Abstract
Tissue type II transglutaminase (TGase) is a member of the TGase family that catalyzes Ca^2+^‐dependent covalent cross‐linking of several amines to the γ‐carboxamide group of protein‐bound glutamine residues. The degree of therapeutic efficacy or toxicity of drugs may be related to their ability to serve as a substrate for TGase and their covalent linkage to glutamine residues of regulatory proteins through the catalytic action of this enzyme. Here, doxorubicin (adriamycin)‐resistant human breast carcinoma MCF‐7~ADR~ cells exhibited 40‐ to 6C‐fold higher TGase activity than control drug‐sensitive MCF‐7~wt~ cells. The same was observed in vivo: a small proportion of tumor cells became positive for TGase after administration of adriamycin‐based chemotherapy to patients with breast carcinoma. Similarly, continuous culture of MCF‐7~WT~, cells in the presence of adriamycin led to the appearance of the drug‐resistant phenotype that was in turn associated with increased expression of TGase. This increase in TGase was specific for adriamycin resistance. Like most known TGases, MCF‐7~ADR~ TGase was completely dependent on the presence of Ca^2+^ for its catalytic activity. Based on its immunoreactivity, the TGase in MCF.7~ADR~ cells was identified as an 85‐kDa tissue‐type TGase and was present only in the soluble form. Immunoblot analysis revealed that the increase in TGase activity was due to accumulation of the protein. Two cytosolic proteins of approximately 20 and 30 kDa in MCF‐7 cells served as suitable acyl donor substrates in TGase‐catalyzed reactions.