Gia proteins

G-protein a-subunits also possess specific residues that can be covalently modified by bacterial toxins. Cholera toxin catalyzes the transfer of ADP-ribose moiety of NAD to a specific arginine residue in certain a-subunits, whereas pertussis toxin ADP-ribosylates those a-subunits that contain a specific cysteine residue near the carboxy-terminus. Modification of the a-subunit by cholera toxin persistently activates these protein by inhibiting their GTPase activity, whereas pertussis toxin inactives Gia protein and thereby results in the uncoupling of receptor from the effector. G-protein a-subunits are regulated by covalent modifications by fatty acids myristate and palmate. These lipid modifications serve to anchor the subunits to the membrane and increase the interaction with other protein and also increase the affinity of the a-subunit for 3y. In this regard, the myristoylation of Gia is required for adenylyl cyclase inhibition in cell-free assay (Taussig et al. 1993). 

The role of MAP kinase and PI3K signaling as well as oxidative stress in Ang II-induced enhanced levels of Gia proteins has also been reported (Ge and... 

Fig. 1.3 Effect of captopril and losartan treatment on blood pressure and the expression of Gia proteins in different models of hypertensive rats (HR). Twelve-week-old SHR and age-matched WKY rats, 1 kidney 1 clip hypertensive rats (1K-1C HR) were treated with captopril (150mg/kg body wt/day) as described earlier (Pandey and Anand-Srivastava 1996 Ge et al. 1999), whereas L-NAME-induced hypertensive rats were treated with losartan (lOmg/kg body wt/day) as described earlier (Hashim and Anand-Srivastava 2004). The blood pressure was monitored by the tail cuff method. The expression of Gia-2 and Gia-3 protein in heart from SHR and L-NAME HR and aorta from 1K-1C HR were determined by Western blotting using antidodies AS/7 and EC/1 against Gia-2 and Gia-3 protein, respectively. The blots are representative of three or four separate experiments. P < 0.01, P < 0.001 versus control/WKY, tt/> < 0.01, mP < 0.001 versus SHR/1K-1C/L-NAME.

L-NAME hypertensive rats may be responsible for the enhanced expression of Gia proteins and also suggest that NO-induced decreased levels of Gia proteins may represent an additional mechanism through which NO decreases the blood pressure (Figure 1.7). 

Fig. 1.7 Possible mechanisms involving angiotensin II, oxidative stress and nitric oxide in enhanced Gi oc protein expression in hypertension. Gi protein expression is enhanced in genetic (SHR) and experimental hypertension including 1 kidney 1 clip (1K1C) and L-NAME-induced hypertension. Inhibition of nitric oxide synthase (NOS) by L-NAME activates renin angiotensin system, and also decreases the level of NO. 1K1C hypertensive rats also exhibit enhanced levels of Ang II. Ang II increases oxidative stress that through increased MAP kinase activity results in enhanced expression of Gi oc proteins and thereby hypertension. On the other hand, increased levels of NO and cGMP decrease the expression of Gia proteins in VSMC which may be an additional mechanism through which NO decreases blood pressure in L-NAME-induced hypertensive rats.

Ge C, Garcia R, Anand-Srivastava MB. 2006. Enhanced expression of Gia proteins and adenylyl cyclase signalling in aortas from 1 kidney 1 clip hypertensive rats. Can J Physiol Pharmacol 84 739-746. 

Hashim S, Anand-Srivastava MB. 2004. Losartan-induced attenuation of blood pressure in L-NAME hypertensive rats is associated with reversal of the enhanced expression of Gia protein. J Hypertens 22 181-190. 

Hyperglycemia-induced enhanced oxidative stress has also been reported in cultured VSMC and different tissues from STZ-diabetic rats (Baynes and Thorpe 1999 Baynes 1991 Cai and Harrison 2000). In addition, the contribution of enhanced production of superoxide anion (02 ) in the decreased expression of Gia proteins has recently been reported in aortic VSMC from STZ-diabetic rats and A10 cells exposed to high glucose (Li et al. 2008). Antioxidants such as a-tocopherol and NAC—scavengers of 02 —and DPI (an inhibitor of NADPH oxidase) that restored the enhanced levels of O2- induced by hyperglycemia also restored the hyperglycemia-induced decreased expression of Gia-2 and Gia-3 to control levels (Li et al. 2008). These studies implicate NADPH oxidase/C>2 in... 

Fig. 9.2 Schematic diagram depicting the possible mechanisms by which hyperglycemia/diabetes decreases the expression of Gia proteins and adenylyl cyclase signaling. Diabetes/hyperglycemia augments the levels of vasoactive peptides including Ang II/ET-1 that enhance the oxidative stress by increasing the levels of superoxide anion (O2 ) and peroxynitrite (ONOO ). O2 and ONOO-decrease the levels of Gi proteins.The treatment with antioxidants and ONOO- scavengers reverses the hyperglycemia-induced decreased expression of Gia proteins and adenylyl cyclase signaling.

Bassil, M., and M.B. Anand-Srivastava. 2006b. Peroxynitrite modulates the expression of Gia protein and adenylyl cyclase signaling in vascular smooth muscle cells. FASEB J. 20 A664. 

Fig. 2. The proposed transition-state intermediate of the ADP-ribosyltransferase reaction catalyzed by PT. The carboxylate group of Glu-129 is proposed to interact with the hydrogen of the ribose 2 -hydroxyl group, thereby promoting the ionization of the diol which weakens the N-glycosidic bond by intramolecular stabilization of an oxocarbonium-like intermediate. The His-35-activated cysteine of the Gia protein may simultaneously exert its nucleophilic attack on the weakened N-glycosidic bond to cleave completely the pyridine-ribosyl bond...

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