Role of the retinoic acid receptor-α in HIV-associated nephropathy.

All-trans retinoic acid protects against the development of HIV-associated nephropathy (HIVAN) in HIV-1 transgenic mice (Tg26). In vitro, all-trans retinoic acid inhibits HIV-induced podocyte proliferation and restores podocyte differentiation markers by activating its receptor-α (RARα). Here, we report that Am580, a water-soluble RARα-specific agonist, attenuated proteinuria, glomerosclerosis, and podocyte proliferation, and restored podocyte differentiation markers in kidneys of Tg26 mice. Furthermore, RARα-/- Tg26 mice developed more severe kidney and podocyte injury than did RARα+/- Tg26 mice. Am580 failed to ameliorate kidney injury in RARα-/- Tg26 mice, confirming our hypothesis that Am580 acts through RARα. Although the expression of RARα-target genes was suppressed in the kidneys of Tg26 mice and of patients with HIVAN, the expression of RARα in the kidney was not different between patients with HIVAN and minimal change disease. However, the tissue levels of retinoic acid were reduced in the kidney cortex and isolated glomeruli of Tg26 mice. Consistent with this, the expression of two key enzymes in the retinoic acid synthetic pathway, retinol dehydrogenase type 1 and 9, and the overall enzymatic activity for retinoic acid synthesis were significantly reduced in the glomeruli of Tg26 mice. Thus, a defect in the endogenous synthesis of retinoic acid contributes to loss of the protection by retinoic acid in HIVAN. Hence, RARα agonists may be potential agents for the treatment of HIVAN.

Reduction of Stat3 activity attenuates HIV-induced kidney injury.

HIV-1 Nef induces podocyte proliferation and dedifferentiation by activating the Stat3 and MAPK1,2 pathways. Activation of Stat3 also occurs in human kidneys affected by HIV-associated nephropathy (HIVAN), but its contribution to the development of HIVAN is unknown. Here, we generated HIV-1 transgenic mice (Tg26) with either 75% Stat3 activity (Tg26-SA/+) or 25% Stat3 activity (Tg26-SA/-). The kidneys of Tg26-SA/+ mice, but not Tg26-SA/- mice, showed increased Stat3 phosphorylation. The Tg26-SA/+ phenotype was not different from Tg26 mice, but Tg26-SA/- mice developed significantly less proteinuria, glomerulosclerosis, and tubulointerstitial injury. Tg26-SA/+ mice exhibited reduced expression of podocyte differentiation markers and increased expression of VEGF and proliferation markers as compared to Tg26-SA/- mice. Primary podocytes isolated from Tg26-SA/+ mice showed increased Stat3 phosphorylation and reduced expression of podocyte differentiation markers. The tubulointerstitial compartment and isolated tubules of Tg26-SA/+ mice also had increased Stat3 phosphorylation and expression of Stat3 target genes. We confirmed that the expression of the HIV-1 transgene and reduction of Stat3 activity did not affect T and B cell development. In conclusion, Stat3 plays a critical role in the pathogenesis of HIVAN.

Knockdown of Stat3 activity in vivo prevents diabetic glomerulopathy.

Recent studies suggest that Stat3, a transcription factor that mediates cytokine signaling, plays a critical role in the pathogenesis of diabetic nephropathy. Complete Stat3 gene knockout is embryonic lethal; therefore, we crossed Stat3+/- mice with Stat3 mutant mice (SA/SA) that lack full Stat3 activity. This strategy generated Stat3SA/- mice (25% activity) and Stat3SA/+ mice (75% activity), which were made diabetic using streptozotocin in order to define the role of Stat3 in diabetic kidney disease. While the glomerular number was not different between these two groups of mice, the diabetic SA/- mice had significantly less proteinuria, mesangial expansion, glomerular cell proliferation, and macrophage infiltration than the diabetic SA/+ mice. The reduction in Stat3 activity did not affect glomerular hyperfiltration seen after the induction of diabetes, as it was increased to the same degree in both groups of mice. Phosphorylation of Stat3 was markedly increased in the glomeruli of diabetic SA/+ mice compared to diabetic SA/- mice. The expression of inflammatory markers, IL-6, MCP-1, and activated NF-kappaB; type IV collagen, TGF-beta, and ICAM-1 mRNA; or type IV collagen and TGF-beta protein, were all found to be significantly less in glomeruli isolated from diabetic SA/- mice, as compared with diabetic SA/+ mice. Our study shows that Stat3 plays a critical role in the regulation of inflammation and abnormal matrix synthesis at an early stage of DN.

Retinoic acid utilizes CREB and USF1 in a transcriptional feed-forward loop in order to stimulate MKP1 expression in human immunodeficiency virus-infected podocytes.

Nef-induced podocyte proliferation and dedifferentiation via mitogen-activated protein kinase 1,2 (MAPK1,2) activation plays a role in human immunodeficiency virus (HIV) nephropathy pathogenesis. All-trans retinoic acid (atRA) reverses the HIV-induced podocyte phenotype by activating cyclic AMP (cAMP)/protein kinase A (PKA) and inhibiting MAPK1,2. Here we show that atRA, through cAMP and PKA, triggers a feed-forward loop involving CREB and USF1 to induce biphasic stimulation of MKP1. atRA stimulated CREB and USF1 binding to the MKP1 gene promoter, as shown by gel shifting and chromatin immunoprecipitation assays. CREB directly mediated the early phase of atRA-induced MKP1 stimulation; whereas the later phase was mediated by CREB indirectly through induction of USF1. These findings were confirmed by a reporter gene assay using the MKP1 promoter with mutation of CRE or Ebox binding sites. Consistent with these findings, the biological effects of atRA on podocytes were inhibited by silencing either MKP1, CREB, or USF1 with small interfering RNA. atRA also induced CREB phosphorylation and MKP1 expression and reduced MAPK1,2 phosphorylation in kidneys of HIV type 1-infected transgenic mice. We conclude that atRA induces sustained activation of MKP1 to suppress Nef-induced activation of the Src-MAPK1,2 pathway, thus returning the podocyte to a more differentiated state. The mechanism involves a feed-forward loop where activation of one transcription factor (TF) (CREB) leads to induction of a second TF (USF1).

HIV-1 upregulates VEGF in podocytes.

HIV-associated nephropathy (HIVAN) is characterized by collapsing FSGS. Because transgenic mice with podocyte-specific overexpression of the vascular endothelial growth factor 164 (VEGF164) isoform also develop collapsing FSGS, we sought to determine whether VEGF plays a role in HIVAN. Compared with controls, immunohistochemistry revealed that kidneys from HIV-1-transgenic mice (Tg26) and from patients with HIVAN had greater expression of both VEGF and its transcriptional regulator, hypoxia-inducible factor 2alpha (HIF-2alpha). Similarly, mRNA and protein levels of VEGF and HIF-2alpha were increased in HIV-infected podocytes in vitro, and this transcriptional upregulation was found to be stimulated by the HIV viral protein Nef in a Src kinase-and Stat3-dependent manner. HIV-1 also upregulated VEGFR2 and its co-receptor neuropilin-1 and suppressed the expression of semaphorin 3a in the podocyte. Exogenous VEGF stimulated proliferation and de-differentiation of podocytes, which are features of collapsing FSGS, and VEGFR2 neutralizing antibodies reversed these features in podocytes infected with HIV-1 or isolated from Tg26 mice. In conclusion, HIV-1 induces VEGF and VEGFR2 expression in podocytes, and this may be a critical step in the pathogenesis of HIVAN.

HIV-1 Nef disrupts the podocyte actin cytoskeleton by interacting with diaphanous interacting protein.

The ability of the human immunodeficiency virus, type 1 (HIV-1) protein Nef to induce cytoskeleton changes in infected host cells is a key event in viral replication. In renal podocytes, we found that Nef induced loss of stress fibers and increased lamellipodia, pathological changes leading to proteinuria in HIV-associated nephropathy. These morphological changes were mediated by Nef-induced Rac1 activation and RhoA inhibition. We identified a new interaction between Nef and diaphanous interacting protein (DIP), a recently described regulator of Rho and Rac signaling. We found that the Src homology 3 binding domain of DIP and the Nef PXXP motif were required for this interaction. Nef also interacts with Vav2 in podocytes. DIP and Vav2 both interact directly with Nef in a competitive manner. DIP interacts with p190RhoGAP, and intact DIP was required for Nef-induced phosphorylation of p190RhoGAP. DIP also interacts with Vav2, and although DIP enhanced baseline phosphorylation of Vav2, it was not required for Nef-induced Vav2 activation. In Nef-infected podocytes, Src kinase induces phosphorylation of DIP, p190RhoGAP, and Vav2, leading to RhoA inhibition and Rac1 activation. Inhibition of the Nef-induced signaling pathway by using a dominant negative of either Src or DIP or siRNA for DIP or p190RhoAGAP restored RhoA activity and stress fiber formation in Nef-infected podocytes, whereas siRNA for Vav2 reduced Rac1 activity and formation of lamellipodia. We conclude that in HIV-infected podocytes, Nef, through the recruitment of DIP and p190RhoAGAP to Nef-Src complex, activates p190RhoAGAP and down-regulates RhoA activity.

Podocytes in HIV-associated nephropathy.

HIV-associated nephropathy (HIVAN) is the leading cause of end-stage renal failure in HIV-1 seropositive patients. The pathologic findings include collapsing focal segmental glomerulosclerosis with proliferation of epithelial cells in Bowman's space. Anatomically, these cells correspond to podocytes and exhibit a unique phenotype with loss of many differentiation markers including synaptopodin and dysregulation of the cell cycle markers consistent with proliferation. Podocyte dysfunction appears to be a direct result of HIV-1 protein expression, specifically Nef and Vpr as well as specific host factors that have yet to be elucidated. The mechanism by which Nef induces podocyte proliferation and dedifferentiation has been traced to its ability to activate several signaling pathways including Src-Stat3 and ras-raf-MAPK1, 2. Activation of the cAMP/PKA pathway with all-trans-retinoic acid appears to modulate these changes and returns podocytes to a differentiated, nonproliferating phenotype.

Retinoic acid inhibits HIV-1-induced podocyte proliferation through the cAMP pathway.

HIV-associated nephropathy is characterized by renal podocyte proliferation and dedifferentiation. This study found that all-trans retinoic acid (atRA) reverses the effects of HIV-1 infection in podocytes. Treatment with atRA reduced cell proliferation rate by causing G1 arrest and restored the expression of the differentiation markers (synaptopodin, nephrin, podocin, and WT-1) in HIV-1-infected podocytes. It is interesting that both atRA and 9-cis RA increased intracellular cAMP levels in podocytes. Podocytes expressed most isoforms of retinoic acid receptors (RAR) and retinoid X receptors (RXR) with the exception of RXRgamma. RARalpha antagonists blocked atRA-induced cAMP production and its antiproliferative and prodifferentiation effects on podocytes, suggesting that RARalpha is required. For determination of the effect of increased intracellular cAMP on HIV-infected podocytes, cells were stimulated with either forskolin or 8-bromo-cAMP. Both compounds inhibited cell proliferation significantly and restored synaptopodin expression in HIV-infected podocytes. The effects of atRA were abolished by Rp-cAMP, an inhibitor of the cAMP/protein kinase A pathway and were enhanced by rolipram, an inhibitor of phosphodiesterase 4, suggesting that the antiproliferative and prodifferentiation effects of atRA on HIV-infected podocytes are cAMP dependent. Furthermore, both atRA and forskolin suppressed HIV-induced mitogen-activated protein kinase 1 and 2 and Stat3 phosphorylation. In vivo, atRA reduced proteinuria, cell proliferation, and glomerulosclerosis in HIV-1-transgenic mice. These findings suggest that atRA reverses the abnormal phenotype in HIV-1-infected podocytes by stimulating RARalpha-mediated intracellular cAMP production. These results demonstrate the mechanism by which atRA reverses the proliferation of podocytes that is induced by HIV-1.

Animal models of HIV-associated nephropathy.

PURPOSE OF REVIEW: HIV-1-associated nephropathy is characterized clinically by proteinuria with azotemia and pathologically by collapsing focal segmental glomerulosclerosis with tubulointerstitial nephritis and microcystic tubular dilatation. This review summarizes the manner in which different transgenic animal models contribute to our knowledge of the pathogenesis of HIV-1-associated nephropathy. RECENT FINDINGS: The most widely studied has been a transgenic mouse model bearing a gag and pol-deleted proviral construct that develops renal disease with many of the clinical and pathologic characteristics seen in HIV-1-associated nephropathy. Studies using this model have helped to highlight the role of HIV-1 viral gene expression in renal cells, podocyte dysregulation, and genetic host factors in the pathogenesis of HIV-1-associated nephropathy. This model has provided the key insights that led to detection of HIV-1 in human kidney epithelial cells. Other transgenic models have helped define critical roles for individual HIV gene products (Nef and Vpr) in the pathogenesis of HIV-1-associated nephropathy. Transgenic mouse models have also provided a method to discover new treatments targeting various steps in the pathogenesis of this disease. SUMMARY: Transgenic animal models of HIV-1-associated nephropathy have contributed greatly to the progress made toward understanding the pathogenesis of this disease.

HIV-associated nephropathy: a brief review.

HIV-associated nephropathy (HIVAN) is an important cause of renal failure in HIV-1 seropositive patients. The disease is characterized by collapsing focal segmental glomerulosclerosis with marked podocyte proliferation, microcystic dilatation of the tubules and interstitial nephritis. Patients generally present with advanced HIV-1 infection, renal insufficiency and marked proteinuria. No serologic markers exist to diagnose HIVAN, and given the broad differential diagnosis for renal failure in these patients, renal biopsy should be performed. Viral infection of renal cells plays a central role in the pathogenesis of HIVAN. There is now compelling evidence that highly active antiretroviral therapy (HAART) is effective in preventing end-stage renal disease in patients affected with HIVAN. The efficacy of angiotensin-converting enzyme (ACE) inhibitors and prednisone has also been evaluated, but larger prospective studies are needed.