title: Farnesyl and geranylgeranyl transferase inhibitors induce G1 arrest by targeting the proteasome.
abstract: Isoprenoid inhibitors are being evaluated as agents for the treatment of cancer. Their antitumor activity is attributed to inhibition of post-translational modification of Ras, which is crucial for its translocation and attachment to the plasma membrane, and ultimate involvement in signal transduction. However, whether blocking of Ras is solely responsible for the observed antitumor activity is unresolved. In this report, we propose an alternate mechanism. Using breast tumor models, we show that agents possessing a lactone moiety, including statins (such as lovastatin) and the isoprenoid inhibitors (such as FTI-277 and GGTI-298), mediate their cell cycle inhibitory activities by blocking the chymotrypsin activity of the proteasome in vitro. This results in the accumulation of cyclin-dependent kinase inhibitors p21 and p27 with subsequent G(1) arrest. Cells devoid of p21 were refractory to the growth-inhibitory activity of lovastatin, FTI-277, and GGTI-298. However, in these p21 null cells, isoprenylation of key substrates of farnesyl transferase (such as Ras) and of geranylgeranyl transferase (such as RAP-1) were inhibited by FTI-277 and GGTI-298, respectively, suggesting that although both these isoprenoid inhibitors reached and inhibited their intended targets, inhibition of the isoprenylation of Ras and RAP-1A are not sufficient to mediate G(1) arrest. We also show that the cell cycle effects can be attributed to the functional lactone moiety of the aforementioned agents. Collectively, our data suggest that FTI and GGTI and other agents containing an active lactone moiety mediate G(1) arrest via inhibition of the proteasome and up-regulation of p21, independent of the inhibition of isoprenylation of Ras or RAP-1.
authors.affiliation: Department of Experimental Radiation Oncology, The University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
authors.forename: Ekem T
authors.initials: ET
authors.lastname: Efuet
authors.forename: Khandan
authors.initials: K
authors.lastname: Keyomarsi
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D001549:Benzamides / Q000494:pharmacology; D001943:Breast Neoplasms / Q000473:pathology*; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000096:biosynthesis / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D016193:G1 Phase / Q000187:drug effects; D006801:Humans; D019344:Lactic Acid / Q000737:chemistry; D008148:Lovastatin / Q000494:pharmacology*; D008715:Methionine / Q000031:analogs & derivatives / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D017368:Protein Prenylation; D014407:Tumor Cells, Cultured; D015854:Up-Regulation; D020827:rap1 GTP-Binding Proteins / Q000378:metabolism; D018631:ras Proteins / Q000378:metabolism
chemical_list: D000970:Antineoplastic Agents; D001549:Benzamides; C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D004791:Enzyme Inhibitors; C096856:FTI 277; C102521:GGTI 298; D019344:Lactic Acid; D008148:Lovastatin; D008715:Methionine; D046988:Proteasome Endopeptidase Complex; D020827:rap1 GTP-Binding Proteins; D018631:ras Proteins