Bexarotene activates the p53/p73 pathway in human cutaneous T-cell lymphoma.
BACKGROUND
Bexarotene is the first synthetic retinoid X receptor-selective retinoid (rexinoid) approved for the treatment of cutaneous T-cell lymphoma (CTCL). However, little is known about the signalling pathways by which it exerts its anticarcinogenic effect.


OBJECTIVE
To characterize the effects of bexarotene in CTCL cell lines and elucidate the underlying molecular pathways of its antineoplastic effect.


METHODS
The cell lines Hut-78, HH and MJ were used. Cell viability was assessed with the XTT assay. The self-renewal potential of cells after bexarotene treatment was studied with the methylcellulose clonogenic assay. Flow cytometry was used to analyse the effects on cell cycle, Ki-67 expression and apoptosis induction. Cell cycle and apoptosis-related protein expression were determined by Western blot and immunofluorescence.


RESULTS
Bexarotene induced a loss of viability and more pronounced inhibition of clonogenic proliferation in HH and Hut-78 cells, whereas the MJ line exhibited resistance. Bexarotene upregulated and activated Bax in sensitive lines, although not enough to signal significant apoptosis. Instead, all data point to the inhibition of proliferation, rather than apoptosis, as the main mechanistic action of the rexinoid. Bexarotene signals both G(1) and G(2)/M arrest by the modulation of critical checkpoint proteins. We further found that bexarotene activates p53 by phosphorylation at Ser15, which influences the binding of p53 to promoters for cell cycle arrest, induces p73 upregulation, and, in concordance, also modulates some p53/p73 downstream target genes, such as p21, Bax, survivin and cdc2. Bexarotene-mediated ataxia telangiectasia mutated protein (ATM) activation in all studied lines suggests that ATM is likely to be the p53/p73 upstream activator.


CONCLUSIONS
Our data indicate for the first time that bexarotene exerts its effect in CTCL mainly by triggering the p53/p73-dependent cell cycle inhibition pathway, probably by upstream ATM activation. Therefore, bexarotene-modulated genes represent potential biomarkers to assess the response to treatment of patients with CTCL.
Nieto-Rementería
N
N
Department of Cell Biology and Histology, School of Medicine and Dentistry, University of the Basque Country, Leioa 48940, Bizkaia, Spain.
Pérez-Yarza
G
G
Boyano
M D
MD
Apraiz
A
A
Izu
R
R
Díaz-Pérez
J L
JL
Asumendi
A
A
D016588:Anticarcinogenic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects; D064007:Ataxia Telangiectasia Mutated Proteins; D000077610:Bexarotene; D002453:Cell Cycle / Q000187:drug effects; D018797:Cell Cycle Proteins / Q000187:drug effects / Q000378:metabolism; D002470:Cell Survival / Q000187:drug effects; D004268:DNA-Binding Proteins / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D016410:Lymphoma, T-Cell, Cutaneous / Q000378:metabolism / Q000473:pathology*; D014411:Neoplastic Stem Cells / Q000187:drug effects; D009687:Nuclear Proteins / Q000187:drug effects / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000187:drug effects / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D012878:Skin Neoplasms / Q000378:metabolism / Q000473:pathology*; D013764:Tetrahydronaphthalenes / Q000494:pharmacology*; D014407:Tumor Cells, Cultured; D000072160:Tumor Protein p73; D016159:Tumor Suppressor Protein p53 / Q000187:drug effects / Q000378:metabolism*; D025521:Tumor Suppressor Proteins / Q000187:drug effects / Q000378:metabolism*
D016588:Anticarcinogenic Agents; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; C000606460:TP73 protein, human; D013764:Tetrahydronaphthalenes; D000072160:Tumor Protein p73; D016159:Tumor Suppressor Protein p53; D025521:Tumor Suppressor Proteins; D000077610:Bexarotene; C576324:ATM protein, human; D064007:Ataxia Telangiectasia Mutated Proteins; D017346:Protein Serine-Threonine Kinases