Chemokine-mediated migration of melanoma cells towards lymphatics--a mechanism contributing to metastasis

The mechanisms that cause tumors such as melanomas to metastasize into peripheral lymphatic capillaries are poorly defined. Non-mutually-exclusive mechanisms are lymphatic endothelial cell (LEC) chemotaxis and proliferation in response to tumor cells (chemotaxis-lymphangiogenesis hypothesis) or LECs may secrete chemotactic agents that attract cancer cells (chemotactic metastasis hypothesis). Using migration assays, we found evidence supporting both hypotheses. Conditioned medium (CM) from metastatic malignant melanoma (MMM) cell lines attracted LEC migration, consistent with the lymphangiogenesis hypothesis. Conversely, CM from mixed endothelial cells or LECs, but not blood endothelial cells, attracted MMM cells but not non-metastatic melanoma cells, consistent with the chemotactic metastasis hypothesis. MMM cell lines expressed CCR7 receptors for the lymphatic chemokine CCL21 and CCL21 neutralizing antibodies prevented MMM chemotaxis in vitro. To test for chemotactic metastasis in vivo tumor cells were xenotransplanted into nude mice approximately 1 cm from an injected LEC depot. Two different MMM grew directionally towards the LECs, whereas non-metastatic melanomas did not. These observations support the hypothesis that MMM cells grow towards regions of high LEC density owing to chemotactic LEC secretions, including CCL21. This chemotactic metastasis may contribute to the close association between metastasizing tumor cells and peri-tumor lymphatic density and promote lymphatic invasion.

Vascular Endothelial Growth Factor–C (VEGF-C) Expression in Normal Human Tissues

OBJECTIVE

To characterize vascular endothelial growth factor-C (VEGF-C) protein expression in normal human tissues by immunohistochemistry (IHC). VEGF-C is a growth factor for lymphatic endothelial cells. VEGF-C mRNA and protein are expressed in a variety of cancerous tissues, but the localization of VEGF-C protein in many normal human tissues has not been clearly demonstrated to date. We therefore performed an immunohistochemical survey of the distribution of intracellular VEGF-C protein in a range of normal human tissue types.

METHODS

Five microm sections were cut from archived human tissues. Sections were dewaxed, rehydrated, and subjected to microwave pretreatment. They were incubated with VEGF-C antibody before detection with biotinylated secondary antibody using 'Elite' avidin-biotin enzyme complex and diaminobenzidine substrate. The primary antibody recognized the C-terminus of the VEGF-C propeptide that is cleaved before secretion and hence only cellular protein was detected. Negative controls used the same concentration of normal goat IgG.

RESULTS

Staining manifested as small punctate cytoplasmic granules. Strong expression was observed in large intestine epithelium, and mammary duct epithelium, skeletal and cardiac muscle, thyroid, ovary, and the prostate. Weaker expression was also detected in the hepatocytes close to the terminal hepatic venules of the liver, vascular smooth muscle, and placenta. No expression was consistently detected in spleen or thymus.

CONCLUSIONS

Intracellular VEGF-C protein is widely expressed in many normal human adult tissues. Its expression in cancer is not therefore per se indicative of a prolymphangiogenic change. To demonstrate the latter, a quantitative change in expression level is required.

Effect of iron deficiency on placental transfer of iron and expression of iron transport proteins in vivo and in vitro

Maternal iron deficiency during pregnancy induces anaemia in the developing fetus; however, the severity tends to be less than in the mother. The mechanism underlying this resistance has not been determined. We have measured placental expression of proteins involved in iron transfer in pregnant rats given diets with decreasing levels of iron and examined the effect of iron deficiency on iron transfer across BeWo cell layers, a model for placental iron transfer. Transferrin receptor expression was increased at both mRNA and protein levels. Similarly, expression of the iron-responsive element (IRE)-regulated form of the divalent metal transporter 1 (DMT1) was also increased. In contrast, the non-IRE regulated isoform showed no change in mRNA levels. Protein levels of DMT1 increased significantly. Iron efflux is thought to be mediated by the metal transporter protein, IREG1/ferroportin1/MTP1, and oxidation of Fe(II) to Fe(III) prior to incorporation into fetal transferrin is carried out by the placental copper oxidase. Expression of IREG1 was not altered by iron deficiency, whereas copper oxidase activity was increased. In BeWo cells made iron deficient by treatment with desferrioxamine ('deferioxamine'), iron accumulation from iron-transferrin increased, in parallel with increased expression of the transferrin receptor. At the same time, iron efflux also increased, showing a higher flux of iron from the apical to the basolateral side. The data show that expression of placental proteins of iron transport are up-regulated in maternal iron deficiency, resulting in an increased efficiency of iron flux and a consequent minimization of the severity of fetal anaemia.