Impact of the anti-diabetic drug metformin on tumor growth in vivo.

摘要

The prevalence of obesity is rapidly increasing in affluent countries and in many urban areas of the developing world. Epidemiologic studies have associated obesity with increased burden of many cancer types but the mechanisms by which obesity induces transformation or promotes neoplastic growth remain to be fully elucidated. Possible mediators include insulin, free fatty acids, increased bio-availability of steroid hormones and inflammation. It is important to gain better understanding of the relationship between these diseases as new insights may provide new opportunities for cancer prevention and treatment.;We wished to expand these studies by using an in vivo model of cancer and diet-induced hyperinsulinemia in order to determine if metformin has anti-neoplastic action and if so, if the indirect (insulin lowering) systemic action of metformin or the direct AMPK mediated effect on neoplastic cells is responsible. We used mouse models of diet induced hyperinsulinemia by providing a high energy/high fat diet and a control diet ad lib in order to induce the desired metabolic phenotypes.;As described in Chapter II, our results reveal that metformin attenuated the stimulatory effect of the high energy diet on growth of LLC1 carcinoma in vivo while having no effect on tumor growth in mice consuming a control diet. This suggested that the indirect, insulin lowering, effects of metformin played an important role in the attenuation of tumor burden, and these effects might be independent of AMPK activation in neoplastic cells.;In Chapter III we show that the effects of diet and metformin on tumor growth described in Chapter II are reproducible in another cell line, MC38 colon carcinoma. In addition to the observed effects of metformin on tumor growth, we report that metformin reduced the cleavage of SREBP-1 and the expression of fatty acid synthase in MC38 colon carcinoma.;Metformin, a biguanide, is a drug often prescribed for treatment of type II diabetes. Recent retrospective epidemiologic data comparing diabetics taking metformin to diabetics taking other therapies suggests that metformin may reduce the risk of developing cancer or the risk of dying from cancer. These data contribute to the rationale for research to study the physiologic links between cancer and diabetes. Preliminary laboratory work has shown that metformin is an indirect activator of AMPK via its inhibitory action on oxidative phosphorylation in the mitochondria. AMPK is a sensor of cellular energy supply and activation of this serine/threonine kinase leads to inhibition of gluconeogenesis in the liver and reduced cell proliferation in transformed cells.;The results presented in Chapters II and III did not separate the 'direct' from the 'indirect' effects of metformin on the attenuation of tumor growth. We address this problem in Chapter IV where we present data from an experiment that allowed us to study these effects independently by using cancer cell lines engineered to be insensitive to the 'direct' AMPK-mediated effects of metformin, grown in mice that were sensitive to metformin. We used shRNA to the decrease expression of LKB1 in two cancer cell lines and observed that these cells were resistant to metformin in vitro but were sensitive to metformin in vivo when grown in animals on either a high fat or control diet. Further analysis revealed that the loss of LKB1, a known tumor suppressor and activator of AMPK, may sensitize transformed cells to metformin under conditions of energy stress.