Inhibition of prostate cancer cell growth by an avocado extract: role of lipid-soluble bioactive substances

https://doi.org/10.1016/j.jnutbio.2004.08.003Get rights and content

Abstract

Although the avocado is known as a rich source of monounsaturated fatty acids, there has been far less attention given to its content of other bioactive substances including carotenoids, which might contribute to cancer preventive properties similar to those attributed to other fruits and vegetables. The yellow-green color of the avocado prompted us to study the carotenoid content of this fruit using established methods in our laboratory. The California Hass avocado (Persea americana Mill.) was selected for study, because it is the most commonly consumed variety in the southwest United States. These avocados were found to contain the highest content of lutein among commonly eaten fruits as well as measurable amounts of related carotenoids (zeaxanthin, α-carotene, and β-carotene). Lutein accounted for 70% of the measured carotenoids, and the avocado also contained significant quantities of vitamin E. An acetone extract of avocado containing these carotenoids and tocopherols was shown to inhibit the growth of both androgen-dependent (LNCaP) and androgen-independent (PC-3) prostate cancer cell lines in vitro. Incubation of PC-3 cells with the avocado extract led to G2/M cell cycle arrest accompanied by an increase in p27 protein expression. Lutein alone did not reproduce the effects of the avocado extract on cancer cell proliferation. In common with other colorful fruits and vegetables, the avocado contains numerous bioactive carotenoids. Because the avocado also contains a significant amount of monounsaturated fat, these bioactive carotenoids are likely to be absorbed into the bloodstream, where in combination with other diet-derived phytochemicals they may contribute to the significant cancer risk reduction associated with a diet of fruits and vegetables.

Introduction

Numerous epidemiological studies suggest that a diet rich in fruits and vegetables is associated with a reduced risk of many common forms of cancer [1], [2], [3]. In support of these observations, several in vitro and in vivo studies have demonstrated the antitumor properties of various fruit and vegetable extracts [4], [5], [6], [7], [8], [9], [10], [11], [12]. These studies have significant implications for the agricultural production of fruits and vegetables that could be geared to enhance these health-promoting activities. Moreover, the same chemicals have been shown to have anti-inflammatory effects in some systems that broaden the potential benefits of these bioactive substances to several chronic diseases including heart disease.

Chemoprevention involving the use of plant chemicals to suppress, block, or reverse the process of carcinogenesis has received considerable attention over several decades. Several types of phytochemicals in vegetables and fruits such as carotenoids, flavonoids, and antioxidative vitamins have been studied as potential chemopreventive agents [13], [14]. In addition, public health recommendations to increase fruit and vegetable consumption have been implemented in several countries around the world [15], [16], [17].

Avocado (Persea americana Mill.) is an oleaginous fruit [18]. This fruit has a lipid content approximating 25% of the edible portion with an energy density similar to chicken breast [19]. The principal components of the lipid fraction are monounsaturated fatty acids [20]. Such monounsaturates have been studied for their potential cardiovascular benefits including effects on serum lipids [21], [22], [23], [24].

In addition to its high content of monounsaturated fats, avocados contain several bioactive phytochemicals. These include some carotenoids [25], [26], [27], B vitamins, vitamins C and E [25], terpenoids [28], d-manno-heptulose [29], β-sitosterol [30], persenone A and B [31], and phenols [32]. The bioactive substances in this fruit and in its extract or individual components have been shown to have antioxidative, radical suppressing [31], [32], [33], acetyl CoA carboxylase inhibitory [34], and antifungal [35], [36] activities.

One previous study reported on the lutein, β-cryptoxanthin, α-carotene, and β-carotene content of avocados from Finland but did not identify the variety studied [26]. Another study reported β-carotene and α-tocopherol contents in Hass and Fuerte varieties but demonstrated no lutein [25]. These studies stimulated us to undertake this research based on the obvious yellow-green color of the avocado, which suggested to us that the predominant carotenoid should be lutein. The antiproliferative effects of a lipid extract of avocado on prostate cancer cells were examined in vitro to assess in preliminary fashion the potential chemopreventive effects of the lipid-soluble bioactive substances in the avocado.

Section snippets

Sampling

Hass avocado samples were provided by Mission Produce (Oxnard, CA) and were shipped from Oxnard, CA to the Analytical Phytochemical Laboratory of the UCLA Center for Human Nutrition. Samples arrived in cartons were immediately sealed hermetically and stored at room temperature to permit slow ripening. They were analyzed within 1–4 days after arrival once ripened. Ripeness was judged based on the softening of the avocado and darkening of its skin [25]. Twelve ripened avocados were randomly

Carotenoid and vitamin E content

The concentrations of the carotenoids, lutein, zeaxanthin, β-cryptoxanthin, α-carotene, and β-carotene in the nine avocado samples from three harvesting times (January, April, and May 2001) are shown in Table 1. In addition to these carotenoids, concentrations of two forms of vitamin E, α-tocopherol and γ-tocopherol, were also measured in the same samples (Table 1). Recovery after chemical extraction and saponification was determined for β-carotene and was 97.3% on average. The results of this

Discussion

Avocado is a fruit typically viewed only as a source of monounsaturated fats in the diet. However, the appearance of the fruit suggested that it might also contain lutein based on its yellow-green color. In the present study, we demonstrated that the California Hass avocado contains several carotenoids and that it is one of the richest sources of lutein among commonly eaten fruits while also containing significant amounts of vitamin E. Previously, Heinonen et al. reported on the lutein,

Acknowledgments

This research was supported by research grants from the UCLA Clinical Nutrition Research Unit (NIH grant CA 42710), UCLA Center for Dietary Supplements Research in Botanicals (NIH grant AT00151), and California Avocado Commission. J. Arteaga and S. Huerta were supported by the Nutrition and Obesity Training Grant (PHS/NIH DK07688).

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