Human ROR1 Activates AKT and Accelerates Leukemia Cell Proliferation

Abstract 3872

ROR1 is an orphan-receptor-tyrosine-kinase-like developmental surface-antigen that is expressed by embryonic cells, chronic lymphocytic leukemia (CLL) cells, and neoplastic cells of many other types of cancer, but not by virtually all normal adult tissues. ROR1 can serve as a receptor for Wnt5a and potentially complex with other surface receptors for growth/survival factors elaborated by the tumor microenvironment or expressed by the tumor itself. In prior studies, we and others found that silencing ROR1 could impair tumor-cell growth and/or survival in vitro and in vivo. Although the CLL cells of ≥96% of all examined patients (N= 800) expressed ROR1, we found that a B-cell line derived from CLL cells, namely MEC1 (Leuk Res 23:127, 1999), lacked expression of this protein. This provided us with an opportunity to assess how introduction of ROR1 could affect the biology of this CLL-cell line, which has relatively slow doubling-time of ≈40 hours. We transfected MEC1 cells with either an expression vector encoding human ROR1 or a control vector, and then selected for stable transfectants in selection medium. MEC1 cells transfected with the ROR1 vector expressed high-levels of ROR1, but not MEC1 cells transfected with the control vector. MEC1 cells made to express ROR1 had higher levels of phosphorylated and activated-AKT and activated cAMP response-element-binding (CREB) protein than non-transfected MEC1 cells or cells transfected with the control vector. Furthermore, MEC1 made to express ROR1 had significantly greater proportions of cells in S/G2/M phase than did control-transfected cells 16 hours after transfer from serum-free medium to complete growth medium, indicating that MEC1 cells made to express ROR1 had increased relative proportions of cells undergoing cell division. Consistent with this, we found that ROR1⁺ MEC1 cells had significantly shorter doubling times in culture than did comparably cultured parental MEC1 cells or MEC1 cells transfected with the control vector. This study indicates that expression of ROR1 in the MEC1 CLL cell line can activate AKT/CREB and accelerate leukemia cell growth, providing us with a model system with which to interrogate the function of ROR1 in vitro.