gms | German Medical Science

Objective: In many cell types retinoic acid (RA) leads to differentiation and growth inhibition. Due to aberrant distribution of the RA binding proteins FABP5 and CRABP2 breast cancer cells do not differentiate but instead enter a prosurvival pathway after RA exposure. Similarly, glioblastoma stem-like cells respond poorly to RA treatment; their capacity for differentiation is compromised. This discovery has prompted us to investigate the state of RA signaling in these cells.

Methods: Mass arrays, knockdown of CRABP2 and FABP5 with lentiviral vectors expressing small interfering RNAs (siRNA), and overexpression of CRABP2 and FABP5 were performed; expression levels were confirmed with quantitative PCR and FACS analysis.

Results: We compared the ratio of FABP5:CRABP2 in normal neuronal stem cells (n=2), astrocytes (n=2), adherently growing glioma cell lines (n=10) GBM stem cells (n=4) and GBM tumor tissue (n=30). The ratio increased successively with astrocytes and normal neuronal stem cells having the lowest ratio followed by adherently growing glioma cells and glioblastoma tumor tissue. GBM stem cells displayed the highest FABP5:CRABP2 ratio. As a possible cause for aberrant expression of RA binding proteins we found a differential methylation pattern of CRABP2 comparing GBM tissue (n=50) with non-tumerous normal brain (n=4). Using overexpression and knockdown experiments we hope to further support our hypothesis that the ratio of FABP5:CRABP2 is essential for determining cell fate in the presence of RA.

Conclusions: Retinoic acid can bind two nuclear receptors. Our observations suggest that partitioning of RA between the two nuclear receptors is important in determining the fate of a cell. We hypothesize that the binding of RA to CRABP2 indirectly promotes differentiation and the binding of RA to FABP5 reduces the differentiation capacity in gliomas. This is consistent with our findings in normal neural stem cells, which have a higher expression level of CRABP2 and differentiate after RA exposure whereas GBM stem-like cells mostly maintain their stem cell features. Abnormal response to RA by GBM stem-like cells is most likely due to deregulation of the levels of RA-binding proteins CRABP2 and FABP5. We hypothesize that RA might be a treatment option in a subgroup of glioma patients that have a high expression level of CRABP2.