The insulin receptor negatively regulates the action of Pseudomonas toxin-based immunotoxins and native Pseudomonas toxin.

SS1P is a recombinant immunotoxin composed of an antimesothelin Fv fragment fused to a truncated portion of Pseudomonas exotoxin A. SS1P targets and kills mesothelin-expressing tumors, which include mesothlioma as well as ovarian, lung, and pancreatic cancers. SS1P is currently in clinical trials in mesothelioma. Because insulin acting through the insulin receptor is a survival factor for many cancer cell lines, we explored how lowering insulin receptor level would affect the cytotoxic action of SS1P. We show here that siRNA knockdown of the insulin receptor enhanced the cytotoxic action of native Pseudomonas exotoxin and enhanced SS1P toxicity on several human cell lines, but did not affect the response to other cytotoxic agents such as TRAIL, etoposide, and cycloheximide. To determine how insulin receptor knockdown enhances SS1P action, we analyzed various steps involved in cell killing. We found that insulin receptor knockdown increases the cleavage of SS1P by furin, which allows more toxin to reach the cytosol and inactivate elongation factor 2. These findings indicate that the insulin receptor negatively regulates immunotoxin action.