Extracellular kinome network reveals secretory kinase FAM20C as a key driver of recurrence in oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) is a highly aggressive malignancy with a recurrence rate of 40-60%, frequently exhibiting locoregional relapse and lymph node metastasis even after definitive treatment. Tumor progression and recurrence are driven not only by intrinsic characteristics of cancer cells but also by complex interactions with the surrounding tumor microenvironment (TME). To explore recurrence-associated regulatory pathways, we performed LC-MS/MS-based phosphoproteomic profiling of plasma from 34 OSCC patients and identified 113 phosphoproteins harboring 261 phosphosites enriched in lipid binding, complement activation, extracellular matrix (ECM) organization, and calcium-binding functions. Principal component and correlation analyses distinguished patients with recurrence from those without, and consensus clustering revealed three phospho-secretory subtypes. Notably, the subtype with the poor prognosis exhibited a enrichment of phospho-serine within the S-x-E motif. This pattern suggested activation of the secreted serine kinase FAM20C, which phosphorylates ER/Golgi-localized secretory proteins and membrane ectodomain substrates. Public datasets confirmed that high FAM20C expression correlates with increased recurrence risk and reduced overall and progression-free survival. To validate these findings, we examined patient-derived organoids (PDOs) and cancer-associated fibroblasts (CAFs) established from our cohort. Both FAM20C expression and secretion were markedly elevated in PDOs derived from recurrent tumors, and CAFs from recurrent patients also exhibited increased FAM20C secretion. Functionally, FAM20C overexpression enhanced invasion, ECM remodeling, and EMT activation, while FAM20C knockdown suppressed mesenchymal markers, reduced TGF-beta-SMAD2/3 signaling, and suppressed stemness-associated genes including SOX2, OCT4, CD44, NANOG, MYC, and CD133. Together, these findings identify FAM20C as a key regulator of recurrence-associated extracellular signaling and highlight its potential as a prognostic biomarker for recurrence in OSCC.