8.2 PAR2 mediates glomerular injury and renal dysfunction in FSGS model
Focal segmental glomerulosclerosis (FSGS) is a glomerular disease that is manifested as elevated proteinuria, podocyte injury, and immune cell infiltrations [86].  A study has investigated the role of PAR2 in mediating focal segmental glomerulosclerosis in drug-induced nephrotoxicity models [87]. In this study, nephropathy was induced by giving adriamycin (ADR) intravenously for six weeks which resulted in elevated PAR2 protein expression in the renal tissues along with the increased expression of phosphokinase A (PKA) and phosphokinase C (PKC). Among various signal transduction pathways, PKA and PKC are members of the cAMP-dependent serine-threonine kinase family that undergoes a series of phosphorylation and initiates a wide range of intracellular target proteins [88]. In turn, activation of these enzymes are likely to be responsible for vascular pathologies along with ECM synthesis and fibrosis, ROS generation, apoptosis, activation of monocytes, cell migration, dysregulation of cytokines accelerating inflammatory cascades in the development of nephropathy [89,90]. Also, past evidence suggests that direct pharmacological inhibition of PKA and PKC pathways attenuates the progression of renal injuries [91,92]. Moreover, ADR injection elevated the levels of pro-inflammatory cytokines in the renal tissues which were significantly attenuated by blocking PAR2 with FSLLRY-NH2. Notably, the elevated expression of PKA, PKC, and inflammation was attenuated upon administration of PAR2 blocker FSLLRY-NH2 (FSL). Furthermore, this study also suggested that PAR2 activation was involved in increasing the renal expression of pro-fibrotic TGF-β1, caspase-9 responsible for apoptosis execution and desmin which is a cytoskeletal muscle-specific protein and key subunit of type III intermediate filament essential for tensile strength and structural integrity of myofibrils [93]. In kidneys, intermediate filament proteins are expressed in glomerular podocytes and desmin acts as an important marker of podocyte injury and excretion of this protein reflects heavy proteinuria as well [94,95]. Moreover, activation of PAR-2 down-regulated podocyte slit-diaphragm structural protein nephrin expression in the renal tissues of adriamycin injected rats. Interestingly, it was observed that administration of PAR2 blocker FSL corrected the levels of these pathological molecular mediators. Studies have reported that TGF-β1 is a key mediator in renal inflammation and fibrosis. It also plays key roles in cell proliferation, differentiation, apoptosis, migration, and ECM production [96]. In glomerular diseases; TGF-β1 is secreted by mesangial cells and is stored and transported to the podocyte surface where it is responsible for mediating inflammation, apoptosis, and podocyte injury via activating downstream signaling pathways such as Smad, MAPK, ERK, and PI3K [97-99]. Thus, in the same study, further investigations were conducted on TGF-β1 treated cultured podocyte cells and it was observed that there was an increase in the level of the caspase-9 enzyme, indicating podocyte apoptosis along with depleted nephrin and elevated desmin levels. However, administration of PAR2 blocker in podocyte cell line has corrected the altered caspase-9, nephrin, and desmin levels, thus indicating the involvement of PAR2 in mediating the TGF-β1 induced podocyte damage. Histological studies also showed that administration of adriamycin has to lead to glomerular injury as manifested by the expansion of the mesangial matrix. Also in adriamycin injected rats elevated urinary proteins and serum creatinine levels, together with increased kidney weight to body weight ratio were noted which are important markers of glomerular injury and hypertrophy. However, all these manifestations were attenuated by the administration of the PAR2 blocker. This study clearly suggested that blocking of PAR2 is beneficial in the prevention of focal segmental glomerulosclerosis by suppression of pro-inflammatory cytokines release and TGF-β1 inactivation. It is worth mentioning here that presently the treatment for FSGS relies on ACE inhibitors or ARBs which may not completely block the development of kidney diseases in FSGS patients [100,101]. Thus it is important to further investigate the therapeutic potential of PAR2 in other FSGS models.