Inhibition of the receptor tyrosine kinase ROR1 by anti-ROR1 monoclonal antibodies and siRNA induced apoptosis of melanoma cells.

Abstract

e22198 Background: The receptor tyrosine kinase ROR1 has important functions during normal embryogenesis. It is overexpressed and of importance for the survival of different malignancies, including lung adenocarcinoma, breast cancer and chronic lymphocytic leukemia (CLL). ROR1 inhibition in CLL cells and cell lines with high expression of ROR1 induced specific apoptosis of the cells. There is however little information on the expression and function of ROR1 in melanoma. Here, we investigated ROR1 expression, phosphorylation and the effects of anti-ROR1 monoclonal antibodies (mAbs) and ROR1 suppressing siRNA on seven melanoma cell lines. Methods: ROR1 expression and phosphorylation was analysed in a series of malignant melanoma cell lines using RT-PCR, immunocytochemistry, flow cytometry and western blot. Annexin V/PI staining, MTT assay, PARP and caspase 8 and 9 cleavage as well as MCL-1 protein were used for detection of apoptosis induced by anti-ROR1 mAbs. The cytotoxic effects of anti-ROR1 mAbs were evaluated in the absence or presence of complement (CDC) or immune effector cells (ADCC). Transfection of melanoma cell lines using ROR1 siRNA was used for gene silencing. Results: ROR1 was shown to be overexpressed to a varying degree at protein level. ROR1 was phosphorylated at tyrosine and serine residues. Three out of four anti-ROR1 mAbs (clones 3H9, 5F1 and 1A8) induced a significant direct apoptosis of the ESTDAB049, ESTDAB112, DFW and A375 cell lines as well as cell death in the presence of complement or immune effector cells. ESTDAB081 and 094 cell lines were resistant to direct apoptosis of the four anti-ROR1 mAbs alone compared to the other melanoma cell lines, but not to complement dependent cytotoxicity (CDC) and antibody dependent cell mediated cytotoxicity (ADCC). ROR1 siRNA transfection induced down-regulation of the ROR1 expression both at the mRNA and protein levels proceeded by a significant apoptosis of the melanoma cells. Conclusions: The results indicate that ROR1 may play an important role in the survival of melanoma cells. The surface expression of ROR1 on melanoma cells may support the notion that ROR1 might be a suitable targeted for mAb therapy.