Experimental study on mechanism and rarity of metastases in skeletal muscle.

OBJECTIVE

To investigate the reasons for the rarity of metastases in skeletal muscle.

METHODS

By injecting tumor cells (Walker256 rat carcinosarcoma) through the iliac artery (experimental group) and the tail vein (control group), animal models of blood-borne metastases were established. The quadriceps femoris muscle and lungs were observed grossly and microscopically. Immunohistochemistry was applied to investigate the expression of vascular cell adhesion molecule-1 (VCAM-1) in the microvascular endothelium of these organs. Primary culture of rat skeletal muscle cells was established and conditioned medium (MCM) was collected. Effects of MCM on several tumor cell lines and the biochemical characteristics of skeletal muscle delivered tumor factor(s) were tested by MTT assay. Apoptosis and morphological examination were carried out to investigate the antitumor mechanisms of MCM.

RESULTS

In the experimental group, there were no definite metastases observed in muscle cells. In the control group, lung metastases were present in the lungs of all rats that were sacrificed at the 14th day or died spontaneously (17 rats in all). There was no significant difference between the increase in VCAM-1 in quadriceps femoris muscle 7 days after iliac artery injection and that in lungs 7 days after tail vein injection (P > 0.05). In vitro studies showed that the proliferation of tumor cell lines of mouse SP2/0 myeloma, rat Walker256 carcinosarcoma or human chronic granulocytic leukemia K562, human acute lymphatic leukemia HL-60, LS-174-T colon adenocarcinoma, PC3-M prostatic carcinoma and lung giant cell carcinoma with different metastatic potency (PLA801-C with low metastatic potency, PLA801-D with high metastatic potency) was significantly inhibited when cultured with MCM (P < 0.01 - 0.05). Proliferation of malignant cells showed a dose-dependent decrease, to a certain degree. Proliferation of normal rabbit joint epiphysial disk cells (RGP-2) were not affected by MCM. Proliferation of lung giant cell carcinoma cells with high metastatic potency showed a significant decrease even when cultured in highly diluted MCM (6.25% of primary MCM), when compared with the strain of low metastatic potency. Following ultrafiltration, boiling at 100 degrees C, and treatment with trypsin, skeletal muscle delivered tumor factor(s) were found to be a low molecular weight (MW <or= 10.0 KDa) component which was trypsin resistant but not heat resistant. The factor(s) did not induce apoptosis in K562 cells but caused direct destruction of the cytoplasmic membrane.

CONCLUSIONS

The rarity of metastases in skeletal muscles, generally accepted in the clinical setting, can be reproduced in an animal model. It does not seem to be related to VCAM-1 expression in the microvessels of these organs. Skeletal muscle delivered factor(s) play a key role in the mechanism of the rarity of metastases in skeletal muscle.