Apoptosis induction in human melanoma cells by inhibition of MEK is caspase-independent and mediated by the Bcl-2 family members PUMA, Bim, and Mcl-1.
OBJECTIVE
Given that inhibitors of mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase (MEK) are being introduced into treatment for melanoma, the present study was carried out to better understand the mechanism by which they may induce apoptosis of melanoma cells.


METHODS
A panel of human melanoma cell lines and fresh melanoma isolates was assessed for their sensitivity to apoptosis induced by the MEK inhibitor U0126. The apoptotic pathways and regulatory mechanisms involved were examined by use of the inhibitor and small interfering RNA (siRNA) techniques.


RESULTS
Inhibition of MEK induced apoptosis in the majority of melanoma cell lines through a mitochondrial pathway that was associated with the activation of Bax and Bak, release of mitochondrial apoptogenic proteins, and activation of caspase-3. However, apoptosis was independent of caspases and instead was associated with mitochondrial release of AIF as shown by the inhibition of apoptosis when AIF was knocked down by siRNA. Inhibition of MEK resulted in the up-regulation of the BH3-only proteins PUMA and Bim and down-regulation of the antiapoptotic protein Mcl-1. These changes were critical for the induction of apoptosis by U0126 as siRNA knockdown of PUMA or Bim inhibited apoptosis, whereas siRNA knockdown of Mcl-1 increased apoptosis particularly in the apoptosis-resistant cell lines.


CONCLUSIONS
Apoptosis of melanoma cells induced by the inhibition of the MEK/ERK pathway is mediated by the up-regulation/activation of PUMA and Bim and down-regulation of Mcl-1. Release of AIF rather than the activation of caspases seems to be the mediator of apoptosis. Our results suggest that cotargeting Mcl-1 and the MEK/ERK pathway may further improve treatment results in melanoma.
Wang
Yu Fang
YF
Immunology and Oncology Unit, Room 443, David Maddison Clinical Sciences Building, Cnr. King and Watt Streets, Newcastle, NSW, Australia.
Jiang
Chen Chen
CC
Kiejda
Kelly Anne
KA
Gillespie
Susan
S
Zhang
Xu Dong
XD
Hersey
Peter
P
D017209:Apoptosis / Q000187:drug effects*; D051017:Apoptosis Regulatory Proteins / Q000502:physiology*; D000072224:Bcl-2-Like Protein 11; D020169:Caspases / Q000502:physiology*; D045744:Cell Line, Tumor; D048049:Extracellular Signal-Regulated MAP Kinases / Q000037:antagonists & inhibitors*; D006801:Humans; D008545:Melanoma / Q000188:drug therapy* / Q000473:pathology; D008565:Membrane Proteins / Q000502:physiology*; D008928:Mitochondria / Q000187:drug effects / Q000502:physiology; D009154:Mutation; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins / Q000502:physiology*; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D011518:Proto-Oncogene Proteins / Q000502:physiology*; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000502:physiology*; D050998:bcl-2 Homologous Antagonist-Killer Protein / Q000502:physiology; D051028:bcl-2-Associated X Protein / Q000502:physiology; D051025:bcl-Associated Death Protein / Q000502:physiology
D051017:Apoptosis Regulatory Proteins; C494819:BAD protein, human; C494772:BAK1 protein, human; C438378:BBC3 protein, human; C000606775:BCL2L11 protein, human; D000072224:Bcl-2-Like Protein 11; D008565:Membrane Proteins; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009363:Neoplasm Proteins; D047428:Protein Kinase Inhibitors; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein; D051025:bcl-Associated Death Protein; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; D048049:Extracellular Signal-Regulated MAP Kinases; D020169:Caspases