Mechanical entrapment is insufficient and intercellular adhesion is essential for metastatic cell arrest in distant organs

Abstract

In this report, we challenge a common perception that tumor embolism is a size-limited event of mechanical arrest, occurring in the first capillary bed encountered by blood-borne metastatic cells. We tested the hypothesis that mechanical entrapment alone, in the absence of tumor cell adhesion to blood vessel walls, is not sufficient for metastatic cell arrest in target organ microvasculature. The in vivo metastatic deposit formation assay was used to assess the number and location of fluorescently labeled tumor cells lodged in selected organs and tissues following intravenous inoculation. We report that a significant fraction of breast and prostate cancer cells escapes arrest in a lung capillary bed and lodges successfully in other organs and tissues. Monoclonal antibodies and carbohydrate-based compounds (anti-Thomsen-Friedenreich antigen antibody, anti-galectin-3 antibody, modified citrus pectin, and lactulosyl-l-leucine), targeting specifically beta-galactoside-mediated tumor-endothelial cell adhesive interactions, inhibited by >90% the in vivo formation of breast and prostate carcinoma metastatic deposits in mouse lung and bones. Our results indicate that metastatic cell arrest in target organ microvessels is not a consequence of mechanical trapping, but is supported predominantly by intercellular adhesive interactions mediated by cancer-associated Thomsen-Friedenreich glycoantigen and beta-galactoside-binding lectin galectin-3. Efficient blocking of beta-galactoside-mediated adhesion precludes malignant cell lodging in target organs.

Figure 1

A significant fraction of intravenously injected cancer cells escapes mechanical entrapment in the first capillary bed encountered (lung) and reaches other organs and tissues. Metastatic deposits of fluorescently labeled cancer cells (DU-145 human prostate carcinoma shown) formed in vivo 3 hours postinjection of 1 x 10⁶ cells in the lungs (A), vertebrae (B), sternum (C), kidney (D), and thyroid gland (E). Scale bar, 500 µm. (F) Metastatic deposit counts in lungs and bones (vertebrae). Bars, mean ± SD.

Figure 2

The effect of anti-TF antigen and anti-galectin-3 antibodies on metastatic deposit formation in the lungs and bones in vivo. Both anti-TF antigen and anti-galectin-3 function-blocking monoclonal antibodies dramatically inhibit metastatic deposit formation in mouse lungs (top panel) and bones (bottom panel) in vivo. Scale bar, 500 µm.

Figure 3

The effect of various function-blocking monoclonal antibodies and carbohydrate-based compounds on subpleural metastatic deposit formation of DU-145 human prostate carcinoma and MDA-MB-435 human breast carcinoma cells. Anti-TF antigen, anti-galectin-3, lactulosyl-l-leucine, and MCP inhibit >90% subpleural metastatic deposit formation of prostate (closed bars) and breast (open bars) cancer cells in vivo, whereas all three of antiselectin antibodies tested fail to affect this process significantly. Bars, mean ± SD.