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Increased aerobic glycolysis and de novo lipid biosynthesis are common characteristics of invasive cancers. UDP-glucuronosyltransferases (UGTs) are phase II drug metabolizing enzymes that in normal cells possess the ability to glucuronidate these lipids and speed their excretion; however, de-regulation of these enzymes in cancer cells can lead to an accumulation of bioactive lipids, which further fuels cancer progression. We hypothesize that UGT2B isoform expression is down-regulated in cancer cells and that exogenous re-introduction of these enzymes will reduce lipid content, change the cellular phenotype, and inhibit cancer cell proliferation. In this study, steady-state mRNA levels of UGT isoforms from the 2B family were measured using qPCR in 4 breast cancer and 5 pancreatic cancer cell lines. Expression plasmids for UGT2B isoforms known to glucuronidate cellular lipids, UGT2B4, 2B7, and 2B15 were transfected into MCF-7 and Panc-1 cells, and the cytotoxic effects of these enzymes were analyzed using trypan blue exclusion, annexin V/PI staining, TUNEL assays, and caspase-3 immunohistochemistry. There was a significant decrease in cell proliferation and a significant increase in the number of dead cells after transfection with each of the 3 UGT isoforms in both cell lines. Cellular lipids were also found to be significantly decreased after transfection. The results presented here support our hypothesis and emphasize the important role UGTs can play in cellular proliferation and lipid homeostasis. Evaluating the effect of UGT expression on the lipid levels in cancer cell lines can be relevant to understanding the complex regulation of cancer cells, identifying the roles of UGTs as “lipid-controllers” in cellular homeostasis, and illustrating their suitability as targets for future clinical therapy development.

Tamoxifen (Tam) is a selective estrogen receptor (ER) modulator used to inhibit breast tumor growth. Tam can be directly N-glucuronidated via the tertiary amine group or O-glucuronidated after cytochrome P450 mediated hydroxylation. In this study, the glucuronidation of Tam and its hydroxylated and/or chlorinated derivatives [4OHTamoxifen (4OHTam), Toremifene (Tor), and 4OHTor] was examined using recombinant human UGTs from the 1A subfamily and human hepatic microsomes. Recombinant UGT1A4 catalyzed the formation of N-glucuronides of Tam and its derivatives and was the most active UGT enzyme toward these compounds. Therefore, it was hypothesized that single nucleotide polymorphisms (SNPs) in the promoter region of UGT1A4 have the ability to significantly decrease the glucuronidation rates of Tam metabolites in the human liver. In vitro activity of 64 genotyped human liver microsomes were used to determine the association between the UGT1A4 promoter and coding region SNPs and the glucuronidation rates of Tam, 4OHTam, Tor, and 4OHTor. Significant decreases in enzymatic activity were observed in microsomes for individuals heterozygous for -163G/A and -217T/G. These alterations in glucuronidation may lead to prolonged circulating half-lives and potentially modify the effectiveness of these drugs in the treatment of breast cancer.

Tamoxifen (Tam) is classified as a selective estrogen receptor modulator (SERM) and is used for treatment of patients with ER-positive breast cancer. However, it has been shown that Tam and its cytochrome P450-generated metabolite 4-hydroxy-Tam (4OH-Tam) also exhibit cytotoxic effects in ER-negative breast cancer cells. These observations suggest that Tam and 4OH-Tam can produce cytotoxicity via estrogen receptor (ER)-independent mechanism(s) of action. The molecular targets responsible for the ER-independent effects of Tam and its derivatives are poorly understood. Interestingly, similar to Tam and 4OH-Tam, cannabinoids have also been shown to exhibit anti-proliferative and apoptotic effects in ER-negative breast cancer cells, and estrogen can regulate expression levels of cannabinoid receptors (CBRs). Therefore, this study investigated whether CBRs might serve as novel molecular targets for Tam and 4OH-Tam. We report that both compounds bind to CB1 and CB2Rs with moderate affinity (0.9-3 μM). Furthermore, Tam and 4OH-Tam exhibit inverse activity at CB1 and CB2Rs in membrane preparations, reducing basal G-protein activity. Tam and 4OH-Tam also act as CB1/CB2R-inverse agonists to regulate the downstream intracellular effector adenylyl cyclase in intact cells, producing concentration-dependent increases in intracellular cAMP. These results suggest that CBRs are molecular targets for Tam and 4OH-Tam and may contribute to the ER-independent cytotoxic effects reported for these drugs. Importantly, these findings also indicate that Tam and 4OH-Tam might be used as structural scaffolds for development of novel, efficacious, non-toxic cancer drugs acting via CB1 and/or CB2Rs.

Drosophila melanogaster larvae require choline for growth but when fed dl-carnitine in place of choline, carnitine is decarboxylated to β-methylcholine and phosphatidyl-β-methylcholine is synthesized to replace phosphatidylcholine. Feeding (−)-carnitine in place of choline results in a moderate deficiency of lecithin-type phospholipid and a corresponding reduction in growth. The capacity of larvae to employ (+)-carnitine for phosphatidyl-β-methylcholine synthesis and subsequently for growth is very limited. Only 41% as much phosphatidyl-β-methylcholine is synthesized by larvae fed 5.7 X 10−4 M (+)-carnitine as is formed when the same concentration of (−)-carnitine is fed. This suggests that (−)-carnitine is the preferred substrate for carnitine decarboxylase, the enzyme that degrades carnitine to form β-methylcholine. Utilization of carnitine for phospholipid synthesis is clearly dependent upon a choline-deficient state. The presence of small quantities of choline, either in body tissues or in the diet, inhibits the incorporation of carnitine derivatives into phospholipid. Intact carnitine is needed for maximum larval growth and adult activity, indicating that D. melanogaster larvae synthesize a suboptimal quantity of carnitine. Thus, dietary carnitine supplements in vivo carnitine synthesis for physiological purposes.