Title: SIGNIFICANCE OF THE INTERACTION BETWEEN CALRETICULIN AND COMPLEMENT PROTEIN C1Q IN BREAST CANCER CELLS TREATED WITH ANTHRACYCLIN
Author(s): Onoriode Oyiborhoro and Oshomoh, Emmanuel Ola
The initiation and maintenance of immunogenic cancer cell death is currently seen as one of the major aims of cancer chemotherapy and radiotherapy. Some chemotherapeutic agents such as anthracyclins mediate a rapid transfer of calreticulin from the lumen of endoplasmic reticulum to the cell surface. Presence of calreticulin on the cell surface can potentially lead to binding of soluble complement proteins such as C1q and thrombospondin, triggering an immune-mediated killing of cancer cells. The calreticulin-C1q interaction might also alter C1q-mediated immune response, since the N- and P- domains of calreticulin interacts with the globular head region of C1q to inhibit C1q-mediated haemolysis of IgM sensitized erythrocytes. However, details concerning the effect of these interactions on C1q binding to chemotherapy-induced cancer cells remain unknown. Using pMal-c2 protein fusion and purification system and solid-phase assay methods, we show here that C1q binds more to the N-domain of calreticulin at high concentrations and more to the P-domain at moderate and low concentrations, with very minimal interaction with the C-domain of calreticulin at all concentrations. We further demonstrate that the binding of C1q to the N- and P- domains of calreticulin increases C1q binding to anthracyclin-treated breast cancer cells, while binding of C1q to the C-domain of calreticulin inhibits C1q binding to anthracyclin-induced breast cancer cells. These findings suggest that the N- and P- domains of calreticulin might bind to a site on C1q (globular head) that differs from that to which the C-domain of calreticulin binds and that the binding site for the C-domain of calreticulin might also be the site via which C1q interacts with drug-induced breast cancer cells. The effect of this on C1q-mediated anti-cancer immune response however, remains unknown.
KEYWORDS: Calreticulin, cancer cells, complement proteins, immune system