Which of the following adhesion molecules is primarily responsible for the initial attachment of circulating tumor cells to the vascular endothelium during the process of metastasis?

Explanation

## Role of Selectins in Tumor Cell Adhesion **Key Point:** Selectins mediate the initial, low-affinity rolling interaction between circulating tumor cells and the vascular endothelium, a critical first step in the metastatic cascade. ### Mechanism of Selectin-Mediated Adhesion Selectins (L-selectin, E-selectin, P-selectin) bind to carbohydrate ligands on tumor cell surfaces and endothelial cells. This interaction is: - **Transient and reversible** — allows tumor cells to "roll" along the vessel wall - **Shear-resistant** — occurs even under high blood flow conditions - **Essential for extravasation** — precedes firm adhesion via integrins ### Comparison of Adhesion Molecules in Metastasis | Molecule | Function | Stage | |----------|----------|-------| | **Selectins** | Initial rolling, low-affinity binding | Early attachment | | **Integrins** | Firm adhesion, transmigration | Secondary adhesion | | **E-cadherin** | Cell–cell cohesion within tissue | Epithelial integrity (lost in EMT) | | **Claudins** | Tight junction proteins | Barrier function | **High-Yield:** Tumor cells often upregulate selectin ligands (sialyl-Lewis^x^, sialyl-Lewis^a^) to enhance endothelial binding and extravasation. **Clinical Pearl:** The selectin-mediated rolling phase is followed by integrin-mediated firm adhesion and transmigration through the endothelial monolayer. ### Why Selectins Are First Selectin-ligand interactions occur at physiologic shear stress and do not require prior activation. Integrins, by contrast, require conformational changes and are responsible for the firm adhesion that follows selectin-mediated rolling.