Michael C. Archer

Cancer cells exhibit altered metabolism characterized by the generation of adenosine triphosphate by glycolysis and generation of fatty acids by de novo synthesis. The majority of genes involved in these pathways have binding sites for specificity protein (Sp) transcription factors in their promoters. Studies showing that Sp transcription factors, particularly Sp1, are involved in the regulation in cancer cells of hexokinase, pyruvate kinase, lactate dehydrogenase, fatty acid synthase, and hypoxia-inducible factor-1α are reviewed. Glycolysis and lipogenesis in cancers are also known to be stimulated by the constitutive activation of the PI3K/Akt signaling pathway. Evidence is presented for the notion that Sp transcription factors may act in concert with Akt to regulate the abnormal metabolism of cancer cells.

There are concerns regarding increased cancer incidence in patients treated with ezetimibe, an inhibitor of the absorption of dietary cholesterol. Here we tested the hypothesis that ezetimibe will accelerate mammary tumorigenesis in rats. The drug was administered at a dose of 1 ppm in an AIN‐93G diet that contained 0.3% cholesterol. This experimental diet and control diets that contained either no additions or cholesterol or ezetimibe only, were fed to groups of 30 Sprague‐Dawley rats 3 days after they were treated with 50 mg/kg methylnitrosourea (MNU). All rats were euthanized 22 weeks after MNU administration. Tumor multiplicity was significantly smaller in rats fed cholesterol than those fed no cholesterol (1.84 ± 0.42 vs. 3.86 ± 0.86 respectively, P < 0.05), but was significantly greater in the cholesterol/ezetimibe group than the group fed only cholesterol (3.48 ± 0.59 vs. 1.84 ± 0.42 respectively, P < 0.04). The average weight of tumors/rat was also significantly larger in the cholesterol/ezetimibe group than those fed cholesterol alone (5.67 ± 1.15 vs. 2.56 ± 0.71 respectively, P < 0.04). As expected, ezetimibe prevented the cholesterol raising effect of the dietary cholesterol. These results show that ezetimibe reverses the inhibitory effect of dietary cholesterol on the development of rat mammary tumors.

Cancers express high levels of fatty acid synthase (FAS) from which they derive fatty acids for membrane biosynthesis to sustain cell proliferation. How cancer cells coordinate de novo lipogenesis and proliferation has not been investigated. Transcription factors Sp1, Sp3 and Sp4 are overexpressed in a variety of cancers and regulate gene expression by interacting with GC‐rich Sp1 binding sites. Genes encoding FAS and cell cycle proteins such as CDC25A contain Sp1 binding sites in their promoters. We demonstrate by RNA interference that Sp1, Sp3 and Sp4 all play a role in regulating CDC25A expression and proliferation in human breast cancer cells. Only Sp1, however, also regulates FAS. Furthermore, mithramycin, which blocks Sp1 binding sites, decreased proliferation, inhibited CDC25A and FAS expression and reduced binding of Sp1 to the promoters of these genes as assessed by ChIP assays. Conversely, 17β‐estradiol (E ₂ ) increased proliferation and CDC25A and FAS expression along with increased binding of Sp1 to the promoters of the 2 genes. In addition, we showed that the expression of sterol regulatory element‐binding protein‐1c (SREBP‐1c), the only transcription factor that has been shown to regulate genes of lipogenic enzymes in cancer cells, is also regulated by Sp1. Finally, we demonstrated that Sp1 plays a role in sustaining proliferation and FAS expression in colon as well as prostate cancer cells. Overall, these observations suggest that Sp1 coordinately regulates de novo lipogenesis and proliferation in cancer cells.

Fatty acid synthase (FAS) is over‐expressed in many human cancers including colon. High levels of FAS expression have also been observed in a number of lesions that are precursors to invasive colorectal cancer. However, FAS expression in aberrant crypt foci (ACF), the earliest identifiable lesions in colon cancer development, has not been investigated. In this study, we treated Fisher rats with a single dose of the colon carcinogen azoxymethane then evaluated ACF 100 days later. We showed that large ACF (≥4 crypts/focus) have a significantly higher level of immunohistochemical staining for FAS than either small ACF (≤3 crypts/focus) or normal crypts. Furthermore, the severity of nuclear atypia in ACF was positively associated with increased expression of FAS. These findings suggest that the genes associated with FAS over‐expression are activated early in the stepwise development of colon cancer, though not until the ACF have reached a critical size with a level of nuclear atypia indicative of dysplasia.

Obesity, particularly visceral adiposity, is an established risk factor for colorectal cancer (CRC) and this is thought to result, at least in part, from insulin resistance and chronic hyperinsulinemia that may be mediated by adipokines. Serum levels of adiponectin, the most abundant protein secreted from adipocytes, are decreased in obesity and are inversely associated with insulin resistance and hyperinsulinemia. The objective of this study was to determine whether elevated circulating adiponectin plays a role in colon carcinogenesis using adiponectin transgenic (AdTg) mice that have 2–3-fold elevated circulating adiponectin but similar body weights as wildtype (WT) littermates used as controls. Eight-week old male and female AdTg and WT mice were treated with 4 weekly injections of the colon-specific carcinogen azoxymethane (AOM). One week following the last dose of AOM, all mice were placed on a high-fat diet and killed 24 weeks later, at 36 weeks of age, for the analysis of colon tumors. Glucose tolerance tests (GTT) were performed by injecting 2g/kg dextrose or 1.25–1.5 g/kg dextrose into all 12-week and 32–35-week-old mice respectively, and measuring blood from the tail vein 15, 30, 60 and 120 min following glucose administration. There were no significant differences in colon tumor incidence, number or size between AdTg and WT mice of either sex. AdTg mice of both sexes displayed resistance to diet-induced decreases in insulin sensitivity. Our results show that constitutively elevated levels of circulating adiponectin in AdTg mice do not confer protection against the development of colon tumors.

Obesity is associated with impaired fatty acid (FA) oxidation and increased de novo hepatic lipogenesis that may contribute to the development of hypertriglyceridemia, an important risk factor for the development of cardiovascular disease. Strategies to improve hepatocyte FA metabolism, including dietary interventions, are therefore important for the prevention of obesity‐associated co‐morbidities. Farnesol is consumed in the diet as a component of plant products. In the present study, we administered farnesol orally to rats for seven days and found significantly reduced serum triglyceride concentrations compared with controls. Potential mechanisms underlying the hypotriglyceridemic effect of farnesol were investigated using clone‐9 cultured rat hepatocytes. Farnesol significantly upregulated expression of peroxisome proliferator‐activated receptor alpha (PPARα) and the PPARα‐regulated genes fatty acyl‐CoA oxidase and carnitine palmitoyl transferase 1a, suggesting that increased hepatic FA oxidation may contribute to serum triglyceride lowering in rats. Farnesol did not change SREBP‐1c mRNA levels, but significantly down‐regulated fatty acid synthase (FAS) mRNA and protein levels and activity, indicating that attenuated lipogenesis may also contribute to hypotriglyceridemic effects of farnesol in vivo. Rescue experiments revealed that down‐regulation of FAS by farnesol was not related to activation of PPARα, but rather was caused by a 9‐cis retinoic acid mediated mechanism that involved down‐regulation of retinoid X receptor β. Diets rich in plant products are associated with a lower risk of cardiovascular disease. Our findings suggest that farnesol may contribute to this protective effect by lowering serum TG levels.

Circulating insulin‐like growth factor I (IGF‐I) is associated with increased risk of colorectal cancer. It is not clear, however, whether IGF‐1 plays a direct causative role in colon carcinogenesis or whether it mediates the known promoting effects of insulin. The objective of this study was to determine the role of IGF‐1 in colon carcinogenesis using liver‐specific IGF‐I deficient (LID) mice that exhibit 70% reductions in circulating IGF‐I. Female and male LID mice were treated with the colon‐specific carcinogen azoxymethane to induce aberrant crypt foci (ACF) or colon tumors. Female LID mice developed significantly fewer ACF and had normal insulin levels compared to controls. Male LID mice, however, were hyperinsulinemic and exhibited no significant differences in ACF development compared to controls. In the tumor study, both male and female LID mice were hyperinsulinemic and had no significant differences in tumor incidence or multiplicity compared to their respective controls. There was a significant 25% reduction in tumor size, however, in both male and female LID mice compared to controls. These data suggest that IGF‐I deficiency attenuates the promoting effect of insulin on colon carcinogenesis and that IGF‐I is an independent promoter of the growth of established tumors. Our findings implicate both IGF‐I and insulin as important promoters of colon cancer development.