Fluorescent mimics of cholesterol that rapidly bind surfaces of living mammalian cells

Surfaceome Profiling Identifies Basigin-Chaperoned Protein Clients

The surface proteome or "surfaceome" is a critical mediator of cellular biology, facilitating cell-to-cell interactions and communication with extracellular biomolecules. Constituents of the surfaceome can serve as biomarkers for changing cell states and as targets for pharmacological intervention. While some pathways of cell surface trafficking are well characterized to allow prediction of surface localization, some non-canonical trafficking mechanisms do not. Basigin (Bsg), a cell surface glycoprotein, has been shown to chaperone protein clients to the cell surface. However, understanding which proteins are served by Bsg is not always straightforward. To accelerate such identification, we applied a surfaceome proximity labeling method that is integrated with quantitative mass spectrometry-based proteomics to discern changes in the surfaceome of hepatic stellate cells that occur in response to the genetic loss of Bsg. Using this strategy, we observed that the loss of Bsg leads to corresponding reductions in the cell surface expression of monocarboxylate transporters MCT1 and MCT4. We also found that these relationships were unique to Bsg and not found in neuroplastin (Nptn), a related family member. These results establish the utility of the surfaceome proximity labeling method to determine clients of cell surface chaperone proteins.

Antibody-Drug Conjugate that Exhibits Synergistic Cytotoxicity with an Endosome-Disruptive Peptide

Antibody-drug conjugates are an important class of cancer therapeutics. These agents generally bind a specific cell surface receptor, undergo receptor-mediated endocytosis, and enter the endosomal-lysosomal system, where the environment in these organelles facilitates the release of a membrane-permeable cytotoxin. By using a membrane-impermeable cytotoxin, we describe here a method that allows the cytotoxicity of an antibody conjugate to be triggered by co-administration with an endosome-disruptive peptide that exhibits low toxicity. This approach was validated by conjugation of an anionic derivative of the tubulin-binding cytotoxin colchinol methyl ether to lysine residues of the HER2-targeting antibody trastuzumab (Herceptin) via a disulfide. When this antibody binds HER2 on SKBR3 breast cancer cells and undergoes endocytosis, the membrane-impermeable cytotoxin is released, but it becomes trapped in endosomes, resulting in relatively low cytotoxicity (IC50 > 1 μM). However, co-administration with an essentially nontoxic (IC50 > 10 μM) cholesterol-linked endosome-disruptive peptide promotes the release of this small molecule into the cytoplasm, conferring subnanomolar cytotoxic potency (IC50 = 0.11 ± 0.07 nM). Studies of a structurally related fluorophore conjugate revealed that the endosome-disruptive peptide does not substantially enhance cleavage of the disulfide (t 1/2 = 8 ± 2 h) within endosomes, suggesting that the mechanism of endosomal escape involves the efflux of some small molecules without facilitating substantial influx of reduced glutathione.