Fatty acid desaturation and triglyceride accumulation protects breast cancer cells against fatostatin induced apoptosis
Authors: Brovkovych Viktor , Danes Jeanne M. , Sakallioglu Isin T. , Atilla‐Gokcumen G. Ekin , Frasor Jonna 
Published: November 2023

It is well established that cancer cells depend on de novo lipid synthesis to provide necessary membrane and signaling lipids for cell proliferation and survival. Sterol regulatory element‐binding protein 1 (SREBP1) is a transcription factor that plays a major role in regulating de novo lipogenesis in lipid‐depleted conditions. SREBP1 is blocked by the antitumor agent fatostatin (FS), which inhibits SREBP cleavage‐activating protein and prevents SREBP activation. Antitumor properties of FS include the inhibition of cancer cell proliferation, invasion, and migration, and activation of apoptosis. Precise mechanisms for these properties of FS are poorly understood. Here, we investigated whether changes in lipid species occur in response to FS treatment and, if so, which lipids may contribute to the anti‐cancer effects of FS. We find that treating MCF‐7 or T47D breast cancer cells with FS in lipid‐depleted media inhibits SREBP translocation into the nucleus and transcription of SREBP‐dependent lipogenic genes. In addition, cell proliferation is inhibited while the number of dead cells increases, based on propidium iodide staining. Surprisingly, treatment of cells with FS in lipid‐rich conditions also caused a decrease in cell proliferation and viability and promoted apoptosis, as measured by caspase 3/7 activity and annexin V staining. A global lipidomic analysis revealed increased production of ceramide in response to FS, which may contribute to its apoptotic effects. Unexpectedly, FS also induces a 2–3 fold increase in long‐chain polyunsaturated fatty acids (PUFAs), which appear to accumulate in triacylglerides (TAGs). In fact, a clear increase in intracellular TAG‐enriched lipid droplets was evident by Nile Red staining after FS treatment. The increase in PUFA levels is associated with an increase in desaturase and elongase gene expression in response to FS. The increase of TAG accumulation is associated with desaturase and diglycerol acyltransferase (DGAT) activity. To determine the functional role of unsaturated fatty acids and TAG accumulation, cells were treated with delta6‐desaturase inhibitor (SC‐26196, 1 uM), or stearoyl‐CoA desaturase‐1 (SCD1) inhibitor (CAY10566, 1 uM) or a mixture of DGAT1 and DGAT2 inhibitors (PF‐04620110 + PF‐06424439, 1 uM + 1 uM) in the presence or absence of FS. Without FS, inhibitors have no effect or slightly increase proliferation whereas co‐treatment with FS further enhanced apoptosis over FS alone, suggesting that fatty acid desaturation and elongation, as well as accumulation of PUFAs in TAGs, represent a cellular defense mechanism against FS triggered lipotoxicity. Moreover, these findings imply that the efficiency of FS as an anticancer drug could be increased by combination therapies with desaturases or DGAT inhibitors, which would represent a novel approach to breast cancer treatment.Support or Funding InformationThis study was supported by the NIH fund CA196930 01