The Role of GREMLIN1, a Bone Morphogenetic Protein Antagonist, in Cancer Stem Cell Regulation

Cancer remains a leading cause of death globally, characterized by uncontrolled cell proliferation, tumor growth and metastasis. Bone morphogenetic proteins (BMPs) and their growth differentiation factor (GDF) relatives are crucial regulators of developmental processes such as limb, kidney and lung formation, cell fate determination, cell proliferation, and apoptosis. Cancer stem cells (CSCs) are a subpopulation of self-renewing cells within tumors that possess stemness properties and a tumor cell-forming capability. The presence of CSCs in a tumor is linked to growth, metastasis, treatment resistance and cancer recurrence. The tumor microenvironment in which CSCs exist also plays a critical role in the onset, progression and treatment resistance in many cancers. Growth factors such as BMPs and GDFs counterbalance transforming growth factor-beta (TGF-β) in the maintenance of CSC pluripotency and cancer cell differentiation. BMP signaling typically functions in a tumor suppressor role in various cancers by inducing CSC differentiation and suppressing stemness characteristics. This differentiation process is vital, as it curtails the self-renewal capacity that characterizes CSCs, thereby limiting their ability to sustain tumor growth. The interplay between BMPs and their secreted antagonists, such as GREM1, Noggin and Chordin, adds another layer of complexity to CSC regulation. Human cancers such as gastric, colorectal, glioblastoma, and breast cancer are characterized by GREMLIN1 (GREM1) overexpression, leading to inhibition of BMP signaling, facilitating the maintenance of pluripotency in CSCs, thus promoting tumorigenesis. GREM1 overexpression may also contribute to CSC immune evasion, further exacerbating patient prognoses. In addition to BMP inhibition, GREM1 has been implicated as a target of fibroblast growth factor (FGF) → Sonic hedgehog (Shh) signaling, as well as the Wnt/Frizzled pathway, both of which may contribute to the maintenance of CSC stemness. The complex role of BMPs and their antagonists in regulating CSC behavior underscores the importance of a balanced BMP signaling pathway. This article will summarize current knowledge of BMP and GREM1 regulation of CSC function, as well as conflicting data on the exact role of GREM1 in modulating CSC biology, tumor formation and cancer. Targeting this pathway by inhibiting GREM1 using neutralizing antibodies or small molecules may hold early-stage promise for novel therapeutic strategies aimed at reducing CSC burden in cancers and improving patient outcomes.