The Role of the TG2-GPR56 Complex in Cutaneous Squamous Cell Carcinoma (CSCC) Aggression and Therapeutic Resistance

Cutaneous squamous cell carcinoma (cSCC) is the second most prevalent skin cancer diagnosed worldwide after basal cell carcinoma. CSCC represents a growing global public health challenge due to its higher potential of local invasion, recurrence, and metastasis. Incidence rates of cSCC are projected to increase due to rising exposures to risks factors. Ultraviolet light exposure is the primary cause, and lighter skin pigmentation, immunosuppressive conditions and skin phototype are the primary risk factors. CSCC typically presents as a red, scaly, flat lesion (in situ tumors) or a red, firm, raised lesion with scale or erosion (invasive tumors). Surgical excision remains the standard-of-care for localized cSCC and is often curative. Although, most patients achieve favorable outcomes, a subset of cSCC exhibits a highly aggressive and metastatic phenotype (postoperative recurrence rates are approximately 5%). Addressing the clinical challenge posed by these high-risk cases requires a more comprehensive understanding of the underlying molecular drivers. This review examines the interaction between transglutaminase 2 (TG2) and the G-protein-coupled receptor 56 (GPR56) as a pivotal driver of the aggressive cSCC phenotype. This molecular axis is particularly significant for its role in the maintenance of epidermal cancer stem (ECS) cells, which contribute to tumor progression and therapy resistance. While the definitive link between the TG2-GPR56 complex and systemic metastasis in cSCC is currently being elucidated, significant evidence from analogous malignancies and in vitro keratinocyte models provides a clear mechanistic roadmap for its involvement in tumor invasion.