Abstract 566: In vivo cardioprotective activity of Sphingosine 1-phosphate mediated through Akt signaling
Left ventricular (LV) remodeling after myocardial infarction (MI) has been associated with a prolonged increase in myocardial apoptosis. The sphingosine 1-phosphate (S1P) signaling cascade inhibits apoptosis in a number of cell types, and has been linked to both the Akt and ERK1/2 pro-survival pathways in cardiac myocytes (CM) in vitro. We have shown previously that protective myocardial S1P generation declines after MI, and that systemic treatment with S1P agonist SEW2871 preserves cardiac function and decreases apoptosis after MI in mice. To further elucidate the mechanism by which S1P promotes CM survival in the post-MI myocardium, we hypothesized that S1P-induced survival signaling, either from pharmacologic S1P receptor agonism or from endogenous S1P generation in cardiac tissue, supports the ability of the myocardium to withstand MI-induced stresses. Gain of S1P signaling function via oral delivery of SEW2871 to male C57/B6 mice during the first 2 weeks after mid-LAD ligation did, in fact, increase activation (i.e. phosphorlyation) of Akt (2.96 ± 0.63 ng/mg protein vs. 1.56 ± 0.29 in vehicle treated controls, P<0.05) and of its downstream intermediate p70S6 kinase (5.1 ± 1.01 vs. 2.69 ± 0.58, P < 0.05) as measured by ELISA. Although apoptotic index was reduced by SEW2871, as has been reported previously, this pro-survival effect was not associated with an increase in activation of ERK1/2. Instead, a decrease in ERK phosphorylation was observed (2.10 ± 0.53 ng/mg protein vs. 3.58 ± 0.43, P < 0.05), possibly reflecting a reduction in cardiac fibroblast proliferation with S1P agonist therapy. Similarly, loss of endogenous S1P generation by sphingosine kinase-1 (SK-1) in SK-1 knockout mice, which we have previously associated with an increase in post-MI apoptosis and a drop in post-MI fractional shortening, led to a decrease in Akt and p70S6 kinase activation (2.18 ± 0.24 vs. 2.94 ±0.24, P<0.06 and 3.5± 0.36 vs. 5.53 ±0.38, P < 0.01, respectively) with no difference in ERK1/2 phosphorylation. These results suggest a central role for Akt signaling, but not the ERK1/2 pathway, in mediating S1P-induced cardioprotection during post-MI LV remodeling in vivo, and may guide ongoing efforts to exploit this novel strategy for prevention of post-MI cardiomyopathy.