Integrin alpha 3 beta 1 Can Function to Promote Spontaneous Metastasis and Lung Colonization of Invasive Breast Carcinoma

Significant evidence implicates alpha 3 beta 1 integrin in promoting breast cancer tumorigenesis and metastasis-associated cell behaviors in vitro and in vivo. However, the extent to which alpha 3 beta 1 is actually required for breast cancer metastasis remains to be determined. We used RNA interference to silence alpha 3 integrin expression by approximately 70% in 4T1 murine mammary carcinoma cells, a model of aggressive, metastatic breast cancer. Loss of alpha 3 integrin reduced adhesion, spreading, and proliferation on laminin isoforms, and modestly reduced the growth of orthotopically implanted cells. However, spontaneous metastasis to lung was strikingly curtailed. Experimental lung colonization after tail vein injection revealed a similar loss of metastatic capacity for the alpha 3-silenced (alpha 3si) cells, suggesting that critical, alpha 3-dependent events at the metastatic site could account for much of alpha 3 beta 1's contribution to metastasis in this model. Reexpressing alpha 3 in the alpha 3si cells reversed the loss of metastatic capacity, and silencing another target, the small GTPase RhoC, had no effect, supporting the specificity of the effect of silencing alpha 3. Parental, alpha 3si, and alpha 3-rescued cells, all secreted abundant laminin alpha 5 (LAMA5), an alpha 3 beta 1 integrin ligand, suggesting that loss of alpha 3 integrin might disrupt an autocrine loop that could function to sustain metastatic growth. Analysis of human breast cancer cases revealed reduced survival in cases where alpha 3 integrin and LAMA5 are both overexpressed.