GCS-100 Targets Apoptosis and Cell Cycle Regulation in Myeloma Cells.

Myeloma is characterised by abnormalities of multiple cellular pathways that result in apoptosis resistance, aberrant signalling and deregulated cell cycle. GCS-100 is a modified citrus pectin that has demonstrated galectin-3 inhibitory activity. It has recently been reported to have clinical activity against CLL (Cotter et al. ASCO 2009) and has previously been observed to have pre-clinical anti-myeloma effect even in the context of bortezomib resistance (Chauhan et al. Canc Res 2005). We have elucidated the mechanisms of action of GCS-100 in the context of myeloma cell biology.

The effects of GCS-100 on myeloma cell proliferation, cell cycle, apoptosis induction, cell death and cell signalling were examined in representative cell lines and primary myeloma cells. Cell viability assay, flow cytometric assessment of apoptosis, DNA content, mitchochondrial transmembrane potential and immunoblotting assessment of protein expression were assessed following GCS-100 exposure.

Exposure of RPMI8226, U266 and OPM2 myeloma cell lines to GCS-100 confirmed that it inhibits proliferation and induces apoptosis with activation of both caspase-8 and -9 pathways. GCS-100 exposure was also associated with accumulation of cells in sub-G1 and G1 phases of cell cycle and a dose and time dependent loss of mitochondrial potential. Primary myeloma cells treated with GCS-100 had reduced viability both in isolation and in a stromal cell co-culture model. Examination of key anti-apoptotic and pro-apoptotic proteins revealed that Mcl-1 and Bcl-x_L levels were reduced by GCS-100 and this was accompanied by a rapid induction of pro-apoptotic Noxa. Bcl-2, Bax, Bak, Bim, Bad, Bid and Puma remained unchanged. Pan-caspase inhibition abrogated Mcl-1 but not Bcl-X_L reduction and inhibited loss of mitochondrial transmembrane potential. GCS-100 treatment also upregulated the cell cycle inhibitor p21^Cip1 with concurrent reduction of the procycling proteins cyclin E2, cyclin D2 and CDK6. Furthermore, there was a reduction in signal transduction in the form of reduced activated IκBα, IKK and Akt as well as prevention of upregulated IκBα and Akt following stimulation with appropriate cytokines suggesting a potential microenvironment effect.

GCS-100 is a potent modifier of myeloma cell biology in a manner suitable for novel myeloma therapy.

No relevant conflicts of interest to declare.