Leukemia inhibitory factor induces multi-lineage differentiation of adult stem-like cells in kidney via kidney-specific cadherin 16

Stem cells: a potential treatment option for kidney diseases

The prevalence of kidney diseases is emerging as a public health problem. Stem cells (SCs), currently considered as a promising tool for therapeutic application, have aroused considerable interest and expectations. With self-renewal capabilities and great potential for proliferation and differentiation, stem cell therapy opens new avenues for the development of renal function and structural repair in kidney diseases. Mounting evidence suggests that stem cells exert a therapeutic effect mainly by replacing damaged tissues and paracrine pathways. The benefits of various types of SCs in acute kidney disease and chronic kidney disease have been demonstrated in preclinical studies, and preliminary results of clinical trials present its safety and tolerability. This review will focus on the stem cell-based therapy approaches for the treatment of kidney diseases, including various cell sources used, possible mechanisms involved, and outcomes that are generated so far, along with prospects and challenges in clinical application.

Adult stem-like cells in kidney

Human pluripotent cells are promising for treatment for kidney diseases, but the protocols for derivation of kidney cell types are still controversial. Kidney tissue regeneration is well confirmed in several lower vertebrates such as fish, and the repair of nephrons after tubular damages is commonly observed after renal injury. Even in adult mammal kidney, renal progenitor cell or system is reportedly presents suggesting that adult stem-like cells in kidney can be practical clinical targets for kidney diseases. However, it is still unclear if kidney stem cells or stem-like cells exist or not. In general, stemness is defined by several factors such as self-renewal capacity, multi-lineage potency and characteristic gene expression profiles. The definite use of stemness may be obstacle to understand kidney regeneration, and here we describe the recent broad findings of kidney regeneration and the cells that contribute regeneration.

The effects of ABCG2 on the viability, proliferation and paracrine actions of kidney side population cells under oxygen-glucose deprivation

Bcrp1/ABCG2 is exclusively expressed in side population (SP) cells, however, it has not been fully elucidated whether it has an impact on the viability, proliferation and paracrine actions in kidney SP cells under oxygen-glucose deprivation (OGD) followed by reoxygenation. In this study, we found that 2-h OGD did not injure SP cells (sub-lethal OGD) but induced SP cells proliferation 48 and 72 h after reoxygenation; whereas 4-h OGD markedly injured the cells (lethal OGD) and led to apoptosis 24-72 h after reoxygenation. Fumitremorgin C, an inhibitor of ABCG2, attenuated both the proliferation and viability of SP cells. Sub-lethal and lethal OGD induced the increase in the secretion of vascular endothelial growth factor, insulin-like growth factor 1, hepatocyte growth factor, and stromal cell-derived factor-1α in kidney SP cells, which was inhibited by Fumitremorgin C. Collectively, these findings provide evidence for a crucial role for the ABCG2 expression in the viability, proliferation and paracrine actions of kidney SP cells after OGD.

Reconstitution of kidney side population cells after ischemia-reperfusion injury by self-proliferation and bone marrow-derived cell homing

The aim of this study was to examine the contribution of side population (SP) cells from kidney and bone marrow for reconstitution of kidney SP pools after ischemia-reperfusion injury (IRI). The SP and non-SP cells in kidneys following IRI were isolated and serially assessed by fluorescence-activated cell sorting. The apoptosis, proliferation, phenotype, and paracrine actions of SP cells were evaluated in vitro and in vivo. Results indicated that the SP cells from ischemic kidney were acutely depleted within one day following renal IRI and were progressively restored to baseline within 7 days after IRI, through both proliferation of remaining kidney SP cells and homing of bone marrow-derived cells to ischemic kidney. Either hypoxia or serum deprivation alone increased apoptosis of SP cells, and a combination of both further aggravated it. Furthermore, hypoxia in vivo and in vitro induced the increase in the secretion of vascular endothelial growth factor, insulin-like growth factor 1, hepatocyte growth factor, and stromal cell-derived factor-1 α in kidney SP but not non-SP cells. In summary, these results suggest that following renal IRI, kidney SP cells are acutely depleted and then progressively restored to baseline levels by both self-proliferation and extrarenal source, that is, bone marrow-derived cell homing.

Basic helix-loop-helix transcriptional factor MyoR regulates BMP-7 in acute kidney injury

MyoR was originally identified as a transcriptional repressor in embryonic skeletal muscle precursors, but its function in adult kidney has not been clarified. In this study, we tried to clarify the functional role of MyoR using MyoR(-/-) mice. Cisplatin induced a significantly higher degree of severe renal dysfunction, tubular injury, and mortality in MyoR(-/-) mice than in wild-type mice. The injection of cisplatin significantly increased the number of apoptotic cells in the kidney tissues of MyoR(-/-) mice, compared with that in wild-type mice. To clarify the mechanism of severe cisplatin-induced damage and apoptosis in MyoR(-/-) mice, we focused on the p53 signaling pathway and bone morphogenic protein-7 (BMP-7). Treatment with cisplatin significantly activated p53 signaling in cultured renal proximal tubular epithelial cells (RTECs) in both wild-type and MyoR(-/-) mice, but no significant difference between the groups was observed. The injection of cisplatin significantly increased the expression of BMP-7 in the kidney tissues of wild-type mice, but no increase was observed in the MyoR(-/-) mice. Treatment with cisplatin significantly increased the expression of BMP-7 in cultured RTECs from wild-type mice but not in those from MyoR(-/-) mice. Moreover, treatment with recombinant BMP-7 rescued the cisplatin-induced apoptosis in RTECs from MyoR(-/-) mice. Taken together, our results demonstrate a new protective role of MyoR in adult kidneys that acts through the regulation of BMP-7.

ABCG2 protects kidney side population cells from hypoxia/reoxygenation injury through activation of the MEK/ERK pathway

Breast cancer resistance protein 1 (BCRP1/ABCG2) is used to identify the side population (SP) within a population of cells, which is enriched for stem and progenitor cells in different tissues. Here, we investigated the role of extracellular signal-regulated kinase (ERK) 1/2 in the signaling mechanisms underlying ischemic/hypoxic conditions in kidney SP cells. Kidney SP cells were isolated using Hoechst 33342 dye-mediated fluorescein-activated cell sorting and then incubated under hypoxia/reoxygenation (H/R) with or without verapamil, a selective BCRP1/ABCG2 inhibitor. ABCG2 expression, ERK activity, cell viability, metabolic activity, and membrane damage were tested after H/R treatment. To evaluate the role of ERK 1/2 on the expression and function of ABCG2, the expression of mitogen-activated protein kinase (MAPK)/ERK kinase (MEK), which preferentially activates ERK, was upregulated by transfection with the recombinant sense expression vector pcDNA3.1-MEK and downregulated by pretreatment with U0126, a specific MEK inhibitor. We found that hypoxia activated ERK activity in the kidney SP cells but not in non-SP cells both in vitro and in vivo. Overexpression of MEK mimicked hypoxia-induced ABCG2 expression. Contrarily, U0126 inhibited hypoxia- and MEK-upregulated ABCG2 expression. Furthermore, H/R induced significant increases in nuclear, metabolic, and membrane damage in both SP cells and non-SP cells; however, this H/R-induced cytotoxicity was much more severe in non-SP cells than in SP cells. Notably, the viability of kidney SP cells was enhanced by MEK overexpression and inhibited by U0126. Verapamil treatment reversed MEK-induced viability of kidney SP cells. When administered systemically into animals with renal ischemia/reperfusion injury, the SP cells significantly improved renal function, accelerated mitogenic response, and reduced cell apoptosis. However, this improved therapeutic potential of SP cells was significantly reduced by pretreatment with verapamil. Collectively, these findings provide evidence for a crucial role for the MEK/ERK-ABCG2 pathway in protecting kidney SP cells from ischemic/hypoxic injury.

GADD45β Determines Chemoresistance and Invasive Growth of Side Population Cells of Human Embryonic Carcinoma

Side population (SP) cells are an enriched population of stem, and the existence of SP cells has been reported in human cancer cell lines. In this study, we performed an SP analysis using 11 human cancer cell lines and confirmed the presence of SP cells in an embryonic carcinoma cell line, NEC8. NEC8 SP cells showed characteristics of cancer stem cells, such as high growth rate, chemoresistance and high invasiveness. To further characterize the NEC8 SP cells, we used DNA microarrays. Among 38,500 genes, we identified 12 genes that were over-expressed in SP cells and 1 gene that was over-expressed in non-SP cells. Among these 13 genes, we focused on GADD45b. GADD45b was over-expressed in non-SP cells, but the inhibition of GADD45b had no effect on non-SP cells. Paradoxically, the inhibition of GADD45b significantly reduced the viability of NEC8 SP cells. The inhibition of ABCG2, which determines the SP phenotype, had no effect on the invasiveness of NEC8 SP cells, but the inhibition of GADD45b significantly reduced invasiveness. These results suggest that GADD45b, but not ABCG2, might determine the cancer stem cell-like phenotype, such as chemoresistance and the high invasiveness of NEC8 SP cells, and might be a good therapeutic target.

Angiotensin II type 1 receptor blockade prevents decrease in adult stem-like cells in kidney after ureteral obstruction

Infusion of renal side population (SP) cells, enriched with adult stem-like cells, can ameliorate acute renal failure. We investigated the effects of an angiotensin II type 1 (AT(1)) receptor antagonist, valsartan on SP cell changes in renal injury by ureteral obstruction. Renal SP fraction was reduced by 38%, and the number of cells expressing CD45, a marker of hematopoietic system, in renal SP cells was increased in obstructed kidneys. Valsartan attenuated renal injury and the associated SP profile changes. Angiotensin AT(1) receptor blockade may exert regenerative effect by preserving adult stem-like cells such as SP cells in the kidney.

Inhibition of histone deacetylase activates side population cells in kidney and partially reverses chronic renal injury

Bone morphogenic protein (BMP)-7 is expressed in the adult kidney and reverses chronic renal injury when given exogenously. Here, we report that a histone deacetylase inhibitor, trichostatin A (TSA), attenuates chronic renal injury, in part, by augmenting the expression of BMP-7 in kidney side population (SP) cells. We induced accelerated nephrotoxic serum nephritis (NTN) in C57BL/6 mice and treated them with TSA for 3 weeks. Compared with vehicle-treated NTN mice, treatment with TSA prevented the progression of proteinuria, glomerulosclerosis, interstitial fibrosis, and loss of kidney SP cells. Basal gene expression of renoprotective factors such as BMP-7, vascular endothelial growth factor, and hepatocyte growth factor was significantly higher in kidney SP cells as compared with non-SP cells. Treatment with TSA significantly upregulated the expression of BMP-7 in SP cells but not in non-SP cells. Moreover, initiation of treatment with TSA after 3 weeks of NTN (for 3 weeks, until 6 weeks) partially but significantly reversed renal dysfunction. Our results indicate an important role of SP cells in the kidney as one of the possible generator cells of BMP-7 and TSA as a stimulator of the cells in reversing chronic renal disease. Disclosure of potential conflicts of interest is found at the end of this article.

Microarray interrogation of human metanephric mesenchymal cells highlights potentially important molecules in vivo

Many molecules have been implicated in kidney development, often based on experimental animal studies with organ cultures and cell lines. There are very few studies, however, that have directly addressed equivalent living human embryonic tissues. We generated renal mesenchymal cell lines from normal human metanephroi and used a microarray strategy to define changes in gene expression after stimulation with growth factors which enhance nephrogenesis in rodents. Changes were observed in 1) genes modulating diverse general cellular processes, such as matrix metalloproteinase 1 and stanniocalcin 1; 2) genes previously implicated in organogenesis e.g., sprouty 4 and midline 1; and 3) genes involved in blood vessel growth, including angiopoietin 1 and 4. Expression of these same genes was subsequently confirmed in vivo. Our novel data have identified several previously unhighlighted genes that may be implicated in differentiation programs within early human nephrogenesis.

Suppression of disease phenotypes of adult mito-mice carrying pathogenic mtDNA by bone marrow transplantation

For directly addressing the issue of gene therapy of adult patients with mitochondrial diseases, we carried out bone marrow transplantation to adult mito-mice with mutated mtDNA and expressing respiration defects for improvement of disease phenotypes. We supposed that bone marrow cells transdifferentiated into various tissues, so that their transplantation would suppress disease phenotypes. The results showed improvement of survival and delayed expression of renal failure. As most mito-mice without a transplant died due to renal failure, we examined whether transplanted bone marrow cells transdifferentiated into renal tissues carrying improved renal function. Histochemical analyses showed that the suppression of disease phenotypes was not due to transdifferentiation, but due to suppression of apoptosis of renal cells. Thus, bone marrow cells possess a novel function of supporting tissues by suppressing apoptosis induced by respiration defects.

Kidney Epithelial Cells

Kidney tubules are an essential component of an organism's blood clearance mechanism, recovering essential metabolites from glomerular filtration by active transport. Tubules are subject to injury, usually as the result of ischemia-reperfusion events that damage the polarized tubular cell layer that coats the tubule basement membrane, causing dysfunction and necrosis that is often associated with acute renal failure. However, tubules are capable of self-repair, forming new proximal tubular cells to replace failing or necrotic cells. The origin of the progenitor cells that give rise to new tubular cells is unknown. At one extreme, it is possible that all or a fraction of tubular cells can undergo a form of dedifferentiation and subsequent mitosis to form new tubular cells, or alternatively, it is possible that tubular regeneration follows the stem cell/transit-amplifying cell paradigm described for more rapidly regenerating organ systems. Regardless of the mechanism employed to generate new tubular cells, human tubular cells are readily grown in primary cultures and can recapitulate many of the metabolic, endocrine, and immunological properties attributable to endogenous renal proximal tubules when engrafted into bioartificial devices.

Stem Cells and Kidney Disease

Functional recovery in acute renal failure is well known, and the adult kidney is generally recognized to have the capacity to regenerate and repair. Several groups have reported the contribution of bone marrow-derived cells in this process, and others have confirmed the existence of adult stem cells in the kidney, including slow-cycling cells, side population cells, CD133+ cells and rKS56 cells. However, recent data demonstrated that in vivo differentiation of bone marrow-derived cells into renal tubular cells may not occur at all, or is at most a minor component of the repair process. Moreover, it is now generally accepted that stem cells and multipotent cells contribute to the regenerative process by producing protective and regenerative factors rather than by directly differentiating to replace damaged cells. Therefore, for clinical regenerative medicine in kidney disease, the focus of stem cell biology will shift from multiple differentiation of cells or cell-therapy to multiple functions of the cells, such as the production of bone morphologic protein-7 and other regenerative factors.

Musculin/MyoR is expressed in kidney side population cells and can regulate their function

Musculin/MyoR is a new member of basic helix-loop-helix transcription factors, and its expression is limited to skeletal muscle precursors. Here, we report that musculin/MyoR is expressed in adult kidney side population (SP) cells and can regulate their function. SP phenotype can be used to purify stem cell–rich fractions. Microarray analysis clarified that musculin/MyoR was exclusively expressed in kidney SP cells, and the cells resided in the renal interstitial space. Musculin/MyoR-positive cells were decreased in acute renal failure, but infusion of kidney SP cells increased musculin/MyoR-positive cells and improved renal function. Kidney SP cells in reversible acute renal failure expressed a high level of renoprotective factors and leukemia inhibitory factor (LIF), but not in irreversible chronic renal failure. In cultured kidney SP cells, LIF stimulated gene expression of renoprotective factors, and down-regulation of musculin/MyoR augmented LIF-induced gene expression. Our results suggest that musculin/MyoR may play important roles not only in developmental processes but also in regenerative processes in adult tissue.