Objective: Oxidized low-density lipoprotein (oxLDL)-induced smooth muscle cell (SMC) proliferation requires the coactivation of various signaling pathways, namely sphingomyelin/ceramide/sphingosine-1-phosphate, epithelial growth factor receptor (EGFR), and phosphoinositide 3-kinase (PI-3K) pathways. This study aimed to clarify the respective role and the hypothetical cross-talk between sphingomyelin/ceramide/sphingosine-1-phosphate, EGFR, and PI-3K/Akt pathways in the balance between mitogenic and cytotoxic effects elicited by oxLDL.
Methods and results: Coimmunoprecipitation experiments and the use of inhibitors and dominant-negative mutant showed that oxLDL-induced PI-3K activation is dependent on EGFR. PI-3K activation is independent of the sphingomyelin/ceramide/sphingosine-1-phosphate pathway, because PI-3K inhibition by LY294002 or dominant-negative Deltap85 mutant does not abrogate sphingomyelin hydrolysis, and, conversely, the use of permeant C2-ceramide and of N,N-dimethyl-sphingosine, a sphingosine kinase inhibitor, does not alter PI-3K activity. Activation of Akt/PKB by oxLDL requires PI-3K, as shown by the inhibition by LY204002 and in Deltap85 SMC. The inhibition of Akt/PKB by PI-3K inhibitor LY204002 or by overexpression of kinase-dead Akt shifted the mitogenic effect of oxLDL toward apoptosis, thus suggesting that the PI-3K/Akt pathway acts as a survival pathway.
Conclusions: SMC proliferation elicited by moderate concentrations of oxLDL involves the sphingomyelin/ceramide/sphingosine-1-phosphate pathway, which leads to extracellular regulated kinase 1/2 activation and DNA synthesis, and the EGFR/PI-3K/Akt pathway, which prevents the apoptotic effect of oxLDL.