Overexpression of the cardiac beta(2)-adrenergic receptor and expression of a beta-adrenergic receptor kinase-1 (betaARK1) inhibitor both increase myocardial contractility but have differential effects on susceptibility to ischemic injury.

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1999-11-26

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Abstract

Cardiac beta(2)-adrenergic receptor (beta(2)AR) overexpression is a potential contractile therapy for heart failure. Cardiac contractility was elevated in mice overexpressing beta(2)ARs (TG4s) with no adverse effects under normal conditions. To assess the consequences of beta(2)AR overexpression during ischemia, perfused hearts from TG4 and wild-type mice were subjected to 20-minute ischemia and 40-minute reperfusion. During ischemia, ATP and pH fell lower in TG4 hearts than wild type. Ischemic injury was greater in TG4 hearts, as indicated by lower postischemic recoveries of contractile function, ATP, and phosphocreatine. Because beta(2)ARs, unlike beta(1)ARs, couple to G(i) as well as G(s), we pretreated mice with the G(i) inhibitor pertussis toxin (PTX). PTX treatment increased basal contractility in TG4 hearts and abolished the contractile resistance to isoproterenol. During ischemia, ATP fell lower in TG4+PTX than in TG4 hearts. Recoveries of contractile function and ATP were lower in TG4+PTX than in TG4 hearts. We also studied mice that overexpressed either betaARK1 (TGbetaARK1) or a betaARK1 inhibitor (TGbetaARKct). Recoveries of function, ATP, and phosphocreatine were higher in TGbetaARK1 hearts than in wild-type hearts. Despite basal contractility being elevated in TGbetaARKct hearts to the same level as that of TG4s, ischemic injury was not increased. In summary, beta(2)AR overexpression increased ischemic injury, whereas betaARK1 overexpression was protective. Ischemic injury in the beta(2)AR overexpressors was exacerbated by PTX treatment, implying that it was G(s) not G(i) activity that enhanced injury. Unlike beta(2)AR overexpression, basal contractility was increased by betaARK1 inhibitor expression without increasing ischemic injury, thus implicating a safer potential therapy for heart failure.

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Lefkowitz

Robert J. Lefkowitz

The Chancellor's Distinguished Professor of Medicine

Dr. Lefkowitz’s memoir, A Funny Thing Happened on the Way to Stockholm, recounts his early career as a cardiologist and his transition to biochemistry, which led to his Nobel Prize win.

Robert J. Lefkowitz, M.D. is Chancellor’s Distinguished Professor of Medicine and Professor of Biochemistry and Chemistry at the Duke University Medical Center. He has been an Investigator of the Howard Hughes Medical Institute since 1976. Dr. Lefkowitz began his research career in the late 1960’s and early 1970’s when there was not a clear consensus that specific receptors for drugs and hormones even existed. His group spent 15 difficult years developing techniques for labeling the receptors with radioactive drugs and then purifying the four different receptors that were known and thought to exist for adrenaline, so called adrenergic receptors. In 1986 Dr. Lefkowitz transformed the understanding of what had by then become known as G protein coupled receptors because of the way the receptor signal for the inside of a cell through G proteins, when he and his colleagues cloned the gene for the beta2-adrenergic receptor. They immediately recognized the similarity to a molecule called rhodopsin which is essentially a light receptor in the retina. This unexpected finding established the beta receptor and rhodopsin as the first member of a new family of proteins. Because each has a peptide structure, which weaves across the cell membrane seven times, these receptors are referred to as seven transmembrane receptors. This super family is now known to be the largest, most diverse and most therapeutically accessible of all the different kinds of cellular receptors. There are almost a thousand members of this receptor family and they regulate virtually all known physiological processes in humans. They include the receptors not only to numerous hormones and neurotransmitters but for the receptors which mediate the senses of sweet and bitter taste and smell amongst many others. Dr. Lefkowitz also discovered the mechanism by which receptor signaling is turned off, a process known as desensitization. Dr. Lefkowitz work was performed at the most fundamental and basic end of the research spectrum and has had remarkable consequences for clinical medicine. Today, more than half of all prescription drug sales are of drugs that target either directly or indirectly the receptors discovered by Dr. Lefkowitz and his trainees. These include amongst many others beta blockers, angiotensin receptor blockers or ARBs and antihistamines. Over the past decade he has discovered novel mechanisms by which the receptors function which may lead to the development of an entirely new class of drugs called “biased agonists”. Several such compounds are already in advanced stages of clinical testing. Dr. Lefkowitz has received numerous honors and awards, including the National Medal of Science, the Shaw Prize, the Albany Prize, and the 2012 Nobel Prize in Chemistry. He was elected to the USA National Academy of Sciences in 1988, the Institute of Medicine in 1994, and the American Academy of Arts and Sciences in 1988.


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