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The Role of G-Proteins in the Pathophysiology of the Cardiovascular Diseases

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The Role of G-Proteins in the Pathophysiology of the Cardiovascular Diseases Evora & Nobre Arq BrasUpdate Cardiol G-proteins in the pathophysiology of cardiovascular diseases volume 72, (nº 2), 1999 The Role of G-Proteins in the Pathophysiology of the Cardiovascular Diseases Paulo Roberto B. Evora, Fernando Nobre Ribeirão Preto, SP - Brazil Questions exist about the mechanisms by which cells This review aims to describe the innermost mecha- of the cardiovascular and other specialized systems nisms of cardiovascular diseases at the biochemical level, recognize and respond to hormones and neurotransmitters. emphasizing the role of G-proteins. The text begins with the The search for answers to these fundamental questions presentation of basic concepts about the interaction about the innermost workings of pathophysiological between “messengers” that are part of the event cascade mechanisms has prompted a tremendous effort in research that leads to intracellular responses of specialized cells. At over the last 30 years and has led to new frontiers of the subsequent stages, the properties of G-proteins are knowledge, which have resulted in more than one Nobel discussed as well as their role on the cAMP and cGMP vias, Prize. During the last 20 years, the use of biochemical and stressing their importance in specific cardiovascular molecular methods has revealed that the beginning of the diseases. This review does not intend to exhaust the cellular response occurs in membrane receptors. Several subject, but to simplify it as much as possible, so that the types and subtypes of these receptors have been identified, readers of the Arquivos Brasileiros de Cardiologia can defining essential regulatory molecules such as acetyl- recycle their knowledge about the innermost mechanisms choline and noradrenaline. For example, so far, five different of cardiovascular diseases. cholinergic receptors and five different adrenergic receptors have been cloned and sequenced. All of these receptors are Basic Concepts glycoproteins that have regions with substance, sequential identity, and seem to be part of a great family of receptors There are mechanisms by which a biochemical signal that share structural similarities. The occupation of the generated by a hormone or neurotransmitter causes a receptors by “primary messengers” such as the catecho- biological effect inside a cell. These mechanisms are lamines and acetylcholine, regulate the activation of one or generically designated as “signal transduction” and can be more systems of effector “secondary messengers” in divided into two basic groups: 1) transduction through responsive cells. These systems include ion channels and intracellular receptors; 2) transduction through cell surface enzymes, such as adenylate cyclase (which forms cAMP) or membrane receptors (table I) 3. and guanylate cyclase (which forms cGMP) 1,2. At the beginning of the 1970s, Rodbell et al specu- Intracellular receptors - In this system, the trans- lated about the presence of more than one component in duction of the biochemical signal is performed through the activation of adenylate cyclase by hormones, after intracellular receptors bound to the cytosol or to the discovering the involvement in a stage where there was nucleus. The complex receptor-agonist binds specific participation by guanosine triphosphate (GTP). The regions of the DNA, causing an increase in the expression of hydrolysis of GTP stimulated by hormones, observed by specific genes. In this case, the effects of the agonists are Cassel and Selinger, was significant. The discovery by not immediate because time is needed for the genic trans- Gilman et al of a membrane factor different from a receptor cription and for the subsequent translation of the mRNA. or from adenylate cyclase led to the identification of this Some examples of this transduction through intracellular element. In 1987, Alfred Gilman extensively described this receptors are: steroid receptors, vitamin D and retinoic acid component as a family of proteins called G-proteins, which 3 bind the family of receptors and the intracellular effector receptors, and thyroxin receptors . molecules 1,2. Membrane receptors - In the transduction systems that use this type of receptor, the signal is transferred to the intracellular processes responsible for the cellular responses Faculdade de Medicina de Ribeirão Preto-USP and Centro Especializado do through a sophisticated system in which the G-proteins that Coração e Pulmão de Ribeirão Preto (CECORP) Mailing address: Paulo Roberto B. Evora - Rua Rui Barbosa, 367 - 7° - 14015-120 act in the cAMP and cGMP systems take part. These proteins - Ribeirão Preto, SP - Brazil represent a family whose members include the following: 1) 220 Arq Bras Cardiol Evora & Nobre volume 72, (nº 2), 1999 G-proteins in the pathophysiology of cardiovascular diseases Gs – stimulate the adenylate cyclase through beta-adrenergic proteins can activate several intracellular processes. The receptors, and have a recently discovered effect of increasing identity of this G-protein and its relation with the G-proteins the conductance of calcium in the heart; 2) Gi – seems to have involving the alpha1-adrenergic stimulation of phospholi- multiple methods of inhibiting adenylate cyclase through the pases C are unknown 3-6. alfa2-adrenergic receptors; 3) Gt – also known as transducin, Unlike the intracellular receptors, the membrane binds the photoreceptor rhodopsin in the retina; 4) Go – can receptors do not directly regulate the genic expression and regulate calcium channels; 5) Gk – regulates the potassium can have indirect effects. Based on signal transduction channels; 6) Gp – not yet well defined, can regulate phospho- mechanism, there are four classes of membrane receptors: lipases (table II) 3-6. 1) neurotransmitters bound with ion channels of nerves and All G-proteins are structurally very similar, being muscles (ex. nicotinic cholinergic receptors, GABA composed of three subunits designated as alpha (a), beta receptors, and receptors of glycine); 2) catalytic receptors (b), and gamma (g). The alpha-subunit is the most characte- related to the enzymatic activity as part of its structure, and ristic of each G-protein. This is the subunit that usually in the majority of the cases the enzymatic activity is a interacts with the receptor, binds GTP, and regulates the tyrosine-specific protein kinase (ex. insulin receptors); 3) effector systems. The G-proteins can be studied by the use receptors related to intracellular secondary messengers, of bacterial toxins, especially the cholera and pertussis which act as real signal amplifiers, because one molecule of toxin. The cholera toxin stimulates the Gs-proteins, and the the receptor activates several molecules responsible for the pertussis toxin inactivates the Gi-proteins. The effects of intracellular effects. In this case, many ion channels are not these toxins have been tested in cellular cultures and in directly coupled with their receptors, but work through G- cellular membranes, and lymphocytes and monocytes of protein action 3. patients with heart disease have been used as substrates. It should be stressed that the idea that G-proteins However, all these methods are very limited for clinical use provide the only system of signal transduction through on a large scale 3-6. receptors is false. Cumulative efforts of several laboratories The activation of the regulatory G-proteins is asso- have contributed to the elucidation of signaling mecha- ciated with the agonist stimulation of the majority of the nisms independent from G-proteins. These mechanisms receptors bound to the cellular membrane. Therefore some involve activation of several signaling membrane mole- agonist-receptor interactions facilitate the GTP binding cules, followed by a sequential stimulation of various with an alpha-subunit of the protein and the G-protein is protein kinases collectively known as MAPK (mitogen- activated. Then, this G-protein dissociates from the recep- activated protein kinase). The MAPK signaling cascade tor causing decrease of affinity between the receptor and amplifies and transmits signals that, eventually, activate the agonist, and the alpha-subunit is released (figure 1). several regulatory molecules in the cytoplasm and in the The distinct alpha-subunits derived from the different G- nucleus to start cellular processes such as proliferation, differentiation, and development. This cascade is not only restricted to the signaling of growing factors. It is also Table I – Signal-transducing mechanisms related to pathways started by phorbol esters, ionophores, and heat shock proteins. 1) Intracellular receptors: steroid receptors, vitamin D receptors, retinoic acid receptors, and thyroxin receptors. The hormones, the neurotransmitters, and the growth 2) Membrane receptors directly coupled with effector molecules. factors can be considered signals, and the receptors A) Receptors bound to ion channels: cholinergic nicotinic GABA detectors of these signals. Each component functions as an receptors. B) Receptors with catalytic activity: insulin receptors. element of communication between extracellular events and 1) Membrane receptors coupled with effector molecules through chemical alterations inside the cell. Molecules considered secondary messengers. “secondary messengers” (so-called because they function A) Receptors coupled with adenylate cyclase: adrenergic receptors, glucagon, and epinephrine. as an element of binding or transduction of the signal B) Receptors coupled with the phosphatidylinositol cycle and originated in the
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