Characterizing the Roles of Dietary Sulforaphane in Human Health Public Deposited


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  • The consumption of cruciferous vegetables is associated with several health benefits, including cancer prevention. Many of these benefits are attributed to the phytochemical, sulforaphane (SFN), which is derived from cruciferous vegetables such as broccoli and broccoli sprouts. These vegetables contain glucoraphanin (GFN), SFN’s precursor, which is converted to SFN by the plant enzyme, myrosinase. Studies have shown that SFN influences a variety of biological pathways that are thought to be critical for maintaining health and preventing disease. For example, SFN has been shown to reduce inflammation and oxidative stress, and promote cancer cell-specific cell cycle arrest and apoptosis, leaving healthy cells intact. While several of the health-promoting effects of SFN may be mediated by the Keap1/nuclear factor (erythroid-derived 2)-like 2 (Nrf2)/antioxidant response element (ARE) pathway, emerging evidence suggests that epigenetic mechanisms involving histone deacetylases (HDAC), DNA methyltransferases (DNMT) and microRNAs (miRNA) may also play a role (Chapter 1). Much of what is known about SFN comes from studies conducted in cell cultures and animal models using high doses of SFN and purified chemical forms. Observations from these studies may not reflect events that occur in humans who obtain SFN from dietary sources, such as broccoli and broccoli sprouts. Only a few studies have been conducted to evaluate the effects of SFN in humans, and results sometimes vary from those seen in preclinical studies. Even fewer studies have evaluated effects of SFN in human tissue. Additionally, there is limited understanding of how dietary form (food form versus supplemental form), doses and dosing regimens may differ with respect to bioavailability and biochemical target. These differences could impact their ability to elicit specific health benefits in humans. Thus, the purpose of this dissertation is to translate mechanistic work with SFN into humans by evaluating responses of genetic and epigenetic molecular targets in human subjects following consumption of controlled doses of dietary SFN. We also evaluated the impacts of dietary form and dosing schedule on SFN absorption and metabolism in humans. The overarching hypothesis for this dissertation is that, in humans, SFN consumption decreases HDAC activity and increases histone acetylation, thereby promoting transcriptional activation of tumor suppressor genes and changes in prognostic biomarkers. We further hypothesize that, in humans, broccoli sprout consumption alters plasma metabolite profiles. In a randomized, placebo-controlled, clinical trial, we evaluated the effects of consuming a broccoli sprout extract containing GFN, the precursor to SFN, on tumor biomarker responses in breast tissue collected from women (N=54) scheduled for breast biopsy. Following 4-8 weeks of supplementation with the sprout extract, we observed a significant reduction in expression of Ki-67 and HDAC 3 protein in benign breast tissue from pre- to post-supplementation, though responses were not significantly different from those in the placebo group. We also observed decreased HDAC activity in peripheral blood mononuclear cells (PBMC). Importantly, this study identified responses in tumor biomarkers in human breast tissue following consumption of a diet-relevant dose of SFN consumed as part of a broccoli sprout extract. As the effects of SFN depend on its ability to be absorbed, metabolized and distributed to tissues, we evaluated SFN absorption and metabolism from a myrosinase-treated broccoli sprout extract containing SFN in its active form. Healthy adults (N=20) were randomized to consume a single dose of 200 μmol SFN equivalents from either fresh broccoli sprouts or the SFN-rich broccoli sprout extract. Approximately 3 times more SFN was absorbed into the plasma and excreted in urine from sprout consumers compared to extract consumers. While this extract delivered higher amounts of SFN than the GFN extract used in the clinical trial, SFN was still relatively more bioavailable from fresh broccoli sprouts. Even though sprouts delivered higher amounts of SFN than the BSE, sprout and BSE consumers had similar changes in PBMC HDAC activity, and circulating levels of heme oxygenase-1 and p21. Furthermore, since SFN metabolites are rapidly metabolized and mostly excreted within 24 hours following a dose, we conducted a second study phase to evaluate the efficacy of a twice-daily dosing regimen on maintaining SFN metabolite levels in the plasma at the 24-hour time point. SFN metabolite levels were higher 24 hours following the divided dose compared to the same time point after subjects consumed a single dose. To discover additional molecular targets of SFN, we used an untargeted metabolomics approach to screen for changes in the human plasma metabolome following consumption of fresh broccoli sprouts in healthy adults (N=10). This investigation revealed decreases in glutathione, glutamine, cysteine, dehydroepiandrosterone (DHEA), and several fatty acids (14:0, 14:1, 16:0, 16:1, 18:0, 18:1). Deoxyuridine monophosphate (dUMP) was increased. These metabolites are associated with antioxidant status and steroid, nucleotide and lipid metabolism and are possible molecular targets of SFN action. This information can aid in studying novel roles of SFN in human health and disease prevention. In conclusion, these data provide the first evidence that SFN may alter cell proliferation and epigenetic mechanisms in human breast tissue. Additionally, we identified several metabolites detectable in human plasma that were altered with the consumption of fresh broccoli sprouts. Further study of pathways associated with these metabolites will improve understanding of existing and novel health benefits of SFN. We also demonstrated acceptable bioavailability of a SFN-containing broccoli sprout extract, providing critical information for future human studies with SFN. Taken together, this work supports that consuming dietary sources of SFN may indeed contribute to improved health and prevention of disease in humans.
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