A pectic polysaccharide isolated from the roots of Bupleurum falcatum L. stimulates the tyrosine phosphorylation of lipid rafts of murine B cells.
Bupleuran 2IIc, a pectic polysaccharide isolated from the roots of Bupleurum falcatum L., was previously characterized as a T cell-independent B cell mitogen. The endo-(1-->4)-alpha-D-polygalacturonase-resistant moiety of bupleuran 2IIc (bupleuran 2IIc/PG-1) was the active site for expression of the activity, and expression of the cyclin D2 gene by bupleuran 2IIc/PG-1 may be mediated via activation of Src family tyrosine kinase, phosphatidylinositol 3-kinase (PI 3-K) and phospholipase C (PLC)-gamma followed by activation of protein kinase C (PKC) and calcium mobilization (Matsumoto et al., Int. Immunopharmacol., 5, 1373-1386 (2005)). Plasma membrane microdomains (lipid rafts) are enriched in signaling molecules and suggested to be involved in numerous cell functions, including membrane traffic and signaling. When B cells were stimulated with bupleuran 2IIc/PG-1, clustering of membrane lipid rafts was observed. To consider whether lipid rafts are implicated in bupleuran 2IIc/PG-1-mediated B cell proliferation, we analyzed the phosphorylation of tyrosine residues of proteins in lipid rafts. When murine B cells were stimulated with bupleuran 2IIc/PG-1, tyrosine phosphorylation of proteins in lipid rafts fraction was observed within 5 min. Tyrosine phosphorylation in lipid rafts fraction by bupleuran 2IIc/PG-1 was inhibited by the Src-family tyrosine kinase inhibitor, PP2. Together with previously published data, the results presented in this study suggest that activation of signaling molecules in lipid rafts by stimulation of bupleuran 2IIc/PG-1 contributes to B cell proliferation as the membrane-proximal signaling event.
Matsumoto
Tsukasa
T
The Kitasato Institute for Life Sciences and Graduate School of Infection Control Sciences, Kitasato University, Minatoku, Tokyo 108-8641, Japan.
Hosono-Nishiyama
Kanako
K
Yamada
Haruki
H
D000818:Animals; D001402:B-Lymphocytes / Q000187:drug effects / Q000378:metabolism*; D029972:Bupleurum / Q000737:chemistry*; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D004591:Electrophoresis, Polyacrylamide Gel; D015151:Immunoblotting; D047468:Immunoprecipitation; D007202:Indicators and Reagents; D021962:Membrane Microdomains / Q000378:metabolism*; D051379:Mice; D008809:Mice, Inbred C3H; D008856:Microscopy, Fluorescence; D008934:Mitogens / Q000494:pharmacology; D010368:Pectins / Q000494:pharmacology*; D010766:Phosphorylation; D018517:Plant Roots / Q000737:chemistry; D011134:Polysaccharides / Q000494:pharmacology*; D013154:Spleen / Q000166:cytology / Q000187:drug effects; D014443:Tyrosine / Q000378:metabolism*
D007202:Indicators and Reagents; D008934:Mitogens; D011134:Polysaccharides; C073829:Bupleuran 2IIc; D014443:Tyrosine; D010368:Pectins