Sex- and species-specific diseases have been reported for decades since the discovery of unleaded gasoline was shown to lead to kidney tumours in male rats but not in females and both sexes of mice [3]. Likewise, the characterisation of alpha 2u-globulin nephropathy in male rats has also been discussed for decades [1–6, 8–10]. Several chemicals and drugs have been demonstrated to induce alpha 2u-globulin nephropathy only in mature male rats in association with neoplasia enhancement. Alpha 2u-globulin is synthesised in the liver under the influence of androgenic hormone and is released into blood circulation. This protein is freely filtered by the glomeruli and is reabsorbed by the P2 segment of the proximal tubule [3, 11–13] with persistent deposition due to its resistance to hydrolytic and proteolytic enzymes in the lysosomes, and approximately half of them are excreted in the urine [11, 12]. The accumulation of alpha 2u-globulin is cytotoxic and leads to single cell necrosis, a nidus for granular cast formation and reversible re-epithelialisation as presented by regenerative tubules [2, 14, 15]. Enhanced cellular proliferation initiates the transformation of proximal tubules to preneoplastic and neoplastic lesions [2, 16]. The primary histopathological change in alpha 2u-globulin nephropathy is intracytoplasmic “hyaline droplet” or “eosinophilic body” deposition in the proximal tubules with a variety of forms from spherical to polyangular [1, 4–6, 9, 10, 14, 16].
STZ, a nitrosourea alkylating agent or anticancer-antibiotic drug, has occasionally been used as a cytotoxic agent for treating some types of human tumours, e.g., lymphoma, sarcomas and Islet of Langerhans cancer [17]. It has also been extensively used for developing rodent models of diabetes and diabetic nephropathy. Interestingly, the present study demonstrates that end-stage nephropathy induced by STZ exhibits alpha 2u-globulin nephropathy in > 80% of the moribund male rats. Levamisole is also an example of an anticancer and antiparasitic drug that causes alpha 2u-globulin nephropathy only in male rats. Similar to previous studies, intracytoplasmic hyaline droplet deposition in PCTs (Fig. 2C,D) leads to cellular degeneration as characterised by the increment in vacuolated degeneration (Fig. 2D,E) and tubular degeneration and regeneration (Fig. 2F,G, respectively). In contrast to other chemicals or drugs that induce 2u-globulin nephropathy, preneoplastic and neoplastic lesions were not observed in end-stage renal kidney disease induced by STZ. Moreover, electron micrographs also show the presence of mitochondrial degeneration and swelling in rats with alpha 2u-globulin nephropathy (Fig. 3D,E). These results clearly suggest that the cytotoxic properties of alpha 2u-globulin cause cellular and organelle damages. Additionally, considering the glomerular filtration capacity in alpha 2u-globulin nephropathic rats, this study demonstrates deterioration of the filtering apparatus, especially pedicels, as shown in the Fig. 3F,G. However, the detail mechanisms involved in this impairment caused by alpha 2u-globulin deposition require further study.
Diabetic nephropathy is a microangiopathic complication present in one-third of diabetes mellitus patients [18]. It has been claimed that dysregulation of the water channel membrane protein “aquaporin; AQP” in the kidney plays an important role in the pathogenesis of several kidney diseases including diabetic nephropathy [18–20]. Eight AQPs, AQP-1–7 and − 11, are expressed in the kidney to maintain normal urine concentration [20]. Several reports indicate that alterations of AQP-1, -2, -4 and − 5 expression are highly associated with renal diseases. AQP-1 functions in hypertonicity formation and is expressed in apical and basolateral membranes of proximal tubules, descending thin limbs of Henle and descending vasa recta [21]. It also localises in the β-laminin of the glomerular basement membrane [19]. AQP-2, a urine concentration regulator under anti-diuretic hormone, is located at the apical membrane of the collecting duct [22]. AQP-4, a water permeability regulator, is located at the basolateral membrane of the collecting duct and exports water into the cytoplasm via AQP-2 [20]. Lastly, AQP-5 is located in type B intercalated cells of the collecting duct with unclear function [23]. Upregulation of glomerular AQP-1 is found in all forms of human renal diseases, probably due to compensation for losing cellular integrity [19]. Upregulation of AQP-2 and − 5 is closely related to the progression of diabetic nephropathy in diabetic patients and are good candidates to use for diagnosis [18, 24]. Recently, Go and Zhang also reported that an increase in AQP-5 in patients with diabetic nephropathy is independently associated with a reduction in the glomerular filtration rate [25]. In addition, a STZ-induced diabetic rat model exhibits a high level of anti-diuretic hormone, leading to upregulation of AQP-2 as a compensatory mechanism [26]. Dysregulation of intrarenal AQP-4 is involved in end-stage renal disease in HIV patients with glomerulosclerosis and renal tubular dysfunction [27]. In the present study, immunohistochemical studies reveal significant upregulation of AQP-1, -2, -4 and − 5 in the alpha 2u-globulin nephropathic rats (Fig. 4). These findings likely indicate that increases in AQP-1, -2, -4 and − 5 responses are (i) compensatory during high cellular and mitochondrial degeneration due to alpha 2u-globulin deposition in the PCTs, (ii) an advanced stage of diabetic kidney disease, (iii) a depletion of glomerular filtration capacity in association with the presence of pedicels disruption and (iv) renal tubule dysfunction, particularly PCTs, DCTs and CD.
According to mitochondrial function and its architecture, mitochondrial dysfunction is a crucial factor in the pathogenesis of diabetic kidney diseases regarding reactive oxygen species overproduction, apoptosis activation and mitophagy defects [28–34]. The kidney is an extreme oxygen consumption organ, which renders it sensitive to mitochondrial dysfunction. A hyperglycaemic environment also contributes to direct damage of renal tubular cells [28]. Dysregulation of essential mitochondrial genes in diabetic kidney diseases has been reported in relevance to the severity of renal pathology, e.g., glomerular endothelial injury, glomerulosclerosis and podocyte defects [30]. A change in the metabolic energy source under diabetic conditions results in increased oxygen consumption in the kidney and leads to renal hypoxia, ischaemia and necrosis [8, 29]. Our recent studies have demonstrated that cellular power synthesis (Haloacid Dehalogenase-Like Hydrolase Domain-Containing [HDHD]-3) and a mitochondrial apoptotic marker (NADH: ubiquinone oxidoreductase core subunit S1 [NDUFS-1]) in liver mitochondria in sericin-fed rats are preserved compared to those of non-treated rats under hypercholesterolemic conditions [35, 36]. In this study, the immunogold labelling technique indicates significant upregulation of HDHD-3 and NDUFS-1 in the alpha 2u-globulin nephropathic rats (Fig. 5). This suggests the high incidence of degenerative mitochondria in the alpha 2u-globulin nephropathic kidney, which attempt to increase energetic protein for the maintenance of renal function and integrity even when high levels of apoptosis were also observed.