BACKGROUND Mesenchymal stem cells (MSCs) transplantation may partially restore heart function in the treatment of acute myocardial infarction (AMI). The aim of this study was to explore the beneficial effects of MSCs modified with heme xygenase-1 (HO-1) on post-infarct swine hearts to determine whether the induction of therapeutic angiogenesis is modified by the angiogenic cytokines released from the implanted cells. METHODS In vitro, MSCs were divided into four groups: (1) non-transfected MSCs (MSCs group), (2) MSCs transfected with the pcDNA3.1-Lacz plasmid (Lacz-MSCs group), (3) MSCs transfected with pcDNA3.1-hHO-1 (HO-1-MSCs group), and (4) MSCs transfected with pcDNA3.1-hHO-1 and pretreatment with an HO inhibitor, tin protoporphyrin (SnPP) (HO-1-MSCs + SnPP group). Cells were cultured in an airtight incubation bottle for 24 hours, in which the oxygen concentration was maintained at < 1%, followed by 12 hours of reoxygenation. After hypoxia/reoxygen treatment, ELISA was used to measure transforming growth factor (TGF-β) and fibroblast growth factor (FGF-2) in the supernatant. In vivo, 28 Chinese mini-pigs were randomly allocated to the following treatment groups: (1) control group (saline), (2) Lacz-MSCs group, (3) HO-1-MSCs group, and (4) HO-1-MSCs + SnPP group. About 1 × 10(7) of autologous stem cells or an identical volume of saline was injected intracoronary into porcine hearts 1 hour after MI. Magnetic resonance imaging (MRI) assay and postmortem analysis were assessed four weeks after stem cell transplantation. RESULTS Post hypoxia/reoxygenation in vitro, TGF-β in the supernatant was significantly increased in the HO-1-MSCs ((874.88 ± 68.23) pg/ml) compared with Lacz-MSCs ((687.81 ± 57.64) pg/ml, P < 0.001). FGF-2 was also significantly increased in the HO-1-MSCs ((1106.48 ± 107.06) pg/ml) compared with the Lacz-MSCs ((853.85 ± 74.44) pg/ml, P < 0.001). In vivo, at four weeks after transplantation, HO-1 gene transfer increased the capillary density in the peri-infarct area compared with the Lacz-MSCs group (14.24 ± 1.66/HPFs vs. 11.51 ± 1.34/HPFs, P < 0.001). Arteriolar density was also significantly higher in HO-1-MSCs group than in the Lacz-MSCs group (7.86 ± 2.00/HPFs vs. 6.45 ± 1.74/HPFs, P = 0.001). At the same time, the cardiac function was significantly improved in the HO-1-MSCs group compared with the Lacz-MSCs group ((53.17 ± 3.55)% vs. (48.82 ± 2.98)%, P < 0.05). However, all these effects were significantly abrogated by SnPP. CONCLUSIONS MSCs provided a beneficial effect on cardiac function after ischemia/reperfusion by the induction of therapeutic angiogenesis, and this effect was amplified by HO-1 overexpression.