CD98hc orchestrates amino acid and glucose availability, redox homeostasis and energetic and nucleotide metabolism = CD98hc sostiene la disponibilidad de aminoácidos y glucosa, la homeóstasis óxido-reducción y el metabolismo energético y de nucleótidos

Abstract

The wide variety of disorders associated with the malfunction of amino acid (AA) transporters reflect the relevant roles they fulfil in human physiology. Heteromeric AA transporters (HATs), one of the eleven families of AA plasma membrane transporters in human cells, are formed by an ancillary heavy subunit, which brings the holotransporter to the plasma membrane and a catalytic light subunit. The heavy subunit CD98hc (also named 4F2hc or SLC3A2) can dimerize with any of six light subunits (LAT1, LAT2, xCT, y+LAT1, y+LAT2 and asc1) that confer substrate specificity to the heterodimer. Cancer cells overexpress CD98hc-transporters in order to meet their increased nutritional and antioxidant demands, since these transporters are crucial to control reactive oxygen species and intracellular AA levels, thus sustaining cell survival and proliferation. CD98hc-xCT is required for cystine uptake, which is rapidly reduced to cysteine inside the cell, a rate-limiting AA in the synthesis of reduced glutathione (GSH). Since GSH is vital for controlling the levels of reactive oxygen species in the cell, loss of CD98hc-xCT in vitro cannot be compensated, leading to cell death by ferroptosis. Supplementation of culture media with β-mercaptoethanol rescues CD98hc-deficient cell survival. Under such conditions cells lacking CD98hc still present increased levels of reactive oxygen species and intracellular AA imbalance, probably due to the loss of CD98hc-LAT1. Consequently, cells lacking CD98hc present alterations in the stress-sensing signalling pathways mTORC1 and Integrated Stress Response- mediated by eIF2α, leading to defective protein synthesis and proliferation. Furthermore, CD98hc regulates the energy metabolism of the cell. Indeed, lack of CD98hc triggers a reduction the glucose uptake and glucose-related metabolic pathways, such as glycolysis and the pentose phosphate pathway, forcing cells to use mitochondrial oxidative metabolism in order to maintain the cellular ATP levels, suggesting a metabolic rewiring. The abrogation of the pentose phosphate pathway flux by CD98hc ablation triggers a dramatic reduction in the levels of ribose-5P, which forms the sugar backbone of all nucleotides. Consequently, these cells present a dramatic reduction in the nucleotide pool, which cause replicative stress, enhanced DNA Damage Response, delayed cell cycle progression during the S-phase and diminished rate of mitosis, all recovered by nucleoside supplementation. In addition, reduction in the intracellular content of the essential branched-chain AAs (leucine, isoleucine and valine) and aromatic AAs (tyrosine, phenylalanine and tryptophan), all transported by CD98hc-LAT1, leads to repressed expression of RRM2, the only enzyme able to catalyse the reduction of ribonucleotides to deoxyribonucleotides, resulting as well in replicative stress. Taking all of these considerations, CD98hc may be a putative target in pathophysiological scenarios, thus, generating drugs against CD98hc could represent a novel option, widening the therapeutic window of cancer therapy by inducing an immediate impairment in: the redox counterbalance capacity, the intracellular AA content for proper protein synthesis and cell proliferation and the pentose phosphate pathway flux required for producing ribose-5P to sustain nucleotide biosynthesis of the cells.

La gran variedad de trastornos asociados con el mal funcionamiento de los transportadores de aminoácidos (AA) evidencia la importancia de las funciones que éstos llevan a cabo en la fisiología humana. Los transportadores heteroméricos de AAs (HATs) están formados por una subunidad pesada, que sitúa el heterodímero en la membrana plasmática, y una subunidad ligera que confiere la capacidad catalítica al transportador. La subunidad pesada CD98hc (también denominada 4F2hc o SLC3A2) puede dimerizar las subunidades ligeras LAT1, LAT2, xCT, y + LAT1, y + LAT2 y asc1, confiriendo especificidad de sustrato al heterodímero. Las células tumorales sobreexpresan los transportadores asociados a CD98hc para satisfacer sus demandas nutricionales y antioxidantes, ya que estos transportadores son cruciales para controlar las especies reactivas de oxígeno y los niveles intracelulares de AA, soportando la viabilidad y la proliferación celular. CD98hc-xCT es el responsable de la captación de cistina, necesaria para sintetizar glutatión y por tanto para mantener la homeostasis óxido-reducción. En la célula. Por este motivo, la eliminación de CD98hc-xCT in vitro da lugar a la muerte celular por ferroptosis si el medio no es suplementado con un agente reductor como el β-mercaptoetanol. En dichas condiciones, las células deficientes de CD98hc presentan estrés oxidativo y un desequilibrio intracelular de AAs, probablemente debido a la pérdida de CD98hc-LAT1. Consecuentemente, la síntesis proteica y la proliferación son defectuosas en estas células. Además, CD98hc regula el metabolismo energético de la célula, de manera que su eliminación causa una reducción de la captación de glucosa y de las vías metabólicas relacionadas con su metabolismo, como la glucólisis y la ruta de las pentosas fosfato, lo que obliga a las células a utilizar el metabolismo oxidativo mitocondrial para mantener los niveles celulares de ATP. El bloqueo de la ruta de las pentosas fosfato provoca una reducción dramática del contenido de ribosa-5P, lo que se ve reflejado en una disminución global del conjunto de nucleótidos, causando estrés replicativo y alteraciones en la progresión del ciclo celular. Previas consideraciones sugieren que la eliminación de CD98hc es una posible diana terapéutica para el tratamiento anti-tumoral.