Early signaling interactions between the insulin and leptin pathways in bovine myogenic cells

Cross-talk between hormone signaling pathways provides mechanisms to facilitate flexibility in the cellular response to extracellular conditions. One function of insulin is to signal high extracellular glucose, while leptin may signal the abundance of extracellular lipid, both energy sources being readily utilized by muscle. The present study reports early signaling events in the insulin and leptin cascades in primary bovine myogenic cells (BMC). BMC were treated with insulin, or leptin for 1, 10, 30 and 120 min, or pretreated with leptin for 10 min followed by insulin for 1, 10, 30 and 120 min. BMC were insulin resistant, showing a significant inhibition of IRS-1 association with the insulin receptor (IR) following insulin stimulation, a corresponding increase in PI 3-kinase association with the IR, and a slow and modest increase in GLUT4 recruitment to the plasma membrane. Pretreatment of BMC for 10 min leptin, followed by insulin time-course, caused IRS-1 recruitment to be unresponsive, but evoked a rapid, phasic response of PI 3-kinase recruitment to the IR and abrogated the response of GLUT4 translocation to the plasma membrane evoked by insulin alone. The lack of insulin response was independent of IR abundance or affinity. JAK-2 association with the ObR and JAK-2 tyrosine phosphorylation were responsive to all three treatments. Insulin alone down-regulated the leptin signaling pathway, JAK-2 association with ObR decreased at all time-points, and JAK-2 phosphorylation decreased similarly. Leptin alone also appeared to down-regulate JAK-2 association with the ObR, but stimulated the down-regulated pathway to signal, JAK-2 tyrosine phosphorylation being increased at later time-points. Pretreatment with leptin followed by insulin time-course showed marked up-regulation of the early leptin signaling pathway, JAK-2 association with the ObR being increased by insulin while JAK-2 tyrosine phosphorylation was also increased. The contrasting responses of BMC to insulin alone, leptin alone and the sequential leptin-insulin treatment may point to the ability of these cells to respond to energy substrate availability, as bovine muscle has evolved to utilize lipids and fatty acids in response to a metabolism which provides only limited glucose. This cross-talk between insulin and leptin signaling pathways points to a better understanding of the mechanisms driving energy substrate utilization in ruminant muscle and may provide a useful model for greater understanding of the molecular mechanisms underlying the development of insulin resistance and Type 2 diabetes in man.