Effects of metabolic stress on lipid metabolism in cancer cells

Effects on gene expression profiles of cancer cells

Fatty acid synthase (FASN) –a key-regulator of de novo fatty acid (FA) synthesis– has been extensively shown to fuel cancer cell proliferation and malignant progression. Expression of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) –the rate-controlling enzyme of the mevalonate pathway– is also up-regulated in cancers. Importantly, inhibition of FA synthesis or cholesterol synthesis pathways results in growth-arrest of lipogenic tumor cells rendering these pathways interesting targets for antineoplastic therapy. Although endogenous FA synthesis has historically been considered the principal source of fatty acids (FAs) in cancer cells, lipolytic phenotypes are also widely recognized. For example, it has been reported that in addition to the markers of de novo synthesis (FASN) different cancer cells also express markers of lipolysis (lipoprotein lipase, LPL) and exogenous FA uptake (CD36). Additional support for coordinated lipolytic and lipogenic metabolism in cancer cells involves the incorporation of endogenously synthesized FAs into cellular neutral lipid stores. It has been proposed that complementary lipolytic pathways are required to release fatty acyl moieties from these lipid reservoirs and have demonstrated a specific role for an intracellular lipase, monoacyl glycerol lipase (MGLL), in promoting tumorigenesis. MGLL provides, by de-esterification, a stream of intracellular free FAs to fuel proliferation, growth, and migration. Taken together, these findings are compatible with the notion that both lipogenesis and lipolysis may be utilized by cancer cells to fulfill their FA requirements. Cancer cells modify the balance between fatty acid (FA) synthesis and uptake under metabolic stress, induced by oxygen/nutrient deprivation. One of the major ongoing projects in the Cancer Biology Lab focuses on studying the metabolic expression profiling of cancer cells growing under metabolic stress. We have recently studied the impact of metabolic stress on the markers of de novo fatty acid synthesis and markers of lipolysis and exogenous FA uptake. We observed differential impact of metabolic stress on expression of FASN and HMGCR. The expression of markers of lipid uptake/degradation (LPL, MGLL and CD36) was also differentially affected by metabolic stress.

Effect of metabolic stress on expression of selected genes from de novo lipid synthesis or lipid uptake/degradation pathway in different cancer cell lines. Box plots showing log2 transformed and median normalized values for (a) FASN  (b) HMGCR (c) MGLL expression levels in KG1, KCL22, KU812, SW480, SW620 and A549. (d) LPL expression level in KU812 and SW480 cells. (e) CD36 expression level in KU812 cells. Cells were cultivated (48 hours) under lipoprotein deficient medium (LPDS serum), low-serum (LS) medium (2% serum), hypoxia (2% O2), or hypoxia in combination with LS medium. The levels of the different transcripts were measured in 3 to 6 samples by qPCR. The results show the distribution of corresponding transcripts relative to GAPDH, with the box indicating the 25th–75th percentiles, with the median indicated. line. The whiskers show the range. Data were normalized to the median expression level of the given transcript under normal conditions for the respective cell line.

Overview of the major lipid metabolism pathways shown to be affected by metabolic stress. The figure highlights all the key lipid metabolism pathways activated in cancer cells. Major pathways are shown as boxes without outlines. The systematic names of these pathways are given at the bottom-right corner of each box. The numbers given in superscript with each protein/metabolite indicate the reference number. For further details see Text, and Supplementary Table 1. Abbreviations: ACACA, acetyl-CoA carboxylase 1; ACACB, acetyl-CoA carboxylase 2; ACSS2, acyl Co-A synthetase-2; ADFP, adipose differentiation protein; ATGL, adipose triglyceride lipase; FA, fatty acids; FABP3, fatty acid binding protein 3; FABP7, fatty acid binding protein 7; FFA, free fatty acids; FASN, fatty acid synthase; H, hypoxia; HIF-1α, hypoxia-inducible factor 1-alpha; HIF-2α, hypoxia-inducible factor 2α; HIG2, hypoxia-inducible gene 2 protein; HMGCR, 3-hydroxy-3-methylglutaryl-CoA reductase; LD, lipid droplet; LCAD, long-Chain Specific acyl-CoA dehydrogenase; MAG, monoacylglycerol ; MCAD, Medium-chain acyl-CoA dehydrogenase; MUFA, monounsaturated fatty acids; PBMCs, peripheral blood mononuclear cells; PC, Phosphatidylcholines; PE, phosphatidylethanolamines; Pcho, propargyl-choline; PI, phosphatidylinositol;  PS, phosphatidylserine; PUFA, polyunsaturated fatty acids; SCD, stearoyl-CoA Desaturase; SFA, saturated fatty acids; SREBP, sterol regulatory element-binding proteins; TG, triglycerides

Effects on lipidomic profiles of cancer cells

The mode of FA acquisition –via de novo synthesis or uptake– may significantly affect the lipidomic profiles of cancer cells. Mammalian cells have a limited ability to synthesize polyunsaturated fatty acids de novo, as they lack the Δ12 desaturase. Therefore, enhanced de novo FA synthesis enriches the cancer cell membranes with saturated and/or mono-unsaturated fatty acids. As these FAs are less prone to lipid peroxidation than polyunsaturated acyl chains, de novo synthesis was proposed to make cancer cells more resistant to oxidative stress-induced cell death. Moreover, as saturated lipids pack more densely their increased levels alter lateral and transverse membrane dynamics that may limit the uptake of drugs, making the cancer cells more resistant to therapy. Hence, the balance between FA synthesis and uptake may have important therapeutic implications. Most studies on the impact of hypoxia on lipidomic profiles of cancer cells are limited to specific lipid classes. Hence, one of the major projects at the Cancer Biology Lab, MMG is to study the effects of metabolic stress on lipidomic profiles of cancer cells. Recently we performed a broad lipidomics assay comprising 244 lipids from six major classes. To this end we identified multiple changes in lipidomic profiles of cancer cells cultivated under low-serum or lipid-deficient conditions. Under hypoxic stress cancer cells displayed alterations in cell proliferation rates and expression profiles of various lipid metabolism associated genes. Interestingly, no robust changes were observed in lipidomic profiles of hypoxic cancer cells indicating that the cells maintain lipid class homeostasis.

Effect of metabolic stress on lipidomic profiles of cancer cells:  Each column shows changes in cellular levels of the six major lipid classes –including CEs, DGs, PCs PPCs, PEs and TG– under a specific stress condition relative to control for KCL22, KG1, KU812, SW620, SW480 and A549 cells. For each lipid class the peak intensities of the subspecies containing similar number of double bonds in their fatty acyl chains (chain containing highest number of double bonds) were summed up and the data were log2 transformed and median normalized. LS+Hyp stress was not tested for SW480 and SW620 cell lines.

Related Articles from Our Lab

  1. Lisec, J., C. Jaeger, and N. Zaidi, Cancer cell lipid class homeostasis is altered under nutrient-deprivation but stable under hypoxia. 2018: p. 382457. doi: https://doi.org/10.1101/382457
  2. Munir, R, Lisec, J, and N. Zaidi, Lipid metabolism in cancer cells under metabolic stress. Under-review at British Journal of Cancer.
  3. Ameer F, Scandiuzzi L, Hasnain S, Kalbacher H, Zaidi N: De novo lipogenesis in health and disease. Metabolism 2014

Related Theses from Our Lab

Level Student’s Name Thesis Title Year
PhD Fatima Ameer Factors affecting lipid and lipidomic profile of malignant and non-malignant hematopoietic cells. Work in progress
Rimsha Munir Studying lipid metabolism in 2D and 3D cell culture systems: developing in vitro models that mimic in vivo tumor physiology. Work in progress
Rida Rashid Saturation index of cellular lipids in hypoxic cancer cells: studying the impact on cell viability, metabolic gene expression and drug-uptake Work in progress
M.Phil. Maria Ramzan Effect of hypoxia on cell proliferation and lipid-load:  A comparison between 2-dimensional (2D) and 3-dimensional (3D) cell culture systems 2016-17
Undergraduate  Hafiza Ishrat Fatima Triglyceride and phospholipid content in cancer cells under metabolic stress 2014-18