Damaged microtubules can inactivate BCL-2 by means of the mTOR kinase.
Rapamycin, a specific inhibitor of the serine/threonine mTOR kinase, markedly inhibited both cell growth and apoptosis in human B-cell lines. Besides arresting cells in G(1) by increasing p27(kip1), rapamycin tripled the cellular level of the BCL-2 protein. The activity was dose-dependent and specific for the p27(kip1) and BCL-2 proteins. Rapamycin did not affect bcl-2 mRNA although it increased cellular BCL-2 concentration by inhibiting phosphorylation, a mechanism initiating the decay process. To add new insight, we combined rapamycin treatment with treatment by taxol, which, by damaging microtubules, can phosphorylate BCL-2 and activate apoptosis. It was found that the mTOR kinase was activated in cells treated with taxol or with nocodazole although it was inhibited in cells pre-treated with rapamycin. BCL-2 phosphorylation, apoptosis and hyperdiploidy were also inhibited by rapamycin. In contrast, taxol-induced microtubule stabilization or metaphase synchronization were not inhibited by rapamycin. Taken together, these findings indicate that mTOR belongs to the enzymatic cascade that, starting from damaged microtubules, phosphorylates BCL-2. By regulating apoptosis, in addition to the control of a multitude of growth-related pathways, mTOR plays a nodal role in signaling G(1) and G(2)-M events.
Calastretti
A
A
Department of Pharmacology, University of Milan, Via Vanvitelli 32, Milan 20129, Italy.
Bevilacqua
A
A
Ceriani
C
C
Viganò
S
S
Zancai
P
P
Capaccioli
S
S
Nicolin
A
A
D017209:Apoptosis / Q000187:drug effects; D015153:Blotting, Western; D002453:Cell Cycle / Q000187:drug effects; D018797:Cell Cycle Proteins / Q000378:metabolism; D002455:Cell Division; D050760:Cyclin-Dependent Kinase Inhibitor p27; D004171:Diploidy; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D005434:Flow Cytometry; D006801:Humans; D008870:Microtubules / Q000187:drug effects / Q000378:metabolism*; D015739:Nocodazole / Q000494:pharmacology; D017239:Paclitaxel / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D011494:Protein Kinases / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins
D018797:Cell Cycle Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D011494:Protein Kinases; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases; D017239:Paclitaxel; D015739:Nocodazole; D020123:Sirolimus