Tetraspanins set the stage for bone marrow microenvironment-induced chemoprotection in hematologic malignancies

Abstract

Despite recent advances in the treatment of hematologic malignancies, relapse still remains a consistent issue. One of the primary contributors to relapse is the bone marrow microenvironment providing a sanctuary to malignant cells. These cells interact with bone marrow components such as osteoblasts and stromal cells, extracellular matrix proteins, and soluble factors. These interactions, mediated by the cell surface proteins like cellular adhesion molecules (CAMs), induce intracellular signaling that leads to the development of bone marrow microenvironment-induced chemoprotection (BMC). Although extensive study has gone into these CAMs, including the development of targeted therapies, very little focus in hematologic malignancies has been put on a family of cell surface proteins that are just as important for mediating bone marrow interactions: the transmembrane 4 superfamily (tetraspanins; TSPANs). TSPANs are known to be important mediators of microenvironmental interactions and metastasis based on numerous studies in solid tumors. Recently, evidence of their possible role in hematologic malignancies, specifically in the regulation of cellular adhesion, bone marrow homing, intracellular signaling, and stem cell dynamics in malignant hematologic cells has come to light. Many of these effects are facilitated by associations with CAMs and other receptors on the cell surface in TSPAN-enriched microdomains. This could suggest that TSPANs play an important role in mediating BMC in hematologic malignancies and could be used as therapeutic targets. In this review, we discuss TSPAN structure and function in hematologic cells, their interactions with different cell surface and signaling proteins, and possible ways to target/inhibit their effects.

Figure 1.

Structure and domain functions of TSPANs. TSPANs are characterized by the presence of 4 transmembrane domains (1-4). There are multiple regions within the protein that mediate key effects. (A) Disulphide-stabilized (green spheres) variable region of the large extracellular loop (EC2) responsible for the unique interactions specific to each TSPAN. (B) The conserved region of the EC2 loop mediates homodimerization. (C) Transmembrane regions are important for the formation of TEMs. (D) Palmitoylation sites are crucial to the proper formation of new TEMs and play a role in signaling. (E) The C-terminus is responsible for interacting with signaling and cytoskeletal proteins.

Figure 2.

TEMs play a key role in TSPAN function. TEMs are dynamic membrane entities that play a key role in mediating interactions with the BM. TSPANs act as scaffolding proteins to bring together many proteins with similar functions such as CAMs (like integrins and immunoglobulin superfamily [IgSF] members) and signaling receptors (G protein–coupled receptors [GPCRs] and receptor tyrosine kinases [RTKs]). The crosslinking of TSPANs creates a large secondary signaling network, which can effectively transduce extracellular stimuli inside to intracellular signaling pathways.

Figure 3.

CD82 mediates leukemic stem cell survival and interactions with the microenvironment. CD82 mediates leukemic stem cell (LSC) interaction with the BM in multiple ways. It increases microenvironment interaction by organizing N-cadherin at the cell surface in high densities via CD82 palmitoylation (green triangles) and glycosylation (red spheres). However, the connection of N-cadherin to CD82-associated intracellular effects is still unknown. CD82-associated STAT5 signaling can combine with increased p-Akt (via inhibition of PTEN by DNA methylation from EZH2 due to downregulation of p38 caused by CD82-associated signaling) to increase the expression of antiapoptotic protein BCL2L12. The combination of these effects leads to increased LSC survival.

Figure 4.

CD37 mediates both prosurvival and prodeath effects from its N- and C-termini. CD37 initiates cell death via phosphorylation of an immune tyrosine-based inhibitory motif (ITIM)-like motif in its N-terminus tail. This leads to a SHP1-dependent cell death. IT can also initiate prosurvival signaling through phosphorylation of its immune tyrosine-based activation motif (ITAM) on the C-terminus tail. This occurs through PI3K/Akt signaling.

Figure 5.

Possible methods to target TSPANs. There have been multiple proposed mechanisms on how to target TSPANs and inhibit their functions. (A) mAbs can be used to antagonize TSPANs either directly, or by inhibiting its lateral interactions. (B) Soluble EC2 loops can be used to disrupt the unique interactions of each TSPAN. (C) Analogues to transmembrane regions can prevent the formation of TEMs, thereby disrupting their effects. (D) Inhibiting palmitoylation can also be used as a mechanism to prevent TEM formation. (E) Many TSPANs have a motif in their C-terminus that can interact with PDZ domains on different signaling proteins. Small molecule inhibitors to these domains can prevent their activation, and therefore the cellular responses.