We reported previously that increased
acid sphingomyelinase (ASMase)-catalyzed hydrolysis of
sphingomyelin, which leads to increases in
ceramide and
sphingosine 1
phosphate (S1P), played a key role in the synergistic upregulation of proinflammatory
cytokines by
palmitic acid (PA), a major
saturated fatty acid, and
lipopolysaccharide (LPS) in macrophages. Since macrophages are vital players in
nonalcoholic steatohepatitis (NASH) and
atherosclerosis, we assessed the effect of ASMase inhibition on NASH and
atherosclerosis cooperatively induced by high-PA-containing high-fat diet (HP-HFD) and LPS in
LDL receptor-deficient (LDLR-/-) mice. LDLR-/- mice were fed HP-HFD, injected with low dose of LPS and treated with or without the ASMase inhibitor
amitriptyline. The neutral
sphingomyelinase inhibitor
GW4869 was used as control. Metabolic study showed that both
amitriptyline and
GW4869 reduced
glucose,
lipids, and
insulin resistance. Histological analysis and
Oil Red O staining showed that
amitriptyline robustly reduced hepatic steatosis while
GW4869 had modest effects. Interestingly, immunohistochemical study showed that
amitriptyline, but not
GW4869, strongly reduced hepatic
inflammation. Furthermore, results showed that both
amitriptyline and
GW4869 attenuated
atherosclerosis. To elucidate the underlying mechanisms whereby
amitriptyline inhibited both NASH and
atherosclerosis, but
GW4869 only inhibited
atherosclerosis, we found that
amitriptyline, but not
GW4869, downregulated proinflammatory
cytokines in macrophages. Finally, we found that inhibition of
sphingosine 1
phosphate production is a potential mechanism whereby
amitriptyline inhibited proinflammatory
cytokines. Collectively, this study showed that
amitriptyline inhibited NASH and
atherosclerosis through modulation of
sphingolipid metabolism in LDLR-/- mice, indicating that
sphingolipid metabolism in macrophages plays a crucial role in the linkage of NASH and
atherosclerosis.