13-cis retinoic acid and complete androgen blockade in advanced hormone-naive prostate cancer patients: report of a phase II randomized study

Retinoids as Chemo-Preventive and Molecular-Targeted Anti-Cancer Therapies

Retinoic acid (RA) agents possess anti-tumor activity through their ability to induce cellular differentiation. However, retinoids have not yet been translated into effective systemic treatments for most solid tumors. RA signaling is mediated by the following two nuclear retinoic receptor subtypes: the retinoic acid receptor (RAR) and the retinoic X receptor (RXR), and their isoforms. The identification of mutations in retinoid receptors and other RA signaling pathway genes in human cancers offers opportunities for target discovery, drug design, and personalized medicine for distinct molecular retinoid subtypes. For example, chromosomal translocation involving RARA occurs in acute promyelocytic leukemia (APL), and all-trans retinoic acid (ATRA) is a highly effective and even curative therapeutic for APL patients. Thus, retinoid-based target discovery presents an important line of attack toward designing new, more effective strategies for treating other cancer types. Here, we review retinoid signaling, provide an update on retinoid agents and the current clinical research on retinoids in cancer, and discuss how the retinoid pathway genotype affects the ability of retinoid agents to inhibit the growth of colorectal cancer (CRC) cells. We also deliberate on why retinoid agents have not shown clinical efficacy against solid tumors and discuss alternative strategies that could overcome the lack of efficacy.

The miR-96 and RARγ signaling axis governs androgen signaling and prostate cancer progression

Expression levels of retinoic acid receptor gamma (NR1B3/RARG, encodes RARγ) are commonly reduced in prostate cancer (PCa). Therefore, we sought to establish the cellular and gene regulatory consequences of reduced RARγ expression, and determine RARγ regulatory mechanisms. RARG shRNA approaches in non-malignant (RWPE-1 and HPr1-AR) and malignant (LNCaP) prostate models revealed that reducing RARγ levels, rather than adding exogenous retinoid ligand, had the greatest impact on prostate cell viability and gene expression. ChIP-Seq defined the RARγ cistrome, which was significantly enriched at active enhancers associated with AR binding sites. Reflecting a significant genomic role for RARγ to regulate androgen signaling, RARγ knockdown in HPr1-AR cells significantly regulated the magnitude of the AR transcriptome. RARγ downregulation was explained by increased miR-96 in PCa cell and mouse models, and TCGA PCa cohorts. Biochemical approaches confirmed that miR-96 directly regulated RARγ expression and function. Capture of the miR-96 targetome by biotin-miR-96 identified that RARγ and a number of RARγ interacting co-factors including TACC1 were all targeted by miR-96, and expression of these genes were prominently altered, positively and negatively, in the TCGA-PRAD cohort. Differential gene expression analyses between tumors in the TCGA-PRAD cohort with lower quartile expression levels of RARG and TACC1 and upper quartile miR-96, compared to the reverse, identified a gene network including several RARγ target genes (e.g., SOX15) that significantly associated with worse disease-free survival (hazard ratio 2.23, 95% CI 1.58 to 2.88, p = 0.015). In summary, miR-96 targets a RARγ network to govern AR signaling, PCa progression and disease outcome.

Effect of the Synthetic Retinoid Fenretinide on Circulating Free Prostate-Specific Antigen, Insulin-Like Growth Factor-I, and Insulin-Like Growth Factor Binding Protein-3 Levels in Men with Superficial Bladder Cancer

PURPOSE

Fenretinide (4-HPR) is a synthetic retinoid that has shown a preventive activity in prostate cancer animal models.

EXPERIMENTAL DESIGN

We measured the changes in total and free prostate-specific antigen (PSA) and its association with insulin-like growth factor I (IGF-I) and IGFBP-3 levels after 1 year of treatment in 24 subjects given 4-HPR and 24 control subjects enrolled in a randomized bladder cancer prevention trial.

RESULTS

No significant effect of 4-HPR was observed on total and free fraction of PSA levels. The median percentage [95 confidence interval (95% CI)] change for % free PSA and total PSA in the 4-HPR and the control group were, respectively, 7.6 (95% CI, -4.0 to 69.3) versus 5.1 (95% CI, -21.4 to 59.8) and -7.8 (95% CI, -18.2 to 52.5) versus -12.3 (95% CI, -44.6 to 9.6). However, in patients ages <60 years, there was a trend to an increase of total free PSA and % free PSA after treatment with 4-HPR that was different from a trend to a decrease in the control group (P = 0.002 and 0.052, respectively). The interaction between age and treatment was statistically significant on free PSA (P = 0.001). A similar pattern was noted with smoking status (P = 0.011 for the interaction on free PSA). No association was observed between PSA levels and IGF-I or IGFBP-3 levels.

CONCLUSIONS

We conclude that 4-HPR has no significant effect on circulating PSA, but it increases significantly free PSA levels in subjects younger than 60 years and in nonsmokers. These effects might support an activity in prostate cancer prevention but further studies are required.

A comparison of the effectiveness of soy protein isolate and fish oil for reducing the severity of retinoid-induced hypertriglyceridemia

The effectiveness of soy protein isolate (SPI) to reduce the severity of retinoid-induced hypertriglyceridemia has been demonstrated in the rat, but not in human subjects. Because fish oil has been demonstrated to be effective at lowering serum triglyceride concentration in human subjects undergoing retinoid therapy, a study was conducted to compare the ability of SPI with that of fish oil to reduce the severity of retinoid-induced hypertriglyceridemia in the rat. Male Fischer 344 rats, n=8/group, were fed one of four isonitrogenous, isoenergetic diets, consisting of a control diet containing 24% casein +20% corn oil (C), and three 13-cis retinoic acid (13cRA)-supplemented diets containing 24% casein +20% corn oil (R), 24% SPI +20% corn oil (SR), and 24% casein +15% fish oil and 5% corn oil (FR). There was no effect of diet on food intake or final body weight. Serum triglyceride concentration for group R was higher (P<0.001) than for groups C, SR, and FR (7.20 vs. 2.50, 2.84, and 2.02 mmol/L, respectively); values for groups SR and FR did not differ for this parameter. The serum concentration of 13cRA for group R did not differ from that for groups SR and FR. Thus, SPI was as effective as fish oil in reducing the severity of retinoid-induced hypertriglyceridemia in an animal model, suggesting that it may be effective for this purpose in human subjects.

Vitamin A, Retinoids and Carotenoids as Chemopreventive Agents for Prostate Cancer

PURPOSE

Evidence is examined for the use of vitamin A, retinoids and carotenoids as chemopreventive agents for prostate cancer.

MATERIALS AND METHODS

Studies in in vitro and animal experimental models as well as in human observational (epidemiologic) and experimental studies are reviewed.

RESULTS

There is little evidence that dietary vitamin A is associated with prostate cancer risk and, thus, it is unlikely that vitamin A or retinyl palmitate, the form most often used in dietary supplements, would be useful as chemoprevention agents. Several pharmaceutical formulations of retinoids show cancer prevention properties in animal experimental models but their high toxicity makes them unlikely candidates for cancer prevention. There is also currently no evidence that dietary carotenoids (except for lycopene and possibly other bioactive compounds found in tomato products) will be useful for prostate cancer prevention. Epidemiological and experimental studies show no association of beta-carotene with prostate cancer risk. There is inconsistent though intriguing evidence from epidemiological studies that tomato products and/or lycopene is associated with reduced prostate cancer risk. However, animal experimental studies of lycopene and prostate cancer are not strongly supportive, and the results of human experimental studies are not interpretable due to poor design.

CONCLUSIONS

There is currently no evidence that vitamin A, synthetic retinoids or carotenoids can be used as prostate cancer chemopreventive agents. Experimental studies are needed to determine whether lycopene or other compounds in tomatoes have chemopreventive properties.