Widespread expression of PICOT in mouse and human tissues with predominant localization to epithelium

The protein kinase C-interacting cousin of thioredoxin (PICOT; also termed glutaredoxin 3) protein was discovered a decade ago as a protein kinase C theta (PKCtheta)-binding protein in human T lymphocytes. PICOT possesses an amino-terminal monothiol thioredoxin-like domain and a carboxy-terminal tandem repeat of a monothiol glutaredoxin-like domain. Nevertheless, the enzymatic activities of PICOT and its potential substrates have not yet been characterized and its biological importance is unknown. Earlier studies reported the presence of PICOT in several different cell lines and tissues, but its expression pattern has not been thoroughly investigated. We performed Northern blot analysis of 19 different human organs and tissues and found the expression of PICOT mRNA in all organs and tissues tested. Western blot analysis confirmed the expression of PICOT at the protein level in all organs and tissues tested and showed, in addition, that PICOT and PKCtheta expression in different tissues only partially overlap. These findings support the involvement of PICOT in biological functions that are independent of PKCtheta. To analyze the in vivo expression pattern of PICOT within cells of different human organs, we performed immunohistochemical staining using PICOT-specific antibodies. Analysis of breast, pituitary, adrenal, pancreas, and kidney sections demonstrated a differential expression of PICOT in various cell types, with a predominant cytosolic staining of epithelial cells and low or undetectable levels of PICOT in the stroma.