Tubulointerstitial disease: role of ischemia and microvascular disease

Anemia management and chronic renal failure progression

Analysis of the biologic effects of erythropoietin and pathophysiology of chronic kidney diseases (CKD) suggests that treatment with erythropoiesis-stimulating agents (ESA) could slow the progression of CKD. By decreasing hypoxia and oxidative stress, it could prevent the development of interstitial fibrosis and the destruction of tubular cells. It could have direct protective effects on tubular cells through its antiapoptotic properties. It could help maintain the integrity of the interstitial capillary network through its effects on endothelial cells. Thus, suggesting that correcting anemia with ESA could slow the progression of CKD is biologically plausible. In patients with CKD, three small prospective studies and a retrospective study have suggested that treatment with ESA may have protective effects. Post-hoc analysis of the Reduction in Endpoints in Noninsulin-dependent Diabetes Mellitus with the Angiotensin II Antagonist Losartan study has also shown that anemia was an independent risk factor for progression of nephropathy in patients with type 2 diabetes. In addition, a large clinical trial, which had to be stopped prematurely because of labeling change for subcutaneous administration of epoetin alfa, suggests that complete normalization of hemoglobin levels is safe in CKD patients not on dialysis and without severe cardiovascular disease. Thus, it seems reasonable to advocate starting a large randomized, prospective study to determine if normalization of hemoglobin concentration can effectively slow the progression of CKD.

Thrombin stimulates proinflammatory and proliferative responses in primary cultures of human proximal tubule cells

BACKGROUND

Fibrin deposition is frequently observed within the tubulointerstitium in various forms of chronic renal disease. This suggests the presence of active components of the coagulation pathway, which may contribute to the progressive deterioration in renal function. The aim of this study was to investigate the proinflammatory and fibroproliferative effects of the coagulation protease thrombin on human proximal tubular cells (PTC) in culture.

METHODS

Primary cultures of PTC were established from normal kidney tissue and grown under serum-free conditions with or without thrombin or the protease-activated receptor (PAR) activating peptides TFLLRN-NH(2), SLIGKV-NH(2), and SFLLRN-NH(2) (100 to 400 micromol/L). DNA synthesis (thymidine incorporation), intracellular Ca(2+) mobilization (fura-2 fluorimetry), fibronectin secretion [enzyme-linked immunosorbent assay (ELISA), immunoblotting], monocyte chemoattractant protein-1 (MCP-1) secretion (ELISA), and transforming growth factor-beta1 (TGF-beta1) secretion (ELISA) were measured. Reverse transcription-polymerase chain reaction (RT-PCR) was used to assess PAR mRNA expression in these cells.

RESULTS

Thrombin enhanced DNA synthesis, fibronectin secretion, MCP-1 secretion, and TGF-beta1 secretion in a concentration-dependent manner. Cell injury [lactate dehydrogenase (LDH) release] and cellular protein levels were unaffected. RT-PCR showed that cultures of PTC expressed mRNA transcripts for the thrombin receptors PAR-1 and PAR-3, but not PAR-4. Thrombin and each of the PAR activating peptides enhanced intracellular calcium mobilization. However, the other effects of thrombin were only fully reproduced by the PAR-2-specific peptide, SLIGKV-NH(2), only partially by SFLLRN-NH(2), (a PAR-1 peptide that can activate PAR-2), and not at all by the PAR-1-specific peptide, TFLLRN-NH(2). Thrombin-induced DNA synthesis, fibronectin, and MCP-1 secretion were unaffected by a TGF-beta neutralizing antibody, the matrix metalloproteinase (MMP) inhibitor, GM6001 and the epidermal growth factor (EGF) receptor kinase inhibitor AG1478.

CONCLUSION

Thrombin initiates both proinflammatory and fibroproliferative responses in human PTC. These responses which are dependent on its protease activity appear not to be mediated by PAR-1 activation, the autocrine action of thrombin-induced TGF-beta1 secretion, MMP activation, or EGF receptor transactivation. The proinflammatory and fibroproliferative actions of thrombin on human PTC may help explain the extent of tubulointerstitial fibrosis observed in kidney diseases where fibrin deposition is evident.

Injury and progressive loss of peritubular capillaries in the development of chronic allograft nephropathy

BACKGROUND

Chronic allograft nephropathy (CAN) remains the most important cause of late renal graft loss. However, the mechanism for graft dysfunction and the process of the development of CAN are not well understood. This study examined the role of microvascular injury in the development of CAN.

METHODS

We studied renal biopsies obtained from grafts with CAN (N= 79) and pretransplant control kidneys (N= 20). Microvascular injury was examined morphologically, and was correlated with interstitial fibrosis, glomerular sclerosis, graft function, and the severity of CAN. The humoral and cellular immunity involved in CAN were examined by C4d, CD3, and TIA-1 staining.

RESULTS

In all the cases of CAN, microvascular injury was evident with or without CD3-positive T cells, TIA-1-positive cytotoxic cells, and/or C4d+ complement deposition. Irrespective of chronic rejection (N= 14), C4d+ chronic humoral rejection (N= 6), or other CAN, the development process of CAN was characterized by injury and progressive loss of identifiable peritubular capillaries (PTCs) accompanied with the development of interstitial fibrosis. Injured PTCs were characterized morphologically by the process of angioregression with the presence of apoptotic cells, lamination of the basement membrane, and loss of PTCs. The low number of PTCs correlated significantly with the severity of CAN (r=-0.74, P < 0.001), the development of interstitial fibrosis (r=-0.75, P < 0.001), graft dysfunction (r=-0.69, P < 0.001), and also correlated weakly with proteinuria (r=-0.45, P < 0.05). In the glomeruli, capillary loss significantly correlated with the degree of glomerular sclerosis (r=-0.66, P < 0.001) and proteinuria (r=-0.65, P < 0.001), but did not correlate with the severity of CAN (r=-0.24, P > 0.05) or graft dysfunction (r=-0.32, P > 0.05).

CONCLUSION

CAN was characterized by progressive injury to the renal microvasculature and the development of renal scarring. In particular, injury, angioregression and progressive loss of the PTC network strongly contributed to the development of interstitial fibrosis and graft dysfunction in CAN, and might play a crucial role in the development of CAN.

Mechanisms of tubulointerstitial fibrosis

PURPOSE OF REVIEW

Tubulointerstitial fibrosis is the final common pathway to end-stage renal disease. Understanding the mechanisms of tubulointerstitial fibrosis is essential in establishing novel therapeutic strategies for the prevention or arrest of progressive kidney diseases. The present review focuses on a newly proposed mechanism of tubulointerstitial fibrosis, one that emphasizes the roles of epithelial-mesenchymal transition and cellular activation.

RECENT FINDINGS

Among the cells that accumulate in the renal interstitium, fibroblasts are the principal effectors mediating tubulointerstitial fibrosis. By contrast, the phagocytosis of extracellular matrix and apoptotic cells by macrophages may actually exert a beneficial effect. Interstitial fibroblasts are more heterogeneous than expected, and during renal fibrosis new fibroblasts are derived mainly through epithelial-mesenchymal transition. The intracellular signaling pathways leading to initiation of epithelial-mesenchymal transition remain largely unknown, though recent studies have identified beta-catenin and Smad3 activation of lymphoid enhancer factor, integrin-linked kinase, and small GTPases and mitogen-activated protein kinases as key components. Transforming growth factor-beta is believed to be a critical fibrogenic factor, but recent studies have also focused on transforming growth factor-beta independent pathways as mechanisms of tubulointerstitial fibrosis. As the mechanisms underlying tubulointerstitial fibrosis leading to epithelial-mesenchymal transition have been identified, so have cytokines that efficiently antagonize renal fibrosis, particularly bone morphogenic protein-7 and hepatocyte growth factor.

SUMMARY

In combination with traditional angiotensin converting enzyme inhibitors, newly identified cytokines may eventually form the basis for new therapeutic strategies aimed at inhibiting the progression of renal disease.

Primary prevention in rheumatology: the importance of hyperuricemia

Hyperuricemia (HU) is present in 5-30% of the general population, although the prevalence is higher among some ethnic groups and seems to be increasing worldwide. Classically, chronic HU has been considered a risk factor for gout or lithiasis and is associated with alcoholism, obesity, hypertension, dyslipidemia, hyperglycemia/diabetes mellitus, renal failure and intake of certain drugs. HU is also associated with cardiovascular diseases such as hypertension, vascular disease, pre-eclampsia, pulmonary arterial hypertension, stroke, heart failure, ischemic heart disease and also metabolic syndrome, renal disease and increased mortality. It is uncertain if these associations are dependent or not, especially cardiovascular and renal diseases. Patients with chronic HU and also those with gout require both medical investigation for associated diseases or drugs as well as nutritional counseling and life-style changes. HU should alert physicians to possible complications.

Novel insights into renal fibrosis

PURPOSE OF REVIEW

Renal fibrosis characterizes a common endpoint of diverse renal diseases which leads to functional impairment ultimately resulting in terminal renal failure.

RECENT FINDINGS

Recent advances in this field led to the discovery of several novel mediators as well as novel aspects of known mediators. Studies on the origin and role of specific renal cell types involved in renal fibrosis identified bone marrow derived mesangial progenitors and offered substantial evidence for the concept of epithelial to mesenchymal transition. Much progress has also been made in better understanding of the interactions between different mediators and between mediators and renal target cells. Compounds designed on the basis of this current knowledge have proven to be potent inhibitors of the development of renal fibrosis or might even induce resolution of renal fibrosis.

SUMMARY

The number and diversity of recent studies in this field offer hope for new treatment regimes in our clinical efforts towards prevention and regression of progressive fibrosing renal diseases.