Exploring the link between cholesterol and cancer pathophysiology: from nuclear receptors to chemoresistance
Transcription factors can have distinct functions in different cells or tissues. In cancer the lines between these roles can become blurred leading to unexpected survival characteristics. This is the discovery we made when we found cholesterol metabolites can drive chemotherapy resistance.
One in seven women will be diagnosed with one of the different breast cancer subtypes at some point in their lifetime . Although many will be cured, survival and recurrence rates depend on whether the tumour is positive or negative for the oestrogen, progesterone and Her2 receptors. If negative for all three of these (triple negative breast cancer) then targeted systemic therapies such as Tamoxifen or Herceptin are not an option and instead cytotoxic chemotherapy is given. Triple negative breast cancer is more likely recur than other subtypes, indicating the therapy has failed . Several factors are thought to influence outcomes of breast cancer patients, such as genetics, mutation profile of the tumour, activity of drug detoxification mechanisms, and body composition parameters such as BMI and cholesterol levels . Predicting which patients will respond to chemotherapy, and those whose tumours are resistant still remains a substantial barrier to curing triple negative breast cancer patients.
Our laboratory has been interested in the role that dietary, nutritional, and metabolic factors play in cancer cell biology, and whether changes to these parameters alter chemotherapy efficacy. In our latest work published in Oncogene, our laboratory examined the link between chemotherapy resistance and cholesterol metabolism. Cholesterol is the precursor of an array of signalling and functional molecules such as hormones and bile acids. However, it is the very first step in these metabolic pathways that we were interested in – the addition of hydroxyl groups to create oxysterols. Lots of work has been done on oxysterols and their role in disease and health are increasingly well understood, in no small part because of the contributions of the European Network for Oxysterol Research (ENOR). Oxysterols are a family of active signalling molecules, several of which function as ligands of the liver x receptor (LXR) members of the nuclear receptor superfamily of transcription factors. The oxysterol-LXR axis typically regulates cholesterol and lipid homeostasis, sometimes activating ATP-binding cassette cholesterol efflux pumps (such as ABCA1). These cholesterol export pumps share structural and regulatory region homology with xenobiotic efflux pumps that can cause chemotherapy resistance in breast cancer (such as ABCB1/P-glycoprotein).
Given the link between high cholesterol and poor outcomes in some breast cancer chemotherapy patients, we set out to investigate whether the oxysterol-LXR axis could confer resistance to the common chemotherapy agents. Through a series of cell and molecular biology experiments we found that chemotherapy drugs were more likely to be exported from triple negative breast cancer cells that had previously been exposed to a specific class of oxysterol (side-chain hydroxycholesterols). We chose these as we had previously found they were potent activators of LXR in triple negative but not of other breast cancer subtypes . Whilst presenting this work at a conference (the 8th ENOR meeting in Bologna) we met Dr Erik Nelson who had previously linked 27-hydroxycholesterol to breast cancer growth . We developed a collaboration with Dr Nelson and soon afterwards his laboratory validated our findings using preclinical models. Indeed, TNBC was less susceptible to epirubicin if the LXR axis had been stimulated.
Part of the funding we secured to run this study allowed one of the group members to train in the laboratory of Dr Hanne Roberg-Larsen, another signed up member of ENOR, to learn the liquid chromatography mass spectrophotometry methods required to detect and distinguish between the various side-chain hydroxycholesterols (they have identical molecular mass and charge, just the position of the -OH group differs) . Joint first author, Alex Websdale spent three dark winter months in the analytical chemistry laboratories in Oslo analysing a patient tumour cohort that had been collected as part of a tissue banking drive and which now, 5-10 years later, is proving invaluable in breast cancer research coming through the University of Leeds and the Leeds Teaching Hospitals Trust.
This body of work has been almost ten years in the making, with the first ideas forming when I was preparing a review article written with my post-doc supervisor and mentor Dr Moray Campbell, now at Ohio State. This review examined how nuclear receptors are involved energy deregulation and the Warburg effect in cancer . Then, under the tutorage of Dr Thomas Hughes (co-corresponding author) the idea developed and we began collecting experimental data to test our hypotheses. We have needed to work internationally and interdisciplinary to address our hypothesis, with collaborations that include groups from the USA in the form of Dr Erik Nelson and Norway with Dr Hanne Roberg-Larsen, and from disciplines of preclinical models, molecular and cell biology, analytical chemistry, and with several clinician scientists. Pulling together data from so many specialties, and collected from so many different laboratories strengthens our findings and gives confidence in the reliability of our methods.
Our advice, as new PI and an early career researcher, is that ultimately, this collaborative approach to science gives a reassurance that you are contributing to the scientific literature in the most robust way possible. Excitingly, we are moving on to explore this and other pathways with a translational view. The research group is now asking questions such as, what is the role of the tumour microenvironment in mediating metabolic and nutrition affects, which other metabolic processes may impinge on or be hijacked to aid tumour development? We think this work opens a path that may one day help to predict before their treatment begins, which patients won’t respond to chemotherapy. We could help patients be assigned to their ‘correct’ treatment regiment, and perhaps even offer tangible evidence-based guidance to patients at a time when they are vulnerable to non-peer reviewed ‘advice’ found through internet searches.
Fully understanding oxysterol signalling in cancer remains challenging. Cholesterol is the precursor of many metabolites, not just the side-chain hydroxycholesterols we have investigated. We know that some oxysterols are oncogenic and can promote growth of some cancers, while others such as dendrogenin A induce lethal autophagy and are tumour suppressors . Our friends in the oxysterol community are continually findings new and exciting roles for these compounds. Our next conference later this year is going to be virtual, get in touch if you are interested in learning more about these exciting molecules.
- BCUK, Breast cancer is the most common cancer in the UK. But over a quarter of cases are preventable. 25/02/2021, 2021. <https://www.breastcanceruk.org.uk/>.
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