Abstract
The homologs EpCAM and TROP2, which both interact with claudin-1 and claudin-7, are frequently coexpressed in epithelia including skin. Intestine uniquely expresses high levels of EpCAM but not TROP2. We previously identified EpCAM as a substrate of the membrane-anchored protease matriptase and linked HAI-2, matriptase, EpCAM and claudin-7 in a pathway that is pivotal for intestinal epithelial cells (IEC) homeostasis. Herein, we reveal that TROP2 is also a matriptase substrate. Matriptase cleaved TROP2 when purified recombinant proteins were mixed in vitro. TROP2, like EpCAM, was also cleaved after co-transfection of matriptase in 293T cells. Neither EpCAM nor TROP2 cleavage was promoted by protease-disabled matriptase or matriptase that harbored the ichthyosis-associated G827R mutation. We confirmed that EpCAM and TROP2 are both expressed in skin and detected cleavage of these proteins in human keratinocytes (HaCaT cells) after the physiologic inhibition of matriptase by HAI proteins was relieved by siRNA knockdown. Knockdown of EpCAM or TROP2 individually had only small effects on claudin-1 and claudin-7 levels, whereas elimination of both markedly diminished claudin levels. HAI-1 knockdown promoted EpCAM and TROP2 cleavage accompanied by reductions in claudins, whereas HAI-2 knockdown had little impact. Double knockdown of HAI-1 and HAI-2 induced nearly complete cleavage of EpCAM and TROP2 and drastic reductions of claudins. These effects were eliminated by concurrent matriptase knockdown. Decreases in claudin levels were also diminished by the lysosomal inhibitor chloroquine and cleaved EpCAM/TROP2 fragments accumulated preferentially. We demonstrate that TROP2 and EpCAM exhibit redundancies with regard to regulation of claudin metabolism and that an HAI, matriptase, EpCAM and claudin pathway analogous to what we described in IECs exists in keratinocytes. This study may offer insights into the mechanistic basis for matriptase dysregulation-induced ichthyosis.