Abstract
The transcription factor homeodomain-only protein X (HOPX) is the smallest member of the homeodomain protein family. Lacking a DNA-binding domain, it acts as a co-effector, interacting with other transcription factors such as serum response factor (SRF) and GATA-binding factor 6 (GATA6) to regulate the differentiation and development of the heart and lung. HOPX exerts a tumor-suppressive function in various types of epithelial-derived carcinoma, while it promotes oncogenic effects in mesenchymal-derived sarcoma, indicating a distinct role of HOPX in the two major types of the malignancy. In addition, accumulating evidence shows that HOPX is expressed in the immune system and involved in the differentiation of immune cells. Recently, the emerging role of HOPX in metabolism has gained attention. This review describes the identification of HOPX in various tissues and discusses its role in carcinogenesis, as well as its functions in tissue differentiation, lipid metabolism, immunity, and the tumor microenvironment. The participation of HOPX in carcinogenesis and immunity implies that it may serve as a potential enhancer in tumor immunotherapy.