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 alpha (CK2 alpha) (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 p110 alpha-free p85 alpha with unphosphorylated phosphatase tensin homolog deleted on chromosome 10 (PTEN) in lipid rafts. The effects on induction of inactive Rac1-p-CK2 alpha (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 CK2 alpha 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, p85 alpha-p110 alpha-GTP-Rac1 complexes, glucosylceramide synthase activity and increase in ceramide and p110 alpha-free p85 alpha-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-kappa B) p65, MMP-2/-9 promoter-driven luciferase, and NF-kappa B-dependent luciferase reporter genes and NF-kappa B specific inhibitors or Rac1 specific inhibitor NSC23766, we confirmed that an attenuation of Rac1 activity by GA confers inhibition of NF-kappa B-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-CK2 alpha (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.