Impaired expression and function of breast cancer resistance protein (Bcrp) in brain cortex of streptozocin-induced diabetic rats

Abstract

The aim of this study was to investigate whether diabetes mellitus (DM) affected breast cancer resistance protein (Bcrp) function and expression in rat brain. 5-week and 8-week diabetic rats were induced by streptozocin (STZ). Bcrp expression and function in brain cortex were assessed by western blot and measuring the brain-to-plasma concentration ratios of two typical substrates prazosin and cimetidine, respectively. The diabetic rats were treated with three different agents insulin, aminoguanidine (AG) and metformin (MET). It was found that the brain-to-plasma ratios of prazosin and cimetidine in diabetic rats were significantly higher than those of control rats, which were dependent on duration of diabetes. Lower levels of Bcrp were found in brain cortex of diabetic rats, which were in parallel with increase of brain-to-plasma ratios. Insulin treatment may attenuate the impairment of Bcrp expression and function induced by diabetes. Aminoguanidine and metformin treatment did not prevent the impairment of Bcrp function and expression in brain cortex of diabetic rats. All results gave a conclusion that STZ-induced DM may induce the impairment of function and expression of Bcrp in brain cortex, and lower levels of insulin may mainly contribute to Bcrp dysfunction in brain.

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.