Impaired expression and function of breast cancer resistance protein (Bcrp) in brain cortex of streptozocin-induced diabetic rats
Introduction
Diabetes mellitus (DM) is associated with the development and progression of pathological changes in various organ systems including the central nervous system. However, the specific effects of diabetes on blood–brain barrier (BBB) remain controversial. Studies utilizing the STZ-induced model of diabetes to assess its effect on the BBB have indicated that the physical barrier remains intact [1], whereas recent studies [2], [3], [4] indicated that BBB permeability to some substances is increased in STZ-induced diabetes. However, there are some reports that showed contradictory outcome [5], [6].
Explanation for these conflicting results is that both barrier and transport components of the BBB function can be attributed to dysfunction of cerebral microvasculature. Paracellular and transcellular permeability of the BBB is differentially regulated in diabetes. Permeability to macromolecules and most small, water-soluble molecules is limited through the paracellular pathway by epithelial-like tight junctions [7] and transcellular movement of lipid-soluble molecules and organic ions is highly regulated by numerous transporters including efflux transporters [8].
The efflux transporters mainly belong to the ABC superfamily including P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP). P-gp was first described as a component of the acquired multidrug resistance (MDR) mechanism of tumor cells, but it is now known to be present physiologically at the luminal membrane of the brain capillary endothelial cells. P-gp actively expels a wide spectrum of amphiphilic drugs from brain into blood, leading to a decrease in their uptake by the brain. Bcrp, which was first called MXR and is now designated ABCG2 on nomenclature, confers resistance to many drugs, including mitoxantrone, prazosin and some camptothecin derivatives, and its range may overlap with the substrates of other P-gp and/or multidrug resistance-associated proteins (MRPs) [9]. BCRP/ABCG2 was identified in human and porcine brain capillary endothelial cells and found to be responsible for the in vitro export of daunorubicin from porcine brain capillary endothelial cells. That ABCG2 is present in brain endothelial cells indicated that Bcrp may influence the efflux of drugs at the BBB [10], [11], [12], [13], [14]. In vivo studies on Bcrp1 knockout mice have shown that Abcg2 is involved in brain penetration of imatinib [15]. A series of studies showed that some diseases including diabetes mellitus could affect expression and function of these efflux transporters, resulting in change of BBB permeability. Our previous study [3] showed that expression and function of P-gp in brain of STZ-induced diabetic rats were damaged, accompanied by increase of brain penetration of its substrates rhodamine 123 and vincristine without marked change of BBB integrity. Similar down-regulation was also found in diabetic mice induced by STZ [4], [16]. In contrast, expression and function of MRP-2 was found to be up-regulated in brain of diabetic rats induced by STZ, accompanied by decrease of fluorescein brain penetration [17]. Bcrp was one of the important efflux transporters which restricted entrance of some substances into brain. However, little is known about its regulation of expression and function in diabetes mellitus.
The purposes of this study were, firstly, to investigate whether DM changed the Bcrp function and expression in brain cortex of streptozotocin-induced diabetic rats, using western blot and two typical substrates prazosin and cimetidine both of which as typical substrates of Bcrp were widely used to evaluate the Bcrp function [14], [18], [19], [20], [21], [22], [23]; secondly, to clarify whether agent treatment reversed the change of Bcrp expression and function induced by DM; thirdly, to study which factors may contribute to change of function and expression of Bcrp in brain cortex.
Section snippets
Animal preparation
Male Sprague–Dawley rats (180–220 g), purchased from Center of Experimental Animal, China Pharmaceutical University were used in the study and housed under controlled room of humidity (50 ± 5%) and temperature (23 ± 1 °C). The rats fasted for 6 h prior to intraperitoneal (i.p.) injection of 65 mg/kg STZ dissolved in 0.1 M sodium citrate buffer at pH 4.5. Following injection, animals were returned to their cages, maintained under standard 12-h light:12-h dark conditions, and given food and water ad
Physiological and biochemical parameters of experimental rats
The physiological parameters of body weight, blood glucose level, insulin level and serum AGEs level in 5-week and 8-week experimental rats were measured and listed in Table 1. Compared with aged-matched normal rats, lower level of insulin, higher level of blood glucose and AGEs were found (p < 0.01) in serum of diabetic rats induced by STZ, accompanied by lower body weight. The treatment of insulin could reverse these changes of physiological and biochemical parameters in diabetic rats.
Discussion
The present study was first designed to explore whether DM affected the Bcrp function and expression in brain cortex of STZ-induced diabetic rats using typical substrates prazosin and cimetidine and western blot analysis. Lower levels of Bcrp in brain cortex of diabetic rats were paralleled with increase of brain-to-plasma ratios of prazosin and cimetidine.
Prazosin as a typical substrate of Bcrp was widely used to evaluate the Bcrp function [14], [18], [19]. Cimetidine was initially recognized
Acknowledgements
This work was supported by the National Science foundation of China (no. 30672499) and the Project of Innovation in Graduate Education, Jiangsu Province (no. 2006).
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