Hodgkin's lymphoma cells exhibit high expression levels of the PICOT protein

PICOT was originally discovered as a protein kinase C (PKC) binding protein in human Jurkat T-lymphocytes in which it was found to modulate PKCtheta-dependent functions. In addition, RT-PCR analysis suggested the expression of PICOT in a wide range of organs and cell types, including cells that are devoid of PKCtheta. We aimed at analyzing the expression of the PICOT protein in mouse lymphoid organs, and to compare them with those of Jurkat T-lymphocytes and other cell lines. We also analyzed whether PICOT expression in T-lymphocytes is dependent on the presence of PKCtheta, and whether it correlates with cell growth rate. Western blot analyses demonstrated PICOT expression in all lymphoid organs and cell lines tested. In addition, similar expression levels were observed in lymphoid organs of wild-type and PKCtheta-null mice, suggesting that PICOT expression in T-lymphocytes is independent of PKCtheta. However, PICOT expression levels were higher in Jurkat T-lymphocytes and other lymphoma cell lines compared to freshly isolated lymphocytes, while T-lymphocyte mitogens, such as concanavalin A, increased PICOT expression concomitantly with the induction of a faster T-lymphocyte growth rate. Finally, immunohistochemistry of freshly-isolated lymph nodes from Hodgkin's lymphoma patients revealed significantly higher levels of PICOT in Hodgkin's cells, compared to the normal surrounding lymphocytes. The present results show a direct correlation between PICOT expression levels and increased cell growth, both in vitro and in vivo, and suggest that immunostaining of PICOT might be useful for in situ identification of transformed cells, such as those of Hodgkin's lymphoma.