title: Paclitaxel induces inactivation of p70 S6 kinase and phosphorylation of Thr421 and Ser424 via multiple signaling pathways in mitosis.
abstract: The 70 kDa ribosomal S6 kinase (p70S6K) is important for cell growth and survival. Activation of p70S6K requires sequential phosphorylation of multiple serine and threonine sites often triggered by growth factors and hormones. Here, we report that paclitaxel, a microtubule-damaging agent, induces phosphorylation of p70S6K at threonine 421 and serine 424 (T421/S424) in a concentration- and time-dependent manner in multiple breast and ovarian cancer cell lines demonstrated by a T421/S424 phospho-p70S6K antibody. Phosphoamino-acid analysis and Western blot analysis by serine-/threonine-specific antibodies further confirms that both serine and threonine residues are phosphorylated in p70S6K following treatment with paclitaxel. Paclitaxel-induced p70S6K(T421/S424) phosphorylation requires both de novo RNA and protein synthesis via multiple signaling pathways including ERK1/2 MAP kinase, JNK, PKC, Ca(++), PI3K, and mammalian target of rapamycin (mTOR). Despite phosphorylation of p70S6K(T421/S424), paclitaxel inactivates this kinase in a concentration- and time-dependent manner as illustrated by in vitro kinase assay. Inhibitors of mTOR, PI3K, and Ca(++) impair p70S6K activity, whereas inhibitors of JNK and PKC stimulate p70S6K activity. Inhibition of PKC and JNK prevents paclitaxel-induced p70S6K inactivation. Moreover, the paclitaxel-induced phosphorylation and low activity of p70S6K mainly occurs during mitosis. In summary, paclitaxel is able to induce p70S6K(T421/S424) phosphorylation and decrease its activity in mitotic cells via multiple signaling pathways. Our data suggest that paclitaxel-induced p70S6K(T421/S424) phosphorylation and kinase inactivation are differentially regulated. Our data also indicate that paclitaxel may exert its antitumor effect, at least in part, via inhibition of p70S6K.
authors.affiliation: Department of Experimental Therapeutics, The University of Texas MD Anderson Cancer Center, Houston 77030, USA.
authors.forename: Xiao-Feng
authors.initials: XF
authors.lastname: Le
authors.forename: Walter N
authors.initials: WN
authors.lastname: Hittelman
authors.forename: Jiaxin
authors.initials: J
authors.lastname: Liu
authors.forename: Amanda
authors.initials: A
authors.lastname: McWatters
authors.forename: Chun
authors.initials: C
authors.lastname: Li
authors.forename: Gordon B
authors.initials: GB
authors.lastname: Mills
authors.forename: Robert C
authors.initials: RC
authors.lastname: Bast
mesh_terms: D000972:Antineoplastic Agents, Phytogenic / Q000494:pharmacology*; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism; D006801:Humans; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors; D008938:Mitosis / Q000187:drug effects*; D017239:Paclitaxel / Q000494:pharmacology*; D051966:Phospholipase C gamma; D010766:Phosphorylation; D011233:Precipitin Tests; D019893:Ribosomal Protein S6 Kinases / Q000037:antagonists & inhibitors*; D012694:Serine / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects*; D013912:Threonine / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors / Q000378:metabolism
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D007527:Isoenzymes; D013912:Threonine; D012694:Serine; D019893:Ribosomal Protein S6 Kinases; D020928:Mitogen-Activated Protein Kinases; D010738:Type C Phospholipases; D051966:Phospholipase C gamma; D017239:Paclitaxel; D002118:Calcium