Leptin upregulates telomerase activity and transcription of human telomerase reverse transcriptase in MCF-7 breast cancer cells
Introduction
Obesity is a risk factor in breast cancer [1]. Obesity breast cancer patients face a higher risk for lymph node metastasis, larger tumor burden, and mortality compared with those non-obese patients. At present, the mechanism of obesity-related breast cancer remains to be defined. Previous studies have shown that overweight is associated with overexpression of many hormones including leptin [2]. Leptin is an adipose-secreted hormone. It functions through binding to its specific receptors. Evidence has accumulated that leptin is a link between obesity and higher incidence of breast cancer. Leptin induces the growth of breast cancer through multiple mechanisms, including regulating the cell cycle, apoptosis, and the extracellular environment [3]. For instance, leptin enhances the expression of cell cycle regulators (Cyclin D, Cyclin G, and cdk2) and anti-apoptotic protein (Bcl-2 and survivin).
Telomerase is a ribonucleoprotein complex composed of the catalytic subunit human telomerase reverse transcriptase (hTERT) and RNA subunit human telomerase RNA [4]. Overactivity of telomerase occurs in about 90% cancer cells whereas normal cells exhibit undetectable level of activity [5]. Activation of telomerase is required for cancer cells to maintain their malignant phenotype and therefore provides a potential target for the treatment of cancer. Importantly, hTERT expression is specific to cancer cells and tightly associated with telomerase activity [6]. Therefore, numerous studies have focused on the cancer-specific regulation of hTERT and its application of tumor diagnosis and treatment. hTERT promoter has numerous transcription factor binding sites such as c-Myc, SP1, NF-kB, MZF2, AP1. Recently, signal transducer and activator of transcription 3 (STAT3) has been identified as a critical regulator of hTERT [7]. STAT proteins are critical mediators of cytokine signaling in cancer cells [8]. After phosphorylation, STAT3 is activated and translocated to the nucleus to regulate target gene. Furthermore, previous studies have shown that leptin signaling is mainly mediated by the Janus kinase 2 (JAK2)/STAT3 pathway [9], [10]. Based on these studies, we postulate that leptin may upregulate the expression of hTERT.
In this study, we aimed (1) to evaluate the effect of leptin on the regulation of telomerase activity in MCF-7 cells, (2) to evaluate the expression of hTERT in response to leptin (3) to elicit the role of STAT3 in leptin-induced expression of hTERT. Above all, we try to understand the mechanism of leptin-induced proliferation in MCF-7 breast cancer cells and provide a new explanation for obesity-related breast cancer.
Section snippets
Materials and methods
Cell culture, reagents and treatments. Human MCF-7 breast cancer cells were obtained from Tianjin Medical University Cancer Institute and Hospital (Tianjin, China) and grown in Dulbecco’s modified Eagle’s medium supplemented with 10% fetal bovine serum (FBS), l-glutamine (2 mM) at 37 °C, under 5% CO2 atmosphere. For treatment, cells were plated at a density of 3 × 105 cells/well in six-well plates containing complete medium and then serum-starved overnight. Subsequently, cells were treated with
Leptin activates telomerase
Telomerase activity was detected in MCF-7 breast cancer cells after treatment with different concentrations of leptin (10, 80, 160 ng/ml) for 24 h. As shown in Fig. 1A, leptin upregulated telomerase activity in a dose-dependent manner. Telomerase activity was increased 1.7-fold (p < 0.01) by 10 ng/ml of leptin and 2.8-fold (p < 0.01) by 160 ng/ml of leptin. For Blocking experiments, a leptin receptor monoclonal antibody, ZMC-2 (kindly gifts from Dr. Zida Wu, University Medicine Berlin) was used to
Discussion
It is generally believed that obesity is a reverse prognostic factor in breast cancer. One compelling study evaluated 495,477 US women over a 16-year period, and demonstrated that the death rate of women from breast cancer in the highest quantile of body mass index (BMI) is 2-fold as high as that in the lowest BMI [12]. Obesity is considered as an endocrine disorder exerting the biological effects through cytokines released by adipocytes. Among these cytokines, leptin has been characterized as
Acknowledgments
This work was supported by the grants from National Natural Science Foundation of China (30701013, 30900596, 30973490, 30901448) and from China Postdoctoral Science Foundation (20090450773).
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These authors contributed equally to this work.