Deleted in colorectal cancer (DCC), a large transmembrane receptor of netrin-1, is critical for mediating netrin-1's cardioprotective function. In the present study we investigated novel mechanisms underlying netrin-1-induced, rapid, and feed-forward up-regulation of DCC, which is believed to sustain nitric oxide (NO) production to potentiate cardioprotection. Intriguingly, NO markedly reduced expression of the E3 ubiquitin ligase seven in absentia homolog (SIAH) that is specific for regulation of protesome-dependent DCC degradation, resulting in accumulation of DCC. The two SIAH isoforms compensate for each other when one is repressed; inhibition of both SIAH1 and SIAH2 using combined siRNAs significantly reduced infarct size while improving cardiac function after ischemia/reperfusion injury of the heart. This effect was absent in DCC-deficient mice. Moreover, in vivo RNAi inhibition of SIAH1/2 further augmented netrin-1's cardioprotective function. In summary, these data identify a novel therapeutic target of SIAH in facilitating NO/netrin-1-dependent cardioprotection, using the DCC receptor. Combination of netrin-1 and SIAH RNAi may prove to be a substantially effective therapy for myocardial infarction.eleted in colorectal cancer (DCC), a single transmembrane receptor of netrin-1, was first discovered in 1990 during the search for candidate tumor suppressor genes in chromosome 18q (1). At present DCC is known to play important roles in the following biological processes: guidance of developing axons (2, 3), conditional control of apoptosis (4), tumorigenesis (5), and angiogenesis (6, 7). DCC-dependent production of nitric oxide (NO) mediates angiogenic responses of endothelial cells (6). We have recently identified a novel and robust effect of netrin-1 in cardioprotection (8, 9). Netrin-1 induces cardioprotection against ischemia/reperfusion (I/R) injury both ex vivo and in vivo, which is mediated by DCC/ERK1/2/eNOS/NO-dependent protection of cardiomyocytes from necrosis and apoptosis, mediated by NOdependent attenuation of oxidative stress and preservation of mitochondrial function (8-10). A deficiency of DCC abolished the protective effects of netrin-1 against cardiac I/R injury, implying a crucial role of DCC in mediating netrin-1-induced cardioprotection (8, 10).Although it is a large transmembrane protein, DCC protein abundance was significantly up-regulated by netrin-1 in I/Rinjured heart within a very short time of 2 h (8). Intriguingly, the DCC accumulation induced by netrin-1 was attenuated by the NOS inhibitor L-NAME, implicating an NO-dependent mechanism (8). This NO-DCC feed-forward regulation, in addition to the initial DCC/ERK1/2/eNOS/NO pathway, may be extremely beneficial in potentiating netrin-1-dependent cardioprotection. It is possible that a proteasome dependent mechanism was involved in this quick action of protein regulation. Protein ubiquitination plays a key role in determining the half-life of proteins in mammalian cells. In ubiquitin-proteasome systems, E3 ubiquitin ligases recogniz...