The involvement of anti-apoptotic Bcl-2 family proteins in vinblastine-induced apoptosis
The efficacy of many chemotherapeutic agents can be attenuated by expression of the anti-apoptotic proteins Bcl-2, Bcl-X and Mcl-1. When ML-1 leukemia cells are incubated with vinblastine, they rapidly upregulate Mcl-1. Suppression of Mcl-1 with inhibitors of MEK-ERK signaling or by expression of shRNA dramatically accelerates the rate of apoptosis with cells dying in 4 hours. However, most leukemia cells appear to rely on Bcl-2 or Bcl-X rather than Mcl-1 to protect themselves from this acute vinblastine-mediated apoptosis. Small molecule inhibitors of Bcl-2 proteins were investigated for their ability to sensitize a range of leukemia cell lines to vinblastine. ABT-737, an inhibitor of Bcl-2 and Bcl-X, acutely sensitized only the NB4 cells which express only Bcl-2. Most of the other cell lines also expressed Mcl-1 which may provide resistance to ABT-737. However, GX15-070, which inhibits Mcl-1, Bcl-2 and Bcl-X only sensitized the ML-1 cells. This was attributed to its ability to inhibit MEK-ERK signaling and consequently suppress Mcl-1 protein levels. This is an off-target effect of this inhibitor, and no on-target effect was confirmed. As an alternative means to suppress Mcl-1, global transcription was suppressed with flavopiridol. Flavopiridol acutely sensitized most of the leukemia lines to vinblastine. Because flavopiridol-mediated sensitization did not correlate with expression of Mcl-1, these results suggest another short-lived protein suppresses this acute apoptosis. It is proposed that vinblastine primes the cells for apoptosis by activating a pro-apoptotic Bcl-2 protein while concurrent inhibition of anti-apoptotic proteins leads to rapid apoptosis. The rapid apoptosis is dependent on vinblastine-mediated activation of JNK which may cause concurrent inhibition of Bcl-2. One exception is the sensitization of NB4 cells to ABT-737, a situation where JNK-mediated inhibition of Bcl-2 would be redundant. Interestingly, OCI-AML1 cells were acutely sensitive to vinblastine alone, which was attributed to their expression of only Bcl-2 and at very low levels. The results have clinical significance as they show that brief administration of these drugs in combination can be sufficient to kill leukemia cells. A future challenge will be to determine which tumors will respond to each drug combination.