33 Our previous studies have shown the direct involvement of Stat3 and Akt signaling in procancerous actions of leptin.8, 11 In this study we show that adiponectin effectively inhibits the oncogenic functions of leptin such as proliferation, cell migration, and invasion. We sought to determine the underlying molecular mechanism by which adiponectin antagonizes the oncogenic actions of leptin. We found that leptin

increased phosphorylation of Stat3 and Akt in comparison to untreated HCC cells, whereas combined treatment with adiponectin significantly reduced leptin-induced Stat3 and Akt phosphorylation (Fig. 4). We previously demonstrated that activation of Stat is upstream of the activation of Akt.11 Activation of these critical downstream effectors

is interdependent such that leptin signaling can be inhibited Dabrafenib mw by up-regulation of an upstream inhibitory molecules, suppressor of cytokine signaling 3 (SOCS3).27 Overexpression of SOCS3 inhibits leptin-mediated tyrosine phosphorylation of JAK2 and subsequently Stat3 activation,27, 34, 35 which can in turn inhibit Akt activation. Thus, we examined whether adiponectin can up-regulate SOCS3 expression. Indeed, adiponectin treatment increased SOCS3 expression GSI-IX concentration in HCC cells (Fig. 5). These results collectively show that adiponectin inhibits components of the signaling machinery used by leptin in addition to up-regulating an important upstream inhibitor. We investigated the physiological relevance of our in vitro findings by evaluating

suppressing effects of adiponectin on leptin-induced development of HCC in vivo. Leptin treatment significantly increased tumor growth as compared to the saline-treated MCE group. Adiponectin treatment (Ad-Adn) inhibited tumor growth, resulting in reduced tumor size compared to saline and adenovirus-luciferase control. Importantly, adiponectin treatment efficiently inhibited leptin-induced tumor growth (Fig. 6A,B). Adiponectin adenovirus-treated tumors showed elevated levels of adiponectin, whereas leptin-treated tumors showed increased staining for leptin as compared to controls. The immunohistochemical assessment of tumor proliferation showed higher MIB1 and PPH3 expression in the leptin-treated group, whereas little if any MIB1 and PPH3 expression was observed in the adiponectin-treated group (Fig. 6C). We further confirmed our in vitro findings regarding important signaling molecules using tumor samples from various treatment groups. Leptin-treated tumors revealed elevated p-Stat3 levels in comparison to saline-treated controls. Adiponectin treatment, on the other hand, inhibited leptin-induced p-Stat3 levels in combined-treatment tumor groups. Adiponectin-treated tumors and leptin-adiponectin combination-treated tumors showed elevated SOCS3 levels (Fig. 6D). Analysis of signaling molecules in tumor samples provided the critical molecular link between p-Stat3 and SOCS3 in leptin-adiponectin crosstalk.