TSC2 is a tumor suppressor gene that encodes a 200 kDa protein called tuberin. TSC2 heterodimerizes with TSC1 to form a complex with GTPase-activating protein (GAP) activity. The C-terminus of TSC2 contains the GAP domain responsible for this catalytic activity. The complex was first discovered through its role in the tumor-forming condition Tuberous Sclerosis. Mutations in TSC1 and TSC2 can either destabilize the complex or compromise the GAP activity. The TSC1-TSC2 complex acts as a GAP for the small G-protein Rheb, expressed ubiquitously throughout the body (1). Rheb inhibits mTOR signaling to downregulate protein synthesis and cell growth. Thus in the absence of TSC1-TSC2, mTOR signaling allows unchecked cellular growth and proliferation. TSC1-TSC2 acts as a nutrient sensor for amino acids, growth factors, hormones, etc. and mediates cell cycle progression. The TSC1-TSC2 complex can be phosphorylated at a number of sites that are either activating or inactivating. Phosphorylation by PI3K/AKT and MAPK/ERK pathways activate the TSC1-TSC2 complex to promote GAP activity (2). Meanwhile, phosphorylation by AMPK attenuates the GAP activity of the TSC1-TSC2 complex.
Jham et. al. used the TSC2 antibody to understand how the TSC/mTOR axis interacts with the KSHV-GPCR (vGPCR) oncogene in Kaposi’s sarcoma (3). To demonstrate that vGPCR secretions regulate the TSC/mTOR axis, the group pretreated cells with inhibitors of AKT, ERK, p38, or IKKβ, then exposed them to media conditioned by either control cells or vGPCR-expressing cells. The group used the phospho-TSC2 antibody to look at levels of TSC2 activation during pharmacologic inhibition of the various kinases. The phospho-TSC2 antibody showed no detectable phospho-TSC2 in the control cell lines with AKT inhibition or IKKβ activation. Meanwhile, the phospho-TSC2 antibody showed increased levels of TSC2 activation in vGPCR cells with AKT inhibition and IKKβ activation, indicating activation of the TSC1-TSC2 complex by the vGPCR. This group also used the phospho-TSC2 antibody to stain murine vGPCR tumors as well as human Kaposi’s sarcoma tumors. Staining with the TSC2 antibody showed significant levels of phospho-TSC2 in both the murine and human samples. This confirmed their hypothesis that the TSC/mTOR axis plays a role in Kaposi’s Sarcoma development in vivo.
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