The primacy of ß1 integrin activation in the metastatic cascade.

ABSTRACT

After neoplastic cells leave the primary tumor and circulate, they may extravasate from the vasculature and colonize tissues to form metastases. ß1 integrins play diverse roles in tumorigenesis and tumor progression, including extravasation. In blood cells, activation of ß1 integrins can be regulated by "inside-out" signals leading to extravasation from the circulation into tissues. However, a role for inside-out ß1 activation in tumor cell metastasis is uncertain. Here we show that ß1 integrin activation promotes tumor metastasis and that activated ß1 integrin may serve as a biomarker of metastatic human melanoma. To determine whether ß1 integrin activation can influence tumor cell metastasis, the ß1 integrin subunit in melanoma and breast cancer cell lines was stably knocked down with shRNA and replaced with wild-type or constitutively-active ß1. When tumor cells expressing constitutively-active ß1 integrins were injected intravenously into chick embryos or mice, they demonstrated increased colonization of the liver when compared to cells expressing wild-type ß1 integrins. Rescue expression with mutant ß1 integrins revealed that tumor cell extravasation and hepatic colonization required extracellular ligand binding to ß1 as well as ß1 interaction with talin, an intracellular mediator of integrin activation by the Rap1 GTPase. Furthermore, shRNA-mediated knock down of talin reduced hepatic colonization by tumor cells expressing wild-type ß1, but not constitutively-active ß1. Overexpression in tumor cells of the tumor suppressor, Rap1GAP, inhibited Rap1 and ß1 integrin activation as well as hepatic colonization. Using an antibody that detects activated ß1 integrin, we found higher levels of activated ß1 integrins in human metastatic melanomas compared to primary melanomas, suggesting that activated ß1 integrin may serve as a biomarker of invasive tumor cells. Altogether, these studies establish that inside-out activation of ß1 integrins promotes tumor cell extravasation and colonization, suggesting diagnostic and therapeutic approaches for targeting of ß1 integrin signaling in neoplasia.