To gain insight into the mechanism(s) by which leptin contributes to mammary tumor (MT) development we investigated the effects of leptin, kinase inhibitors, and/or leptin receptor antagonists (LPrA2) on 4T1 mouse mammary cancer cells in vitro and LPrA2 on 4T1-MT development in vivo. Leptin increases the expression of vascular endothelial growth factor (VEGF), its receptor (VEGF-R2), and cyclin D1 through phosphoinositide 3-kinase, Janus kinase 2/signal transducer and activator of transcription

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... 3, and/or extracellular signal-activated kinase 1/2 signaling pathways. In contrast to leptin-induced levels of cyclin D1 the changes in VEGF or VEGF-R2 were more dependent on specific signaling pathways. Incubation of 4T1 cells with anti-VEGF-R2 antibody increased leptin-mediated VEGF expression suggesting an autocrine/paracrine loop. Pretreatment of syngeneic mice with LPrA2 prior to inoculation with 4T1 cells delayed the development and slowed the growth of MT (up to 90%) compared with controls. Serum VEGF levels and VEGF/VEGF-R2 expression in MT were significantly lower in mice treated with LPrA2. Interestingly, LPrA2-induced effects were more pronounced in vivo than in vitro suggesting paracrine actions in stromal, endothelial, and/or inflammatory cells that may impact the growth of MT. Although all the mechanism(s) by which leptin contributes to tumor development are unknown, it appears leptin stimulates an increase in cell numbers, and the expression of VEGF/VEGF-R2. Together, these results provide further evidence suggesting leptin is a MT growthpromoting factor. The inhibition of leptin signaling could serve as a potential adjuvant therapy for treatment of breast cancer and/or provide a new target for the designing strategies to prevent MT development. Elevated serum leptin levels are often correlated with obesity and associated with the development of breast cancer in postmenopausal women (1-4) reinforcing a long recognized causal link between obesity and increased risk for some types of cancer. The levels of leptin and its receptor (OB-R) 3 are correlated with distant metastasis, reoccurrence, and survival outcome. Furthermore, expression of leptin and OB-R levels are increased within human breast cancer tissue (2, 5). Similar to obese women, mice and rats that have high leptin levels are more likely to develop mammary tumors (6, 7). Leptin-deficient mice (ob Ϫ /ob Ϫ ) or mice lacking a functional OB-R (db Ϫ /db Ϫ ) do not develop mammary tumors (MT). Moreover, genetically engineered mouse models of mammary neoplasia exhibit a decreased propensity to develop MT when they are crossed with leptin/OB-R-deficient mice (8, 9) . The sequence of leptin is highly conserved among species. However, OB-R has several isoforms with diverse signaling capabilities. These OB-R isoforms have the same extracellular sequence but differ in the length of the cytoplasmic tail. It is believed these variants are derived by alternate splicing (10). The full-length isoform (OB-RL, cytoplasmic tail, 303 amino acid residues) is mainly expressed in the hypothalamus. A short isoform (OB-Ra, cytoplasmic tail, 34 amino acids residues) is found in higher concentrations in peripheral tissues (11). In endometrial cancer cells an increase of the relative concentrations of OB-Ra to OB-RL has been reported (5). However, the exact implication of these findings in leptin-mediated cancer events is unknown. OB-R belongs to the superfamily of cytokine receptor I characterized by a lack of autophosphorylation capabilities (11). Upon leptin binding, OB-R homodimerizes and signals through phosphorylation of Janus kinase 2 (JAK2) and signal transducer