6.1 Role of PAR1 in progression of diabetic nephropathy
Experimental studies have suggested that PAR1 plays a pivotal role in diabetic kidney disease, where it is responsible for activation of fibroblast proliferation and extracellular matrix production resulting in progression of renal injury [2, 5]. The pathological role of the PAR1 receptor has been investigated in both type-1 and type-2 diabetic animal models [5, 12]. In the streptozotocin (STZ) induced type-1 diabetic mice model, the renal expression of PAR1 mRNA was higher in comparison to non-diabetic mice [4]. Subsequently, this study also revealed that cultured MES13 (mouse mesangial cells) cells exposed to high glucose medium showed higher transcripts of PAR1 than low glucose medium environment exposure. Furthermore, this study showed that in MES13 addition of thrombin resulted in the mesangial proliferation and fibronectin production mediated by PAR1 dependent activation of MEK and p38 signaling pathways. MEK and p38 are important components of the MAPK signaling pathway [50]. Interestingly, western blot analysis also showed that their expressions were higher in diabetic rats as compared to non-diabetic rats [51]. Moreover, studies have documented that MEK and P38 signaling are activated by high glucose levels, oxidative stress, and also inflammation and are responsible for various pathological events including cell proliferation, differentiation, and apoptosis, which further promote the progression of diabetic nephropathy [50,51]. Notably, in STZ induced type-1 diabetic mice model, the fibrin-mediated mesangial proliferation was abolished upon co-administration of PAR1 antagonist (p1pal-12) or by direct inhibition of MEK/p38 signaling indicating that thrombin and PAR1 are interlinked in the development of glomerular injury. Additionally, immunohistochemical evidence from PAR1 deficient diabetic mice showed a reduction in mesangial expansion, proliferation, fibronectin deposition, and also the absence of tubular atrophy compared to wild-type (WT) diabetic mice. Moreover, in WT diabetic mice increased plasma cystatin C levels and also proteinuria was observed due to hyperglycemia-induced hyperfiltration, podocyte apoptosis, and glomerular filtration barrier damage. So far studies have reported that the generation of plasma thrombin (PAR1 agonist) is increased in patients suffering from diabetes [52-54]. This evidence suggests that inhibition of thrombin-mediated PAR1 activation could be a novel therapeutic target for the prevention of type-1 diabetic nephropathy.
Another experimental study has tested the pathological involvement of PAR1 in the development of type-2 diabetic nephropathy [12]. BTBRob/ob mice are leptin-deficient obese mice that served as a diabetic group, whereas wild types (WT) are considered as a non-diabetic control group. BTBRob/ob diabetic mice were treated with vorapaxar, a PAR1 antagonist which produced an increased body weight. In this study, the type-2 diabetic mice exhibited renal pathological changes such as increased kidney weight, albuminuria, neutrophil-gelatinase-associated lipocalin (NGAL) excretion, and also increased plasma insulin levels. Moreover, BTBRob/obdiabetic mice showed glomerular injury with mesangial expansion, capillary dilation, and glomerulosclerosis. Notably, vorapaxar treatment did not correct the glomerular damage, but mesangial expansion was significantly reduced in comparison to non-treated diabetic BTBRob/ob mice. During the disease progression in BTBRob/ob mice, diabetic nephropathy was accompanied by inflammation as illustrated by increased IL-6, IL-1β, TNF-α, and MCP-1 in both vorapaxar treated and non treated diabetic mice. Importantly, these findings are in contrast to a previous study in which inhibition of PAR1 receptor prevented nephropathy in type 1 diabetes model [55]. The most reasonable explanation for distinct outcomes observed due to PAR1 blockade in the above studies could be based upon etiological differences in the development of type 1 and type 2 diabetic kidney diseases. In diabetic nephropathy, both hemodynamic and structural changes are interlinked with each other. Studies showed that in type-1 diabetic nephropathy, the earliest hemodynamic abnormality observed is renal hyperfiltration which leads to increased intra-glomerular pressure, followed by glomerular injury with podocyte effacement and also tubular dilation. These pathological events are also accompanied by microalbuminuria and a progressive decline in glomerular filtration rate (GFR) [56]. However, type-2 diabetic nephropathy is an inflammatory prominent disease, where other pathogenic factors including obesity, hypertension with compensatory hyperinsulinemia could exacerbate the metabolic disturbance [57]. Moreover, renal hypertrophy is observed post-development of glomerulosclerosis and tubulointerstitial fibrosis in DN (diabetic nephropathy). Of note, hemodynamic changes are commonly observed in both type 1 and 2 DN patients [58]. Thus it might be possible that PAR 1 inhibition could not influence the pathological manifestations accompanying the type-2 DN due to the complex etiological basis of this disease type. However, in type 1 DN, the PAR 1 inhibition exhibited renoprotection by improving renal dysfunction and ameliorating proliferative changes in glomerular regions.