Oncogenic BRAFV600E inhibits BIM expression to promote melanoma cell survival

RA Cartlidge, GR Thomas, S Cagnol… - Pigment cell & …, 2008 - Wiley Online Library
RA Cartlidge, GR Thomas, S Cagnol, KA Jong, SA Molton, AJ Finch, M McMahon
Pigment cell & melanoma research, 2008Wiley Online Library
Somatic activating mutations of BRAF are the earliest and most common genetic abnormality
detected in the genesis of human melanoma. However, the mechanism (s) by which
activated BRAF promotes melanoma cell cycle progression and/or survival remain unclear.
Here we demonstrate that expression of BIM, a pro‐apoptotic member of the BCL‐2 family,
is inhibited by BRAF→ MEK→ ERK signaling in mouse and human melanocytes and in
human melanoma cells. Trophic factor deprivation of melanocytes leads to elevated BIM …
Summary
Somatic activating mutations of BRAF are the earliest and most common genetic abnormality detected in the genesis of human melanoma. However, the mechanism(s) by which activated BRAF promotes melanoma cell cycle progression and/or survival remain unclear. Here we demonstrate that expression of BIM, a pro‐apoptotic member of the BCL‐2 family, is inhibited by BRAF→MEK→ERK signaling in mouse and human melanocytes and in human melanoma cells. Trophic factor deprivation of melanocytes leads to elevated BIM expression. However, re‐addition of trophic factors or activation of a conditional form of BRAFV600E leads to rapid inhibition of BIM expression. In both cases, inhibition of BIM expression was dependent on the activity of MEK1/2 and the proteasome. Consistent with these observations, pharmacological inhibition of BRAFV600E or MEK1/2 in human melanoma cells (using PLX4720 and CI‐1040 respectively) led to a striking elevation of BIM expression. Re‐activation of BRAF→MEK→ERK signaling led to phosphorylation of BIM‐EL on serine 69 and its subsequent degradation. Interestingly, endogenous expression of BIM in melanoma cells was insufficient to induce apoptosis unless combined with serum deprivation. Under these circumstances, inhibition of BIM expression by RNA interference provided partial protection from apoptosis. These data suggest that regulation of BIM expression by BRAF→MEK→ERK signaling is one mechanism by which oncogenic BRAFV600E can influence the aberrant physiology of melanoma cells.
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