Research Interests: Overview

Chemoprevention Through Dietary Vegetables

Consumption of fruits and vegetables has been associated with reduced incidence of cancer, especially in the gastrointestinal tract. The Brassica vegetables, which include broccoli, spinach, cabbage, cauliflower, Brussels sprouts, kale, collard greens, pak choi and kohlrabi, are rich sources of glucosinolates (β-thioglucoside-N-hydroxysulfates, eg. glucoraphanin (1), Figure 1); evidence suggests that these phytochemicals are indirectly responsible for the observed cancer chemopreventive properties of cruciferous vegetables. Glucosinolates are substrates of the enzyme myrosinase (β-thioglucoside glucohydrolase, EC 3.2.3.1). Although several myrosinase isoenzymes have been isolated from various organisms, they are generally promiscuous with respect to the glucosinolate aglycone. Both glucosinolate-producing plants and intestinal bacteria are sources of myrosinase, a feature which allows humans to degrade dietary glucosinolates when plant myrosinase is inactivated through cooking. Myrosinase catalyzes the hydrolysis of the glucosinolate thioglucosidic linkage to form an unstable intermediate (2) that degrades to a variety of bioactive products. At physiological pH, this intermediate predominantly undergoes a Lossen rearrangement to form the respective isothiocyanate (ITC, eg.L-sulforaphane, 3).

Figure 1

Biological Activities of Isothiocyanates

Although glucosinolates themselves have no known bioactivity, many of their corresponding organic ITCs are well-documented chemopreventive agents. ITCs can induce phase II enzymes such as glutathione S-transferases, NAD(P)H:quinone reductase, epoxide hydrolase, and UDP-glucuronosyl-transferases, and can block carcinogen activation by reducing expression levels of phase I enzymes and stimulating apoptosis of damaged cells. Many of these effects are believed to result from the Nrf2-mediated activation of the antioxidant response element (ARE). Under basal conditions, Nrf2 is sequestered in the cytoplasm by Keap1, a multidomain, cysteine-rich protein bound to the actin cytoskeleton. The majority of ARE inducers are electrophiles capable of modifying cysteine, suggesting that Keap1 cysteines are targeted by these compounds in signaling ARE induction. Modification of specific Keap1 cysteines by ITCs is believed to disrupt the Keap1-Nrf2 complex, resulting in Keap1 saturation and subsequent Nrf2 nuclear accumulation. ITC bioactivity may also, in part, result from inhibition of histone deacetylase (HDAC). However, the specific molecular interactions responsible for these observed activities are not yet fully known.

Projects Overview

Our research group is interested in exploiting the myrosinase/glucosinolate enzymatic processes as a means of achieving selective drug or drug candidate activation. This multi-disciplinary research program will bridge the boundaries between chemistry and biology and offer students the opportunity for training in several fields of research. Multiple student projects are available with differential and cross-emphases including multi-step organic synthesis, cell culture, molecular biology, enzymology, and computational modeling. Interested students should email Dr. Mays for more information.