Trop2 and its overexpression in cancers: regulation and clinical/therapeutic implications

Abstract

Trop2 is a transmembrane glycoprotein encoded by the Tacstd2 gene. It is an intracellular calcium signal transducer that is differentially expressed in many cancers. It signals cells for self-renewal, proliferation, invasion, and survival. It has stem cell-like qualities. Trop2 is expressed in many normal tissues, though in contrast, it is overexpressed in many cancers and the overexpression of Trop2 is of prognostic significance. Several ligands have been proposed that interact with Trop2. Trop2 signals the cells via different pathways and it is transcriptionally regulated by a complex network of several transcription factors. Trop2 expression in cancer cells has been correlated with drug resistance. Several strategies target Trop2 on cancer cells that include antibodies, antibody fusion proteins, chemical inhibitors, nanoparticles, etc. The in vitro studies and pre-clinical studies, using these various therapeutic treatments, have resulted in significant inhibition of tumor cell growth both in vitro and in vivo in mice. A clinical study is underway using IMMU-132 (hrS7 linked to SN38) in patients with epithelial cancers. This review describes briefly the various characteristics of cancer cells overexpressing Trop2 and the potential application of Trop2 as both a prognostic biomarker and as a therapeutic target to reverse resistance.

Figure 1. Trop2-Regulated Intramembrane Proteolysis (RIP)

RIP is required for Trop2 activity in prostate cancer. Trop2 is cleaved by the TNF-α converting enzyme (TACE), followed by y-secretase. The cleavage is mediated by the enzymes PS-1 and PS-2 in the complex. PS-1 is the dominant enzyme. Two products are made: the extracellular domain (ECD) is shed and is found on the plasma membrane and in the cytoplasm, and the intracellular domain (ICD) is released from the membrane and accumulates in the nucleus (although some is found on the membrane). The ICD is the functionally dominant part of Trop2 in prostate cancer. Within prostate cancer regions, β-catenin colocalizes with the ICD in the nucleus, leading to Trop2 proliferation. This process could possibly occur in other cancers. This colocalization causes upregulation of the downstream targets cyclin D1 and c-myc, which leads to cell growth [10].

Figure 2. PLC Cleavage of PIP2 via Trop2 S303 Phosphorylation

If the cytoplasmic tail of Trop2 is bound to PIP₂, it might be concentrating Trop2 for hydrolysis by phospholipase C (PLC). Once position S₃₀₃ on the Trop2's cytoplasmic tail is phosphorylated by protein kinase C (PKC), PIP₂ is exposed for cleavage by PLC₃₀₃ It is uncertain whether S phosphorylation by PKC comes before increased Ca²⁺ concentration from Trop2 signaling or after or whether this phosphorylation itself releases PIP₂ [4]. When PLC cleaves PIP₂, this results in an increase of IP₃ (inositol 1,4,5-triphosphate) in the cytoplasm and DAG (deacylglycerol) in the plasma membrane. IP₃ causes Ca²⁺ release from the endoplasmic reticulum (ER) [82]. The increase in free Ca²⁺ and DAG could activate more PKC in a positive feedback mechanism. This increase in PKC could lead to further phosphorylation of Trop2 and activation of the Raf and NF-κB pathways [4]. Ca2+release stimulates MAPK signaling and cell cycle progression [8].

Figure 3. mTrop2 Cell Signaling and Resulting Activities

mTrop2 expression increases the expression of the proliferation marker Ki-67 and causes Ca²⁺ to be mobilized from internal stores. mTrop2 expression downregulates p27 (cyclin-dependent kinase inhibitor 1B). mTrop2 expression activates MAPK signaling, which increases levels of phosphorylated ERK1 and ERK2. MAPK signaling and cell cycle progression can be further stimulated by Ca²⁺. mTrop2 increases levels of cyclin D1 and cyclin E, which help mediate ERK1/2 cell cycle progression (an increased percentage of cells enter the S phase). ERK signaling leads to induction of the AP-1 transcription factor [8]. It is a central regulator of tumor-associated target genes during carcinogenesis. AP-1 causes angiogenesis via VEGF (vascular endothelial growth factor), cell proliferation via the cyclins and CDKs, apoptosis via pro-apoptotic bcl-2 (B-cell lymphoma 2) or FasL (Fas ligand), and causes cell invasion and metastasis via MMPs (matrix metalloproteinases), Pdpn (podoplanin), Ezrin, and CD44, and it causes the epithelial to mesenchymal transition (EMT) via Pdpn. The EMT allows for the nuclear translocation of β-catenin, which causes cell growth via β-catenin's downstream effectors [26]. Heightened ERK activity could induce phosphorylation of FOXO3a at residues S294, S344, and S245, which can lead to ubiquitination by MDM2 (mouse double minute 2) and subsequent cytoplasmic localization and proteasomal degradation [8]. FOXO3a can induce cell death, therefore, its degradation could help promote cell survival in cancer [39].