Cell cycle regulatory proteins in renal disease: role in hypertrophy, proliferation, and apoptosis

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Insulin inhibits dexamethasone effect on angiotensinogen gene expression and induction of hypertrophy in rat kidney proximal tubular cells in high glucose

The present studies investigated whether insulin inhibits the stimulatory effect of dexamethasone (DEX) on angiotensinogen (ANG) gene expression and induction of hypertrophy in rat immortalized renal proximal tubular cells (IRPTCs) in a high-glucose milieu. Rat IRPTCs were cultured in monolayer. ANG and ANG mRNA expression in IRPTCs were quantified by a specific RIA for rat ANG and by RT-PCR assay, respectively. A fusion gene containing the full length of the 5'-flanking region of the rat ANG gene linked to a chloramphenicol acetyl transferase reporter gene was introduced into IRPTCs. The level of fusion gene expression was determined by cellular chloramphenicol acetyl transferase enzymatic activity. Cellular hypertrophy was assessed by flow cytometry, cellular p27(Kip1) protein expression, and protein assay. Our results showed that high glucose (i.e. 25 mM) and DEX (10(-7) M) additively stimulated ANG gene expression and induced IRPTC hypertrophy. Insulin inhibited the effect of high glucose and DEX on these parameters. The inhibitory effect of insulin was reversed by PD 98059 (a MAPK inhibitor) but not by wortmannin (a phosphatidylinositol-3-kinase inhibitor). These results demonstrate that insulin is effective in blocking the stimulatory action of high glucose and DEX on ANG gene expression and induction of IRPTC hypertrophy, suggesting its important role in preventing local intrarenal renin-angiotensin system activation and renal proximal tubular cell hypertrophy induced by hyperglycemia and glucocorticoids in vivo.

Connective tissue growth factor and regulation of the mesangial cell cycle: role in cellular hypertrophy

Connective tissue growth factor (CTGF) is now considered to be one of the important driver molecules for the pathogenesis of diabetic nephropathy (DN) and possibly many other fibrotic disorders. However, the molecular mechanisms by which CTGF functions remain to be established. In an attempt to define these mechanisms, this study was designed to investigate whether CTGF has any effect on the cell cycle of human mesangial cells (HMC), which are known to undergo hypertrophy in DN. This report provides the first evidence that CTGF is a hypertrophic factor for HMC. CTGF stimulates HMC to actively enter the G(1) phase from G(0), but they do not then progress further through the cell cycle. The molecular mechanisms underlying this G(1) phase arrest appear to be due to the induction of the cyclin-dependent kinase inhibitors (CDKI) p15(INK4), p21(Cip1), and p27(Kip1), which are known to bind and inactivate cyclinD/CDK4/6 and the cyclin E/CDK2 kinase complexes. This could account for the maintenance of pRb protein in a non- or very low-phosphorylated state, preventing cell cycle progression. Using CTGF antisense oligonucleotides, the results also indicate that the previously identified transforming growth factor-beta (TGF-beta)-induced hypertrophy in mesangial cells is CTGF-dependent. Mesangial cell hypertrophy is one of the earliest abnormalities of diabetic nephropathy; therefore, therapeutic strategies targeting CTGF may be beneficial in controlling DN.

Metanephric mesenchyme contains embryonic renal stem cells

Renal epithelial cells derive from either cells of the metanephric mesenchyme or ureteric bud cells, but the origin of other renal cells is unclear. To test whether metanephric mesenchymal cells generate cells other than epithelial, we examined the developmental potential of a metanephric mesenchymal cell line (7.1.1 cells) and of primary cultures of metanephric mesenchymal cells. 7.1.1 Cells express both mesenchymal and epithelial markers and, on confluence, form well-defined monolayers expressing epithelial junctional proteins. However, 7.1.1 cells as well as primary cultures of metanephric mesenchymal cells also generate spindle-shaped cells that are positive for alpha-smooth muscle actin, indicating that they are myofibroblasts and/or smooth muscle; this differentiation pathway is inhibited by collagen IV and enhanced by fetal calf serum or transforming growth factor-beta(1). Transforming growth factor-beta(1) also induces expression of smooth muscle proteins, indicating that the cells differentiate into smooth muscle. 7.1.1 Cells as well as primary cultures of metanephric mesenchymal cells also express vascular endothelial growth factor receptor 2 and Tie-2, suggesting that the metanephric mesenchymal cells that generate epithelia may also differentiate into endothelial cells. The pluripotency of the 7.1.1 cells is self-renewing. The data suggest that the metanephric mesenchyme contains embryonic renal stem cells.

HIV-1 expression induces cyclin D1 expression and pRb phosphorylation in infected podocytes: cell-cycle mechanisms contributing to the proliferative phenotype in HIV-associated nephropathy

Background

The aberrant cell-cycle progression of HIV-1-infected kidney cells plays a major role in the pathogenesis of HIV-associated nephropathy, however the mechanisms whereby HIV-1 induces infected glomerular podocytes or infected tubular epithelium to exit quiescence are largely unknown. Here, we ask whether the expression of HIV-1 genes in infected podocytes induces cyclin D₁ and phospho-pRb (Ser780) expression, hallmarks of cyclin D1-mediated G₁ → S phase progression.

Results

We assessed cyclin D₁ and phospho-pRb (Ser780) expression in two well-characterized models of HIV-associated nephropathy pathogenesis: HIV-1 infection of cultured podocytes and HIV-1 transgenic mice (Tg26). Compared to controls, cultured podocytes expressing HIV-1 genes, and podocytes and tubular epithelium from hyperplastic nephrons in Tg26 kidneys, had increased levels of phospho-pRb (Ser780), a target of active cyclin D₁/cyclin-dependent kinase-4/6 known to promote G₁ → S phase progression. HIV-1-infected podocytes showed markedly elevated cyclin D₁ mRNA and cyclin D₁ protein, the latter of which did not down-regulate during cell-cell contact or differentiation, suggesting post-transcriptional stabilization of cyclin D₁ protein levels by HIV-1. The selective suppression of HIV-1 transcription by the cyclin-dependent kinase inhibitor, flavopiridol, abrogated cyclin D₁ expression, underlying the requirement for HIV-1 encoded products to induce cyclin D₁. Indeed, HIV-1 virus deleted of nef failed to induce cyclin D₁ mRNA to the level of other single gene mutant viruses.

Conclusions

HIV-1 expression induces cyclin D₁ and phospho-pRb (Ser780) expression in infected podocytes, suggesting that HIV-1 activates cyclin D1-dependent cell-cycle mechanisms to promote proliferation of infected renal epithelium.

Angiotensin II induces apoptosis in rat glomerular epithelial cells

ANG II has been shown to modulate kidney cell growth and contribute to the pathobiology of glomerulosclerosis. Glomerular visceral epithelial cell (GEC) injury or loss is considered to play a pivotal role in the initiation and progression of glomerulosclerosis. In the present study, we investigated the effect of ANG II on GEC apoptosis. Rat GECs were incubated with increasing doses of ANG II for variable time periods. Apoptosis was evaluated by cell nucleus staining and DNA fragmentation assay. ANG II induced GEC apoptosis in a dose- and time-dependent manner. The proapoptotic effect was attenuated by the ANG II receptor type 1 antagonist losartan or the ANG II receptor type 2 antagonist PD-123319 and was completely blocked by incubation with the combined antagonists. Moreover, ANG II stimulated transforming growth factor (TGF)-beta1 production as measured by ELISA. GECs exposed to TGF-beta1 demonstrated a dose- and time-dependent increase in apoptosis. ANG II-induced apoptosis was significantly inhibited by addition of anti-TGF-beta1 antibody. ANG II also upregulated the expression of Fas, FasL, and Bax and downregulated the expression of Bcl-2 in GECs. These studies suggest that ANG II induces GEC apoptosis by a mechanism involving TGF-beta1 expression that may, importantly, contribute to the pathogenesis of glomerulosclerosis.

The hypertrophic effect of transforming growth factor-beta is reduced in the absence of cyclin-dependent kinase-inhibitors p21 and p27

Transforming growth factor-beta (TGF-beta) has both antiproliferative and hypertrophic effects on mesangial cells (MC). However, it is not known if these processes are independent or if they share common signaling pathways. Proliferation and hypertrophy are regulated by specific cell-cycle regulatory proteins, where the cyclin-dependent kinase (CDK) inhibitors inhibit target cyclin-CDK complexes. This study examined whether the growth regulatory effects of TGF-beta were determined by the CDK inhibitors p21 and p27. Accordingly, cultured MC from wild type (+/+) and single and double null (-/-) p21 and p27 mice were grown in 5% serum in the presence or absence of TGF-beta1 (2 ng/ml). Proliferation ([(3)H]-thymidine incorporation, cell number, cell cycle) and hypertrophy ([(3)H]-leucine incorporation, total protein content, forward light scatter) were measured after 24 h, 48 h, and 96 h. TGF-beta inhibited proliferation in +/+ and p21/p27 double -/- MC to a similar extent. TGF-beta induced hypertrophy in +/+ MC (18.0% increase at 48 h), and to lesser extent in p21 -/- (12.8%) and p27 -/- MC (11.5%) measured by forward light scatter analysis. In p21/p27 double -/-, the hypertrophic effects of TGF-beta were significantly reduced (3.9% at 48 h). Similar results were obtained by measuring hypertrophy by total protein and [(3)H]-leucine incorporation. In conclusion, the CDK inhibitors p21 and p27 are not required for the antiproliferative effects of TGF-beta. However, the hypertrophic growth effects of TGF-beta are reduced in the absence of both p21 and p27. These data suggest that the regulation of the antiproliferative and hypertrophic effects of TGF-beta may be distinct processes.

The genetics of hypertension modifies the renal cell replication response induced by experimental diabetes

To investigate whether the genetics of hypertension modifies renal cell responses in experimental diabetes, we studied the renal cell replication and its regulation by two cyclin-dependent kinase (Cdk) inhibitors, p27(Kip1) and p21(Cip1), in prehypertensive spontaneously hypertensive rats (SHR) and their genetically normotensive counterparts, Wistar Kyoto (WKY) rats, with and without streptozotocin-induced diabetes. In diabetic SHR, the number of proliferating glomerular (0.6 +/- 0.3 positive cells/50 glomeruli) and tubulointerstitial (2.8 +/- 0.6 positive tubulointerstitial cells/50 grid fields) cells assessed by the bromodeoxyuridine technique was significantly (P = 0.0002) lower than in control SHR (13.2 +/- 1.7 and 48.6 +/- 9.7, respectively) and control (14.0 +/- 1.8 and 63.9 +/- 10.6) and diabetic (14.3 +/- 3.5 and 66.4 +/- 11.5) WKY rats. Proliferating cell nuclear antigen, another marker of cell proliferation, was significantly reduced in replicating glomerular (P = 0.0002) and tubulointerstitial (P < 0.0001) cells in diabetic SHR. In freshly isolated glomeruli, the level of p27(Kip1) detected by Western blotting was significantly higher in diabetic SHR than in nondiabetic SHR (1.52 +/- 0.14 vs. 1.00 +/- 0.10% of control, P = 0.014). The expression of p21(Cip1) in isolated glomeruli did not differ among the groups of rats. In conclusion, the response of renal cell replication to diabetes differs markedly between prehypertensive SHR and their WKY control rats. The decreased glomerular cell proliferation in prehypertensive diabetic SHR is at least partly mediated by a higher expression of the Cdk inhibitor p27(Kip1).

The subcellular localization of cyclin dependent kinase 2 determines the fate of mesangial cells: role in apoptosis and proliferation

Apoptosis is closely linked to proliferation. In this study we showed that inducing apoptosis in mouse mesangial cells with ultraviolet (UV) irradiation was associated with increased cyclin A-cyclin dependent kinase (CDK) 2 activity. Inhibiting CDK2 activity with Roscovitine or dominant negative mutant reduced apoptosis. Because apoptosis typically begins in the cytoplasm, we tested the hypothesis that the subcellular localization of CDK2 determines the proliferative or apoptotic fate of the cell. Our results showed that cyclin A-CDK2 was nuclear in proliferating cells. However, inducing apoptosis in proliferating cells with UV irradiation was associated with a decrease in nuclear cyclin A and CDK2 protein levels. This coincided with an increase in protein and kinase activity for cyclin A-CDK2 in the cytoplasm. Translocation of cyclin A-CDK2 also occurred in p53-/- mesangial cells. Finally, we showed that caspase-3 activity was significantly reduced by inhibiting CDK2 activity with Roscovitine. In summary, our results show that apoptosis is associated with an increase in cytoplasmic cyclin A-CDK2 activity, which is p53 independent and upstream of caspase-3. We propose that the subcellular localization of CDK2 determines the proliferative or apoptotic fate of the cell.

Role of the microvascular endothelium in progressive renal disease

The role of the vascular endothelium in progressive renal disease is not well understood. This review presents evidence that progressive renal disease is characterized by a progressive loss of the microvasculature. The loss of the microvasculature correlates directly with the development of glomerular and tubulointerstitial scarring. The mechanism is mediated in part by a reduction in the endothelial proliferative response, and this impairment in capillary repair is mediated by alteration in the local expression of both angiogenic (vascular endothelial growth factor) and antiangiogenic (thrombospondin 1) factors in the kidney. The alteration in balance of angiogenic growth factors is mediated by both macrophage-associated cytokines (interleukin-1beta) and vasoactive mediators. Finally, there is intriguing evidence that stimulation of angiogenesis and/or capillary repair may stabilize renal function and slow progression and that this benefit occurs independently of effects on BP or proteinuria. Therefore, angiogenic agents may represent a novel therapeutic approach for slowing the progression of renal disease.

Mechanical stress reduces podocyte proliferation in vitro

BACKGROUND

Mechanical stretch, a consequence of capillary glomerular hypertension, is thought to be the common final pathway for glomerulosclerosis in systemic hypertension, diabetes, reduced nephron number and focal segmental glomerulosclerosis. However, the effects of stretch on podocyte growth and the mechanisms that underlie this have not been elucidated.

METHODS

Mouse podocyte growth (3H-thymidine, MTT-assay, FACS) was measured following the application of mechanical stretch created by vacuum. The expression of specific cell cycle regulatory proteins was examined by RNAse protection assay and Western blot analysis. Control cells were grown under similar conditions, but were not exposed to stretch.

RESULTS

Mechanical stretch decreased DNA-synthesis (3H-thymidine incorporation) and cell number (MTT-assay) in podocytes at 24, 48 and 72 hours (P < 0.001 vs. control non-stretched cells), which was not due to apoptosis (Hoechst staining) nor cell detachment. Stretch decreased the mRNA and protein levels of cyclins D1, A and B1 within 24 hours. Stretching cells decreased the activity of Cdk2 (measured by histone H1 kinase assay) at 48 and 72 hours and Cdc2 at 72 hours. In contrast, stretch increased the protein levels of the cyclin dependent kinase inhibitors (CKI) p21Cip/Kip/Waf (p21) and p27Kip1 (p27) within the first 24 hours, and increased the mRNA levels of p57Kip2 (p57) at 72 hours. To examine the role of p21 in inhibiting proliferation induced by stretch, we studied p21-/- podocytes in culture. Stretch did not reduce proliferation in p21-/- podocytes (P> 0.05 vs. non-stretched podocytes; P < 0.001 vs. stretched p21+/+ podocytes).

CONCLUSIONS

In contrast to mesangial cells, mechanical stretch decreases the growth of podocytes. This effect is mediated through the regulation of specific cell cycle regulatory proteins. These events may explain the apparent lack of podocyte proliferation in diseases correlated with capillary glomerular hypertension.

Cellular overexpression of heme oxygenase-1 up-regulates p21 and confers resistance to apoptosis

BACKGROUND

Induction of heme oxygenase-1 (HO-1) protects against diverse insults in the kidney and other tissues. We examined the effect of overexpression of HO-1 on cell growth, expression of p21, and susceptibility to apoptosis.

METHODS

LLC-PK1 cells were genetically engineered to exhibit stable overexpression of HO-1. The effects of such overexpression on cell growth, the cell cycle, and the cell cycle-inhibitory protein, p21, were assessed; additionally, the susceptibility of these HO-1 overexpressing cells to apoptosis induced by three different stimuli (TNF-alpha/cycloheximide, staurosporine, or serum deprivation) was evaluated by such methods as the quantitation of caspase-3 activity, phase contrast microscopy, and the TUNEL method.

RESULTS

HO-1 overexpressing LLC-PK1 cells demonstrated cellular hypertrophy, decreased hyperplastic growth, and growth arrest in the G0/G1 phase of the cell cycle. HO-1 overexpressing cells were markedly resistant to apoptosis induced by TNFalpha/cycloheximide or staurosporine as assessed by the caspase-3 activity assay. Such overexpression also conferred resistance to apoptosis induced by serum deprivation as evaluated by the TUNEL method; in these studies, inhibition of HO attenuated the resistance to apoptosis. Expression of the cyclin dependent kinase inhibitor, p21CIP1, WAF1, SDI1, as judged by Northern and Western analyses, was significantly increased in HO-1 overexpressing cells, and decreased as HO activity was inhibited. Moreover, this reduction in expression of p21 attendant upon the inhibition of HO activity in HO-1 overexpressing cells paralleled the loss of resistance of these cells to apoptosis when HO activity is inhibited. The pharmacologic inducer of HO-1, hemin, increased expression of p21 in wild-type cells and decreased apoptosis provoked by TNF-alpha/cycloheximide.

CONCLUSION

Cellular overexpression of HO-1 up-regulates p21, diminishes proliferative cell growth, and confers marked resistance to apoptosis. We speculate that such up-regulation of p21 contributes to the altered pattern of cell growth and resistance to apoptosis. Our studies uncover the capacity of HO-1 to markedly influence the cell cycle in renal epithelial cells. In light of the profound importance of the cell cycle as a determinant of cell fate, we speculate that the inductive effect of HO-1 on p21 and the attendant inhibitory effect on the cell cycle provide a hitherto unsuspected mechanism underlying the cytoprotective actions of HO-1.

Atrial natriuretic peptide attenuates ANG II-induced hypertrophy of renal tubular cells

ANG II arrests LLC-PK1 cells in the G1 phase of the cell cycle and induces hypertrophy, an effect mediated by induction of p27Kip1. We studied whether atrial natriuretic peptide (ANP) may modulate ANG II-induced hypertrophy and p27Kip1 expression in tubular LLC-PK1 cells. ANP, through its fragments 3---28 and 4---27, prevented ANG II-induced cell cycle arrest. ANP inhibited >80% of ANG II-induced p27Kip1 protein expression (Western blots). ANP stimulated expression of MKP-1, a phosphatase involved in dephosphorylation of p44/42 mitogen-activated protein (MAP) kinase, up to 12 h. ANP prevented the ANG II-mediated phosphorylation peak of MAP kinase after 12 h of stimulation. 8-Bromo-cGMP mimicked all the effects of ANP. Transfection with MKP-1 antisense, but not sense, oligonucleotides abolished the modifying role of ANP on ANG II-mediated cell cycle arrest. The effect of ANP on ANG II-mediated hypertrophy of LLC-PK1 cells is regulated on the level of MAP kinase phosphorylation, a key step in the induction of p27Kip1. Although ANP and ANG II both stimulate generation of reactive oxygen species, ANP additionally induces expression of MKP-1, leading to interference with ANG II-mediated MAP kinase phosphorylation.

Angiotensin type 2 receptor is expressed in the adult rat kidney and promotes cellular proliferation and apoptosis

BACKGROUND

Angiotensin II (Ang II) is associated with cell proliferation and apoptosis. The role of the angiotensin type 2 receptor (AT2R) in these processes remains controversial. Conventional radioligand binding of 125I-Sar1, Ile8 Ang II in adult kidney has failed to demonstrate the binding for the AT2R.

METHODS

The presence of the AT2R was explored in adult rat kidney by in vitro and in vivo autoradiography using the selective AT2R radioligand 125I-CGP 42112B. The roles of the angiotensin type 1 receptor (AT1R) and the AT2R in mediating cellular proliferation and apoptosis were assessed using selective AT1R or AT2R antagonists in Ang II-infused Sprague-Dawley (SD) rats.

RESULTS

125I-CGP 42112B binding was demonstrated by in vitro and in vivo autoradiography techniques in the glomeruli and proximal tubules of SD rats. This binding could be displaced by Ang II and the AT2R antagonist PD123319 but not by the AT1R antagonist valsartan. Subcutaneous infusion of Ang II for 14 days in eight-week-old SD rats induced proliferation of proximal tubular epithelial cells, as assessed by a twofold increase in proliferating cell nuclear antigen (PCNA)-positive cells and apoptosis, as assessed by a threefold increase in terminal dUTP nick end labeling (TUNEL)-positive cells. The administration of the AT2R antagonist PD123319 or the AT1R antagonist valsartan was associated with attenuation of the increases in both PCNA- and TUNEL-positive cells following Ang II infusion. Ang II infusion was associated with increased osteopontin gene and protein expression, which could be reduced by treatment with either valsartan or PD123319.

CONCLUSION

These findings indicate that there is significant expression of the AT2R in the adult kidney, and that the AT2R has a role in mediating Ang II-induced proliferation and apoptosis in proximal tubular epithelial cells and expression of osteopontin.