Blocking Pleiotrophin Activity Inhibits Multiple Myeloma (MM) Cell Growth In Vitro and in a Severe Combined Immunodeficient (SCID)-hu Murine Model of Human MM.

Pleiotrophin (PTN) is a heparin-binding growth factor that binds CD138 and stimulates angiogenesis, tumor growth and metastasis in some solid tumors. Recently, we have shown that this factor is highly produced by multiple myeloma (MM) cell lines including RPMI8226 and U266 and fresh malignant plasma cells, and is secreted into the culture medium following short-term culture of bone marrow from MM patients. We investigated the effects of PTN on MM growth in vitro and in vivo using a SCID-hu murine MM model. We determined the anti-proliferative effects of suppressing PTN by cloning a whole PTN sense or anti-sense cDNA construct containing the green fluorescent protein (GFP) gene into the MM cell lines RPMI8226 and U266. Cells transduced with sense PTN showed markedly increased proliferation compared to cells transduced with vector alone whereas the anti-sense-containing MM cells showed reduced cell numbers. In addition, we treated RPMI8226 and U266 cells with a polyclonal anti-PTN antibody and evaluated its effect on MM growth. These cells were cultured for 48 hours in the presence of the anti-PTN antibody at a concentration of 100 micrograms/ml or a control antibody, and effects on cell growth assessed with an MTT assay. Marked anti-MM effects were observed with the anti-PTN antibody compared to the control antibody in both cell lines [RPMI8226 (p < 0.01) and U266 (p < 0.001)]. In order to further define the importance of PTN in the growth of MM in a more clinically relevant in vivo setting, we determined whether this polyclonal anti-PTN antibody could suppress tumor growth and human paraprotein secretion using our SCID-hu murine model of human myeloma LAGλ-1. LAGλ-1 has been previously shown by our group to produce large amounts of PTN as measured in mouse serum by ELISA and by RT- PCR analysis on freshly isolated LAGλ-1 tumor cells. Thirty SCID mice (n = 5 mice/group) were implanted with a 0.4 – 0.6 cm³ LAGλ-1 tumor fragment into the left hind limb muscle. Fourteen days following implantation, mice were randomized into treatment groups, and received treatment intraperitoneally (IP) with anti-PTN antibody at doses of 0.1, 0.3, 1.0, 3.0 or 10 mg/kg or vehicle alone twice weekly. Mice receiving anti-PTN antibody at the highest doses (3.0 and 10 mg/kg) showed marked inhibition of tumor growth [3.0 mg/kg (p < 0.03), 10 mg/kg (p < 0.008)] as well as decreases in levels of human paraprotein [3.0 mg/kg (p < 0.004), 10 mg/kg (p < 0.003)]. Notably, immunohistochemical staining with an anti-CD138 antibody showed a marked reduction in cells with CD138 positivity in the LAGλ-1 tumors from animals treated with anti-PTN antibody compared to mice treated with vehicle alone. These in vitro and in vivo results demonstrate that PTN may be a potential new target for the treatment of MM. The effects of this therapy on angiogenesis and cell signaling are currently under investigation.