The link between lipid droplets and autophagy in cancer cell stress response

Rakave celice imajo izredno sposobnost prilagajanja na težke razmere v okolju. Prilagajanje na omenjene razmere jim omogočajo spremembe v presnovi, ki so lahko posledica mutacij. Vpliv na presnovo rakavih celic ima tudi razpoložljivost hranil v okolju, ki lahko spremeni celično sporočanje in spodbudi tvorbo tumorjev. Tako se pri pomanjkanju hranil spodbuja metabolizem v smeri sinteze lipidov, preživetje rakavih celic pa je odvisno od uspešnosti privzema lipidov iz okolja in recikliranja lastnih lipidnih zalog. Ugotovili smo, da stradanje visoko invazivnih in tumorigenih celic raka dojke (MDA-MB-231) v odsotnosti aminokislin spodbudi tvorbo lipidnih kapljic, hkrati pa se aktivira proces avtofagije oziroma poveča avtofagni pretok, ki narašča s časom stradanja. V našem laboratoriju smo pokazali, da se izgradnja lipidnih kapljic poveča po 12 h stradanja najbolj izrazito v gojišču brez aminokislin. Po 24 h sinteza lipidnih kapljic doseže najvišjo točko, količina lipidnih kapljic pa nato začne upadati. Naraščanje količine lipidnih kapljic v celicah MDA-MB-231 tekom 24-urnega stradanja sovpada z naraščanjem avtofagnega pretoka, kar nakazuje na medsebojno povezanost sinteze lipidnih kapljic in avtofagije. Pokazali smo tudi, da je z diacliglicerol aciltransferazo (DGAT) posredovana sinteza lipidnih kapljic pomembna za preživetje celic raka dojke in materničnega vratu (HeLa), pri čemer so slednje bolj občutljive na pomanjkanje aminokislin in zaviranje sinteze lipidnih kapljic. Z uporabo inhibitorjev avtofagije smo ugotovili, da je avtofagija potrebna za sintezo lipidnih kapljic, ni pa ključna za preživetje celic raka dojke pri akutnem stradanju v odsotnosti aminokislin. Naše delo bo pomembno prispevalo k razjasnitvi kompleksnih povezav med lipidnimi kapljicami in avtofagijo ter k razumevanju njune vloge pri odpornosti rakavih celic na stres.Changes in the metabolism of cancer cells drive their remarkable adaptability to difficult environmental conditions. The inconsistent supply of nutrients in the cancer microenvironment alters cell signalling pathways and promotes metabolic adaptation. Stressed cancer cells often become dependent on lipid synthesis, uptake and recycling of lipid reserves to survive in nutrient-poor conditions. Here we found that lipid droplet (LD) biogenesis and autophagy are both elevated in MDA-MB-231 breast cancer cells during growth in a medium lacking amino acids (herein referred to as starvation). The increase in the cellular LD content is apparent after 12 h of starvation, it reaches its maximum after 24 h and begins to drop at later time points. The temporal dynamics of the increase in neutral lipid amount during the 24 h period of starvation correlate well with changes in autophagic flux, which indicates a connection between autophagy and LD biogenesis. Our experiments showed that diacylglycerol acyltransferase (DGAT)-mediated LD biogenesis contributes to the survival of MDA-MB-231 breast and HeLa cervical cancer cells during starvation. The latter are more sensitive to amino acid starvation and inhibition of LD biogenesis. Finally, our experiments with autophagy inhibitors showed that autophagy is important for LD biogenesis, but not essential for the survival of breast cancer cells during acute starvation in the absence of amino acids. Our work will contribute to the understanding of the complex interplay between LDs and autophagy and help define their roles in the resistance of cancer cells to stress