Dissecting cardiac hypertrophy and signaling pathways: evidence for an interaction between multifunctional g proteins and prostanoids.

Over the past two decades, the ability to produce genetically engineered animal models has been widely used to unravel complex biological pathways involved in both physiology and disease. Although uniquely powerful and inherently elegant, these models, while often providing surprising new insights, have not uncommonly raised more questions than they have answered. A case in point is a transgenic mouse model developed to probe the in vivo role of the G protein, Gh, in cardiac signaling, and its consequences.1 Unlike “traditional” heterotrimeric G proteins, Gh is decidedly atypical, being a multifunctional protein with both GTPase and transglutaminase (TGase) activity, and showing no sequence identity with other GTP-binding proteins. Moreover, although similar mouse models developed to study the role of Gq (a heterotrimeric G protein that, like Gh, mediates 1-adrenergic receptor ( 1-AR) signaling, as well as that by other Ca -mobilizing receptors) provided clear evidence for its critical involvement in pressure-overload hypertrophy via a mechanism involving protein kinase C (PKC ) and phospholipase C (PLC) activation,1,2 that for Gh demonstrated a mild form of hypertrophy by a mechanism that remained elusive. Evidence for PKC and PLC activation was lacking, and it was suggested, largely by default, that the hypertrophy in the cardiac Gh animals was due not to its signaling activity but to TGase-mediated protein crosslinking. In this issue of Circulation Research, FitzGerald and coworkers3 have reexamined this issue using an independently developed model of cardiac-restricted Gh overexpression. Interestingly, these studies provide evidence linking two major biological systems, transglutaminases (TGs) and prostanoids, in the cardiac dysfunction and hypertrophy observed in this transgenic model, and potentially contributing to these disorders in humans. TGs are a family of Ca and thiol-dependent enzymes that catalyze the covalent posttranslational modification of proteins, either by formation of an isopeptide bond between the -carboxamide group of glutamine residues and the -amino group of intrachain lysines or by modification of glutamine residues by polyamines.4 Isopeptide bond formation stabilizes the resulting supramolecular structure and confers resistance to proteolysis—a reaction that is critical for hemostasis, for example, involving the crosslinking of fibrin clots by the TG, factor XIIIa. In addition to these proteinmodifying transamidation reactions, TGs can also esterify and deamidate proteins, reactions critical, for example, to the barrier function of skin and to the pathophysiology of gluten-induced enteropathy (celiac disease), respectively.4 Specialized noncatalytic actions of TGs have also been identified, such as scaffolding of the cytoskeleton to maintain membrane integrity (mediated by a catalytically inactive member of the TG family, band 4.2), cell adhesion and, as detailed below, potentially, signal transduction.4 Of the mammalian TGs, encoded by nine distinct genes in the human genome, tissue TG (also known as TG2 or TG-C) was the first to be identified and, yet, paradoxically, despite extensive study, remains one of the least well understood. It is expressed ubiquitously in most organs and tissues, and although localized predominantly in the cytosol (80%), is also found in plasma (10% to 15%) and the nuclear membranes (5%), as well as being secreted from the cell by an unknown mechanism where it localizes to the cell surface and extracellular matrix. In 1987, Greenberg and colleagues showed that, unlike other TGs, TG2 could bind GTP and that when it did, its TGase crosslinking activity was inhibited.4 Subsequently, we showed that a 74-kDa G protein, named Gh (h to indicate its high molecular weight compared with heterotrimeric G protein -subunits that are in the 40to 45-kDa range), previously shown to couple specifically to 1-ARs, was identical to TG2.5 Further, evidence was provided to suggest an elegant regulatory pathway whereby Gh exchanges GTP for GDP, once activated by 1-AR-stimulation, which, as a consequence, inactivates the TGase activity of Gh. In the GTP-bound form, Gh then activates PLC to increase inositol trisphosphate and intracellular Ca . Subsequent hydrolysis of GTP by the GTPase activity of Gh, or the increase in intracellular Ca , which inhibits nucleotide binding by Gh, terminates signaling and restores its TGase activity. Nevertheless, direct in vivo evidence for its role in cell signaling is still lacking, and null mutant mice, lacking TG2/Gh expression, although showing subtle phenotypic changes,6 have not, as yet, confirmed its role as a mediator of receptor-coupled signaling. Additional studies have shown that Gh heterodimerizes with a 50-kDa -subunit that stabilizes the GDP-bound form of Gh (the 74-kDa subunit thus being referred to as Gh ); that TG2/Gh mediates activation of the 1 isoform of PLC and of maxi-K channels, as well as inhibition of adenylyl cyclase; that it supports signaling not only of certain 1-AR subtypes but also of certain oxytocin The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Molecular Cardiology Program, Victor Chang Cardiac Research Institute, Darlinghurst, Sydney, New South Wales, Australia. Correspondence to Robert M. Graham, FAA, MD, Victor Chang Cardiac Research Institute, 384 Victoria St, Darlinghurst, NSW 2010, Australia. E-mail [email protected] (Circ Res. 2003;92:1059-1061.) © 2003 American Heart Association, Inc.