gms | German Medical Science

Objective: Measuring concentrations of the differentiation-promoting hormone retinoic acid (RA) in glioblastoma tissues would help to understand why RA treatment has been inefficient in clinical trials involving brain tumor patients.

Method: We apply a HPLC-based extraction and measurement protocol to screen glioblastoma tissues for levels of the RA precursor retinol and biologically active RA. We combine this approach with mRNA analyses of 27 tumors and 6 normal brains as well as with methylation and ChiP analyses of glioblastoma cell cultures.

Results: We report a multifaceted disturbance of RA synthesis in glioblastoma, involving multiple ALDH1A and RDH enzymes. Through database studies and methylation analyses we narrow down chromosomal deletions and aberrant promoter hypermethylation as potential mechanisms accounting for these alterations. Employing ChiP analyses and cell culture studies, we further show that chromatin at RA target genes is poised to RA substitution, but most glioblastoma cell cultures are completely resistant to RA treatment. Through in vitro studies we show that this paradoxical RA response is unrelated to alternative RA signaling through the FABP5/PPARD axis. Instead, we link RA resistance to activation of AKT as well as to inactivation of the pro-apoptotic downstream targets BAD and GSKβ3.

Conclusions: Our data uncover a multifaceted disturbance of RA synthesis in glioblastoma and encourages us to reconsider current RA treatment strategies.