Proximity Labeling, Quantitative Proteomics, and Biochemical Studies Revealed the Molecular Mechanism for the Inhibitory Effect of Indisulam on the Proliferation of Gastric Cancer Cells.

Indisulam exhibits antitumor activity against several cancer cells. Although the DCAF15-indisulam-RBM39 axis has been well documented in the inhibition of cancer cell growth, it is unknown whether RBM39 degradation alone is the mechanism of action of indisulam. Here, we verified the inhibitory effect of indisulam on the proliferation of gastric cancer cells and its dependence on DCAF15. Proximity-dependent biotin labeling with TurboID and quantitative proteomics revealed that indisulam indeed promoted the interaction between DCAF15 and RBM39. Immunoblotting and immunofluorescence also revealed that indisulam promoted the ubiquitin-mediated RBM39 degradation and RBM39 colocalized with DCAF15 in the nucleus. DCAF15 knockdown almost completely abolished the indisulam-mediated RBM39 reduction. Further knockdown of RBM39 eliminated the effect of DCAF15 on the proliferation of gastric cancer cells upon indisulam treatment. Immunoblotting of gastric tumor tissues confirmed the downregulation of RBM39 by indisulam. Database analysis unveiled that RBM39 was highly expressed in gastric cancer tissues and its high expression significantly shortened the survival time of gastric cancer patients. Taken together, we demonstrated that indisulam enhanced RBM39 ubiquitination and degradation by promoting its interaction with DCAF15, thus inhibiting the proliferation of gastric cancer cells. This work may provide valuable information for drug discovery through proteolysis targeting chimeras. MS data were deposited in ProteomeXchange (Dataset identifier: PXD024168).