Mechanisms by which the IKK-related Kinases Affect Energy Expenditure.

Abstract

Numerous studies have implicated an inflammatory link between obesity and type 2 diabetes. We applied a multidisciplinary approach spanning from in vivo animal physiology to in vitro cell culture and biochemistry to address the role of obesity-related inflammation in adipose tissue.

The inhibitor of κB kinase β (IKKβ) has previously been linked to insulin resistance. High-fat diet dramatically increased protein and kinase activity levels of the two noncanonical IKK family members, IKKε and TBK1, in adipose tissue. Genetic ablation of IKKε or pharmacologic inhibition of IKKε and TBK1, using the selective protein kinase inhibitor amlexanox, resulted in many metabolic improvements, including reduced weight gain in mice on a high fat diet. The improvements in IKKε knockout and amlexanox-treated mice on a high fat diet were correlated with increased energy expenditure, core body temperature, adipogenesis, and the proton uncoupling protein, UCP1 protein levels in adipose tissue.

Studies with 3T3-L1 adipocytes elucidated the mechanism by which IKKε and TBK1 regulate cAMP and β-adrenergic signaling in 3T3-L1 adipocytes. Expression of IKKε in 3T3-L1 adipocytes decreased UCP1 expression in response to β-adrenergic stimulation. The rate of lipolysis, levels of cAMP, and phosphorylation of PKA substrates such as hormone sensitive lipase (HSL) were also diminished in response to isoproterenol or forskolin by overexpression of IKKε or TBK1 in 3T3-L1 adipocytes. IKKε and TBK1 are induced by inflammatory stimuli, such as tumor necrosis α (TNFα) and Poly(I:C), and blockade of these kinases reversed the diminution of β-adrenergic signaling, cAMP, and lipolysis. The reduction of cAMP levels in 3T3-L1 adipocytes expressing IKKε or TBK1 was reversed by the phosphodiesterase 3B inhibitor, zardaverine. IKKε and TBK1 were found to bind to and phosphorylate PDE3B at serine 318, resulting in its activation and 14-3-3β binding. These studies suggest that reduced cAMP production through phosphorylation and activation of PDE3B by IKKε and TBK1 is responsible for mediating the effects of IKKε and TBK1 in adipocytes.