Acid Sphingomyelinase–Deficient Human Lymphoblasts and Mice Are Defective in Radiation-Induced Apoptosis

Abstract

Stress is believed to activate sphingomyelinase to generate ceramide, which serves as a second messenger in initiating the apoptotic response. Conclusive evidence for this paradigm, however, is lacking. In the present study, we used a genetic approach to address this issue directly. We show that lymphoblasts from Niemann-Pick patients, which have an inherited deficiency of acid sphingomyelinase activity, fail to respond to ionizing radiation with ceramide generation and apoptosis. These abnormalities are reversible up on restoration of acid sphingomyelinase activity by retroviral transfer of human acid sphingomyelinase cDNA. Acid sphingomyelinase knockout mice also expressed defects in radiation-induced ceramide generation and apoptosis in vivo. Comparison with p53 knockout mice revealed that acid sphingomyelinase-mediated apoptosis and p53-mediated apoptosis are likely distinct and independent. These genetic models provide definitive evidence for the involvement of acid sphingomyelinase in one form of stress-induced apoptosis.

Stress signals for apoptosis: ceramide and c-Jun kinase

“…Since sphingomyelin is preferentially located in the outer lea¯et of the plasma membrane, S-SMase may be the form of ASMase responsible for ceramidemediated apoptosis. Endothelium appear to be a particularly rich source of S-SMase (Marathe et al, 1998) (Figure 3), and ASMase knockout mice display a de®cit in radiation-and endotoxin/TNF-induced ceramide generation and apoptosis in endothelium, but no defect in p53-mediated death of thymocytes (Haimovitz-Friedman et al, 1997;Santana et al, 1996) (Figure 4). ASMase involvement in apoptosis is not restricted to the endothelium, however, since B cells from patients with Niemann-Pick disease (NPD), another genetic model of SMase de®ciency, fail to respond to ionizing radiation with ceramide generation or apoptosis, and retroviral transfer of the ASMase gene restores both events (Santana et al, 1996).…”

“…Endothelium appear to be a particularly rich source of S-SMase (Marathe et al, 1998) (Figure 3), and ASMase knockout mice display a de®cit in radiation-and endotoxin/TNF-induced ceramide generation and apoptosis in endothelium, but no defect in p53-mediated death of thymocytes (Haimovitz-Friedman et al, 1997;Santana et al, 1996) (Figure 4). ASMase involvement in apoptosis is not restricted to the endothelium, however, since B cells from patients with Niemann-Pick disease (NPD), another genetic model of SMase de®ciency, fail to respond to ionizing radiation with ceramide generation or apoptosis, and retroviral transfer of the ASMase gene restores both events (Santana et al, 1996). Two groups have reported that ASMase signaling may occur in caveolae, which may represent transient membrane structures capable of forming as a physical response of membranes to diverse stresses (Dobrowsky and Carter, 1998;Liu and Anderson, 1995).…”

The yins and yangs of ceramide

“…Ceramide may also be produced via the action of several distinct enzymes involved in the cleavage of SM, namely acidic (a) and neutral (n) sphingomyelienases (aSMase and nSMase, respectively), which have been characterized based on their pH-optima and biological effects [10]. Absence of aSMase (pH optimum 4.5-5.0), originally described as a lysosomal enzyme defective in patients with Neimann-Pick (NP) syndrome [11] and more recently in aSMasedeficient mice [12], results in the inability to signal apoptosis. Acid SMase in both humans and mice have been cloned and determined to be the product of a conserved gene [13].…”

“…High amounts of ceramide usually lead to apoptosis and this destructive property can be used as a powerful weapon for killing tumor cells. Studies in aSMase knockout mice and NP patients have revealed the absence of apoptosis following massive exposure to chemoand radiation-therapy [12]. Furthermore, restoration of aSMase activity in NP patients with retroviral transfer of human aSMase cDNA was able to reverse such effects [12].…”

“…Studies in aSMase knockout mice and NP patients have revealed the absence of apoptosis following massive exposure to chemoand radiation-therapy [12]. Furthermore, restoration of aSMase activity in NP patients with retroviral transfer of human aSMase cDNA was able to reverse such effects [12]. Numerous reperts have revealed that raising intracellular ceramide levels via activation of SMases by chemotherapeutic agents resulte in rapid death of cancer cells [5], [54].…”