Why anti-Bcl-2 clinical trials fail: a solution

Abstract

The alteration in expression of B cell lymphoma-2 (Bcl-2) family of protein members in cancer is involved mainly in the regulation of apoptosis. Bcl-2 family proteins are currently used as major targets in the development of methods to improve treatment outcomes for cancer patients that underwent clinical trials. Although many agents have been developed for targeting Bcl-2 in the past decade, some previous attempts to target Bcl-2 have not resulted in beneficial clinical outcome for reasons unknown. Here, we propose that this was due in part for not considering the cellular level of a different antiapoptotic protein, i.e., galectin-3 (Gal-3). Gal-3 is a member of the β-galactoside binding protein family and a multifunctional oncogenic protein which regulates cell growth, cell adhesion, cell proliferation, angiogenesis, and apoptosis. Gal-3 is the sole protein that contains the NWGR anti-death motif of the Bcl-2 family and inhibits cell apoptosis induced by chemotherapeutic agents through phosphorylation, translocation and regulation of survival signaling pathways. It is now established that Gal-3 is a candidate target protein to suppress antiapoptotic activity and anticancer drug resistance. In this review, we describe the role and relevance of Gal-3 and Bcl-2 protein family in the regulation of apoptosis and propose a novel combination therapy modality. Combination therapy that targets Gal-3 could be essential for improvement of the efficacy of Bcl-2 targeting therapy in cancers and should be studied in future clinical trials. Otherwise, not considering Gal-3 cellular level could lead to trial failure.

Fig. 1

The role of the Bcl-2 family of proteins in apoptosis. 1 The extrinsic pathway is activated by Fas ligand or TRAIL, subsequently activating caspase 8. 2 Caspase 8 transforms Bid into truncated Bid (tBid). In addition, caspase 8 initiates a cascade of caspase activation. 3 Diverse forms of extracellular stress, (DNA damage, cytotoxic drugs, and cytokine withdrawal) initiate the intrinsic pathway. 4 BH3-only proteins (Bim, Bid, Bad, Noxa, and Puma) engage with antiapoptotic Bcl-2 family proteins (Bcl-2, Bcl-XL, Bcl-w, and Mcl-1) to relieve their inhibition of Bax and Bak to activate them. 5 Bax and Bak are oligomerized and activated, leading to mitochondrial outer membrane permeabilization (MOMP). 6 Once mitochondrial membranes are permeabilized, cytochrome c is released into the cytoplasm, resulting in activation of caspase 9, subsequently caspase 3, which is the initiation step for the cascade of caspase activation

Fig. 2

Model for the regulation of apoptosis by Gal-3. Anticancer drugs can induce DNA damage, which causes Gal-3 phosphorylated by Casein Kinase 1 (CK1) translocate from the nuclear to cytoplasm. Gal-3 upregulates the ERK pathway and induces Bad phosphorylation, leading to mitochondrial stabilization. Akt activated by Gal-3 inhibits apoptosis by blocking transformation of Bid into tBid, which is essential for cytochrome c release from the mitochondrial intermembrane space. After treatment of proapoptotic agents, Gal-3 in cytoplasm decreases Bad expression and attenuates the depolarization of the mitochondrial membrane. Consequently, the stabilization of the mitochondrial membrane prevents cytochrome c release and subsequent caspase activation, resulting in suppression of apoptosis and resistance to chemotherapeutic agents

Fig. 3

A novel combination therapy model. a ABT-263 (Navitoclax) is developed for use in the clinic and an oral version of ABT-737. It binds to and inhibits Bcl-2, Bcl-XL, and Bcl-w with higher affinities than any of other Bcl-2 inhibitors, resulting in effective enhancement of the cytotoxicity of several chemotherapy agents. b However, resistance against ABT-263 is observed in cancer cells that express Mcl-1 because ABT-263 does not bind to Mcl-1. On the other hand, Gal-3 that highly expresses in diverse cancers have multiple roles in the regulation of apoptosis (as shown in Fig. 2). Gal-3 may replace or mimic Bcl-2 by binding to Bax through the NWGR motif, which is critical for the antiapoptotic function of both Bcl-2 and Gal-3 proteins. c We propose a novel combination therapy model. Combination therapy targeting Gal-3 using sugar-binding antagonist with the conventional anti-Bcl-2 treatment leads to suppression of antiapoptotic function of Gal-3 itself. Interestingly, GCS-100, which binds to Gal-3 as an antagonist, is shown to reduce Mcl-1 expression and regulate the other bcl-2 family proteins. In addition, targeting the CRD of Gal-3 containing the NWGR motif may induce release of Bax from Gal-3, subsequently resulting in MOMP