Syk Inhibition Sensitizes Skin Squamous Carcinoma Cells to TRAIL-Induced Apoptosis

Spleen tyrosine kinase (Syk) regulates immune responses and has been implicated in haematopoietic and epithelial cancers. Although our previous studies showed that Syk modulates keratinocyte differentiation and UVB-induced inflammation, its role in UVB-related cutaneous squamous cell carcinoma (cSCC) remains unclear. Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) selectively induces apoptosis in cancer cells and is expressed in healthy skin but reduced in non-melanoma skin cancers and sun-exposed skin. This study investigated the role of Syk in cSCC and TRAIL-induced apoptosis. Immunohistochemistry revealed elevated phosphorylated and total Syk levels in SCC tissues compared with normal skin. In SCC12 cells, TRAIL induced dose-dependent apoptosis, which was enhanced by pharmacological Syk inhibition or Syk silencing. Combination treatment with TRAIL and Syk inhibitors increased annexin V positivity and cleavage of caspase-3, PARP1 and Bid, accompanied by reduced Mcl-1 expression. Mechanistically, TRAIL induced Src and Syk phosphorylation. TRAIL-induced Syk activation was suppressed by Src inhibition but not by EGFR inhibition, indicating that Syk activation is Src-dependent but EGFR-independent in this pathway. Syk inhibition further reduced TRAIL-activated Akt and MAPK signalling, while Akt activation may play a role in regulating SCC12 survival. These results suggest that targeting Syk sensitizes TRAIL-mediated apoptosis and may offer a promising therapeutic strategy for treating cutaneous SCC.