Effect of insulin-like growth factor-1 on sphingosine kinase activity and subcellular localization. Serum-starved myoblasts were incubated with 50 ng/ml insulin-like growth factor-1 (IGF-1) for the indicated time intervals. (A) Aliquots of cell extracts (40 μg) were used to determine sphingosine kinase (SK) activity. Data represent the mean ± SEM of at least three independent experiments, each performed at least in duplicate. The effect of IGF-1 in challenged versus unchallenged cells was statistically significant by Student’s t test (*P < 0.05). (B) Western analysis of SK1 (upper panel) and SK2 (lower panel) were performed in membrane and cytosolic fractions. Blots representative of at least three independent experiments are shown. The histograms represent densitometric analysis of three independent experiments. Data reported are expressed as -fold increase of the membrane:cytosol ratio. The increase of SK1 and SK2 membrane content induced by IGF-1 was statistically significant by Student’s t-test (*P < 0.05).