Defining the role of B7-H3 (CD276) in epithelial homeostasis and cancer progression

Abstract

B7-H3 (CD276) is a member of the B7 superfamily and shares up to 30% amino acid identity with other B7 family members. It is expressed in a wide range of cell types and tissues and is overexpressed in many cancers. B7-H3 expression has been linked to poor prognosis in several tumour types, including lung cancer, and is suggested to play both co-inhibitory and co-stimulatory roles in tumour-immune modulation. Thus B7-H3 is an appealing target for therapeutic agents to prevent tumorigenesis. However, the physiological functions of B7-H3 in both lung epithelial cells and tumour cells remains poorly understood, and ligands and specific binding partners for this receptor remain unknown. The purpose of this study was to define the effects of overexpression or knockdown of B7-H3 on normal lung epithelial and cancer cell proliferation, migration and invasion as well as to determine the role of B7-H3 in cell-cell adhesion and signalling. Data revealed that B7-H3 localises to cell-cell adhesions in all cell lines and co-localises with E-cadherin and β-catenin. Depletion of B7-H3 lead to reduced levels of the tight junction protein ZO-1 and reduced active GTPases that are known to control F-actin cytoskeleton dynamics. Moreover, B7-H3 depletion led to enhanced proliferation and invasion of lung cancer cells in 3D spheroid assays. Further analysis revealed that B7-H3 in normal lung epithelial cells localises to IMPDH2-positive structures called rods and rings, as well as to a subset of primary cilia. Both structures were absent in lung cancer cells that express high levels of B7-H3, but their appearance increased upon B7-H3 knockdown. Conversely, cilia and rods and rings were reduced upon B7-H3 overexpression in normal lung epithelial cells. Finally, NMR and proteomics analysis revealed that B7-H3 contributes to maintenance of metabolite levels leading to suppression of oxidative stress, potentially through co-regulation of IMPDH2 activity and cilia assembly. The data arising from this study provides novel insight into B7-H3 function in epithelial and lung cancer cells and how this receptor contributes to tumorigenesis.