Differential Roles of the TRAF3 Adapter Protein in Adipogenesis and Glucose Metabolism
The main goal of this project is to delineate the roles and mechanisms of constitutive type II nuclear factor-kappa B (NF-kB) activation on adipogenesis and glucose metabolism. Our laboratory has shown that the tumor necrosis factor (TNF) receptor-associated factor 3 (TRAF3) is a critical negative modulator of type II NF-kB and TRAF3 knockout (TRAF3-/-) have constitutive activation of non canonical or type II NF-kB pathway. However, TRAF3-/- pups die within two weeks after birth and the function of TRAF3 remain largely elusive. TRAF3 null mice have greatly reduced serum glucose levels, elevated serum corticosterone and minute amount of fat. On the other hand, the development of Smac mimetic compounds (SM), inhibitors of cellular inhibitors of apoptosis (cIAPs), has been a major goal in advancing cancer therapeutics, as cIAPs are believed to antagonize activated caspases and are often amplified in human cancers. Two reports using two different second generation Smac (SM) mimetic compounds, including LBW242 showed that inhibition of cIAPs can activate NF-kB noncanonically. TRAF3 recruits nuclear factor B-inducing kinase (NIK) and TRAF2 recruits cIAP1/2 to TRAF3/NIK, and then promote NIK ubiquination and degradation. On the other hand, SM can prevent NIK degradation and initiate constitutive processing of type II NF-kB. Here, we have shown that TRAF3-/- murine embryonic fibroblasts (MEFs) and bone marrow mesenchymal stem cells (BMs) do not differentiate into adipocytes compared to WT, p100-/- and TRAF3-/-p100-/- counterparts. Treatment with SM can also prevent adipogenesis in WT MEFs and BMs as well as 3T3-L1 and 3T3-F422A preadipocytes. We have found that different generations of SM as well as agonistic lymphotoxin-beta receptor (LTbR) antibody can inhibit PPARg, C/EBPa and downstream adipocyte specific genes at both mRNA and protein levels. We have demonstrated that this inhibition by constitutive p52 is due to upregulation of GATA3, the inhibitor of adipogenesis, which directly inhibit PPARg and C/EBPa. As TRAF3 null mice have low serum glucose level and compound loss of TRAF3 and p100 can rescue this phenotype, we investigated effect of TRAF3 null on glucose related metabolic pathways. Here, we demonstrated that TRAF3-/- MEFs and 3T3-L1 cells treated with SM have higher insulin sensitivity. In addition, liver isolated from TRAF3-/- mice have increased expression of genes involved in gluconeogenesis such as PEP carboxykinase (PEPCK) and pyruvate dehydrogenase kinase (PDK) as well as lactate dehydrogenase (LDH). Investigations presented in this thesis help elucidate novel mechanisms responsible for the crosstalk of non-canonical NF-kB with adipocyte differentiation and host metabolism.