Targeting the Warburg Effect May Pave Way for New Class of Anticancer Agents

Otto Warburg, MD, PhD

Otto Warburg, MD, PhD

Researchers from Saint Louis University have developed a new class of anticancer drugs that potently reduce tumor growth and survival by targeting two energy sources, the Warburg effect (glycolysis) and lipogenesis, according to a University press release.¹Results of the study have been published in the July 13, 2015 issue ofCancer Cell.²Unlike conventional targeted therapies, aimed at specific genetic mutations associated with different types of cancer, these drugs target an aspect of tumor metabolism common to most types of cancer.¹

The ability of tumors to obtain their energy from glycolysis (once termed fermentation) was first described by Otto Warburg, MD, PhD in 1923, using pieces of tumor in vitro. He then greatly expanded the idea in a landmark paper published in 1927, and the preferential use of glycolysis by tumors to obtain energy, became known as the Warburg effect.³

“Targeting cancer metabolism has become a hot area over the past few years, though the idea is not new,” said senior author Thomas P. Burris, PhD, William Beaumont Professor and chair of pharmacological and physiological science at St Louis University. “The Warburg effect ramps up energy use in the form of glucose to make chemicals required for rapid growth, and cancer cells also ramp up another process, lipogenesis, which lets them make their own fats that they need to rapidly grow,” he said.¹

Burris et al at the Scripps Institute designed a liver X receptor (LXR) inverse agonist named SR9243, and have reported study results of the agent’s anticancer properties in cell lines and animal models.²LXRs are a nuclear receptor family, involved in the regulation of lipid and glucose metabolism.⁴SR9243 targets LXR and achieves a down regulation of LXR-mediated gene expression to below basal levels. SR9243 is highly selective, having no effect on other nuclear receptors. At nanomolar concentrations, SR9243 dose dependently suppresses LXR-α and LXR-ߗdependent transcription.²

SR9243 Induces Apoptosis in Cancer Cells

Cancer cells are dependent upon the Warburg effect for energy and progress to apoptosis following inhibition of glycolysis. The viability of prostate, lung, and colorectal cancer cells lines was greatly reduced by SR9243, while having no effect upon nonmalignant cells from those organs, even though they expressed similar levels of the LXRs. Colony formation was inhibited, again, in a dose-dependent nanomolar range. Further clarifying the target of the drug, knockdown of LXR α and LXR ß using short interfering RNAs restored the viability of cancer cells exposed to SR9243.²

The drug proved effective in reducing cancer cell viability, so the team combined it with cisplatin and 5-fluorouracil to test whether or not it would lead to enhanced cytotoxicity. In all cancer cell types tested, the addition of SR9243 to either agent resulted in enhanced efficacy of the chemotherapy.²

SR9243 Targets the Warburg Effect

The team confirmed that the molecule was indeed directly targeting the Warburg effect in cancer cells by showing that the drug caused a down-regulation of glycolytic gene expression (GCK1,PFK2,PFK1, andLDH) and reduced levels of the glycolytic metabolites, pyruvate, lactate, and hexose phosphates. However, SR9234 was without effect on glycolytic gene expression or on the concentrations of glycolytic metabolites in normal cells. Glycolytic enzymes are used in respiration, but SR9243 had no effect on the rate of oxygen consumption in either cancer or normal cells.²

SR9243 Targets Lipogenesis

The expression of LXR directly regulated lipogenesis genes (FASN,SREBP1-c,SCD1), was also greatly reduced by treatment with SR9243, in all the cancer cell lines used in the study. In normal cells,FASNandSCD1gene expression were reduced but notSREBP1-c, which was profoundly reduced in cancer cells. Cholesterol export was affected in both cancer and normal cells where SR9234 treatment reduced expression ofABCA1, the cholesterol transport gene. In cancer cells, SR9243 reduced intracellular levels of the important end-products of lipogenesis, namely palmitate, stearate, palmitoleate, and myristoleate. However, supplementation of culture media with individual fatty acids did not restore the viability of cancer cells. Survival of the cancer cells only improved when several fatty acids were added to the culture media, suggesting SR9243 targeted multiple loci in the lipogenesis cascade. Consequently, the authors pointed out that SR9243 should display therapeutic activity in tumors that are highly glycolytic or highly lipogenic.²

SR9243 in Animal Models

Once again, SR9243 had a dose-dependent effect on tumor growth, evident in a mouse colon cancer xenograft model. The mice treated with SR9243 showed no evidence of weight loss, and within the tumors, there was a dose-dependent reduction in glycolytic and lipogenic enzyme expression and an increased number of apoptotic tumor cells. Key markers of glycolysis, pyruvate, and glycerate, together with lipid content were also greatly reduced in the treated tumors.²

Further experiments with tumor xenografts in which SR9243 had a limited impact on glycolytic gene expression (DU-145 tumors) demonstrated that the dramatic reduction in tumor growth was achieved by the inhibition of lipogenesis (SREBP-1c,SCD1, andFASNmarkedly suppressed). “This implies that because of its dual pathway activity, SR9243 should have efficacy in cancer cells that display variable glycolytic activity within the metabolically heterogenous tumor environment,” the authors concluded. Overall the in vivo experiments showed no evidence of hepatotoxicity, or inflammatory reaction.²

Referring to the experiments with human tumors grown in animals, Burris explained that, “It worked very well on lung, prostate, and colorectal cancers, and it worked to a lesser degree in ovarian and pancreatic cancers.” “Some [tumor types] are more sensitive to it [SR9243]  than others. In several of these pathways, cells had been reprogrammed by cancer to support cancer cell growth. This [SR9243] returns the metabolism to that of more normal cells.”¹

References

1. Newswise (press release) St. Louis University.http://www.newswise.com/articles/view/636377/?sc=sphr&xy=10013055. Accessed July 4, 2015.

2. Flaveny CA, Griffett K, El-Gendy BE, et al. Broad anti-tumor activity of a small molecule that selectively targets the Warburg Effect and lipogenesis.Cancer Cell. 2015 Jun 24. pii: S1535- 6108(15)00183-X. doi: 10.1016/j.ccell.2015.05.007.

3. Warburg O, Wind F, Negelein E. The metabolism of tumors in the body.J Gen Physiol.1927;8:519-530.

4. Lin CY, Gustafsson JA. Targeting liver X receptors in cancer therapeutics.Nat Rev Cancer.2015;15:216-24. doi: 10.1038/nrc3912.