The renin-angiotensin system is a central player in multiple mechanisms responsible for the progression of renal fibrosis 
. Timely blockade with ACE inhibitors, renin inhibitors or AT1 receptor antagonists can promote reversal of renal lesions and ultimately prevent the evolution towards end-stage organ failure in experimental models of renal disease 
. However, renin-angiotensin system blockers are inconstantly efficient in preventing chronic renal disease progression, particularly in humans, although the factors involved in this variable efficacy remain unclear 
In this study we analyzed the reversal of L-NAME hypertensive nephropathy with losartan, and focused on the identification of markers of a non-return point of renal disease reversal. Our results show that losartan ameliorated renal hemodynamic alterations, proteinuria, and vimentin expression in all groups treated, compared to L-NAME 6w and 10w groups. Since these variables are classical determinants of renal disease progression, it could have been expected that the latter effects would be associated with a global protection against the functional and structural alterations ultimately induced by NO-deficiency hypertension. Instead, in spite of these beneficial effects, the No Reg group presented severe functional and structural disease, characterized by elevated plasma creatinine and vascular fibrosis similar to LN 10w group. Important additional factors implicated in renal disease progression and influenced by losartan are therefore responsible for the differential evolution between the Reg and No Reg groups. The reason why these important factors were different between the two groups is uncertain. In spite of the standardized cut-off point chosen to introduce losartan, we cannot exclude that the heterogeneity observed between Reg and No Reg groups may be due to subtle differences in the evolution of the renal disease around the non-return point, when losartan was introduced. Alternatively, preexisting heterogeneity in the regulation of pro- or anti-fibrotic genes between the Sprague Dawley rats, an outbred strain in which genome is not totally identical between animals, could account for the differences between the Reg and No Reg groups. We took advantage of these original experimental conditions to perform a transcriptomic analysis of selected markers associated with the progression of hypertensive nephropathy.
In the Reg group, rats presented reduced renal fibrosis compared to the No Reg group. Renal fibrosis is characterized by the accumulation of extracellular matrix, including fibrillar collagen. Since collagen III production, evaluated by Col3A1 RT-qPCR, was not different between the Reg and No Reg groups, the histological differences observed may be due to increased degradation of fibrillar collagen, as previously described in our laboratory 
L-NAME hypertension is associated with features of endothelial dysfunction 
. Our results show early endothelial activation indicated by increased expression of ET-1 propeptide and E-selectin in LN 6w group. We and others have previously identified ET-1, a potent profibrotic vasoconstrictor, as a mediator of renal fibrosis in hypertensive nephropathy 
. E-selectin is an endothelial inducible adhesion molecule notably involved in the pathophysiology of atherosclerosis and in renal ischemia-reperfusion, but its potential implication in hypertensive nephropathy has not been reported previously. Although the present study shows early increase in renal ET-1 and E-selectin mRNA, their similar expressions in Reg and No Reg groups suggest that these markers of endothelial activation are not major determinants of the reversal of renal disease induced by AT1 receptor blockade. Similarly, vimentin, a classical marker of mesenchymal cells associated with fibrogenesis, was reduced by losartan treatment, irrespective of the reversal of renal disease.
Periostin is an extracellular matrix protein first identified in the periosteum and the periodontal ligament 
. Angiotensin II can induce periostin expression in fibroblasts and vascular smooth muscle cells, via Ras/p38 MAPK/CREB and ERK1/2/TGF-beta1 pathways and via phosphatidylinositol-3-kinase signaling, respectively 
. Accordingly, periostin is induced in models of ischemic, hypertensive and hypertrophic cardiomyopathies, and an AT1 receptor antagonist decreases the cardiac expression of periostin 
. In the kidney, experimental studies evaluating the implication of periostin in physiology and disease are scarce. Periostin is transiently expressed during renal development 
, and the expression in the normal adult kidney is low. Recent studies observed overexpression of periostin in animal models of diabetic, renal subtotal ablation and ureteral obstruction nephropathies 
. In humans, periostin staining was observed in biopsies of glomerular diseases and within the cysts in autosomal dominant polycystic kidney disease 
. In addition, increased urinary excretion rates of periostin were reported in a limited number of proteinuric and non-proteinuric CKD patients 
. Whether periostin is implicated in hypertensive nephropathy and in the progression/reversal of chronic kidney disease remained unknown.
Our results identified a progressive induction of periostin in the kidney with the progression of the hypertensive nephropathy. Regression analyses found a strong association between periostin mRNA expression and robust functional markers of kidney disease. Importantly, these associations held true when systolic blood pressure was added as a covariate, which suggests that periostin is correlated to renal injury independently of the degree of hypertension. Immunohistochemical analyses revealed that the localization of periostin was predominantly perivascular, in areas where important deposits of extracellular matrix occur in this model. We also observed an intense predominantly extracellular staining for periostin in the injured fibrotic tubulo-interstitial regions of chronic allograft nephropathy, which further demonstrates overexpression of periostin in human kidney disease. Identification of the main cells responsible for the interstitial accumulation of periostin requires further investigation. Data from previous studies suggests that fibroblasts, smooth muscle cells and tubular epithelial cells may be involved in periostin expression in this setting 
We found that after the onset of hypertensive renal disease, curative treatment with losartan was associated with diminished periostin expression in Reg, not in No Reg group, which suggests that the reduction of periostin may be implicated in the mechanisms of angiotensin II-related disease progression and that reduction of periostin expression may be a critical determinant of disease reversal. Interestingly, the analysis of histological fibrosis scores shows that the difference between Reg and No Reg groups was most evident for perivascular fibrosis, in accordance with periostin distribution in experimental hypertensive nephropathy. These original data suggest that periostin may be related to the pathophysiology of extracellular matrix accumulation at the site of renal injury.
Together, the results of this work identify periostin as a previously unrecognized marker associated with hypertensive nephropathy. Further research is necessary to precise the potential of renal, plasma and urine periostin as prognostic biomarkers to monitor the progression and therapeutic control of human chronic kidney diseases.