Abstract. Obesity is a significant risk factor for post-menopausal women to develop and die from breast cancer. Leptin, an adipokine is produced in high levels in obese individuals, and its receptor is overexpressed in breast tumors and lymph node metastases. Previously, we demonstrated that leptin stimulates breast cancer cell invasion, which is correlated with breast cancer metastasis. Programmed cell death 4 (PDCD4) has been shown to block cancer cell invasion. However, whether PDCD4 blocks leptin-induced breast cancer cell invasion is not known. Here, we report the novel findings that leptin failed to induce invasion in MCF-7 breast cancer cells overexpressing PDCD4 (MCF-7/PDCD4). Tissue inhibitor of metalloproteinase-2 (TIMP-2) was essential to the anti-invasive effect of PDCD4, as leptin stimulated the invasion of MCF-7/ PDCD4 cells pretreated with TIMP-2 siRNA. Furthermore, TIMP-2 knockdown allowed leptin to augment phosphorylation of extracellular signal-regulated kinases 1,2 and signal transducer and activator of transcription 3, but not that of Jun N-terminal kinases. These data indicate that PDCD4 utilizes TIMP-2 to exert its anti-invasive effect by suppressing leptininduced activation of extracellular signal-regulated kinases 1,2 and signal transducer and activator of transcription 3. Novel therapeutic strategies aiming at enhancing PDCD4 expression in breast tumors may be able to stop obesity-related breast tumor progression and prolong the life of patients.
IntroductionBreast cancer is the most frequently diagnosed cancer in women. In 2010, about 207,090 new cases of invasive breast cancer were expected to be diagnosed among women, and about 39,840 women were expected to die from breast cancer. One of the major risk factors for postmenopausal women to develop breast cancer is obesity. Tumors from breast cancer patients with high body mass index are typically of the hormoneresponsive type (1-3). Leptin, a 16-kDa hormone that was discovered as a regulator of body weight and energy balance acting in the hypothalamus (4), is an important hormonal link between obesity and breast cancer. As body weight and fat mass increase, the level of circulating leptin increases (5,6).High expression of leptin and the long/signaling form of leptin receptor have been found in breast tumors (7-11). Leptin-deficient mice have decreased incidence of oncogeneinduced mammary tumors (12). An inhibitor targeted to the leptin receptor suppresses breast tumor formation in vivo (13). High levels of leptin in breast tumors have been correlated with poorer patient prognosis and increased incidence of metastasis (7,10,11). Breast cancer metastasis is directly associated with breast cancer cell invasion. Recently, we showed that leptin stimulates invasion of MCF-7 breast cancer cells by activating Jun N-terminal kinases (JNK), which increase matrix metalloproteinase-2 activity (14).Leptin has also been associated with the development of anti-estrogen resistance. Leptin is a potent modulator of the estrogen receptor signaling pathwa...