Dietary tomato carotenoids and β-carotene-15,15'- dioxygenase (Bco1) genotype alter androgen metabolism, lipid metabolism, and prostate cancer progression in mice
β-carotene-15,15'-dioxygenase (BCO1) cleaves dietary carotenoids at the central 15,15' double bond, most notably acting on β-carotene to yield retinal. Much work has gone into the description of enzymatic carotenoid cleavage activities of the enzyme, but although pleiotropic effects of Bco1 loss on lipid and cholesterol metabolism in vivo have occasionally been described, few investigators have reported extensively on this. Furthermore, given the intersections of lipid metabolism, cholesterol/steroid metabolism, and carotenoid transport and distribution, functions of BCO1 within these interdependent realms is of clear physiological relevance. Moreover, since observational data in humans have repeatedly demonstrated that tomato carotenoid intake is associated with a reduced risk of primary prostate cancer (PCa), fundamental knowledge of the role of BCO1 on steroid and lipid metabolism is imperative for proper interpretation of epidemiological and experimental data demonstrating a protective role of dietary tomato. Thus, using animal models, we sought to: elucidate the carotenoid cleavageindependent role of Bco1 on circulating testosterone and prostatic androgen signaling; determine the impact of Bco1 on lipid, cholesterol, and retinoid metabolism in the liver, a key organ in the metabolism and biodistribution of dietary carotenoids; and evaluate the effects of dietary tomato feeding in an animal model of PCa and determine its efficacy in preventing disease progression. In Chapter 2, using the Bco1-/- mouse model, we sought to probe the effects of Bco1 ablation on testicular steroidogenesis, serum testosterone levels, and prostatic androgen signaling. Male wild-type (WT) and Bco1-/- mice were raised on carotenoid-free AIN-93G diets before sacrifice between 10-14 weeks of age. We observed a significant reduction in serum testosterone levels due to Bco1-/- genotype, accompanied by significantly reduced weights of the prostate and seminal vesicles. This reduction in serum testosterone may have been driven by significant reductions in the expression of a testicular steroidogenic gene (Hsd17b3) and/or Leydig cell populations. Concomitantly, we observed reductions in markers of androgen signaling, proliferation, and cell cycle progression in the prostate. Together, these data support a hypothesis that Bco1-/- mice exhibit reduced testicular testosterone synthesis, concomitant with a lowering of circulating testosterone, resulting in depressed prostatic androgen signaling, proliferation, and organ weight. In Chapter 3, in order to further probe the impacts of Bco1 loss on lipid and retinoid metabolism, we again utilized the carotenoid-free Bco1-/- mouse model. We found that, first, adult male Bco1-/- mice accumulate 2-6-fold more hepatic retinyl esters than do WT mice and that this accumulation is highly dependent on developmental age, as juvenile mice demonstrated significant decreases in retinyl esters as a result of Bco1 loss. Additionally, patterns of hepatic lipid esterification are altered by Bco1 loss in adult mice, as evidenced by alterations in unique fatty acid species, most notably being a two-fold increase in cholesteryl palmitate. Finally, PPAR/RXR signaling may explain at least a portion of the hepatic lipid phenotype observed in Bco1-/- mice, including the steatosis reported by several independent investigators. In summary, we report that Bco1 loss interacts with developmental age to alter hepatic retinyl ester accumulation, even when maintained on a carotenoid-free, vitamin A-sufficient diet. This effect may in turn result in altered RXR/PPAR signaling and a disruption of hepatic lipid metabolism. In Chapter 4, we sought to determine whether dietary tomato treatments could reduce the PCa progression in a mouse model of lethal PCa. This was provoked by the observation that although epidemiological studies continue to find strong protection against advanced and lethal PCa with increased tomato consumption, no animal studies have examined the effects of dietary tomato on castration-resistant prostate cancer (CRPC), which constitutes the late and lethal stage of human PCa. Therefore, we used a mouse model, the transgenic adenocarcinoma of the mouse prostate (TRAMP), to probe this gap in the literature. TRAMP mice (n=30/treatment) were treated with either control AIN-93G diet (CON) or 10% w/w lyophilized tomato paste (TP) from 4 weeks of age until euthanization. A third group, modeling adjuvant dietary intervention, consumed CON diet from 4 weeks of age until 12 weeks of age, and then 10% w/w lyophilized tomato paste from week 12 until euthanization (TP-I). To induce castration-resistant tumors, all animals were castrated at 12 weeks of age. Beginning at 10 weeks of age, mice were monitored longitudinally with biweekly ultrasound scans for tumor detection and measurement of tumor growth rate. Using this experimental design, we have shown for the first time that dietary tomato can reduce CRPC progression in an animal model. CRPC tumor volume and rate of CRPC tumor growth were significantly reduced nearly 50% by adjuvant dietary tomato intervention following surgical castration. In addition, compared to control feeding, adjuvant tomato treatment reduced distant gross metastasis to the lungs, and increased gonadal adipose depot and body weights. Importantly, CRPC progression was reduced with achievable levels of dietary tomato intake, and through the use of a common tomato food source that is already commercially available to consumers. These effects should be replicated and extended in future animal studies to assess their potential for clinical translation in men with CRPC. In conclusion, this work has revealed that BCO1, a key enzyme involved in carotenoid metabolism, impacts steroid, lipid, and retinoid metabolism independent of its role in carotenoid cleavage. Further work in animal models and humans is needed, but we have shed additional light on a somewhat unexamined corner of the carotenoid field. Future work should utilize our findings in mice and assess their relevance in humans. Secondly, we have demonstrated that a relevant level and source of dietary tomato can reduce cancer progression in a mouse model of human CRPC. This work requires confirmation and further expansion in order to be considered for translation to humans. In total, this dissertation has contributed novel findings to our understanding of carotenoid metabolism and bioactivity in vivo.
tomato, Bacon, prostate cancer, castration-resistant prostate cancer, carotenoids, lipid, retinoid, retinol, retinal ester, liver, hepatic, testosterone, androgen
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign