title: Increased expression of Mcl-1 is required for protection against serum starvation in phosphatase and tensin homologue on chromosome 10 null mouse embryonic fibroblasts, but repression of Bim is favored in human glioblastomas.
abstract: Inactivating mutations in the tumor suppressor gene phosphatase and tensin homologue on chromosome 10 (PTEN) result in elevated levels of phosphatidylinositol (3,4,5)-trisphosphate, activation of protein kinase B (PKB), and protection against apoptotic insults such as withdrawal of survival factors. Protection may arise through the inhibition of the pro-apoptotic protein Bim, which is normally repressed by a PKB-dependent mechanism. Here we show that PTEN-/- immortalized mouse embryonic fibroblasts (MEFs) exhibit elevated PKB phosphorylation and are resistant to serum withdrawal-induced death, but exhibit normal Bim expression following withdrawal of serum. In contrast, expression of Mcl-1, a prosurvival member of the Bcl-2 family, was elevated in PTEN-/- MEFs. Transient or stable overexpression of Mcl-1 in PTEN+/- MEFs conferred resistance to serum withdrawal, whereas ablating expression of Mcl-1 in PTEN-/- MEFs, using RNA interference, abolished their resistance to serum withdrawal-induced apoptosis. To determine if Mcl-1 is selected for overexpression in human tumors we examined human glioblastoma cell lines but found that loss of PTEN had no effect on Mcl-1 expression. In contrast, two of three PTEN-/- glioblastoma cell lines exhibited low expression of Bim, which was refractory to serum withdrawal. These results indicate that the resistance of PTEN-/- MEFs to serum withdrawal is largely due to the up-regulation of Mcl-1 but that loss of PTEN in tumor cell lines is more complex and may favor de-regulation of different apoptotic regulators such as Bim.
authors.affiliation: Laboratory of Molecular Signalling, The Babraham Institute, Babraham Research Campus, Cambridge CB2 4AT, England. mark.austin@bbsrc.ac.uk
authors.forename: Mark
authors.initials: M
authors.lastname: Austin
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Cook
mesh_terms: D000818:Animals; D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism*; D000072224:Bcl-2-Like Protein 11; D015153:Blotting, Western; D002461:Cell Line, Transformed; D045744:Cell Line, Tumor; D004622:Embryo, Mammalian; D005347:Fibroblasts / Q000378:metabolism*; D005786:Gene Expression Regulation; D015972:Gene Expression Regulation, Neoplastic; D005909:Glioblastoma / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D019656:Loss of Heterozygosity; D008565:Membrane Proteins / Q000378:metabolism*; D051379:Mice; D008817:Mice, Mutant Strains; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins / Q000378:metabolism*; D010766:Phosphorylation; D011487:Protein Conformation; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D034622:RNA Interference; D012333:RNA, Messenger / Q000378:metabolism; D034741:RNA, Small Interfering / Q000378:metabolism
chemical_list: D051017:Apoptosis Regulatory Proteins; C000606775:BCL2L11 protein, human; D000072224:Bcl-2-Like Protein 11; C000606776:Bcl2l11 protein, mouse; C579094:Mcl1 protein, mouse; D008565:Membrane Proteins; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; D034741:RNA, Small Interfering