Vascular invasion is one of the major negative prognostic factors in patients with hepatocellular carcinoma (HCC), leading to cancer recurrence. To invade, HCC cells must penetrate the vessel wall, consisting of endothelial cells and extracellular matrix components, including fibronectin and fibrinogen. Employing invasive and noninvasive HCC cells, we studied the mechanism underlying vascular invasion. We show that HCC cells invade blood vessels viaα5β1, that is equally expressed in invasive and noninvasive cells. However, in the former, the intracytoplasmic tail of β1 integrin is constitutively phosphorylated at threonine 788-789 and the extracellular part is conformationally activated. In noninvasive cells,β1 integrin is not activated. Transforming growth factor (TGF)-β1 specifically phosphorylates β1 integrin (threonine 788-789) via Smad-2 and Smad-3, causing a conformational change of the extracellular component with an inside-out mechanism. This leads noninvasive HCC cells to behave like invasive cells. A selective TGF-βRI inhibitor inhibits phosphorylation of the β1 integrin intracytoplasmic tail, and blocks invasion of HCC cells, both constitutively invasive and with acquired invasive properties. In human HCC tissues with microvascular invasion, phospho-β1 integrin was detected as well as TGF-β1, p-Smad-2, and E-cadherin. Conclusion: TGF-β1 promotes vascular invasion by activating β1 integrin. This suggests a rationale for targeting TGF-βRI in future clinical trials.
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