Mesothelin: An Immunotherapeutic Target beyond Solid Tumors

Abstract

Modern targeted cancer therapies rely on the overexpression of tumor associated antigens with very little to no expression in normal cell types. Mesothelin is a glycosylphosphatidylinositol-anchored cell surface protein that has been identified in many different tumor types, including lung adenocarcinomas, ovarian carcinomas, and most recently in hematological malignancies, including acute myeloid leukemia (AML). Although the function of mesothelin is widely unknown, interactions with MUC16/CA125 indicate that mesothelin plays a role in the regulation of proliferation, growth, and adhesion signaling. Most research on mesothelin currently focuses on utilizing mesothelin to design targeted cancer therapies such as monoclonal antibodies, antibody-drug conjugates, chimeric antigen receptor T and NK cells, bispecific T cell engaging molecules, and targeted alpha therapies, amongst others. Both in vitro and in vivo studies using different immunotherapeutic modalities in mesothelin-positive AML models highlight the potential impact of this approach as a unique opportunity to treat hard-to-cure AML.

Keywords: acute myeloid leukemia; immunotherapy; mesothelin.

Figure 1

Mesothelin processing produces mature cell-surface mesothelin and megakaryocyte potentiating factor (MPF). The MSLN gene is located at 16p13.3 and encodes for the MSLN mRNA. The pre-pro-mesothelin consists of a signaling peptide at the N-terminus (yellow), mature MPF (blue), furin cleavage site (red), mature mesothelin (orange), and a GPI-anchor sequence (gray). Along the pre-pro-mesothelin, there are four glycosylation sites (green), three of which exist on the mature mesothelin domain. Once on the surface, mesothelin gets docked to the cell membrane with a glycosylphosphatidylinositol (GPI) anchor. Mesothelin processing continues when furin (or other applicable proteases) cleaves the pre-pro-mesothelin to result in soluble MPF and a mature mesothelin. Finally, mesothelin can undergo membrane shedding and result in a truncated mesothelin protein still left on the cell surface as well as soluble mesothelin.

Figure 2

Signaling pathways associated with mesothelin. Through interactions with soluble or cell surface mesothelin, mesothelin can trigger Akt, ERK1/2, and JNK signaling pathways. Downstream effects of Akt and ERK1/2 signaling include inhibition of Bim, Bad, and Bax, which then inhibits apoptosis and/or stimulate Bcl-xl/Bcl-2 leading to cell survival. The Akt, ERK1/2, and JNK pathways are shown to increase expression of matrix metalloprotease 7 (MMP-7), which leads to increased rates of cellular migration and invasion. MMP-7 pathway can also be triggered through SGK3/FOXO3 and p38 pathways. SGK3/FOXO3 signaling has also been shown to cause the downregulation and expression of DKK1, thus increasing cell migration. In general, overexpression of mesothelin also causes p38 signaling activation, as well as NF-kB and STAT3 signaling. NF-kB downstream effect increases expression of IL-6 and contributes to cell survival and proliferation, while STAT3 signaling has been indicated in Cyclin E/CDK2 complex formation, pushing the cell through the cell cycle, and increasing cell proliferation.

Figure 3

Mesothelin-targeted therapies in preclinical development for pediatric AML. Anetumab ravtansine is an antibody–drug conjugate (ADC) that is an scFv for mesothelin attached to DM4 (red star), a tubulin inhibitor, via a linker (yellow). Once bound to mesothelin, DM4 is internalized and binds to tubulin, which disrupts microtubule assembly and cell cycle is inhibited, which results in cell death. Chimeric antigen receptor (CAR) therapy uses a variety of host immune cells to elicit a tumor-targeted response using a construct designed to recognize a target antigen. In AML, CAR-NK cells (yellow), and CAR T cells (orange) have been investigated. Upon the recognition of the target antigen, NK and T cells activate and elicit a killing mechanism through the secretion of perforin and granzymes (blue), resulting in tumor cell lysis. Bispecific T cell-engaging molecules (BiTEs) work mechanistically by recruiting T cells to tumor cells by engaging with a tumor-associated antigen (shown in blue), while the other side of the molecule engages with a CD3 on a T cell (shown in green). Once bound, the T cell becomes activated and kills through the same mechanisms at the CAR cell lysis.