The incidence of IS in young adults is increasing gradually, which occurred with a considerably wider range of risk factors than older patients. More than 150 causes of early-onset IS has been identified, but the specific pathogenesis remains unclear[24]. This study intends to explore molecular mechanism and screen biomarkers of youth stroke via metabolomics data analysis based on LC-MS/MS system. Totally, 40 differential metabolites were selected, including 14 with VIP value> 2 and Student's t-test P value <0.05, which mainly were ceramides, lipids, and amino acids. After database search and article analysis, elevated plasma level of lactose ceramide and abnormal sphingolipid metabolism pathway were considered the most likely mechanism associated with the onset of stroke in young patients.
Experimental evidence is accumulating which suggesting a crucial role of sphingolipids in the pathogenesis of IS. Glycosphingolipids (GSLs) are important structural molecules constituting the cell membrane and siganl transduction regulators producing a variety of different biological functions[25]. Ceramides has been proved not only to regulate cell proliferation, differentiation, senescence and apoptosis, but also to participate in cellular stress responses[26]. In particular, several different animal models of ischemia/reperfusion injury have shown that ceramide accumulation in ischemic tissue might be a crucial trigger of apoptosis.
Endogenous ceramides are generated by three different biochemical pathways[27]: 1) Ceramides can be produced via the de novo synthesis involving several catalytic steps which fianlly N-acylation of sphinganine to convert ceramide; 2) Ceramide can also be formed via the salvage pathway by re-acylation of sphingoid long chain bases, such as sphingosin; 3) Ceramides can be generated through hydrolysis of complex sphingolipids. In our study, dihydrosphingosine and 2-amino-1,3,4-octadecanetriol (phytosphingosine), both synthetic precursors of ceramide[28], were significantly reduced in young patients, whereas C16 lactosyl ceramide was significantly increased. It indicates more sphingolipid precursors might be used to synthesize lactoceramide (LacCer) in junior group than senior group, which may contribute to the onset of youth IS.
Edsfeldt, Andreas et al. analyzed several GSLs in homogenates from 200 human carotid plaques using mass spectrometry and discovered that glucosylceramide, lactosylceramide, ceramide, dihydroceramide, sphingomyelin, and sphingosine-1-phosphate (S1P) were all significantly increased in symptom-associated plaques compared with plaques from asymptomatic patients[29]. Their study also revealed that increased level of ceramides were correlated with inflammatory cytokines and served as histological markers of plaque instability. However, unstable plaques were considered to be highrisk and prone to thrombotic complication, such as cerebral emboli caused ischemic events[30].
It has already been shown that critical effects of ceramides in various neurodegenerative and inflammatory diseases are physiological condition-exerted and chain length-specific[31]. Alterations of long chain ceramides (≧C16), as well as their respective dihydro-ceramides and precursors, were regulators in apoptotic cell death[32]. Elevated plasma levels of ceramides were predictors of both risk and severity at admission in IS patients, which was confirmed by the study of 202 age and sex matched control patients[33]. Moreover, the serum level of LacCer was strong correlated with arterial stiffness and could be used to indicate vascular dysfunction. Rabbits fed high fat and high cholesterol diet showed a marked increase of LacCer accompanied by extensive atherosclerosis, which was prevented by treatment of the glycosphingolipid glycosyltransferase inhibitor D-PDMP[34]. Kim, Minjoo et al. revealed LacCer were independent predictors of increased arterial stiffness in middle-aged individuals[35].
Further study has elucidated that LacCer stimulates the GTP load on the Ras to initiate cell proliferation via a series of signal transductions which results in the phosphorylation of MAPK p44 [36]. In human smooth muscle cell (SMC), oxidative LDL (ox-LDL) could dose-dependently activate the synthesis of LacCer by rapid phosphorylation of LacCer synthetase[37]. It was reported that D-PDMP reduced ox-LDL guided SMC multiplication[38], indicating that the activation of LacCer synthetase and the increased of LacCer were essential to ox-LDL induced arterial SMC proliferation.
Deguchi, Hiroshi et al. reported a new mechanism for cross-talk between sphingolipid metabolism and thrombin generation[39], which implied sphingosine disrupted interactions between factor Va and the Gla domain of factor Xa in the prothrombinase complex. Thus, certain sphingolipids might be bioactive lipid mediators of thrombin generation to modulate cell growth and death, blood coagulation, and inflammation. Certain factors induced hypercoagulability were high risk factors of thromboembolic diseases[40]. The role of down-regulatied sphingosine in coagulation state and in onset of youth IS need to be furhter studied.
Generally, the onset of youth IS maybe caused by up-regualted lactoceramide and abnormal sphingolipid metabolism, which are correlate with plaque instability, arterial stiffness, cell proliferation, high sensitive of ox-LDL and hypercoagulability. Our result indicates a potential biomarker for the early onset of IS, but the potential mechanism needs more further studies for confirmation.
Limitation
The present study has some limitations. The sample size was relatively small, which might limit the generalizability of the results. This study is a prospective cohort study of single-center and the current findings are preliminary, which needs further validation in more control cohort studies. The relationship between lactose ceramide and the mechanism of young stroke still needs to be verified by further precise studies.