Mycopathologia 157: 339–347, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 339 Inhibition of serine palmitoyltransferase by myriocin, a natural mycotoxin, causes induction of c-myc in mouse liver Quanren He 1 , Victor J. Johnson 2 , Marcin F. Osuchowski 1 & Raghubir P. Sharma 1 1 Department of Physiology and Pharmacology, College of Veterinary Medicine, The University of Georgia, Athens, GA 30602-7389, USA; 2 Current address: Toxicology and Molecular Biology Branch, Health Effects Laboratory Division, National Institute for Occupational Safety and Health, Morgantown, WV 26505, USA Received 11 December 2002; accepted in final form 21 July 2003 Abstract Myriocin, a fungal metabolite isolated from Myriococcum albomyces, Isaria sinclairi, and Mycelia sterilia, is a potent inhibitor of serine palmitoyltransferase (SPT), a key enzyme in de novo synthesis of sphingolipids. To evaluate the biological effects of myriocin in vivo, we investigated the levels of free sphingoid bases and expression of selected genes regulating cell growth in mouse liver. Male Balb/c mice, weighing 22 g were injected intraperi- toneally with myriocin at 0, 0.1, 0.3, and 1.0 mg kg −1 body weight daily for 5 days. Animals were euthanized 24 hours after the last treatment. Levels of plasma alanine aminotransferase and aspartate aminotransferase were not significantly altered by the treatment. A dose-dependent decrease in free sphinganine but not sphingosine was detected by high performance liquid chromatography in both liver and kidney. The decrease of free sphinganine paralleled the decrease in SPT activity. Reverse transcriptase polymerase chain reaction analysis on liver mRNA revealed an increase in expression of c-myc, but no changes in tumor necrosis factor α, transforming growth factor β , and hepatocyte growth factor. Results showed that myriocin blocked de novo synthesis of sphingolipids in vivo by SPT inhibition and induced c-myc expression in liver. Introduction Myriocin (ISP-1, thermozymocidin, Figure 1a) was initially isolated as an antibiotic and immunosuppress- ant from Myriococcum albomyces, Isaria sinclairi, and Mycelia sterilia. This natural product potently inhibits proliferation of lymphocytes in mouse al- logeneic mixed lymphocyte reaction (MLR), T-cell- dependent antibody production, and the generation of allo-reactive cytotoxic T lymphocytes in mice with a potency of 10- to 100-fold greater than that of cyc- losporin, a clinically prescribed immunosuppressant [8]. The growth inhibition of myriocin on mouse cyto- toxic T cell line, CTLL-2, was due to its induction of apoptosis, and the apoptotic effect of myriocin on these cells could be reversed by addition of sphin- gosine to the cultures [22]. The structure of myriocin resembles that of sphingosine (Figure 1a), it has been revealed in vitro that myriocin inhibited the activity of serine palmitoyltransferase (SPT), a rate-limiting en- zyme in de novo biosynthesis of sphingolipids, in a mouse cytotoxic T cell line, CTLL-2 (Figure 1b) [21]. Sphingolipids form specialized membrane struc- tures, mediate cell-cell interactions, regulate the be- haviour of cellular proteins and receptors, and signal transduction [17]. Bioactive intermediates of sphin- golipid metabolism, such as ceramide, sphingosine, sphinganine, and sphingosine-1-phosphate, modulate cellular signaling functions and regulate cell growth, differentiation, apoptosis and proliferation [18, 19]. The discovery of myriocin and fumonisin as specific inhibitors of SPT and ceramide synthase, respectively, the two important enzymes in sphingolipid biosyn- thesis pathway, has helped in identifying key roles for the de novo biosynthesis in sphingolipid-mediated cellular signaling functions [22, 3, 4]. Fumonisin B 1 , a mycotoxin from Fusarium verticillioides, increases accumulation of free sphinganine and sphingosine but decreases de novo synthesis of ceramide as a res- ult of ceramide synthase inhibition (Figure 1b), [31].