PKA Activity In Cells Research Articles

Abstract 460: β 1 -Adrenergic Receptor O-glycosylation Regulates Receptor N-terminal Cleavage and Signaling Responses in Cardiomyocytes

β1-adrenergic receptors (β1ARs) mediate catecholamine actions in cardiomyocytes by coupling to both Gs/cAMP-dependent and Gs-independent/growth-regulatory pathways. Recent efforts to structurally characterize β1ARs have focused on the ligand binding sites in transmembrane helices and effector docking sites in the intracellular loops and C-terminus. The notion that the β1AR extracellular N-terminus (which is a substrate for both N- and O-linked glycosylation and a target for proteolytic cleavage) functions as a structural determinant of β1AR activation has never been considered. This study takes advantage of site-directed mutagenesis and expression studies in Chinese Hamster ovary cells that harbor a reversible glycosylation defect to map β1ARs O-glycosylation sites to Ser37/Ser41 and show that O-glycosylation at these sites prevents β1AR N-terminal cleavage. We then used an adenoviral overexpression approach to show that both full-length fully-glycosylated β1ARs (β1AR-FL) and N-terminally truncated glycosylation-defective β1ARs (β1AR-Δ52) increase cAMP accumulation and activate the ERK-MAPK signaling cascade in cardiomyocytes. However, isoproterenol-dependent cAMP accumulation is markedly enhanced, and ERK activation is markedly attenuated, in cardiomyocytes that express β1AR-Δ52, compared with β1AR-FL cultures. The conclusion that β1ARs are stabilized in a conformation that is biased toward the cAMP pathway as a result of N-terminal truncation gains further support from additional studies showing that agonistic β1AR autoantibodies induce a more rapid and robust increase in membrane PKA activity in cells that express glycosylation-defective β1ARs, compared to cells that express full-length glycosylated β1ARs. These results identify a novel role for O-glycosylation as a structural determinant of β1AR responsiveness in the heart.

Apparent PKA activity responds to intermittent hypoxia in bone cells: a redox pathway?

We studied hypoxia-induced dynamic changes in the balance between PKA and PKA-counteracting phosphatases in the microfluidic environment in single cells using picosecond fluorescence spectroscopy and intramolecular fluorescence resonance energy transfer (FRET)-based sensors of PKA activity. First, we found that the apparent PKA activity in bone cells (MC3T3-E1 cells) and endothelial cells (bovine aortic endothelial cells) is rapidly and sensitively modulated by the level of O(2) in the media. When the O(2) concentration in the glucose-containing media was lowered due to O(2) consumption by the cells in the microfluidic chamber, the apparent PKA activity increases; the reoxygenation of cells under hypoxia leads to a rapid ( approximately 2 min) decrease of the apparent PKA activity. Second, lack of glucose in the media led to a lower apparent PKA activity and to a reversal of the response of the apparent PKA activity to hypoxia and reoxygenation. Third, the apparent PKA activity in cells under hypoxia was predominantly regulated via a cAMP-independent pathway since 1) changes in the cAMP level in the cells were not detected using a cAMP FRET sensor, 2) the decay of cAMP levels was too slow to account for the fast decrease in PKA activity levels in response to reoxygenation, and 3) the response of the apparent PKA activity due to hypoxia/reoxygenation was not affected by an adenylate cyclase inhibitor (MDL-12,330A) at 1 mM concentration. Fourth, the immediate onset of ROS accumulation in MC3T3-E1 cells subjected to hypoxia and the sensitivity of the apparent PKA activity to redox levels suggest that the apparent PKA activity change during hypoxia and reoxygenation in this study can be linked to a redox potential change in response to intermittent hypoxia through the regulation of activities of PKA-counteracting phosphatases such as protein phosphatase 1. Finally, our results suggest that the detection of PKA activity could be used to monitor responses of cells to hypoxia in real time.

Disruption of the RIIβ subunit of PKA reverses the obesity syndrome of agouti lethal yellow mice

Agouti lethal yellow (A(y)) mice express agouti ectopically because of a genetic rearrangement at the agouti locus. The agouti peptide is a potent antagonist of the melanocortin 4 receptor (MC4R) expressed in neurons, and this leads to hyperphagia, hypoactivity, and increased fat mass. The MC4R signals through Gs and is thought to stimulate the production of cAMP and activation of downstream cAMP effector molecules such as PKA. Disruption of the RIIbeta regulatory subunit gene of PKA results in release of the active catalytic subunit and an increase in basal PKA activity in cells where RIIbeta is highly expressed. Because RIIbeta is expressed in neurons including those in the hypothalamic nuclei where MC4R is prominent we tested the possibility that the RIIbeta knockout might rescue the body weight phenotypes of the A(y) mice. Disruption of the RIIbeta PKA regulatory subunit gene in mice leads to a 50% reduction in white adipose tissue and resistance to diet-induced obesity and hyperglycemia. The RIIbeta mutation rescued the elevated body weight, hyperphagia, and obesity of A(y) mice. Partial rescue of the A(y) phenotypes was even observed on an RIIbeta heterozygote background. These results suggest that the RIIbeta gene mutation alters adiposity and locomotor activity by modifying PKA signaling pathways downstream of the agouti antagonism of MC4R in the hypothalamus.

Decrease in cAMP-dependent protein kinase activity in suspension cultures of porcine thyroid cells exposed to TSH or forskolin

Suspension cultures of porcine thyroid cells were used to study the action of TSH and forskolin (Fk) on cAMP-dependent (PKa) and Ca 2+-phospholipid-dependent (PKc) protein kinase — enzymes which represent the key step in the transduction of extracellular signals. The PKa activity in cells cultured for 2 days in the presence of TSH was decreased to about 50% of control level with a TSH dose of 0.1 mU/ml. This decrease is dose dependent; only traces of PKa activity remained at very high doses of TSH (50 mU/ml). Similar results were obtained with Fk (10 −5 M), the adenylate cyclase activator. It decreased the PKa activity to the level obtained with 0.1–1.0 mU/ml TSH. The loss of the PKa activity was parallel in cytosol and particulate fractions, suggesting that there is no translocation of enzymes under the action of either TSH or Fk. Neither TSH nor Fk had any effect on PKc, which became the predominant activity in cells exposed to either of the regulators. The cAMP-dependent phosphorylation of endogenous proteins was lower in TSH- or Fk-treated cells than in controls, and was dependent, like the PKa activity, on the dose of TSH. Polyacrylamide gel electrophoresis (PAGE) revealed the specific substrates of PKa in cultured thyroid cells. Proteins of 28, 30 and 33 kDa were regularly found, while 58 kDa protein was not present in all experiments. PAGE patterns showed that the decrease in endogenous phosphorylation in TSH- and Fk-treated cells was due to decreased labelling of PKa-specific substrates. The observed down-regulation of PKa activity could have an influence on the expression of thyroid cell differentiation. cAMP-dependent protein kinase; Thyroid cell culture, porcine; TSH; Forskolin; Protein phosphorylation