Retinoids represent a popular group of differentiation inducers that are successfully used in oncology for treatment of acute promyelocytic leukemia in adults and of neuroblastoma in children. The therapeutic potential of retinoids is based on their key role in the regulation of cell differentiation, growth, and apoptosis, which provides a basis for their use both in cancer therapy and chemoprevention. Nevertheless, patients treated with retinoids often exhibit or develop resistance to this therapy. Although resistance to retinoids is commonly categorized as either acquired or intrinsic, resistance as a single phenotypic feature is usually based on the same mechanisms that are closely related or combined in both of these types. In this review, we summarize the most common changes in retinoid metabolism and action that may affect the sensitivity of a tumor cell to treatment with retinoids. The availability of retinoids can be regulated by alterations in retinol metabolism or in retinoid intracellular transport, by degradation of retinoids or by their efflux from the cell. Retinoid effects on gene expression can be regulated via retinoid receptors or via other molecules in the transcriptional complex. Finally, the role of small-molecular-weight inhibitors of altered cell signaling pathways in overcoming the resistance to retinoids is also suggested.

IntroductionRetinoids represent a popular group of differentiation inducers that are successfully used in oncology for treatment of neuroblastoma (NBL) in children. However. differentiation therapy has some limitations including toxicity and intrinsic or acquired resistance to retinoids observed in many patients. Therefore, seeking for molecular markers able to predict therapeutic response to retinoid administration is undoubtedly an important aspect of their use in clinical practice. Several putative biomarkers indicating sensitivity or resistance of NBL cells to retinoids were reported in recent studies. The main aim of our study was to analyse the expression of five candidate proteins (PBX1, HOXC9, HMGA1, HMGA2 and DDX39A) in one experimental cohort (NBL cell lines; relevant FFPE tumour samples).Material and methodsIn this study, 20 patient-derived NBL cell lines were used for the experiments. Sensitivity or resistance to natural (all-trans retinoic acid, 13-cis retinoic acid, 9-cis retinoic acid) and synthetic (fenretinide, bexarotene) retinoids was determined by MTT assay. Endogenous expression of the candidate biomarkers was analysed both on mRNA (RT-PCR) and protein (immunoblotting) levels in cell lines and on protein level (immunohistochemistry) in FFPE tumour samples. Changes in expression of these markers after treatment with retinoids were also analysed.Results and discussionsNBL cell lines resistant to retinoids showed either presence of HMGA2 or increased expression of HMGA1 together with PBX1. Cell lines without a detectable expression of HOXC9 is on both mRNA and protein level are resistant to retinoids. Increase of expression of HOXC9 protein after retinoid treatment was detected in sensitive cell lines only. Very strong expression of PBX1 protein was found in tumour samples taken from patients showing resistance or poor clinical outcome after treatment with retinoids.ConclusionOur experimental study confirmed the usefulness of selected putative markers indicating sensitivity or resistance of NBL cells to retinoids in one experimental cohort consisting of patient-derived cell lines and respective tumour samples.This study was supported by the project AZV MZCR 15-34621A.