Regulatory role of sphingomyelin metabolites in hypoxia-induced vascular smooth muscle cell proliferation

Vascular cell adaptive response to hypoxic stress includes enhanced production of sphingomyelin metabolites that regulate cell growth. Here, we examined the vascular smooth muscle (VSM) cell adaptive response to hypoxia (2 and 5% O2) and demonstrated that acute (⩽16 h) hypoxic stress significantly stimulated VSM cell growth compared to cells grown under normoxic (21% O2) conditions. This stimulatory effect of hypoxia on VSM cell growth was significantly inhibited by pretreatment of cells with d-erythro-N,N-dimethylsphingosine, an inhibitor of sphingosine kinase. These results suggest a mechanism by which sphingosine 1-phosphate (S-1-P), a promitogenic sphingolipid-derived second messenger, may play a key role in hypoxia-induced VSM cell growth. Supporting this, S-1-P formation was significantly increased in VSM cells subjected to hypoxia. The hypoxia-induced increase in S-1-P level correlated with the decrease in total cellular ceramide content, a sphingolipid metabolite associated with inhibition of cell growth. The activity of sphingomyelinase was also significantly inhibited in hypoxia-treated VSM cells, likely further contributing to a decrease in total intracellular content of ceramide. As a decrease in ceramide content may play a role in hypoxia-induced VSM growth, we next examined the effects of ceramide in VSM cell growth. Elevating intracellular ceramide content through exogenous (C6-ceramide) or endogenous (ceramidase inhibition) manipulations led to a decrease in hypoxia-induced VSM cell growth. In contrast, hypoxia-induced VSM cell growth was further enhanced by S-1-P treatment. Together, our study indicates that hypoxia-induced VSM cell growth may be modulated by sphingomyelin metabolism that results in reduction of total intracellular ceramide level with concomitant increase in S-1-P formation.