Cardiac alpha 1-adrenoceptors stimulate a high-affinity GTPase activity in sarcolemmal membranes from rabbit atrial and ventricular myocytes

Increased cardiac alpha-1-adrenoceptor density in rats following treatment with amiodarone

This study was undertaken to investigate the interaction between amiodarone and alpha-1-adrenoceptors in rat cardiac cells. The level (Bmax) and affinity (Kd) of alpha-1-adrenoceptors in heart cells were determined by [3H]prazosin radioligand binding following amiodarone treatment. In cultured intact cardiocytes treated for 48 h with 10 microM amiodarone, [3H]prazosin binding increased by 31% compared with the control cells (p<0.05). The increase was both dose and time dependent and was found to be specific because no significant change occurred in creatine kinase activity. Additionally, under the same conditions, an increase in [3H]prazosin binding to cultured cardiocyte cell membranes was also obtained. Oral gavage of amiodarone to rats for 8 d resulted in a 25% increase in [3H]prazosin binding to isolated ventricle membranes compared with control rats (p<0.05). We conclude that amiodarone treatment can increase the response to alpha-1-adrenoceptors agonist in the heart due to an increase in the density of alpha-1-adrenoceptors.

Alpha 1B-adrenoceptor interacts with multiple sites of transglutaminase II: characteristics of the interaction in binding and activation

We previously reported that a novel GTP binding protein (G alpha h) is tissue type transglutaminase (TGII) and transmits the alpha 1B-adrenoceptor (AR) signal to phospholipase C (PLC) through its GTPase function. We have also shown that PLC-delta 1 is the effector in TGII-mediated signaling. In this study, interaction sites on TGII for the alpha 1B-AR were identified using a peptide approach and site-directed mutagenesis, including in vivo reconstitution of TGIIs with the alpha 1B-AR and PLC-delta 1. To identify the interaction sites, 11 synthetic peptides covering approximately 132 amino acid residues of the C-terminal domain of TGII were tested. The studies with the peptides revealed that three peptides, L547-I561, R564-D581, and Q633-E646, disrupted formation of an alpha 1-agonist-alpha 1B-AR-TGII complex and blocked alpha 1B-AR-mediated TGase inhibition in a dose-dependent manner, indicating that these peptide regions are involved in recognition and activation of TGII by the alpha 1B-AR. These three regions were further evaluated with full-length TGIIs by constructing and coexpressing each site-directed mutant with the alpha 1B-AR and PLC-delta 1 in COS-1 cells. Supporting the findings with these peptides, these TGII mutants lost 56-82% the receptor binding ability and reduced by 29-68% the level of alpha 1B-AR-mediated IP3 production via PLC-delta 1 as compared to those with wild-type TGII. The results also revealed that the regions of R564-D581 and Q633-E646 were the high-affinity binding sites of TGII for the receptor and critical for the activation of TGII by the receptor. Taken together, the studies demonstrate that multiple regions of TGII interact with the alpha 1B-AR and that the alpha 1B-AR stimulates PLC-delta 1 via TGII.

The core domain of the tissue transglutaminase Gh hydrolyzes GTP and ATP

Tissue transglutaminase (TGase II) catalyzes the posttranslational modification of proteins by transamidation of available glutamine residues and is also a guanosinetriphosphatase (GTPase) and adenosinetriphosphatase (ATPase). Based on its homology with factor XIIIA, an extracellular transglutaminase, the structure of TGase II is likely composed of an N-terminal beta-sandwich domain, an alpha/beta catalytic core, and two C-terminally located beta-barrels. Here we used a domain-deletion approach to identify the GTP and ATP hydrolytic domains of TGase II. Full-length TGase II and two domain-deletion mutants, one retaining the N-terminal beta-sandwich and core domains (betaSCore) and the other retaining only the core domain, were expressed as glutathione S-transferase (GST) fusion proteins and purified. GST-Full and GST-betaSCore exhibited calcium-dependent TGase activity, whereas GST-Core had no detectable TGase activity, indicating the beta-sandwich domain is required for TGase activity but the C-terminal beta-barrels are not. All three GST-TGase II fusion proteins were photoaffinity-labeled with [alpha-32P]-8-azidoGTP and were able to bind GTP-agarose. The GTPase activity of GST-betaSCore was equivalent to that of GST-Full, whereas the ATPase activity was approximately 40% higher than GST-Full. GST-Core had approximately 50% higher GTPase activity and approximately 75% higher ATPase activity than GST-Full. The GTPase and ATPase activities of each of the GST-TGase II fusion proteins were inhibited in a dose-dependent manner by both GTPgammaS and ATPgammaS. These results demonstrate that the GTP and ATP hydrolysis sites are localized within the core domain of TGase II and that neither the N-terminal beta-sandwich domain nor the C-terminal beta-barrels are required for either GTP or ATP hydrolysis. Taken together with previous work [Singh, U. S., Erickson, J. W., & Cerione, R. A. (1995) Biochemistry 34, 15863-15871; Lai, T.-S., Slaughter, T. F., Koropchak, C. M., Haroon, Z. A., & Greenberg, C. S. (1996) J. Biol. Chem. 271, 31191-31195] the results of this study indicate that the GTP and ATP hydrolysis sites are localized to a 5. 5 kDa (47 amino acid) region at the start of the core domain.

Noradrenaline stimulation of high-affinity GTPase activity in membranes from rat aorta and caudal artery

The ability of noradrenaline (NA) to stimulate increases in high-affinity GTPase activity in sarcolemma-enriched rat aorta and caudal artery membranes was examined in the present study. In aortic membranes, NA significantly (P < 0.05; N = 5) increased the Vmax from a basal value of 103 +/- 29 to 156 +/- 38 pmol Pi/min/mg protein, but did not affect the Km which was 0.32 +/- 0.08 microM in the absence and 0.58 +/- 0.16 microM in the presence of NA. However, in caudal artery membranes, NA significantly (P < 0.05; N = 6) increased both the Vmax and the Km from basal values of 69 +/- 12 pmol Pi/min/mg protein and 0.24 +/- 0.05 microM, respectively, to 205 +/- 54 pmol Pi/min/mg protein and 1.01 +/- 0.25 microM, respectively. Removing the endothelium from both artery preparations did not alter significantly basal GTPase activity or the magnitude of the increase stimulated by NA. Prazosin significantly inhibited NA-stimulated increases in GTPase activity in membranes from endothelium-denuded caudal artery and aorta, and in endothelium-intact caudal artery membranes. However, yohimbine significantly inhibited NA-stimulated increases in GTPase activity only in preparations from endothelium-intact caudal arteries. Therefore, in endothelium-intact caudal artery membranes, NA stimulated increases in GTPase activity that were apparently mediated by both alpha 1-adrenoceptors and alpha 2-adrenoceptors, while in endothelium-denuded aortic and caudal artery membranes this increase was mediated solely by alpha 1-adrenoceptors. Western blotting of these arteries confirmed the presence of both Gi alpha 2,3 and Gq/11 alpha, which are candidates for mediating the alpha 1-adrenoceptor-stimulated increases in GTPase activity.

Alpha 1-adrenergic receptor coupling with Gh in the failing human heart

BACKGROUND

We recently demonstrated that Gh, which transfers the signal from the alpha 1-adrenergic receptor to the 69-kD phospholipase C, is the previously identified tissue-type transglutaminase (TGase II). The alpha 1-adrenergic receptor mediates actions of the sympathetic nervous system, including cardiac, arteriolar, and smooth muscle contractions. In human cardiac tissue, the expression of the alpha 1-adrenergic receptor is increased under pathophysiological conditions, but changes in the physiological response are small. Therefore, it has been suggested that the other components involved in the alpha 1-adrenergic receptor-mediated signaling pathway are probably altered.

METHODS AND RESULTS

Immunological and biochemical studies with nonfailling and failing human heart tissues revealed that the GTP-binding and TGase activities of human heart TGase II (hhG alpha n) are downregulated in both ischemic and dilated cardiomyopathic human heart. In ischemic cardiomyopathy, the alpha 1-adrenergic receptor number increased twofold (27.0 fmol/mg) compared with the nonfailing (12.8 fmol/mg) and the dilated cardiomyopathic (15.6 fmol/mg) heart tissues, but the coupling of hhG alpha h with the alpha 1-adrenergic receptor did not increase. The intrinsic activity of hhG alpha h, was greatly decreased in membrane fractions, whereas the cytosolic TGase activity was not changed. In the dilated cardiomyopathic human heart, these intrinsic enzyme activities of hhG alpha h were also downregulated in the membrane fraction, whereas the amount of hhG alpha h protein was greatly increased (2.8-fold) compared with the nonfailing heart.

CONCLUSIONS

The results of the present study clearly demonstrate that the alpha 1-adrenergic receptor in human heart couples with Gh (TGase II) and indicate that downregulation of hhG alpha h activity is associated with human cardiac failure but that the mechanism differs between ischemic and dilated cardiomyopathies.