Effects of endoglycoceramidase or D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol on glucose uptake, glycolysis, and mitochondrial respiration in HL60 cells

Enforced C-Src Activation Causes Compartmental Dysregulation of PI3K and PTEN Molecules in Lipid Rafts of Tongue Squamous Carcinoma Cells by Attenuating Rac1-Akt-GLUT-1-Mediated Sphingolipid Synthesis

Pharmacologic intervention to affect the membrane lipid homeostasis of lipid rafts is a potent therapeutic strategy for cancer. Here we showed that gallic acid (GA) caused the complex formation of inactive Ras-related C3 botulinum toxin substrate 1 (Rac1)-phospho (p)-casein kinase 2 α (CK2α) (Tyr 255) in human tongue squamous carcinoma (TSC) cells, which disturbed the lipid raft membrane-targeting of phosphatidylinositol 3-kinase (PI3K)-Rac1-protein kinase B (Akt) signal molecules by inducing the association of p110α-free p85α with unphosphorylated phosphatase tensin homolog deleted on chromosome 10 (PTEN) in lipid rafts. The effects on induction of inactive Rac1-p-CK2α (Tyr 255) complex formation and attenuation of p-Akt (Ser 473), GTP-Rac1, glucose transporter-1 (GLUT-1) lipid raft membrane-targeting, and cell invasive activity by GA were counteracted either by CK2α short hairpin RNA or cellular-Src (c-Src) inhibitor PP1. PP1 treatment, GLUT-1 or constitutively active Rac1 ectopic-expression blocked GA-induced decreases in cellular glucose, sphingolipid and cholesterol of lipid raft membranes, p85α-p110α-GTP-Rac1 complexes, glucosylceramide synthase activity and increase in ceramide and p110α-free p85α-PTEN complex levels of lipid raft membranes, which reversed the inhibition on matrix metalloproteinase (MMP)-2/-9-mediated cell invasion induced by GA. Using transient ectopic expression of nuclear factor-kappa B (NF-κB) p65, MMP-2/-9 promoter-driven luciferase, and NF-κB-dependent luciferase reporter genes and NF-κB specific inhibitors or Rac1 specific inhibitor NSC23766, we confirmed that an attenuation of Rac1 activity by GA confers inhibition of NF-κB-mediated MMP-2/-9 expression and cell invasion. In conclusion, GA-induced c-Src activation is a key inductive event for the formation of inactive Rac1-p-CK2α (Tyr 255) complexes, which disturbed lipid raft compartment of PI3K and PTEN molecules by impairing Akt-regulated GLUT-1-mediated sphingolipid synthesis, and finally resulting in inhibition of TSC cell invasion.

Glucose availability and glycolytic metabolism dictate glycosphingolipid levels

Cancer therapeutics has seen an emergence and re-emergence of two metabolic fields in recent years, those of bioactive sphingolipids and glycolytic metabolism. Anaerobic glycolysis and its implications in cancer have been at the forefront of cancer research for over 90 years. More recently, the role of sphingolipids in cancer cell metabolism has gained recognition, notably ceramide's essential role in programmed cell death and the role of the glucosylceramide synthase (GCS) in chemotherapeutic resistance. Despite this knowledge, a direct link between these two fields has yet to be definitively drawn. Herein, we show that in a model of highly glycolytic cells, generation of the glycosphingolipid (GSL) glucosylceramide (GlcCer) by GCS was elevated in response to increased glucose availability, while glucose deprivation diminished GSL levels. This effect was likely substrate dependent, independent of both GCS levels and activity. Conversely, leukemia cells with elevated GSLs showed a significant change in GCS activity, but no change in glucose uptake or GCS expression. In a leukemia cell line with elevated GlcCer, treatment with inhibitors of glycolysis or the pentose phosphate pathway (PPP) significantly decreased GlcCer levels. When combined with pre-clinical inhibitor ABT-263, this effect was augmented and production of pro-apoptotic sphingolipid ceramide increased. Taken together, we have shown that there exists a definitive link between glucose metabolism and GSL production, laying the groundwork for connecting two distinct yet essential metabolic fields in cancer research. Furthermore, we have proposed a novel combination therapeutic option targeting two metabolic vulnerabilities for the treatment of leukemia.

Inhibition of sodium-calcium exchange by ceramide and sphingosine

Na(+)/Ca(2+) exchange activity in Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger was inhibited by the short chain ceramide analogs N-acetylsphingosine and N-hexanoylsphingosine (5-15 micrometer). The sphingolipids reduced exchange-mediated Ba(2+) influx by 50-70% and also inhibited the Ca(2+) efflux mode of exchange activity. The biologically inactive ceramide analog N-acetylsphinganine had only modest effects on exchange activity. Cells expressing the Delta(241-680) and Delta(680-685) deletion mutants of the Na(+)/Ca(2+) exchanger were not inhibited by ceramide; these mutants show defects in both Na(+)-dependent and Ca(2+)-dependent regulatory behavior. Another mutant, which was defective only in Na(+)-dependent regulation, was as sensitive to ceramide inhibition as the wild-type exchanger. Inhibition of exchange activity by ceramide was time-dependent and was accelerated by depletion of internal Ca(2+) stores. Sphingosine (2.5 micrometer) also inhibited the Ca(2+) influx and efflux modes of exchange activity in cells expressing the wild-type exchanger; sphingosine did not affect Ba(2+) influx in the Delta(241-680) mutant. The effects of the exogenous sphingolipids were reproduced by blocking cellular ceramide utilization pathways, suggesting that exchange activity is inhibited by increased levels of endogenous ceramide and/or sphingosine. We propose that sphingolipids impair Ca(2+)-dependent activation of the exchanger and that in cardiac myocytes, this process serves as a feedback mechanism that links exchange activity to the diastolic concentration of cytosolic Ca(2+).

G-protein signaling abnormalities mediated by CD95 in salivary epithelial cells

Salivary epithelial cells from patients with primary Sjögren's syndrome (SS) undergo Fas-mediated apoptosis. Bcl-2 and Bcl-xL are apoptosis suppressing oncogenes. Very little is known about the role of these oncogene molecules in salivary epithelial cells. To investigate the possible prevention of salivary glandular destruction in SS by Bcl-2 and Bcl-xL, stable transfectants expressing these molecules were made from HSY cells, a human salivary epithelial cell line. HSY cells were transfected with an expression vector for human Bcl-2 or Bcl-xL. Stable transfectants were selected and apoptosis was induced by anti-Fas antibody. Apoptosis was quantified by propidium iodide staining followed by flow cytometry. Caspase activity was detected by immunohistochemical analysis and enzyme cleavage of DEVD-AMC, a fluorescent substrate. Response to carbachol, a muscarinic receptor agonist, and EGF was measured by Ca2+ mobilization and influx. Fas-mediated apoptosis was significantly inhibited in Bcl-2 and Bcl-xL transfectants compared to wild-type and control transfectants (empty vector). Surprisingly, caspase activity was not inhibited in Bcl-2 and Bcl-xL transfectants. Activation of the Fas pathway in the Bcl-2 and Bcl-xL transfectants by antibody also inhibited carbachol and EGF responsiveness (i.e., Ca2+ mobilization and/or influx) by 50-60%. This Fas-mediated inhibition of cell activation was partially or completely restored by specific peptide interference of caspase enzyme activity. The prevention of Fas-mediated apoptosis by the overexpression of Bcl-2 and Bcl-xL in salivary gland epithelial cells results in injured cells expressing caspase activity and unable to respond normally to receptor agonists. Such damaged cells may exist in SS patients and could explain the severe dryness out of proportion to the actual number of apoptotic cells seen on salivary gland biopsy.