RESUMEN
BACKGROUND: Fibroblast growth factor (FGF) 21, a novel member of the FGF family, plays a role in a variety of endocrine functions, including regulation of glucose and lipid metabolism. The role of FGF21 in skeletal muscle is currently not known. METHODS: Serum levels and skeletal muscle mRNA of FGF21 were determined in normal glucose tolerant (n = 40) and type 2 diabetic (T2D; n = 40) subjects. We determined whether FGF21 has direct effects on glucose metabolism in cultured myotubes (n = 8) and extensor digitorum longus skeletal muscle. RESULTS: Serum FGF21 levels increased 20% in T2D versus normal glucose tolerant subjects (p < 0.05), whereas skeletal muscle mRNA expression was unaltered. Fasting insulin, homeostatic model assessment of insulin resistance (HOMA-IR), waist circumference, and body mass index (BMI) significantly correlated with serum FGF21 levels in T2D (p < 0.01), but not in normal glucose tolerant subjects. Serum FGF21 concentrations were greater in T2D patients in the highest tertile of fasting insulin (p < 0.05) and BMI (p < 0.05). Stepwise regression analysis identified BMI as the strongest independent variable correlating with FGF21. FGF21 exposure increased basal and insulin-stimulated glucose uptake in human myotubes, coincident with increased glucose transporter 1 mRNA, and enhanced glucose transporter 1 abundance at the plasma membrane. In isolated extensor digitorum longus muscle, FGF21 potentiated insulin-stimulated glucose transport, without altering phosphorylation of Akt or AMP-activated protein kinase. CONCLUSIONS: Plasma FGF21 is increased in T2D patients, and positively correlated with fasting insulin and BMI. However, FGF21 has direct effects in enhancing skeletal muscle glucose uptake, providing additional points of regulation that may contribute to the beneficial effects of FGF21 on glucose homeostasis. Whether increased plasma FGF21 in T2D is a compensatory mechanism to increase glucose metabolism remains to be determined.
Asunto(s)
Factores de Crecimiento de Fibroblastos/fisiología , Músculo Esquelético/metabolismo , Animales , Índice de Masa Corporal , Diabetes Mellitus Tipo 2/sangre , Femenino , Factores de Crecimiento de Fibroblastos/biosíntesis , Glucosa/metabolismo , Transportador de Glucosa de Tipo 1/biosíntesis , Humanos , Insulina/fisiología , Masculino , Ratones , Persona de Mediana Edad , Músculo Esquelético/efectos de los fármacos , Obesidad/sangre , ARN Mensajero/metabolismo , Transducción de SeñalRESUMEN
BACKGROUND: Recombinant adenovirus vectors and transfection agents comprising cationic lipids are widely used as gene delivery vehicles for functional expression in cultured cells. Consequently, these tools are utilized to investigate the effects of functional over-expression of proteins on insulin mediated events. However, we have previously reported that cationic lipid reagents cause a state of insulin unresponsiveness in cell cultures. In addition, we have found that cultured cells often do not respond to insulin stimulation following adenovirus treatment. Infection with adenovirus compromises vital functions of the host cell leading to the activation of protein kinases central to insulin signalling, such as protein kinase B/Akt. Therefore, we investigated the effect of adenovirus infection on insulin unresponsiveness by means of Akt activation in cultured cells. Moreover, we investigated the use of baculovirus as a heterologous viral gene delivery vehicle to circumvent these phenomena. Since the finding that baculovirus can efficiently transduce mammalian cells, the applications of this viral system in gene delivery has greatly expanded and one advantage is the virtual absence of cytotoxicity in mammalian cells. RESULTS: We show that infection of human neuroblastoma SHSY-5Y and liver C3A cells with recombinant adenovirus results in the activation of Akt in a dose dependent manner. In addition, this activation makes treated cells unresponsive to insulin stimulation as determined by an apparent lack of differential phosphorylation of Akt on serine-473. Our data further indicate that the use of recombinant baculovirus does not increase the phosphorylation of Akt in SHSY-5Y and C3A cells. Moreover, following infection with baculovirus, SHSY-5Y and C3A cells respond to insulin by means of phosphorylation of Akt on serine-473 in the same manner as uninfected cells. CONCLUSION: Widely-used adenovirus vectors for gene delivery cause a state of insulin unresponsiveness in human SHSY-5Y and C3A cells in culture due to the activation of central protein kinases of the insulin signalling pathway. This phenomenon can be avoided when studying insulin signalling by using recombinant baculovirus as a heterologous viral expression system. In addition, our data may contribute to an understanding of the molecular mechanisms underlying baculovirus infection of human cells.