The Molecular Mechanisms behind Advanced Breast Cancer Metabolism: Warburg Effect, OXPHOS, and Calcium

Abstract

Altered metabolism represents a fundamental difference between cancer cells and normal cells. Cancer cells have a unique ability to reprogram their metabolism by deviating their reliance from primarily oxidative phosphorylation (OXPHOS) to glycolysis, in order to support their survival. This metabolic phenotype is referred to as the "Warburg effect" and is associated with an increase in glucose uptake, and a diversion of glycolytic intermediates to alternative pathways that support anabolic processes. These processes include synthesis of nucleic acids, lipids, and proteins, necessary for the rapidly dividing cancer cells, sustaining their growth, proliferation, and capacity for successful metastasis. Triple-negative breast cancer (TNBC) is one of the most aggressive subtypes of breast cancer, with the poorest patient outcome due to its high rate of metastasis. TNBC is characterized by elevated glycolysis and in certain instances, low OXPHOS. This metabolic dysregulation is linked to chemotherapeutic resistance in TNBC research models and patient samples. There is more than a single mechanism by which this metabolic switch occurs and here, we review the current knowledge of relevant molecular mechanisms involved in advanced breast cancer metabolism, focusing on TNBC. These mechanisms include the Warburg effect, glycolytic adaptations, microRNA regulation, mitochondrial involvement, mitochondrial calcium signaling, and a more recent player in metabolic regulation, JAK/STAT signaling. In addition, we explore some of the drugs and compounds targeting cancer metabolic reprogramming. Research on these mechanisms is highly promising and could ultimately offer new opportunities for the development of innovative therapies to treat advanced breast cancer characterized by dysregulated metabolism.

Keywords: JAK/STAT; OXPHOS; ROS; TNBC; Warburg effect; glycolysis; hexokinase; metabolic reprogramming; miRNAs; mitochondrial Ca²⁺.

Fig. 1.. Schematic representing the differences observed in glucose metabolism between normal cells and cancer cells.

In normal cells, glucose that enters the cells undergoes glycolysis to generate pyruvate, nicotinamide adenine dinucleotide (NAD+), and a modest amount of adenosine triphosphate (ATP). When O₂ is available, pyruvate gets converted to Acetyl-CoA in the mitochondria, entering the tricarboxylic acid (TCA) cycle and cellular respiration continues through to oxidative phosphorylation (OXPHOS), generating an abundant amount of ATP. However, if O₂ is unavailable, the cell will direct pyruvate to generate lactate, allowing the regeneration of NAD+ to cycle back into glycolysis. In the Warburg effect or aerobic glycolysis, cancer cells will preferentially generate lactate from glucose, regardless of the availability of O₂ in the cell, rather than prioritizing the production of ATP through OXPHOS. The continuous regeneration of NAD+ helps it loop back and resupply glycolysis.

Fig. 2.. Diagram of the key metabolic features and mechanisms influencing the metabolic reprogramming of triple-negative breast cancer (TNBC).

Overexpression of glucose transporters (GLUTs) increases glucose amount and flux into the cell. TNBC cells preferentially use glycolysis for fueling alternative metabolic pathways that are necessary for building cellular biomass and continue proliferation (dashed arrows). Nicotinamide adenine dinucleotide phosphate (NADPH) can be generated through different pathways in the cell and two of those are shown here-the pentose phosphate pathway (PPP) cycle and the tricarboxylic acid (TCA) cycle. NADPH is crucial in maintaining redox homeostasis by acting as an antioxidant to inhibit surplus reactive oxygen species (ROS) damaging the cell. Moderate amounts of ROS are necessary for cancer cell survival and stress adaptation. Gene regulation occurs in the cell through microRNAs (miRNAs), which are shown here in the cytoplasm, regulating gene expression post-transcriptionally of their target mRNA. The triad of transcription factors (TFs) are shown in the nucleus with their relative expressions in TNBC. STAT3, a recognized transcription factor can function canonically or non-canonically by localizing to the mitochondria. Focusing on the diagram of the mitochondria, the Ca²⁺ entering the mitochondria supports mitochondrial metabolism by the regulation of certain TCA cycle enzymes. A simplified schematic shows how hexokinase 2 (HK2) interacts with the mitochondria at the outer mitochondrial membrane (OMM) by binding voltage-dependent anion channel (VDAC). This interaction can inhibit the release of pro-apoptotic factors and inhibit apoptosis (right). On the left, HK2 release from the mitochondria-associated-membranes (MAMs) (endoplasmic reticulum not shown) causes an influx of Ca²⁺ in the mitochondria, causing an overload and leading to cell death. Red arrows (up/down) represent the current knowledge of the expression patterns of the pathway/protein/metabolite. Dashed arrows represent alternative pathways. Monocarboxylate transporters (MCTs), lactate dehydrogenase (LDH), phosphofructokinase-1 (PFK-1) and the mitochondrial calcium uniporter (MCU) are generally upgregulated in TNBC.