Pifithrin-alpha inhibits p53 signaling after interaction of the tumor suppressor protein with hsp90 and its nuclear translocation.
Pifithrin-alpha (PFTalpha) was originally thought to be a specific inhibitor of signaling by the tumor suppressor protein p53. However, the laboratory that discovered pifithrin recently reported that the compound also inhibits heat shock and glucocorticoid receptor (GR) signaling, and they suggested that PFTalpha targets a factor common to all three signal transduction pathways, such as the hsp90/hsp70-based chaperone machinery (Komarova, E. A., Neznanov, N., Komarov, P. G., Chernov, M. V., Wang, K., and Gudkov, A. V. (2003) J. Biol. Chem. 278, 15465-15468). Because it is important for the mechanistic study of this machinery to identify unique inhibitors of chaperone action, we have examined the effect of PFTalpha on transcriptional activation, the hsp90 heterocomplex assembly, and hsp90-dependent nuclear translocation for both p53 and the GR. At concentrations where PFTalpha blocks p53-mediated induction of p21/Waf-1 in human embryonic kidney cells, we observed no inhibition of GR-mediated induction of a chloramphenicol acetyl transferase reporter in LMCAT cells. PFTalpha did, however, cause a left shift in the dexamethasone dose response curve by increasing intracellular dexamethasone concentration, apparently by competing for dexamethasone efflux from the cell. The assembly of p53 or GR heterocomplexes with hsp90 and immunophilins was not affected by PFTalpha either in vivo or in vitro and did not affect the nuclear translocation of either transcription factor. Thus, we conclude that PFTalpha does not inhibit GR-mediated induction or the function of the chaperone machinery, and, as originally thought, it may specifically inhibit p53 signaling by acting at a stage after p53 translocation to the nucleus.
Murphy
Patrick J M
PJ
Department of Pharmacology, The University of Michigan Medical School, Ann Arbor, Michigan 48109, USA. pjmurphy@umich.edu
Galigniana
Mario D
MD
Morishima
Yoshihiro
Y
Harrell
Jennifer M
JM
Kwok
Roland P S
RP
Ljungman
Mats
M
Pratt
William B
WB
D021581:Active Transport, Cell Nucleus; D000818:Animals; D052160:Benzothiazoles; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D015500:Chloramphenicol O-Acetyltransferase / Q000378:metabolism; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000378:metabolism; D003907:Dexamethasone / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D018841:HSP90 Heat-Shock Proteins / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D020104:Immunophilins / Q000737:chemistry; D051379:Mice; D011485:Protein Binding; D015398:Signal Transduction; D013844:Thiazoles / Q000494:pharmacology*; D013997:Time Factors; D014050:Toluene / Q000031:analogs & derivatives* / Q000494:pharmacology*; D014158:Transcription, Genetic; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*; D025521:Tumor Suppressor Proteins / Q000378:metabolism*
D052160:Benzothiazoles; C493956:CDKN1A protein, human; C493957:Cdkn1a protein, mouse; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D018840:HSP70 Heat-Shock Proteins; D018841:HSP90 Heat-Shock Proteins; D013844:Thiazoles; D016159:Tumor Suppressor Protein p53; D025521:Tumor Suppressor Proteins; D014050:Toluene; D003907:Dexamethasone; C121565:pifithrin; D015500:Chloramphenicol O-Acetyltransferase; D020104:Immunophilins