Trans retinoic acid inhibits in vivo tumour growth of C6 glioma in rats: effect negatively influenced by nerve growth factor.

Retinoic acid (RA) and nerve growth factor (NGF) are both differentiation factors for central nervous system tumours. Mouse-derived NGF inhibits proliferation of C6 glioma cells in vivo in the absence of serum. Retinoic acid inhibits in vivo growth of C6 gliomas in the subcutaneous tissue of rats. This study evaluated the response of C6 cells implanted in the rat cortex to NGF, RA, or a combination of the two in 89 rats. Tumour size, cellular density and morphology were analysed using light microscopy. Treatment with RA alone resulted in tumour volumes that were 38% of control and 48% of NGF-treated groups. There was no significant difference in the tumour volumes or in cell morphology in C6 cells treated with NGF alone compared to controls. Tumours treated with a combination of RA and NGF were larger however, than tumours treated with RA alone. This suggests that despite the growth inhibitory effects of NGF in vitro, NGF acts to prevent the growth inhibitory effect of RA in vivo.