Nephroprotective action of tocotrienol-rich fraction (TRF) from palm oil against potassium dichromate (K₂Cr₂O₇)-induced acute renal injury in rats

Abstract

Industrial and occupational exposure to chromium compounds, particularly hexavalent chromium (Cr(VI))-containing compounds are often known to cause acute renal injury (ARI) in humans and animals. Its nephrotoxicity is associated with an increased formation of reactive oxygen species and lipid peroxidation in renal tissue. Recent studies suggest that antioxidants of the vitamin E family have protective effects against metal toxicity. Tocotrienols are known to have greater antioxidant activity than tocopherols and protect more efficiently against some free radical-related diseases than does tocopherols. In the present study, ARI induced by potassium dichromate (K₂Cr₂O₇) has been used as a model to investigate the possible nephroprotective effect of tocotrienol-rich fraction (TRF) from palm oil. Wistar male rats having an average body weight (bw) of 210 g were divided into four groups. The first group was taken as control and injected with vehicle alone while the second group was drug control and ingested with TRF (200 mg/kg, bw, orally, once daily for 21 days); the third group served as toxicant and was pre-treated with saline, followed by a single subcutaneous (SC) injection of K₂Cr₂O₇ (15 mg/kg bw). The fourth group was pre-treated with TRF and subsequently injected with K₂Cr₂O₇ (same dose as for the third group). Renal functions, oxidative and nitrosative stress were evaluated on days 0, 1, 2, 4, 7, 11 and 14 after treatment with K₂Cr₂O₇. The results revealed altered proximal tubular function; decreased glomerular filtration accompanied by oxidative damage 48 h after exposure to dichromate; while in the TRF-treated group proximal reabsorptive function, glomerular function and the cellular redox status were sustained. These results were further supported and confirmed by histological findings. The study suggests that TRF is effective in preventing K₂Cr₂O₇-induced acute renal injury, but more studies are needed to confirm the effects of TRF as a nephroprotective agent.

Introduction

Hexavalent chromium Cr(VI) is stable in the ambient environment and known to cause toxicity in humans and animals [1]. It is a strong oxidizing agent, causes dermatotoxicity, immunotoxicity, neurotoxicity, genotoxicity, cytotoxicity, mutagenesis and carcinogenicity [2]. The main site of Cr induced nephropathy in rats [3] and humans [4] is the proximal tubule, although other segments of the nephron might also be affected. Cr(VI) can easily enter many types of cells through SO₄²⁻ and HPO₄²⁻ channels [5]. The intracellular reduction of chromium(VI) via H₂O₂ produces reactive oxygen species (ROS). The most important mechanisms of oxygen activation by Cr(VI) involve Fenton-like reaction to generate the powerful DNA-damaging hydroxyl radical (HO) [2]. C(VI) causes nitrosative stress by reducing nitric oxide (NO) bioavailability [6], [7] and lipid peroxidation through the overproduction of (ROS) [8]. The role of oxidative stress in the renal damage induced by K₂Cr₂O₇ has been supported by the fact that some antioxidants ameliorate K₂Cr₂O₇-induced nephrotoxicity and oxidative damage [9], [10], [11], [12], whereas the inhibition of glutathione biosynthesis enhances it [13].

We have shown in our previous study, that Pycnogenol pre-treatment has a protective role in K₂Cr₂O₇-induced nephrotoxicity [14]. Therefore, antioxidants have been considered for prevention of nephropathy progression by reducing oxidative stress and/or preserving NO bioavailability. It has been shown that vitamin E is a naturally occurring, potent lipid-soluble, chain-breaking antioxidant that scavenges ROS and lipid peroxyl radicals both in vitro and in vivo[9], [15]. It protects the integrity of membrane by inhibiting lipid peroxidation and augmenting the activity of antioxidant enzymes in the kidneys of diabetic rats [16] and is also shown to suppress oxidative stress and glomerulosclerosis in rat remnant kidney [17] along with protecting against cisplatin-induced nephrotoxicity in developing rats [18]. Vitamin E is the generic term for a group of four tocopherols (TOC) and four tocotrienols (T3) (Fig. 1). T3 qualitatively exhibit the biological activity of α-tocopherol. TOC and T3 share structural features, namely a common chromanol head and a side chain at the C-2 position. The difference between TOC and T3 is that the former have a saturated phytyl tail, whereas the later possess an unsaturated isoprenoid (farnesyl) side chain. TOC and T3 are further separated into eight individual compounds (α-, β-, γ-, δ-TOC or T3) differing in the number and position of methyl substitutions on the chromanol ring. T3 have been poorly studied as compared to TOC [19]. Several investigators reported that T3 have greater antioxidant activity than TOC and protect more efficiently against some free radical-related diseases than does TOC [20], [21], [22], [23], [24]. At nanomolar concentration, α-T3, not α-TOC, prevents stroke-related neurodegeneration [24].

Palm oil contains more than 70% T3, which consists of α-, β-, γ-, δ-tocotrienols [25]. Although, the pharmacotherapeutic properties of tocotrienol-rich fraction (TRF) from palm oil are well known, the nephroprotective role of TRF has not previously been investigated. The aim of this study was to examine the protective effect of TRF against acute renal injury (ARI) caused by the acute exposure to potassium dichromate. The study also analysed the alterations in the function of the different segments (glomeruli and proximal tubules) of the nephron.