THE INFUSION OF MESENCHYMAL STROMAL CELLS INCREASES MYOCARDIAL S100A6 AND LIMITS ADVERSE LEFT VENTRICULAR REMODELING POST MYOCARDIAL INFARCTION

Abstract

The beneficial effects originally attributed to the ability of bone-marrow derived mesenchymal stromal cells BM-MSCs to differentiate into cardiomyocytes, both in vitro and in vivo experimental models has been questioned due to the transient presence of donor cells at injury site following myocardial infarction (MI) suggesting that the MSC-induced improvement in hemodynamic function may be attributable to paracrine effects. We have shown that S100A6, a 20 kDa EF-hand Ca++-binding dimer, is upregulated and secreted following MI and forced expression post-MI attenuated myocyte hypertrophy, decreased fibrosis and apoptosis and was beneficial to the preservation of cardiac function. The aim of this study was to determine whether the beneficial effects of infused BM-MSCs may be related to the secretion of S100A6. In 12 week old Balb/c mice, saline or green fluorescence protein (GFP)-marked BM-MSCs transfected with either a scrambled or S100A6 siRNA were infused intravenously 3-4 hrs post coronary artery ligation. Balb/c murine cultured GFP-marked BM-MSCs express cytoplasmic and nuclear S100A6 at baseline and in response to a hypoxia (5%C02/95% N2) for 1 hr increase S100A6 mRNA and protein approximately 2-3 fold, and release S100A6 (1 nM) in the culture media, responses inhibited in BM-MSCs transfected with S100A6 siRNA. Treatment of neonatal Balb/c cardiac myocytes with human recombinant S100A6 (1nM) for 1-24 hrs attenuated baseline apoptosis (30 per cent decrease in BAX/BCL2 ratio), induced cyclin-dependent kinase 1(CDK1) mRNA 1.5 fold, miR199a 2 fold and myocyte proliferation 2.5 fold, the latter inhibited in the presence of anti-miR 199a. In Balb/c mice, 3-4 days post infusion, the GFP-marked cells were confined strictly to ischemic areas and represented approximately 10 % of total cellularity and coexpressed collagen type IV and troponin T or myosin heavy chain, characteristic of MSCs and cardiomyocytes, respectively, and were CD45(-). Despite the absence of donor cells in the infarcted myocardium 21 days after infusion, mice that have received MSCs alone compared to MSCs transfected with an S100A6 siRNA or saline alone showed a 6-fold increase in S100A6 in peri-infarcted myocardium, attenuated myocyte hypertrophy, decreased fibrosis and apoptosis and preservation of cardiac function. The secretion of S100A6 by infused BM-MSCs may contribute in limiting adverse left ventricular remodeling post-MI.