Tetraspanins as therapeutic targets in hematological malignancy: a concise review

Abstract

Tetraspanins belong to a family of transmembrane proteins which play a major role in the organization of the plasma membrane. While all immune cells express tetraspanins, most of these are present in a variety of other cell types. There are a select few, such as CD37 and CD53, which are restricted to hematopoietic lineages. Tetraspanins associate with numerous partners involved in a diverse set of biological processes, including cell activation, survival, proliferation, adhesion, and migration. The historical view has assigned them a scaffolding role, but recent discoveries suggest some tetraspanins can directly participate in signaling through interactions with cytoplasmic proteins. Given their potential roles in supporting tumor survival and immune evasion, an improved understanding of tetraspanin activity could prove clinically valuable. This review will focus on emerging data in the study of tetraspanins, advances in the clinical development of anti-CD37 therapeutics, and the future prospects of targeting tetraspanins in hematological malignancy.

Keywords: CD37; CD53; TSPAN; tetraspanin.

Figure 1

Structural features of tetraspanins. Several common features of tetraspanins are depicted here. They possess 4 transmembrane domains (which are highly conserved), two short cytoplasmic tails, and two extracellular portions known as the EC1 domain (small extracellular loop) and EC2 domain (large extracellular loop). Portions of the EC2 domain are conserved between various tetraspanins, but it also contains a highly variable region (shown in red). One of the features of this segment is the presence of 2–4 disulfide bonds (yellow lines) formed between cysteine residues (yellow circles), the number of which depend on the particular tetraspanin. The variable region of the EC2 domain contains binding sites for interactions with partner proteins and is frequently where epitopes for anti-tetraspanin antibodies are found. Many tetraspanins undergo palmitoylation at cysteine residues located near the intracellular border of the four transmembrane portions. Additionally, most tetraspanins also experience N-linked glycosylation at extracellular asparagine residues (not depicted).

Figure 2

Signaling pathway associated with CD37 ligation by SMIP-016. Lapalombella et al. described a number of cytoplasmic proteins which can associate with CD37, as depicted in the diagram. Ligation by SMIP-016 leads to phosphorylation Tyr¹³ within an ITIM-like motif found in the N-terminal cytoplasmic tail, which associates with a complex of proteins that includes Syk, Lyn, and SHP1 (which likewise become phosphorylated). In addition, SMIP-016 induces phosphorylation of an ITAM-like motif (containing Tyr²⁷⁴ and Tyr²⁸⁰) located in the C-terminal cytoplasmic tail that recruits PI3Kδ. Mutational studies suggest that the events requiring the N-terminal ITIM drive apoptosis, while the C-terminal tail has a role in promoting cell survival. The proposed mechanism of anti-CD37 induced cellular death involves a balance between these signals, with preferential SHP1 activation driving apoptosis. SHP1 is capable of inactivating both PI3K and Akt. SMIP-016 decreases the nuclear localization of Akt, preventing phosphorylation of FoxO3a (and promoting retention in the nucleus) to allow transcription of pro-apoptotic BIM. An opposing signal is transduced through PI3Kδ recruited to the C-terminal ITAM, activating Akt and resulting in the downstream phosphorylation of GSK3β (which permits nuclear translocation of pro-survival β-catenin). However, the contribution of PI3Kδ to survival can be eliminated by either combination with a PI3K inhibitor or deletion of the ITAM-containing C-terminal domain of CD37. While both pro-survival and pro-apoptotic signaling pathways that are activated upon ligation by anti-CD37 SMIP-016, those that promote cellular death predominate. Several other CD37-targeted antibodies directly induce leukemia cell death, presumably in a similar fashion as SMIP-016/TRU-016. However, they do not require additional receptor crosslinking (by use of anti-Fc antibody to amplify the signal) as was observed with SMIP-016.

Figure 3

CD37-Targeted antibody therapeutics. Several anti-CD37 therapies under clinical development are shown. Left: Otlertuzumab is an ADAPTIR™ molecule, constructed from an anti-CD37 single-chain variable fragment (scFv; a binding domain formed by linking the heavy and light chain variable regions of an immunoglobulin) which has been fused to the hinge region and Fc domain of human IgG1. Middle: mAb 37.1 is an Fc-engineered IgG1 with specific amino acid substitutions within the Fc region to increase ADCC mediated by effectors such as NK cells and macrophages. Right: IMGN529 is a humanized anti-CD37 IgG1 (K7153A) conjugated to 3–4 molecules of cytotoxic drug (DM1) by stable thioether bonds. mAb, monoclonal antibody; CHO, Chinese hamster ovary; VH, heavy chain variable region; VL, light chain variable region; CH, heavy chain constant region (1, 2, or 3); CL, light chain constant region; D (orange circles), DM1; SMCC, N-succinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate.