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BCL2 limits crizotinib efficiency in ALk+ ALCL
levels of BCL2, respectively.23 We found low concentra- tions of venetoclax to have no impact either on cell viabil- ity or autophagy. Conversely, high doses were found to be highly toxic, even in the absence of crizotinib, a result most likely attributable to off-target effects (data not shown). Thus, in accordance with literature highlighting the efficiency of this compound in the treatment of tumors harboring high expression of BCL2 but its uncommon use in lymphoma, we found that the pharmacological inhibition of BCL2 with venetoclax in KARPAS-299 ALK positive cells did not recapitulate the effects of BCL2 targeted downregulation, a result that should stimulate further investigation.
Our team previously demonstrated that treatment of ALK-positive ALCL cells with crizotinib induces an autophagic flux endowed with pro-survival properties.30 In the present study, we further demonstrated that crizo- tinib-induced autophagy was associated with an upregu- lation of BCL2, which limits the cytotoxic effects of the drug. Indeed, BCL2 knockdown combined to crizotinib
treatment led to a profound loss of cell viability that was found to be due to an increase in apoptosis. This did not, however, exclude the occurrence of another type of cell death. Indeed, in addition, our data showed that crizo- tinib/BCL2 knockdown treatments also led to a significant potentiation of autophagic flux (Figure 7). Several lines of evidence clearly indicate that autophagy is a multifaceted regulator of cell death.12-14 However, controversy remains as to whether autophagy directly executes cell death or how it interferes with other forms of death, including apoptosis and necrosis/necroptosis.15 In the present study, the contribution of excessive autophagy in the process of cell death was first evidenced by the fact that impairing the autophagic machinery (by the molecular targeting of ULK1) resulted in a complete reversal of the potent loss of cell viability in BCL2-knocked down cells. Secondly, our in vivo data further revealed LC3B and p62 stainings con- sistent with increased autophagy activity in tumor tissues harvested from ALK-positive ALCL cells xenografted mice submitted to crizotinib and miR34a-mediated BCL2
ABC
BCL2 knockdown
Figure 7. Proposed model of the combined ALK and BCL2 inhibitions on the fate of ALK-positive anaplastic large cell lymphoma (ALCL) cells. (A) An ALK-dependent BCL2 repression mechanism is at work In ALK-positive ALCL cells. (B) Strategies based on the inhibition of ALK activity, such as crizotinib treatment, impair this repression mechanism. This leads to an increase in BCL2 levels that, in turn, limits both cell death and autophagy induction. (C) Blocking crizotinib-induced BCL2 elevation results in a potentiation of the cytotoxic effects of the drug, through both overactivation of autophagic flux and an increase in cell death (including apoptosis and, potentially, other cell death modalities).
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