Pharmacologic inhibition of fibronectin accumulation suppresses bone metastasis growth

Introduction:

Fibronectin is an extracellular matrix protein that is expressed in almost all types of cancers and plays a crucial role in tumor growth promotion, survival and resistance to therapy. We have shown that deleting fibronectin genetically in mice leads to smaller bone metastasis. Furthermore, higher fibronectin expression in breast and prostate cancer lesions shortens survival in affected patients (von Au et al. Neoplasia). Under special circumstances fibronectin suppresses the ability of myeloid cells to fight cancer in vivo in mice (Rossnagl et al. PLoS Biology). In a model of liver fibrosis we had successfully used a peptide that prevents fibronectin fibril formation and hence matrix accumulation (Altrock et al. J Hepatology). We therefore hypothesized that pharmacologically decreasing fibronectin accumulation in cancer will not only slow down cancer progression by suppressing angiogenesis but may additionally diminish cancer size by stimulating the immune response.

Results:

To test this we first deleted fibronectin (knockdown: Kd) in human breast cancer cells (MDA-MB-231) and found a decrease in cancer growth in mice (Conrtrol (CT): 7 ± 1 vs. Kd: 1 ± 0.3 (x106 RLU); p < 0.0001). This was associated with an increase in macrophages (CT: 0.8 ± 0.2 vs. Kd: 1.5 ± 0.3%, n = 16/11, p < 0.05) in tumors. We therefore used the peptide that prevents fibronectin accumulation in cultures of breast cancer cells and found a decrease in fibronectin accumulation in the matrix when the peptide was added over three days as shown by ELISA (CT: 31 ± 6 vs: pUR4: 19 ± 12 ng/mg; p < 0.05; n = 22/17). We therefore tested the effect of the peptide in mice in which bone lesions of breast cancer were induced. Administration of this peptide for 10 days slowed down growth of the tumors resulting in tumors half the size of control lesions (CT: 7 ± 1 × 106 RLU vs. pUR4: 4 ± 1 × 106 RLU, n = 22/19, p < 0.05). Suppressing fibronectin accumulation also led to an increase in macrophages (CT: 1.0 ± 0.1% vs. pUR4: 1.8 ± 0.4%, p < 0.05) similar to that seen in MDA-Kd tumors compared to control tumors. Macrophages from tumors of peptide-treated mice were isolated and found to express lower levels of arginase mRNA (> 10-fold, p < 0.05). This suggests that this inhibitor exerts an immune regulatory function that ultimately suppresses tumor growth.

Discussion:

Taken together, these data suggest that inhibition of fibronectin accumulation diminishes cancer growth through stimulation of the macrophage mediated immune response. Thus, matrix manipulation offers new possibilities in controlling cancer growth.