Despite the known anti-proliferative and tumor-suppressive effects seen with retinoic acid (RA), treatment of metastatic renal cell carcinoma (RCC) failed to meet the initial expectations. As the exact mechanisms of action of RA and especially the role of the cellular RA binding proteins (CRABP) have not been elucidated yet, we investigated the expression of CRABP-I and its potential influence on RA response in RCC. Real-time RT-PCR analysis disclosed a significant lack of CRABP-I expression in four RCC cell lines and 12 primary RCC samples; in contrast, high expression levels were found in the respective adjacent normal kidney tissue. To further investigate the impact of CRABP-I on RA response in RCC, A-498 RCC cells were employed as a cellular model system. CRABP-I was stably transfected into A-498 cells which consequently displayed substantial resistance to all-trans (ATRA) and 9-cis RA compared to vector controls lacking CRABP-I. Comparison of gene expression profiles of ATRA-treated CRABP-I-expressing A-498 cells and vector controls revealed specific regulation of 54 of ∼20,000 genes tested on a selected human CodeLink^TM UniSet Bioarray, with a prominent modulation of genes involved in transcriptional control, signaling, apoptosis, cell cycle regulation and metabolism. The genetic changes reported here contribute to a better understanding of the role of RA in RCC. They also provide new insights into CRABP-I-mediated signaling and gene expression.

No abstract

Introduction: Retinoic acid (RA) and its derivates possess antiproliferative and tumor-suppressive abilities and are successfully used in the treatment of various malignancies. However, in metastatic renal cell carcinoma (RCC), its application did not meet first expectations. As the exact mechanisms of RA action and especially the role of the cellular retinoic acid-binding proteins (CRABP) still remain unclear, we studied the expression of CRABP-II and its potential influence on RA response in RCC. Materials and Methods: We used the real-time RT-PCR methodology to investigate CRABP-II expression in 12 RCC samples and corresponding normal kidney tissue. Moreover, CRABP-II was cloned and overexpressed in CAKI-2 RCC cells. CRABP-II (un)transfected CAKI-2 cells were stimulated with all-trans RA (ATRA) and 9-cis RA, and their antiproliferative effects were evaluated using ³H-thymidine-proliferation assays. Results: Using RPS9 and RPLP0 to normalize its expression, the median tumor/kidney ratio for CRABP-II expression was 0.16 and 0.12, respectively. Using proliferation assays, CRABP-II overexpressing CAKI-2 cells did not exhibit a significant change in RA sensitivity, but appeared to be less sensitive toward RA-stimulation compared to CAKI-2 cells expressing naturally low levels of CRABP-II (maximum difference, 59% at 3 µM ATRA). Conclusions: We were able to demonstrate a downregulation of CRABP-II expression in primary RCC tumor samples compared to the corresponding normal kidney tissue. However, CRABP-II overexpression in CAKI-2 RCC cells did not significantly influence RA associated antiproliferative actions. Further experiments are necessary to define the exact role of CRABP-II and its downregulation in RCC including its influence and dependence on other molecules involved in RA signalling and metabolism.

No abstract