Dysregulation of glucose homeostasis (metabolic syndrome and diabetes) and fatty acid metabolism (obesity, particularly of the upper body) have reached epic proportions worldwide, increasing the risk of both cardiovascular disease and cancer. These alterations promote breast cancer in two major risk subgroups, post‐menopausal women (who develop ER+ breast cancers (1)) and premenopausal women of color (who are more likely to present with advanced stage disease, have a poorer prognosis and develop the ‘basal’ subtype of cancer (2)). Because obesity and type II diabetes are potentially modifiable risk factors for breast cancer incidence and outcome, further understanding of these processes should provide insights that can be used to devise interventional or pharmacologic strategies. Reported mechanisms by which these chronic diseases induce breast cancer are diverse, including but not limited to: upregulation of the insulin/insulin‐like growth factor (IGF) system (3), shifts in cytokines and adipokines, which act as endocrine, paracrine and autocrine factors (4), alterations of energy sensing mechanisms involving the AMP‐activated protein kinases (AMPK (5)), metabolic reprogramming of stem and progenitor cells involving “aerobic glycolysis, de novo lipid biosynthesis and glutamine‐dependent anaplerosis” (6, 7), upregulation of estrogen and androgen associated signaling (particularly in peri‐ and post menopausal women), and alterations of molecular/genetic factors which induce shifts in cellular (lipid and glucose) metabolism (8).
Hormonal factors, especially estrogen and progesterone (with or without binding to cognate receptors) promote breast cancer and may act through induction of cellular proliferation or effects on stem cell growth (9). Insulin and insulin‐like growth factors (which are typically secreted at levels higher than insulin) are typically upregulated early on in type II diabetes and in patients with metabolic syndrome and obesity. Receptors for these important growth hormones are expressed ubiquitously in human cells, and frequently upregulated in breast cancers of all molecular subtypes. Insulin and IGF‐1 have been shown to activate tyrosine kinase growth receptor pathways, upregulate the insulin regulated substrate (IRS1), activate the Ras‐Raf‐mitogen‐activated protein (MAP) kinase cascade and phosphoinositol‐3 kinase/Akt (PI3K‐Akt) signaling (3).
Cancerous cells have long been known to have higher rates of glucose uptake (from the systemic and localized microenvironment) and an increase in aerobic glycolysis, resulting in local production of lactate and ATP (known as the Warburg effect). In 1956, Oto Warburg postulated that alterations of glucose metabolism may represent the “origin of cancer” (10). He later compared the increased glycolytic activity of cancer cells to that of early embryonic cells, setting the stage for modern theories of cancer stem cells (11). Ironically, tissue culture media typically contains high levels of glucose (∼17mM); as compared to physiological (5mM), metabolic syndrome (7mM), and diabetic (10mM) concentrations. This means that our culture systems drive aerobic glycolysis and may select subpopulations that thrive under these conditions. This “metabolic reprogramming” enables cancer cell growth and metastasis, through several critical mechanisms: 1) aerobic glycolysis enables energy production and survival under hypoxic conditions; 2) the production of lactate facilitates tumor invasion and suppresses anti‐cancer immunity; 3) the production of NADPH enhances anti‐oxidant defenses against chemotherapy; and 4) glycolytic pathway intermediates promote cell proliferation providing a growth advantage.
Epidemiological data indicates that diabetic and pre‐diabetic women treated with metformin (but not other anti‐diabetic agents) have a lower incidence of breast cancer, whereas diabetic patients treated with other agents do not (12). In a recent study of breast cancer patients administered neoadjuvant chemo‐therapy, diabetic patients who also received metformin had the highest pathologic complete response rate (pCR 24%), as compared to non‐diabetic patients (pCR 16%) and diabetic patients treated with agents other than metformin (pCR 8%) (13).
We have investigated the biological and molecular effects of hyperglycemia, with or without the addition of metformin, insulin or leptin, using cell lines representing all molecular subtypes of breast cancer. The anti‐diabetic drug metformin induces growth inhibition (S phase arrest) and apoptosis of basal (ER−, PR−, HER2−, EGFR+, CK5+) breast cancer cell lines in vivo and in vitro (14). In contrast, metformin is less effective in growth inhibition (it induces a partial G1 arrest at higher doses) and does not induce apoptosis in other breast cancer subtypes (e.g. luminal A (ER+, PR+, HER2−); luminal B (ER+, PR+, HER2+) or HER2 (ER−, PR, HER2+) (15). Luminal A and basal breast cancer cell lines showed the most proliferation in response to supraphysiologic glucose, compared to luminal B and HER2 lines which were not differentially responsive. Metformin abrogated glucose induced mitogenesis across all glucose concentrations, with the most in basal cell lines derived from African American patients. Metformin was less effective in basal cells derived from Caucasians and in luminal A cells. In basal cells, glucose increased EGFR, pEGFR, IGF1R, pIGF1R, AKT and pAKT, IRS2, the cyclins D1, E an A and decreased AMPK, pAMPK, pMAPK in a dose dependent manner. The addition of metformin to glucose containing media blocked the aforementioned changes in EGFR, pEGFR, IGF1R, pIGF1R, AKT, MAPK, pMAPK, AMPK, pAMPK and Cyclin D1. Metformin had a variable effect on the other molecular breast cancer subtypes grown under high glucose conditions. All basal breast cancer cell lines were uniformly resistant to the mitogenic effects of leptin and insulin (despite expressing the cognate receptors), across a wide range of glucose concentrations. In contrast, both leptin and insulin promoted the growth of luminal A and to a lesser extent luminal B and HER2 cancer cells. Metformin partially abrogated the growth induced by insulin and leptin.
In conclusion, basal breast cancer cell lines showed the most significant mitogenesis in response to glucose, whereas they were uniformly resistant to leptin and insulin. This effect of both glucose and metformin were most pronounced in lines derived from minority patients. Luminal A type cell lines also showed marked growth induction by glucose, but unlike basal cells they also showed growth induction with leptin and insulin. These data are consistent with the molecular subtype and ethnicity associated variables linking obesity and diabetes to breast cancer risk and outcomes‐because the molecular subtypes of luminal A and basal are increased in women with these metabolic defects.
Citation Information: Cancer Prev Res 2010;3(1 Suppl):CN13-04.