TUMOR-NECROSIS-FACTOR-ALPHA STIMULATES THE MATURATION OF STEROL REGULATORY ELEMENT-BINDING PROTEIN-1 IN HUMAN HEPATOCYTES THROUGH THE ACTION OF NEUTRAL SPHINGOMYELINASE

The mechanism by which genes involved in cholesterol biosynthesis and import are preferentially up-regulated in response to sterol depletion was elucidated with the cloning of sterol regulatory element binding protein-1 (SREBP-1), SREBP-1 is a transcription factor whose entry int o the nucleus is gated by sterol-regulated proteolysis. We have invest igated the role of tumor necrosis factor-alpha (TNF-alpha) as a mediat or of SREBP-1 maturation in human hepatocytes. TNF-alpha is capable of inducing SREBP-1 maturation in a time-and dose-dependent manner that is consistent with the kinetics of TNF-alpha-mediated activation of ne utral sphingomyelinase (N-SMase). Antibodies to N-SMase inhibit TNF-al pha-induced SREBP-1 maturation suggesting that N-SMase is a necessary component of this signal transduction pathway, Ceramide, a product of sphingomyelin hydrolysis, is also capable of inducing SREBP-1 maturati on, The mature form of SREBP-1 generated by TNF-alpha, sphingomyelinas e or ceramide treatment translocates to the nucleus and binds the ster ol regulatory element. This promotes transcription of the gene upstrea m of the sterol regulatory element. A unique finding of our studies is that ceramide stimulated SREBP-1 maturation even in the presence of c holesterol and 25-hydroxycholesterol both of which are known suppresso rs of SREBP-1 maturation, Our findings indicate that ceramide-mediated maturation of SREBP-1 maturation is a novel sterol-independent mechan ism by which cholesterol homeostasis may be regulated.