Alterations in the homeostasis of several adhesion GPCRs (aGPCRs) have been observed in cancer. The main cellular functions regulated by aGPCRs are cell adhesion, migration, polarity, and guidance, which are all highly relevant to tumor cell biology. Expression of aGPCRs can be induced, increased, decreased, or silenced in the tumor or in stromal cells of the tumor microenvironment, including fibroblasts and endothelial and/or immune cells. For example, ADGRE5 (CD97) and ADGRG1 (GPR56) show increased expression in many cancers, and initial functional studies suggest that both are relevant for tumor cell migration and invasion. aGPCRs can also impact the regulation of angiogenesis by releasing soluble fragments following the cleavage of their extracellular domain (ECD) at the conserved GPCR-proteolytic site (GPS) or other more distal cleavage sites as typical for the ADGRB (BAI) family. Interrogation of in silico cancer databases suggests alterations in other aGPCR members and provides the impetus for further exploration of their potential role in cancer. Integration of knowledge on the expression, regulation, and function of aGPCRs in tumorigenesis is currently spurring the first preclinical studies to examine the potential of aGPCR or the related pathways as therapeutic targets.

Soluble factors such as growth factors and cytokines present in the tumor microenvironment regulate a variety of genes associated with malignant properties of tumor cells such as growth, migration, invasion, and metastatic capacities. CD44 is a multi-functional adhesion molecule involved in cell to cell and cell to extracellular matrix interaction, the trapping of growth factors and cytokines, and the regulation of cell traffic. Growth factors and cytokines modify the expression, selective isoform splicing and functions of CD44, resulting in changes in the biological properties of the cells. These include adhesion of circulating tumor cells to endothelium and body cavities, and survival in response to growth factors presented by the CD44 molecule. The modification of CD44 on both tumor and host cells by growth factors may play an important role in tumor progression.