Mcl-1, Bcl-x_L and Cell Cycle Inhibition by GCS-100: A Novel Therapy for Myeloma.

The marrow stromal cells in multiple myeloma activate multiple signalling pathways via stimulation by the cytokines they secrete. This leads to upregulation of anti-apoptosis, drug resistance, cell cycle and metabolic pathways. Disruption of these signalling networks can lead to myeloma cell death and make them attractive targets for novel therapies. In addition, Bcl-2 family proteins, mcl-1 and bcl-x_L are important for the survival of myeloma cells. The modified citrus pectin, GCS-100 has been used safely in Phase I studies for solid tumors, can induce cell death in lymphoma via blocking galectin-3 / Bcl-2 dimerisation and in myeloma cells by mechanisms not fully understood. GCS-100 as well as pro-apoptotic effects, also demonstrates anti-proliferative and anti-angiogenic properties, all of which may be useful in the therapy of myeloma. This study was aimed at elucidating the mechanism of action of this novel agent so as to apply it most effectively to myeloma therapy. Myeloma cell lines RPMI8226 and U266 treated with GCS-100 show induction of apoptosis in a dose and time dependent manner with a loss of cells in the G1 and S phase of cell cycle and a significant increase in G0 / sub-G1 cells. Treatment of RPMI8226 led to a marked reduction of mcl-1 and bcl-x_L after 24 hours of treatment. There was no change observed in protein levels of bcl-2 or galectin-3. Activation of NFκB in myeloma cells is associated with proliferation and adhesion molecule upregulation. GCS-100 treatment led to a time dependent reduction in activated IκBα and p65NFκB. Furthermore pre-treatment of myeloma cells with GCS-100 prior to stimulation with TNFα, a known activator of IκBα, inhibited this activation. Similarly, treatment with GCS-100 led to a reduction in the amount of activated Akt (a regulator of cell cycle) and inhibited IGF-1 associated activation of Akt. The effect of GCS-100 on cell cycle regulatory proteins revealed downregulation of cyclin D1, p16^INK4A and CDK6 at 24hrs, however, there was no change in expression of CDK4 and p15^INK4B. The p21^CIP1 was upregulated with a corresponding decrease in protein levels of cyclin E2 and CDK2 but there was no change in p27^KIP1. GCS-100 clearly inhibits progression through G1 / S phase of cell cycle. Studies are currently ongoing for primary cells. In conclusion GCS-100 is a novel complex carbohydrate that is effective for the induction of myeloma cell death. It has multi faceted effects in reducing proliferation and induces apoptosis by down regulating crucial anti-apoptotic proteins, cell cycle regulators and signalling proteins and may also act by interfering with the myeloma cell microenvironment. Phase I studies in myeloma have been commenced.