| Cytokeratin 19 (KRT19) is a type I intermediate filament protein of the keratin family that forms part of the cytoskeletal network in epithelial cells and acts as a structural scaffold that contributes to cell shape, mechanical resilience and organization of signaling complexes. The protein contains the conserved central α‑helical rod domain flanked by non‑helical head and tail regions typical of keratins, allowing incorporation into heteropolymeric filaments with type II keratins and providing multipurpose platforms for anchoring membrane receptors and signaling proteins. Beyond its role as a structural element, KRT19 participates directly in signaling: in HER2‑overexpressing breast cancer cells, KRT19 expression is induced at the transcriptional level by ERK downstream of HER2, and Akt phosphorylates KRT19 on Ser35, promoting its remodeling from filamentous to granular structures and driving translocation to the plasma membrane. Phosphorylated KRT19 binds HER2 at the cell surface and stabilizes the receptor by inhibiting proteasome-mediated HER2 degradation; silencing KRT19 increases HER2 ubiquitination and destabilization, while antibody targeting of KRT19 reduces HER2 abundance and cell viability, demonstrating that KRT19 directly modulates HER2 receptor turnover and sustains HER2-driven signaling. In breast cancer models that normally lack KRT19, forced expression of CK19 causes cell-cycle arrest, reduced motility and increased drug resistance, and detailed analysis shows that CK19 expression upregulates p38/PERK/p‑eIF2α and GRP78 while downregulating focal adhesion kinase, indicating that KRT19 influences endoplasmic reticulum stress signaling and focal adhesion dynamics. Proteomic and transcript analyses in these CK19-expressing breast cancer cells identify ER protein 29 (ERp29) as a downregulated target; pharmacologic inhibition or siRNA-mediated knockdown of p38 or XBP‑1 reveals that p38/XBP‑1 signaling negatively regulates ERp29 expression, placing CK19 upstream of a p38/XBP‑1–ERp29 axis that contributes to cell survival and dormancy under stress. In hepatocellular carcinoma and breast tumors, CK19 expression is associated with more aggressive features and poorer prognosis, and CK19‑positive tumor cells frequently show increased invasiveness and metastatic potential, consistent with its roles in modulating receptor stability and ER stress pathways. CK19 also participates in epithelial–mesenchymal transition-related phenotypes: knockout of K19 in MCF‑7 cells alters the expression of other keratins, reduces proliferation and migration, and changes nuclear and cell morphology, supporting a role for KRT19 in controlling cytoskeletal organization, cell mechanics and EMT-linked behaviors. |
