The objective of this proposal is to understand how receptors that transmit their signals through the Gaq protein regulate phosphatidylinositol (PI) 3-kinase and its downstream effector Akt. The hypothesis to be evaluated is that activation of Gq-coupled receptors inhibits growth factor stimulation of the PI 3-kinase/Akt signal transduction pathway. Activation of PI 3-kinase and Akt plays an important role in insulin regulation of glucose metabolism. Inhibition of PI 3-kinase and Akt might explain why many hormones that act through Gq-coupled receptors counter the effect of insulin action. Since insulin resistance is a prominent feature in type II diabetes mellitus, discovering the mechanisms used by Gq-coupled receptors to inhibit this signaling pathway will increase our understanding of diabetes.
The specific aims are to determine if Gq-coupled receptors inhibit growth factor and insulin activation of PI 3-kinase and to explore the mechanisms of this inhibitory effect. In addition, the inhibitory effect of Gq-coupled receptors on insulin action will be examined in vivo in order to better understand the pathophysiology of insulin resistance.
Specific Aim 1 will use Gq-coupled receptors in three diverse cell types to assess the generality of the hypothesis. Adenoviral expression of constitutively active Gaq will indicate if Gaq is sufficient to inhibit PI 3-kinase and Akt. A Gaq/11-null cell line will be used to determine if Gaq is necessary for this inhibitory effect.
Specific Aim 2 will focus on determining whether tyrosine dephosphorylation of receptor tyrosine kinases or their substrates is a mechanism utilized by Gaq to inhibit PI 3-kinase activation.
Specific Aim 3 will first determine if acute stimulation of a1 adrenergic receptors in rat liver with a pharmacologic agonist blocks insulin activation of the PI 3-kinase signaling pathway.
Specific Aim 3 will also investigate if insulin resistance in the liver of fructose-fed rats is due to chronic activation of a1 adrenergic receptors that leads to inhibition of PI 3-kinase/Akt signaling. Successful completion of this proposal may lead to a better understanding of and treatment for diabetes mellitus in humans.
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