Leptin Enhances Melanoma Tumor Growth by Increasing Endothelial Progenitor Cells

Epidemiological studies propose that obesity increases the risk of several cancers, including melanoma. Leptin is a peptide predominantly produced by adipocytes. Obesity increases the expression of leptin. On the other hand, several recent experiments have suggested that tumor growth to be dependent on endothelial progenitor cells (EPCs) which are effective in generation of new blood vessels. Our objectives in the present study were to examine the effects of leptin on melanoma growth and circulating EPCs number. Melanoma tumors were induced by subcutaneous injections of 2 × 106 B16F10 melanoma cells to 32 C57BL6 mice. The mice were randomly divided into 4 groups of 8 on the 8th day. The first two groups received intraperitoneal (IP) injections of either phosphate-buffered saline (PBS) or recombinant murine leptin (1 μg/g initial body weight) twice daily. The other two groups received IP injections of either 9F8 (an anti leptin receptor antibody) or the control mouse IgG at 50 μg/mouse every 3 consecutive days. By the end of the second week, the animals were euthanized and blood samples were saved for computation with flow cytometry. The tumors were also analyzed. The EPC numbers in leptin, PBS, 9F8, and IgG groups were 222.66 ± 36.5, 133.33 ± 171, 23.33 ± 18, 132.66 ± 27.26 per ml of blood, respectively. Moreover, the corresponding values for tumor weights were 3.2 ± 0.6, 1.7 ± 0.3, 1.61 ± 0.2, 1.7 ± 0.3 g. Tumors weight and size, and circulating EPC numbers were significantly more in the leptin group than 9F8 and both control groups (P < 0.05). In conclusion, our observations indicated that leptin caused melanoma growth likely through increased circulating EPC numbers and consequently vasculogenesis.