Inhibition of TGF-β induced lipid droplets switches M2 macrophages to M1 phenotype.

Lipid droplets (LD) are newly characterized dynamic cytoplasmic organelle which is the storehouse of different immunosuppressive cytokines and enzymes like cyclooxygenase and lipoxygenase. Tumors are known to modulate the immune system by immune-editing the microenvironment. Immuno-editing comprises of three steps namely cancer immune-surveillance, tumor dormancy and finally escape leading to tumor development. The latency of the tumor microenvironment is greatly contributed by the M2 polarized macrophages and TGF-β is a prime culprit. Modulating M2 macrophages to M1 can be a strategy against tumor progression. We found that tumor-conditioned medium or recombinant TGF-β was efficient to induce LD formation in Raw264.7 cells and the inhibition of LD was associated with the switch of M2 to M1 phenotype involving MEK1/2 axis. Signature molecules of M2 polarized macrophages like CD206 were also downregulated while co-stimulatory molecules like CD80, CD86 were up-regulated along with enhanced surface expression of MHCII when these macrophages were subjected to C75 treatment to reduce the LD formation. The level of pro-inflammatory cytokine, as well as ROS and NO generation, were also increased when TGF-β treated macrophages were subjected to C75 treatment. This study is probably the first report of this kind and can be used in the future in cancer treatment.