitivity when compared to WT mice. Furthermore utilizing microarray gene expression analysis, c-Ski was shown to alter the mRNA expression of multiple genes involved in lipogenesis, glucose metabolism and insulin signalling in both white and brown adipose, such as PPARγ, Srebp1c and the CCAAT/enhancer-binding proteins C/EBPα and C/EBPβ, both important early regulators of adipogenesis and lipogenesis.
Taken altogether these results reveal a significant role for Ski in skeletal muscle and adipose tissue differentiation and metabolism through the modulation of several key genes involved in regulating lipid and glucose homeostasis, insulin signalling and metabolism. Delving further into the mechanisms by which Ski functions in skeletal muscle and adipose tissue will allow the elucidation of novel gene networks and biomarkers regulated by c-Ski. This may provide novel insights into the metabolic actions of the c-Ski gene through its effects on adipocyte and skeletal muscle development. Such insights may potentially identify novel therapeutic targets for intervention to prevent or treat obesity and its metabolic complications, including type 2 diabetes and the metabolic syndrome.