title: Transforming growth factor-beta inhibition of proteasomal activity: a potential mechanism of growth arrest.
abstract: Although the proteasome plays a critical role in the controlled degradation of proteins involved in cell cycle control, the direct modulation of proteasomal function by growth regulatory signaling has not yet been demonstrated. We assessed the effect of transforming growth factor (TGF)-beta, a potent inhibitor of cell growth, on proteasomal function. TGF-beta selectively decreased hydrolysis of the proteasomal substrate Cbz-Leu-Leu-Leu-7-amido-4-methyl-coumarin (z-LLL-AMC) in a concentration-dependent manner but did not inhibit hydrolysis of other substrates Suc-Leu-Leu-Val-Tyr-AMC (suc-LLVY-AMC) or Cbz-Leu-Leu-Glu-AMC (z-LLE-AMC). An increase in intracellular oxidative injury occurred during incubation with TGF-beta. Furthermore, in vitro hydrolysis of z-LLL-AMC, but not suc-LLVY-AMC, was decreased by hydrogen peroxide. TGF-beta did not increase cellular expression of heat shock protein (HSP)90, a potent inhibitor of z-LLL-AMC hydrolysis in vitro. The physiological relevance of TGF-beta inhibition of proteasomal activity was studied by assessing the role of z-LLL-AMC hydrolysis on cyclin-dependent kinase inhibitor expression and cell growth. TGF-beta increased expression of p27KIP1 but did not alter expression of p21WAF1 or p16INK4A. The peptide aldehyde Cbz-Leu-Leu-leucinal (LLL-CHO or MG132) potently inhibited z-LLL-AMC hydrolysis in cell extracts as well as increasing p27KIP1 and decreasing cell proliferation. Thus growth inhibition by TGF-beta decreases a specific proteasomal activity via an HSP90-independent mechanism that may involve oxidative inactivation or modulation of proteasomal subunit composition and results in altered cellular expression of key cell cycle regulatory proteins such as p27KIP1.
authors.affiliation: Department of Internal Medicine, Scott and White Clinic, Texas A&M University System Health Science Center College of Medicine, Temple, TX 76508, USA.
authors.forename: Laura
authors.initials: L
authors.lastname: Tadlock
authors.forename: Yoko
authors.initials: Y
authors.lastname: Yamagiwa
authors.forename: James
authors.initials: J
authors.lastname: Hawker
authors.forename: Carla
authors.initials: C
authors.lastname: Marienfeld
authors.forename: Tushar
authors.initials: T
authors.lastname: Patel
mesh_terms: D018797:Cell Cycle Proteins / Q000187:drug effects / Q000378:metabolism; D002455:Cell Division / Q000187:drug effects / Q000502:physiology*; D002460:Cell Line; D050760:Cyclin-Dependent Kinase Inhibitor p27; D003546:Cysteine Endopeptidases / Q000187:drug effects / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005057:Eukaryotic Cells / Q000187:drug effects / Q000201:enzymology*; D018841:HSP90 Heat-Shock Proteins / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D006868:Hydrolysis / Q000187:drug effects; D007976:Leupeptins / Q000494:pharmacology; D009097:Multienzyme Complexes / Q000187:drug effects / Q000378:metabolism*; D018384:Oxidative Stress / Q000187:drug effects / Q000502:physiology; D010446:Peptide Fragments / Q000187:drug effects / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D021381:Protein Transport / Q000187:drug effects / Q000502:physiology*; D016212:Transforming Growth Factor beta / Q000378:metabolism* / Q000494:pharmacology; D025521:Tumor Suppressor Proteins / Q000187:drug effects / Q000378:metabolism
chemical_list: D018797:Cell Cycle Proteins; D018841:HSP90 Heat-Shock Proteins; D007976:Leupeptins; D009097:Multienzyme Complexes; D010446:Peptide Fragments; D016212:Transforming Growth Factor beta; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde