A novel assay for evaluating glycogenolysis in rat adipocytes and the inability of insulin to antagonize glycogenolysis in this cell type

Identification and characterization of D-AKAP1 as a major adipocyte PKA and PP1 binding protein

Protein kinase A (PKA) plays an important role in the regulation of lipid metabolism in adipocytes. The activity of PKA is known to be modulated by its specific location in the cell, a process mediated by A-kinase anchoring proteins (AKAPs). In order to examine the subcellular localization of PKA in this tissue we performed a search for AKAP proteins in adipocytes. We purified a 120 kDa protein which can bind both the regulatory subunit of PKA as well as the catalytic subunit of protein phosphatase 1 (PP1). This protein was found to be enriched in the lipid droplet fraction of primary adipocytes and was identified as D-AKAP1. This protein may play an important role in the regulation of PKA in adipocytes.

Differential regulation of functional responses by beta-adrenergic receptor subtypes in brown adipocytes

Brown adipose tissue contains both beta(1)- and beta(3)-adrenergic receptors (beta-ARs), and whereas both receptor subtypes can activate adenylyl cyclase, recent studies suggest that these subtypes have different pharmacological properties and may serve different signaling functions. In this study, primary brown adipocyte cultures were used to determine the role of beta-AR subtypes in mediating lipolysis and uncoupling protein-1 (UCP1) gene expression, elicited by the physiological neurohormone norepinephrine (NE). NE increased both lipolysis and UCP1 mRNA levels in brown adipocyte cultures; the beta(1)-receptor-selective antagonist CGP-20712A strongly antagonized the increase in UCP1 gene expression but had little effect on lipolysis. The beta(3)-receptor-selective agonist CL-316243 (CL) also increased lipolysis and UCP1 mRNA levels, yet CL was more potent in stimulating lipolysis than UCP1 gene expression. NE also increased the phosphorylation of cAMP response element-binding protein (CREB) and perilipin (PL), both of which are protein kinase A substrates that are differentially targeted to the nucleus and lipid droplets, respectively. beta(1)-receptor blockade inhibited NE-stimulated phosphorylation of CREB but not PL. The results suggest that beta-AR subtypes regulate different physiological responses stimulated by NE in brown adipocyte cultures in part by differentially transducing signals to subcellular compartments.

The activation of p38 MAPK by the beta-adrenergic agonist isoproterenol in rat epididymal fat cells

Here we report that the beta-adrenergic agonist isoproterenol increases the activity of the stress-activated kinase p38 MAPK over 10-fold in freshly isolated rat epididymal fat cells. Stimulation of the kinase was rapid, sustained for at least 60 min and sensitive to the specific p38 MAPK inhibitor, SB 203580. Half-maximal stimulation of p38 MAPK by isoproterenol occurred at 13 nM isoproterenol. The cell permeable cyclic AMP analogue, chlorophenylthio-cyclic AMP increased p38 MAPK activity to a similar extent to isoproterenol, suggesting that the effect of the beta-adrenergic agonist is mediated via increases in the activity of cyclic-AMP dependent protein kinase. Although it had little or no effect on the activity of c-Jun N-terminal kinase, isoproterenol and a number of other treatments which activated p38 MAPK were found to stimulate AMP-activated protein kinase in fat cells. Activation of AMPK and p38 MAPK were not, however, found to be directly linked.