Mcl-1 determines the imiquimod-induced apoptosis but not imiquimod-induced autophagy in skin cancer cells.
BACKGROUND
Imiquimod had been shown to induce apoptosis and autophagy in several skin cancer cells, especially basal cell carcinoma (BCC) cells.


OBJECTIVE
We evaluate the molecular mechanisms of imiquimod-induced apoptosis and autophagy in skin cancer cell lines.


METHODS
The Mcl-1, Bcl-2 and Bcl-xL proteins were determined by immunoblotting. The Mcl-1 mRNA level was examined by RT-PCR and real-time PCR. The mechanisms of imiquimod-induced decrease in Mcl-1 protein were evaluated by addition of cycloheximide, MG132 proteasome inhibitor or pan-caspase inhibitor. The phosphorylation of eIF4E, 4E-BP1 and eEF2 in imiquimod treated cells were examined by immunoblotting. The imiquimod-induced apoptosis and autophagy were evaluated in Mcl-1-overexpressing cells by XTT test, mitochondrial membrane potential measurement, DNA content assay, LC3 immunoblotting, EGFP-LC3 puncta formation and quantification of acidic vesicular organelle with acridine orange staining.


RESULTS
The decrease in the Mcl-1 protein level was faster and stronger than the decrease in Bcl-2 and Bcl-xL in imiquimod-treated skin cancer cells. The imiquimod-induced decrease in Mcl-1 protein was not caused by blocked transcription or the promotion of degradation but was associated with inactivation of translation factors in BCC cells. The Mcl-1-overexpressing BCC cells were more resistant to intrinsic cellular apoptosis than control BCC cells during imiquimod treatment. Mcl-1 overexpression in BCC cells resulted in the basal activation of autophagy but did not modulate imiquimod-induced autophagy or rescue imiquimod-induced autophagic cell death in BCC cells.


CONCLUSIONS
Imiquimod may rapidly downregulate Mcl-1 protein levels by inhibiting translation in skin cancer cells. Mcl-1 may act to protect against apoptosis but not autophagy and autophagic cell death during imiquimod treatment in skin cancer cells.
Institute of Biomedical Sciences, National Chung Hsing University, Taichung, Taiwan.
Shi-Wei
SW
Huang
Chia-Che
CC
Chang
Chi-Chen
CC
Lin
Jaw-Ji
JJ
Tsai
Yi-Ju
YJ
Chen
Chun-Ying
CY
Wu
Kuang-Ting
KT
Liu
Jeng-Jer
JJ
Shieh
D000634:Aminoquinolines / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D001343:Autophagy / Q000187:drug effects*; D014408:Biomarkers, Tumor / Q000378:metabolism; D002280:Basal Cell Carcinoma / Q000378:metabolism / Q000473:pathology; D045744:Cell Line, Tumor; D015536:Down-Regulation / Q000187:drug effects; D006801:Humans; D000077271:Imiquimod; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D012878:Skin Neoplasms / Q000378:metabolism* / Q000473:pathology*; D015854:Up-Regulation / Q000187:drug effects; D051020:bcl-X Protein / Q000378:metabolism
D000634:Aminoquinolines; D014408:Biomarkers, Tumor; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; D051020:bcl-X Protein; D000077271:Imiquimod