Croatt, Anthony J.

TGF-β1 contributes to chronic kidney disease, at least in part, via Smad3. TGF-β1 is induced in the kidney following acute ischemia, and there is increasing evidence that TGF-β1 may protect against acute kidney injury. As there is a paucity of information regarding the functional significance of Smad3 in acute kidney injury, the present study explored this issue in a murine model of ischemic acute kidney injury in Smad3+/+ and Smad3−/− mice. We demonstrate that, at 24 h after ischemia, Smad3 is significantly induced in Smad3+/+ mice, whereas Smad3−/− mice fail to express this protein in the kidney in either the sham or postischemic groups. Compared with Smad3+/+ mice, and 24 h following ischemia, Smad3−/− mice exhibited greater preservation of renal function as measured by blood urea nitrogen (BUN) and serum creatinine; less histological injury assessed by both semiquantitative and qualitative analyses; markedly suppressed renal expression of IL-6 and endothelin-1 mRNA (but comparable expression of MCP-1, TNF-α, and heme oxygenase-1 mRNA); and no increase in plasma IL-6 levels, the latter increasing approximately sixfold in postischemic Smad3+/+ mice. We conclude that genetic deficiency of Smad3 confers structural and functional protection against acute ischemic injury to the kidney. We speculate that these effects may be mediated through suppression of IL-6 production. Finally, we suggest that upregulation of Smad3 after an ischemic insult may contribute to the increased risk for chronic kidney disease that occurs after acute renal ischemia.

Neointimal hyperplasia contributes to failure of hemodialysis arteriovenous fistulas (AVFs). Increased expression of matrix metalloproteinase (MMP)-9 occurs in AVFs, and MMP-9 is implicated in neointimal hyperplasia and vascular injury. Recent studies demonstrate that MMP-9, by degrading N-cadherin, leads to increased expression of β-catenin and β-catenin-dependent proliferation of smooth muscle cells. The present study examined this pathway in the venous limb of a murine AVF model. Western analyses demonstrate that, in this model, there is diminished expression of N-cadherin accompanied by increased expression of β-catenin, c-Myc, and proliferating cell nuclear antigen (PCNA). By immunohistochemistry, β-catenin and c-Myc localized to proliferating smooth muscle cells in the venous limb of the AVF. Increased expression of β-catenin was accompanied by augmented expression of phosphorylated (p)-glycogen synthase kinase (GSK)-3β, GSK-3β, and integrin-linked kinase. The administration of doxycycline suppressed MMP-9 expression but did not reduce venous histological injury in the AVF, or increase AVF patency assessed 6 wk after its creation. Doxycycline did not influence expression of β-catenin, c-Myc, GSK-3β, or integrin-linked kinase. Thus, in this vascular injury model, the upregulation of β-catenin cannot be readily attributed to MMP-9 upregulation; increased β-catenin expression may reflect either the upregulation of p-GSK-3β, GSK-3β, or integrin-linked kinase. This study provides the first exploration of β-catenin in an AVF, demonstrating substantial upregulation of this mitogenic signaling molecule and uncovering possible mechanisms that may account for such upregulation.

ANG II causes renal injury through hemodynamic and other effects, and pressor doses of ANG II induce heme oxygenase-1 (HO-1) as a protective response. The present studies examined the hemodynamic effects of more clinically relevant, lower doses of ANG II and the role of HO activity in influencing these effects. Under euvolemic conditions, ANG II increased arterial pressure and renal vascular resistance. ANG II did not induce oxidative stress, inflammation/injury-related gene expression, or proteinuria and did not alter extrarenal vascular reactivity. At these doses, ANG II failed to increase HO-1 or HO-2 mRNA expression or HO activity. Inhibiting HO activity in ANG II-treated rats by tin mesoporphyrin further increased renal vascular resistances, decreased renal blood flow, and blunted the rise in arterial pressure without inducing oxidative stress or altering expression of selected vasoactive/injury/inflammation-related genes; tin mesoporphyrin did not alter vasorelaxation of mesenteric resistor vessels. We conclude that in this model renal vasoconstriction occurs without the recognized adverse effects of ANG II on glomerular filtration rate, renal blood flow, oxidative stress, vascular reactivity, proteinuria, and injury-related gene expression; renal HO activity is essential in preserving perfusion of the ANG II-exposed kidney. These findings represent an uncommon example wherein function of a stressed organ (by ANG II), but not that of the unstressed organ, requires intact renal HO activity, even when the imposed stress neither induces HO-1 nor HO activity. These findings may be germane to conditions attended by heightened ANG II levels, ineffective renal perfusion, and susceptibility to acute kidney injury.

Acute ischemic insults to the kidney are recognized complications of human sickle cell disease (SCD). The present study analyzed in a transgenic SCD murine model the early renal response to acute ischemia. Renal hemodynamics were profoundly impaired following ischemia in sickle mice compared with wild-type mice: glomerular filtration rate, along with renal plasma flow and blood flow rates, were markedly reduced, while renal vascular resistances were increased more than threefold in sickle mice following ischemia. In addition to these changes in renal hemodynamics, there were profound disturbances in renal signaling processes: phosphorylation of members of the MAPK and Akt signaling proteins occurred in the kidney in wild-type mice after ischemia, whereas such phosphorylation did not occur in the kidney in sickle mice after ischemia. ATP content in the postischemic kidney in sickle mice was less than half that observed in wild-type mice. Examination of the expression of candidate genes uncovered changes that may predispose to increased sensitivity of the kidney in sickle mice to ischemia: increased expression of inducible nitric oxide synthase and decreased expression of endothelial nitric oxide synthase, and increased expression of TNF-α. Inducibility of anti-inflammatory, cytoprotective genes, such as heme oxygenase-1 and IL-10, was not impaired in sickle mice after ischemia. We conclude that the kidney in SCD is remarkably vulnerable to acute ischemic insults. We speculate that such sensitivity of the kidney to ischemia in SCD may underlie the occurrence of acute kidney injury in patients with SCD and may set the stage for the emergence of chronic kidney disease in SCD.