BCL2/BCL-XL inhibition induces apoptosis, disrupts cellular calcium homeostasis, and prevents platelet activation

Abstract

Apoptosis in megakaryocytes results in the formation of platelets. The role of apoptotic pathways in platelet turnover and in the apoptotic-like changes seen after platelet activation is poorly understood. ABT-263 (Navitoclax), a specific inhibitor of antiapoptotic BCL2 proteins, which is currently being evaluated in clinical trials for the treatment of leukemia and other malignancies, induces a doselimiting thrombocytopenia. In this study, the relationship between BCL2/BCL-X L inhibition, apoptosis, and platelet activa- IntroductionAll nucleated cells in multicellular organisms are genetically programmed to undergo apoptosis to remove unnecessary or damaged cells from the whole organism. This program has been recognized as the central mechanism of platelet production from megakaryocytes. 1 However, the role of apoptosis in anuclear, mature platelets is less well characterized, with apoptotic-like changes seen in both aging platelets and in the formation of procoagulant microparticles after agonist stimulation.Two main pathways lead to the execution of apoptosis: the extrinsic and the intrinsic (or mitochondrial) pathways. Both converge into the activation of caspases, which are proteases that cleave Ͼ 500 cellular targets and induce typical morphologic changes associated with apoptosis in nucleated cells. A critical step in the intrinsic pathway is the loss of mitochondrial membrane potential (MMP) and the release of cytochrome c into cytosol, where it triggers the activation of caspase-9. Therefore, the release of cytochrome c from mitochondria needs to be tightly regulated: a function that is fulfilled by the BCL2 protein family, which consists of proapoptotic and antiapoptotic members that promote or block the release of cytochrome c, respectively. 2,3 The proapoptotic family members BAX and BAK play an essential role in directly mediating the release of cytochrome c by forming a pore in the outer mitochondrial membrane. Antiapoptotic BCL2 proteins, including BCL2, BCL-X L , BCL-w, MCL1, and BCL2A1, prevent the activation of BAX and BAK. Besides their function in regulating mitochondrial cytochrome c release, BCL2 proteins have also been implicated in the regulation of intracellular calcium homeostasis at the endoplasmic reticulum (ER), possibly by interacting with inositol triphosphate receptors. 4,5 Because of their key role, the antiapoptotic BCL2 proteins are attractive targets for anticancer therapy, with several small molecule inhibitors currently in preclinical testing or early clinical trials. 6,7 Among these, the most promising and specific inhibitors are ABT-263 (Navitoclax) and 9 ABT-737 shows promising antitumor activity in animal models of leukemia and lymphoma. A related compound, ABT-263, is metabolically more stable and currently in phase 1 and 2 clinical trials for leukemia and other malignancies. 10 Both compounds have often been regarded as interchangeable because they bind with high affinity to BCL2, BCL-X L , and BCL-w but do not inhibit MCL1 or BCL2A1. 11 Early results from the clinica...

Mcl-1 and Bcl-xL regulate Bak/Bax-dependent apoptosis of the megakaryocytic lineage at multistages

“…However, these drugs have been considered to affect only circulating platelets, unlike the chemotherapy-induced thrombocytopenia acting through myelosuppression, and increase the proportion of reticulated platelets in circulation. 27,28 Our present results indicated that Mcl-1 has an indispensable role in the survival of megakaryocytes and reticulated platelets in the absence of Bcl-xL, which prevents severe hemorrhagic complications from these BH3 mimetics.…”

“…Inhibition of Bcl-xL by ABT-737 administration slightly induced apoptosis in reticulated platelets of the wild-type mice (Figure 6f) but increased their proportion in circulation (Figure 6e) similar to the Bcl-xL knockout mice (Figure 6b), probably due to a higher susceptibility of non-reticulated platelets to ABT-737-induced apoptosis. 27,28 Meanwhile, ABT-737 treatment induced moderate apoptosis of reticulated platelets in the Mcl-1 knockout mice (Figure 6f) and their circulating proportion was significantly lower than the wild-type reticulated platelets (Figure 6e). These results suggested that the presence of either Bcl-xL or Mcl-1 may be important for the survival of reticulated platelets like mature megakaryocytes.…”

Oxidation resistance 1 is a novel senolytic target

“…Indeed, we and others have recently discovered that the upregulation of the antiapoptotic Bcl‐2 family proteins is primarily responsible for the resistance of SCs to apoptosis, and Bcl‐2/xl/w inhibitors such as ABT‐263 are potent senolytic agents (Chang et al., 2016; Yosef et al., 2016; Zhu et al., 2016). Because long‐term treatment with a senolytic drug may be required to prevent/treat age‐related diseases and extend lifespan, there is a concern that Bcl‐2/xl/w inhibitors might be not safe for humans because of their known on‐target (thrombocytopenia) and off‐target toxicities (Roberts et al., 2011; Rudin et al., 2012; Vogler et al., 2011). Thus, it is important that we continue searching for novel senolytic targets to develop safer senolytic agents.…”

Synergistic Bcl-2 inhibition by ABT-737 and cyclosporine A

“…The moderate thrombocytopenia previously described after ABT-737 treatment was significantly increased in combination with CsA (Table 1) and may represent a relevant side effect in a clinical context. ABT-737 dependent thrombocytopenia is the result of Bcl-XL inhibition in platelets and megakaryocytes [31] and CsA enhanced platelet activation [32]. Further investigations are required to clarify the synergistic effect of ABT-737 and CsA in this particular setting.…”