BackgroundCoronary artery disease leading to myocardial ischemia is the most common cause of heart failure. Apelin (APLN), the endogenous peptide ligand of the APJ receptor, has emerged as a novel regulator of the cardiovascular system.Methods and ResultsHere we show a critical role of APLN in myocardial infarction (MI) and ischemia‐reperfusion (IR) injury in patients and animal models. Myocardial APLN levels were reduced in patients with ischemic heart failure. Loss of APLN increased MI‐related mortality, infarct size, and inflammation with drastic reductions in prosurvival pathways resulting in greater systolic dysfunction and heart failure. APLN deficiency decreased vascular sprouting, impaired sprouting of human endothelial progenitor cells, and compromised in vivo myocardial angiogenesis. Lack of APLN enhanced susceptibility to ischemic injury and compromised functional recovery following ex vivo and in vivo IR injury. We designed and synthesized two novel APLN analogues resistant to angiotensin converting enzyme 2 cleavage and identified one analogue, which mimicked the function of APLN, to be markedly protective against ex vivo and in vivo myocardial IR injury linked to greater activation of survival pathways and promotion of angiogenesis.ConclusionsAPLN is a critical regulator of the myocardial response to infarction and ischemia and pharmacologically targeting this pathway is feasible and represents a new class of potential therapeutic agents.

Rationale: Myocardial infarction (MI) results in remodeling of the myocardium and the extracellular matrix (ECM). Tissue inhibitors of metalloproteinases (TIMPs) are critical regulators of ECM integrity via inhibiting matrix metalloproteinases (MMPs). TIMP2 is highly expressed in the heart and is the only TIMP that, in addition to inhibiting MMPs, is required for cell surface activation of pro-MMP2. Hence, it is difficult to predict the function of TIMP2 as protective (MMP-inhibiting) or harmful (MMP-activating) in heart disease. Objective: We examined the role of TIMP2 in the cardiac response to MI. Methods and Results: MI was induced in 11-to 12-week-old male TIMP2؊/؊ and age-matched wild-type mice. Cardiac function was monitored by echocardiography at 1 and 4 weeks post-MI. ECM fibrillar structure was visualized using second harmonic generation and multiphoton imaging of unfixed/unstained hearts. Molecular analyses were performed at 3 days and 1 week post-MI on flash-frozen infarct, periinfarct, and noninfarct tissue. Membrane type 1 (MT1)-MMP levels and activity were measured in membrane protein fractions. TIMP2؊/؊ -MI mice exhibited a 25% greater infarct expansion, markedly exacerbated left ventricular dilation (by 12%) and dysfunction (by 30%), and more severe inflammation compared to wild-type MI mice. Adverse ECM remodeling was detected by reduced density and enhanced disarray of fibrillar collagen in TIMP2 ؊/؊ -MI compared to wild-type MI hearts. TIMP2 deficiency completely abrogated MMP2 activation but markedly increased collagenase activity, particularly MT1-MMP activity post-MI. Key Words: myocardial infarction Ⅲ extracellular matrix Ⅲ remodeling Ⅲ tissue inhibitor of metalloproteinase Ⅲ matrix metalloproteinase C oronary artery disease is the most common cause of heart failure in Western nations. 1 Myocardial infarction (MI) results in adverse remodeling of the myocardium including the extracellular matrix (ECM). 2 ECM is a dynamic structure which undergoes constant turnover, and its intact structure is essential for optimal cardiac structure and function. Tissue inhibitors of metalloproteinases (TIMPs) play a critical role in regulating the activity of the ECM-degrading enzymes matrix metalloproteinases (MMPs). [3][4][5] Four TIMPs have been identified so far (TIMP1 to TIMP4), with TIMP2 and TIMP3 being the highly expressed TIMPs in the heart and TIMP4 showing a tissue-specific expression pattern which includes the heart and the brain. 6 TIMP2 levels have been shown to be altered in patients with different types of heart disease such as pressure overload 7 and atrial fibrillation. 8 In patients with ischemic or idiopathic dilated cardiomyopathy, myocardial protein levels of TIMP2 remained unaltered 9,10 or increased in end-stage dilated cardiomyopathy patients, 11 whereas plasma analyses revealed a rise in TIMP2 levels in late stages of MI. 12 TIMP2 is unique among TIMPs because, in addition to inhibiting a number of MMPs, it mediates MMP activation. TIMP2 is required for cell surface activati...

The balance of matrix metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinases (TIMPs) determines the integrity of the extracellular matrix. TIMP3 is the most highly expressed tissue inhibitor of metalloproteinase (TIMP) in the kidney, but its function in renal disease is incompletely understood. In this study, TIMP3 Ϫ/Ϫ mice demonstrated an age-dependent chronic tubulointerstitial fibrosis. After unilateral ureteral obstruction (UUO), young TIMP3 Ϫ/Ϫ mice exhibited increased renal injury (tubular atrophy, cortical and medullary thinning, and vascular damage) compared with wild-type mice. In addition, TIMP3 Ϫ/Ϫ mice had greater interstitial fibrosis; increased synthesis and deposition of type I collagen; increased activation of fibroblasts; enhanced apoptosis; and greater activation of MMP2, but not MMP9, after UUO. TIMP3 deficiency also led to accelerated processing of TNF␣, demonstrated by significantly higher TACE activity and greater soluble TNF␣ levels by 3 d after UUO. The additional deletion of TNF␣ markedly reduced inflammation, apoptosis, and induction of a number of MMPs. Moreover, inhibition of MMPs in TIMP3 Ϫ/Ϫ /TNF␣ Ϫ/Ϫ mice further abrogated postobstructive injury and prevented tubulointerestitial fibrosis. In humans, TIMP3 expression increased in the renal arteries and proximal tubules of subjects with diabetic nephropathy or chronic allograft nephropathy. Taken together, these results provide evidence that TIMP3 is an important mediator of kidney injury, and regulating its activity may have therapeutic benefit for patients with kidney disease.