Sphingosine-1-Phosphate

Sphingolipids are a class of bioactive lipid mediators that include sphingosine-1-phosphate (S1P), sphingomyelin, and ceramide (Figure). Until recently, sphingolipids were considered only as important structural components of cell membranes. It is now well established that they are also important intracellular and extracellular signaling molecules. Enzymatic breakdown of the abundant membrane phospholipid sphingomyelin by the family of sphingomyelinases serves as the primary source of ceramides and sphingosine.¹ Alternatively, the de novo synthesis of ceramide begins with the condensation of serine and palmitoyl-CoA by the enzyme serine palmitoyl transferase (Figure). The sequential metabolism of ceramide by the enzymes ceramidase and sphingosine kinase (SK1 and SK2) produces S1P, a potent mediator of a variety of biological functions. S1P can be either dephosphorylated by S1P phosphatase or can undergo irreversible cleavage at C2-3 by S1P lyase to yield a long chain aldehyde and ethanolamine phosphate.^2,3

The physiological roles played by ceramide and S1P in cell proliferation are often in opposition, thereby putting the fate of a cell in a life and death balance. Whereas ceramide exhibits cancer preventative properties (growth inhibition, apoptosis, and differentiation), S1P has the opposite, cancer-promoting, effect of cell proliferation, transformation, cell motility, and angiogenesis.⁴ In addition to its cancer-promoting effects, S1P plays an active role in the vasculature, particularly in angiogenesis, and is being recognized as a critical mediator in the growth and maintenance of the vascular system.⁵ Enzymes and receptors involved in the formation and signaling of these molecular species seem to be poised as prime targets for cancer intervention.

S1P elicits its actions through 5 different G-protein coupled receptors, previously known as Endothelial Differential Genes (EDG) – S1P1 (EDG-1), S1P2 (EDG-5), S1P3 (EDG-3), S1P4 (EDG-6), and S1P5 (EDG-8). This receptor family extends to include 3 receptors for the structurally related molecule lysophosphatic acid (LPA). The recently proposed nomenclature for the receptor family adopts the S1P1-5 and LPA1-3 system in favor of the EDG1-8 terminology.⁶ S1P1-3 have wide tissue distribution; S1P1 is abundant in the vascular system, particularly endothelial cells; S1P4 is found mainly in lymphoid tissue; and S1P5 is expressed predominately in the nervous system.

Interest in S1P signaling increased recently with the discovery that FTY720, a highly efficacious immunosuppressant, signals via S1P receptors.⁷ FTY720 is a derivative of ISP-1 (myriocin), a fungal metabolite of the Chinese herb Iscaria sinclarii as well as a structural analog of sphingosine. It is a novel and highly promising immune modulator, currently in Phase III clinical trials, that prolongs allograft transplant survival in numerous models by inhibiting lymphocyte emigration from lymphoid organs.⁸ FTY720 is phosphorylated by sphingosine kinase, and then acts as a potent agonist at 4 of the S1P receptors (S1P1, S1P3, S1P4, and S1P5).⁷ Systemic administration of FTY720 in mice depletes circulating T and B lymphocytes, sequestering them in peripheral lymph nodes and Peyer’s patch, an effect that appears to be predominately mediated by S1P1.^9,10 Agonism at S1P3, on the other hand, accounts for toxicity of FTY720. Selective S1P1 receptor agonists have therefore recently become a target of drug discovery efforts.^11,12

References

1. Liu, B., et al. Cell Dev. Biol. 8, 311-322 (1997).
2. Saba, J.D. and Hla, T. Circ.Res. 94, 724-734 (2004).
3. Yopp, A.C., et al. J. Immunol. 171, 5-10 (2003).
4. Ogretmen, B. and Hannun, Y.A. Nature Reviews Cancer 4, 604-616 (2004).
5. Chae, S.-S., et al. J. Clin. Invest. 114, 1082-1089 (2004).
6. Chun, J., et al. Pharmacol. Rev. 54, 265-269 (2002).
7. Brinkmann, V., et al. J. Biol. Chem. 277(24), 21453-21457 (2002).
8. Brinkmann, V., et al. Transplantation 72, 764-769 (2001).
9. Honig, S.M., et al. J. Clin. Invest. 111(5), 627-637 (2003).
10. Matloubian, M., et al. Nature 427, 355-360 (2004).
11. Hale, J.J., et al. J. Med. Chem. 47, 6662-6665 (2004).
12. Yan, L., et al. Bioorg. Medicinal Chem. Letters 14, 4861-4866 (2004).

For over 100 years, man has known of the existence of a protein substance called renin, released into the blood by the kidney, and capable of inducing vasoconstriction and hypertension. Renin itself has little biological activity, circulating in blood primarily as the 47 kD prorenin glycoprotein. The active 40 kD renin aspartyl protease, secreted by the renal macula densa, supplies angiotensin I substrate to the master kininase ACE (Angiotensin Converting Enzyme). It is the cleavage of 2 amino acids from Angiotensin I by ACE to produce angiotensin II that generates the hypertensive substance triggered by renin release. This supporting role of renin was discovered 65 years ago, but only in the last year has active, recombinant human renin become available for research uses. The renin enzyme provided by Cayman is the active, 340 amino acid protease derived from expression in E. coli.

ACE is a zinc-containing carboxypeptidase that stands at the center of the metabolic pathway for activiation and inactivation of small vasoactive peptides. Homologous C- and N-domains of ACE process a number of peptides, including bradykinin and angiotensin.

Ghrelin is a 28-amino acid peptide secreted by the stomach in a daily rhythm that closely mirrors actual food intake.¹ Plasma ghrelin levels peak just before each of the three normal daily meals, and again at night about 5 hours after dinner. Total plasma ghrelin is higher across the board in normal subjects compared to the obese, who also have a blunted nocturnal ghrelin response.^2,3 Active ghrelin contains a unique post-translational modification in the form of an octanoic acid acylation of serine residue 3 of the mature peptide. This acylation is required for both appetite-stimulating and growth hormone-releasing activities, the two most prominent biological actions associated with ghrelin. Interestingly, most measurements of plasma ghrelin, including those cited here, relied on assay methods incapable of distinguishing the active, octanoylated ghrelin from the inactive, de-acylated form. The human and rat active Ghrelin EIA Kits being introduced with this newsletter will allow researchers to investigate for the first time whether active, acylated ghrelin follows the same diurnal pattern as total plasma ghrelin.

References

1. Kojima, M., et al. Nature 402, 656-660 (1999).
2. Cummings, D.E., et al. N. Engl. J. Med. 346(21), 1623-1630 (2002).
3. Yildiz, B.O., et al. Proc. Natl. Acad. Sci. USA 101(28), 10434-10439 (2004).