87-OR: A PKA to mTORC1 Signaling Axis Controls Metabolic Homeostasis via Phosphorylation of Raptor

As critical mediators of cellular signaling, the second messenger cAMP and its associated protein kinase A (PKA) can function by regulating the activity of the mechanistic target of rapamycin complex 1 (mTORC1). However, the signaling mechanism and physiological significance underpinning this regulation have not been fully defined. Using the adipocyte as a model, we show that PKA phosphorylation of Ser791 in Raptor, the defining component of mTORC1, is required for mTORC1 activation by β₃-adrenergic receptor (β₃AR) stimulation. Mice with a phospho-resistant Raptor Ser791 to Ala (S791A) mutation specifically in adipocytes (adS791A) show impaired adipose browning after β₃AR agonist CL-316,243 (CL) treatment. They develop adipose inflammation, fatty liver, and insulin resistance on high-fat diets. In follow-up hyperinsulinemic-euglycemic clamp studies, they manifest reduced glucose infusion rates, increased hepatic glucose production, and decreased skeletal muscle glucose uptake. Mechanistically, the adipocyte S791A mutation blunts CL-induced mTORC1 activity and leads to degradation of the PPARγ protein. In addition to PPARγ, the level of β₃AR protein is also reduced in Raptor adS791A mice. We have identified PPARγ binding sites along with a known critical C/EBPα site in the β₃AR promoter, suggesting PPARγ, with C/EBPα, could be a transcriptional regulator of β₃AR expression. In contrast, expression of a constitutively active phospho-mimetic Raptor Ser791 to Asp (S791D) mutant in brown adipocytes results in transcriptomic changes indicative of greater PPARγ activity and enhanced adipocyte thermogenesis gene program. These data indicate that Raptor S791 phosphorylation might act through a PPARγ-dependent mechanism to control adipose tissue function. Taken together, our study reveals the functional consequence for the PKA phosphorylation of Raptor in adipocyte metabolism and presents a novel mechanism by which the cAMP-PKA pathway controls the mTORC1 signaling network.