Activin A: autocrine regulator of kidney development and repair

The activin-follistatin system: Key regulator of kidney development, regeneration, inflammation, and fibrosis

Activins, multifunctional cytokines of the transforming growth factor-beta superfamily, play critical roles in the regulation of growth and differentiation in multiple biological systems. Activin activity is finely regulated by the endogenous antagonist follistatin. Early studies reported that activins are involved in renal organogenesis, but subsequent research demonstrated that activins also play a significant role in kidney regeneration following injury. The results of more recent studies suggest activins play roles in both inflammatory kidney diseases and renal fibrosis, conditions that often culminate in end-stage renal disease. Given these findings, the inhibition of activin activity represents a promising therapeutic approach for treating a range of kidney disorders. This review discusses the latest discoveries concerning the role of the activin-follistatin system in renal development and pathophysiology and explores the potential therapeutic implications of targeting this system in the management of kidney diseases.

Diminished vasculogenesis under inflammatory conditions is mediated by Activin A

Severe inflammatory stress often leads to vessel rarefaction and fibrosis, resulting in limited tissue recovery. However, signaling pathways mediating these processes are not completely understood. Patients with ischemic and inflammatory conditions have increased systemic Activin A level, which frequently correlates with the severity of pathology. Yet, Activin A's contribution to disease progression, specifically to vascular homeostasis and remodeling, is not well defined. This study investigated vasculogenesis in an inflammatory environment with an emphasis on Activin A's role. Exposure of endothelial cells (EC) and perivascular cells (adipose stromal cells, ASC) to inflammatory stimuli (represented by blood mononuclear cells from healthy donors activated with lipopolysaccharide, aPBMC) dramatically decreased EC tubulogenesis or caused vessel rarefaction compared to control co-cultures, concurrent with increased Activin A secretion. Both EC and ASC upregulated Inhibin Ba mRNA and Activin A secretion in response to aPBMC or their secretome. We identified TNFα (in EC) and IL-1β (in EC and ASC) as the exclusive inflammatory factors, present in aPBMC secretome, responsible for induction of Activin A. Similar to ASC, brain and placental pericytes upregulated Activin A in response to aPBMC and IL-1β, but not TNFα. Both these cytokines individually diminished EC tubulogenesis. Blocking Activin A with neutralizing IgG mitigated detrimental effects of aPBMC or TNFα/IL-1β on tubulogenesis in vitro and vessel formation in vivo. This study delineates the signaling pathway through which inflammatory cells have a detrimental effect on vessel formation and homeostasis, and highlights the central role of Activin A in this process. Transitory interference with Activin A during early phases of inflammatory or ischemic insult, with neutralizing antibodies or scavengers, may benefit vasculature preservation and overall tissue recovery.

Immunostaining of βA-Activin and Follistatin Is Decreased in HPV(+) Cervical Pre-Neoplastic and Neoplastic Lesions

The activin-follistatin system regulates several cellular processes, including differentiation and tumorigenesis. We hypothesized that the immunostaining of βA-activin and follistatin varies in neoplastic cervical lesions. Cervical paraffin-embedded tissues from 162 patients sorted in control (n = 15), cervical intraepithelial neoplasia (CIN) grade 1 (n = 38), CIN2 (n = 37), CIN3 (n = 39), and squamous cell carcinoma (SCC; n = 33) groups were examined for βA-activin and follistatin immunostaining. Human papillomavirus (HPV) detection and genotyping were performed by PCR and immunohistochemistry. Sixteen samples were inconclusive for HPV detection. In total, 93% of the specimens exhibited HPV positivity, which increased with patient age. The most detected high-risk (HR)-HPV type was HPV16 (41.2%) followed by HPV18 (16%). The immunostaining of cytoplasmatic βA-activin and follistatin was higher than nuclear immunostaining in all cervical epithelium layers of the CIN1, CIN2, CIN3, and SCC groups. A significant decrease (p < 0.05) in the cytoplasmic and nuclear immunostaining of βA-activin was detected in all cervical epithelial layers from the control to the CIN1, CIN2, CIN3, and SCC groups. Only nuclear follistatin immunostaining exhibited a significant reduction (p < 0.05) in specific epithelial layers of cervical tissues from CIN1, CIN2, CIN3, and SCC compared to the control. Decreased immunostaining of cervical βA-activin and follistatin at specific stages of CIN progression suggests that the activin-follistatin system participates in the loss of the differentiation control of pre-neoplastic and neoplastic cervical specimens predominantly positive for HPV.

Tubule-Derived Follistatin Is Increased in the Urine of Rats with Renal Ischemia and Reflects the Severity of Acute Tubular Damage

Activin A, a member of the TGF-beta superfamily, is a negative regulator of tubular regeneration after renal ischemia. Activin action is controlled by an endogenous antagonist, follistatin. However, the role of follistatin in the kidney is not fully understood. In the present study, we examined the expression and localization of follistatin in normal and ischemic rat kidneys and measured urinary follistatin in rats with renal ischemia to assess whether urinary follistatin could serve as a biomarker for acute kidney injury. Using vascular clamps, renal ischemia was induced for 45 min in 8-week-old male Wistar rats. In normal kidneys, follistatin was localized in distal tubules of the cortex. In contrast, in ischemic kidneys, follistatin was localized in distal tubules of both the cortex and outer medulla. Follistatin mRNA was mainly present in the descending limb of Henle of the outer medulla in normal kidneys but was upregulated in the descending limb of Henle of both the outer and inner medulla after renal ischemia. Urinary follistatin, which was undetectable in normal rats, was significantly increased in ischemic rats and peaked 24 h after reperfusion. There was no correlation between urinary follistatin and serum follistatin. Urinary follistatin levels were increased according to ischemic duration and were significantly correlated with the follistatin-positive area as well as the acute tubular damage area. These results suggest that follistatin normally produced by renal tubules increases and becomes detectable in urine after renal ischemia. Urinary follistatin might be useful to assess the severity of acute tubular damage.

Effect of NAD+ boosting on kidney ischemia-reperfusion injury

Acute kidney injury (AKI) is associated with a very high mortality and an increased risk for progression to chronic kidney disease (CKD). Ischemia-reperfusion injury (IRI) is a model for AKI, which results in tubular damage, dysfunction of the mitochondria and autophagy, and in decreased cellular nicotinamide adenine dinucleotide (NAD⁺) with progressing fibrosis resulting in CKD. NAD⁺ is a co-enzyme for several proteins, including the NAD⁺ dependent sirtuins. NAD⁺ augmentation, e.g. by use of its precursor nicotinamide riboside (NR), improves mitochondrial homeostasis and organismal metabolism in many species. In the present investigation the effects of prophylactic administration of NR on IRI-induced AKI were studied in the rat. Bilateral IRI reduced kidney tissue NAD⁺, caused tubular damage, reduced α-Klotho (klotho), and altered autophagy flux. AKI initiated progression to CKD, as shown by induced profibrotic Periostin (postn) and Inhibin subunit beta-A, (activin A / Inhba), both 24 hours and 14 days after surgery. NR restored tissue NAD⁺ to that of the sham group, increased autophagy (reduced p62) and sirtuin1 (Sirt1) but did not ameliorate renal tubular damage and profibrotic genes in the 24 hours and 14 days IRI models. AKI induced NAD⁺ depletion and impaired autophagy, while augmentation of NAD⁺ by NR restored tissue NAD⁺ and increased autophagy, possibly serving as a protective response. However, prophylactic administration of NR did not ameliorate tubular damage of the IRI rats nor rescued the initiation of fibrosis in the long-term AKI to CKD model, which is a pivotal event in CKD pathogenesis.

[Involvement of Notch1 and ALK4/5 Signaling Pathways in Renal Tubular Cell Death: Their Application to Clarification of Cadmium Toxicity]

Renal tubular cell death is caused by various extracellular stresses including toxic amounts of cadmium, an occupational and environmental pollutant metal, and is responsible for renal dysfunction. While cadmium exposure disrupts many intracellular signaling pathways, the molecular mechanism underlying cadmium-induced renal tubular cell death has not yet been fully elucidated. We have recently identified two important intracellular signaling pathways that promote cadmium-induced renal tubular cell death: the Notch1 signaling and activin receptor-like kinase (ALK) 4/5 signaling (also known as the activin-transforming growth factor β receptor pathways). In this review paper, we introduce our previous experimental findings, focusing on Notch1 and ALK4/5 signaling pathways, which may uncover the molecular mechanisms involved in cadmium-induced renal tubular cell death.

Urinary Activin A is a novel biomarker reflecting renal inflammation and tubular damage in ANCA-associated vasculitis

Activin A, a member of the transforming growth factor-beta superfamily, is a critical modulator of inflammation and plays a key role in controlling the cytokine cascade that drives the inflammatory response. However, the role of activin A in inflammatory kidney diseases remains unknown. To address this issue, we examined here whether activin A can be detected in the kidney and/or urine from patients with antineutrophil cytoplasmic antibody (ANCA) -associated vasculitis (AAV). Fifty-one patients who had been diagnosed with AAV and were treated in our department between November 2011 to March 2018 were included in this study. Forty-one patients had renal complications (renal AAV). Serum and urinary activin A levels were measured by enzyme-linked immunosorbent assay. Correlation of urinary activin A concentration with clinical parameters was analyzed. Urinary activin A was undetectable in healthy volunteers. In contrast, urinary activin A concentration was significantly increased in patients with renal AAV but not in those with non-renal AAV. Urinary activin A concentration decreased rapidly after immunosuppressive treatment. There was a significant correlation of urinary activin A level with urinary protein, L-FABP, and NAG. Histologic evaluation revealed that urinary activin A levels were significantly higher in patients with cellular crescentic glomeruli than in those lacking this damage. In situ hybridization demonstrated that the mRNA encoding the activin A βA subunit was undetectable in normal kidneys but accumulated in the proximal tubules and crescentic glomeruli of the kidneys of patients with renal AAV. Immunostaining showed that activin A protein also was present in the proximal tubules, crescentic glomeruli, and macrophages infiltrating into the interstitium in the kidneys of patients with renal AAV. These data suggested that urinary activin A concentration reflects renal inflammation and tubular damage in AAV and may be a useful biomarker for monitoring renal AAV.

Circadian rhythm of activin A and related parameters of mineral metabolism in normal and uremic rats

Activin A is a new fascinating player in chronic kidney disease-mineral and bone disorder (CKD-MBD), which is implicated in progressive renal disease, vascular calcification, and osteodystrophy. Plasma activin A rises early in the progression of renal disease. Disruption of circadian rhythms is related to increased risk of several diseases and circadian rhythms are observed in mineral homeostasis, bone parameters, and plasma levels of phosphate and PTH. Therefore, we examined the circadian rhythm of activin A and CKD-MBD-related parameters (phosphate, PTH, FGF23, and klotho) in healthy controls and CKD rats (5/6 nephrectomy) on high-, standard- and low-dietary phosphate contents as well as during fasting conditions. Plasma activin A exhibited circadian rhythmicity in healthy control rats with fourfold higher values at acrophase compared with nadir. The rhythm was obliterated in CKD. Activin A was higher in CKD rats compared with controls when measured at daytime but not significantly when measured at evening/nighttime, stressing the importance of time-specific reference intervals when interpreting plasma values. Plasma phosphate, PTH, and FGF23 all showed circadian rhythms in control rats, which were abolished or disrupted in CKD. Plasma klotho did not show circadian rhythm. Thus, the present investigation shows, for the first time, circadian rhythm of plasma activin A. The rhythmicity is severely disturbed by CKD and is associated with disturbed rhythms of phosphate and phosphate-regulating hormones PTH and FGF23, indicating that disturbed circadian rhythmicity is an important feature of CKD-MBD.

Activin A: a novel urinary biomarker of renal impairment in multiple myeloma

Renal impairment (RI) is a common complication of multiple myeloma (MM) that significantly affects treatment efficacy and mortality. However, no useful biomarkers for early detection of renal damage in MM exist. Reports indicate that activin A, a multifunctional cytokine of the TGF-β superfamily, is involved in the development and progression of various kidney diseases. In the present study, we measured urinary activin A levels in patients with newly diagnosed MM (NDMM) (n=41), smoldering MM (SMM) (n=10), and monoclonal gammopathy of undetermined significance (MGUS) (n=28), including monoclonal gammopathy of renal significance (MGRS), and assessed the correlation between urinary activin A and several clinical parameters. Urinary activin A, undetectable in healthy volunteers, was significantly increased in NDMM patients but not in patients with SMM and MGUS (97.3, 25.0, and 6.61 mg/gCr, respectively, P<0.05). In all patients with NDMM, urinary activin A levels were significantly reduced after initial treatment regardless of the therapy regimen. There was a significant correlation of urinary activin A with spot urinary protein level (P<0.001) and serum M-protein (P=0.029) but not with estimated glomerular filtration rate (eGFR), serum creatinine (Cr), N-acetyl-glucosaminidase (NAG), and serum activin A level. Histological analysis using renal biopsy samples revealed that activin A, which was absent from normal kidneys, was detected in the renal tubular cells of patients with MGRS. These data suggest that urinary activin A reflects tubular injury in MM and might aid the early detection of RI in plasma cell neoplasms.

Blockade of ALK4/5 signaling suppresses cadmium- and erastin-induced cell death in renal proximal tubular epithelial cells via distinct signaling mechanisms

Various types of cell death, including apoptosis, necrosis, necroptosis, and ferroptosis, are induced in renal tubular epithelial cells following exposure to environmental stresses and toxicants such as osmotic stress, ischemia/reperfusion injury, cisplatin, and cadmium. This is known to cause renal dysfunction, but the cellular events preceding stress-induced cell death in renal tubules are not fully elucidated. The activin receptor-like kinase (ALK) 4/5, also known as activin-transforming growth factor (TGF) β receptor, is involved in stress-induced renal injury. We, therefore, studied the role of ALK4/5 signaling in HK-2 human proximal tubular epithelial cell death induced by cisplatin, cadmium, hyperosmotic stress inducer, sorbitol, and the ferroptosis activator, erastin. We found that ALK4/5 signaling is involved in cadmium- and erastin-induced cell death, but not sorbitol- or cisplatin-induced apoptotic cell death. Cadmium exposure elevated the level of phosphorylated Smad3, and treatment with the ALK4/5 kinase inhibitors, SB431542 or SB505124, suppressed cadmium-induced HK-2 cell death. Cadmium-induced cell death was attenuated by siRNA-mediated ALK4 or Smad3 silencing, or by treatment with SIS3, a selective inhibitor of TGFβ1-dependent Smad3 phosphorylation. Furthermore, ALK4/5 signaling activated Akt signaling to promote cadmium-induced HK-2 cell death. In contrast, siRNA-mediated Inhibin-bA silencing or treatment with TGFβ1 or activin A had little effect on cadmium-induced HK-2 cell death. On the other hand, treatment with SB431542 or SB505124 attenuated erastin-induced ferroptosis by hyperactivating Nrf2 signaling in HK-2 cells. These results suggest that blockade of ALK4/5 signaling protects against cadmium- and erastin-induced HK-2 cell death via Akt and Nrf2 signaling pathways, respectively.

Fibrodysplasia Ossificans Progressiva (FOP): A Segmental Progeroid Syndrome

Segmental progeroid syndromes are commonly represented by genetic conditions which recapitulate aspects of physiological aging by similar, disparate, or unknown mechanisms. Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease caused by mutations in the gene for ACVR1/ALK2 encoding Activin A receptor type I/Activin-like kinase 2, a bone morphogenetic protein (BMP) type I receptor, and results in the formation of extra-skeletal ossification and a constellation of others features, many of which resemble accelerated aging. The median estimated lifespan of individuals with FOP is approximately 56 years of age. Characteristics of precocious aging in FOP include both those that are related to dysregulated BMP signaling as well as those secondary to early immobilization. Progeroid features that may primarily be associated with mutations in ACVR1 include osteoarthritis, hearing loss, alopecia, subcutaneous lipodystrophy, myelination defects, heightened inflammation, menstrual abnormalities, and perhaps nephrolithiasis. Progeroid features that may secondarily be related to immobilization from progressive heterotopic ossification include decreased vital capacity, osteoporosis, fractures, sarcopenia, and predisposition to respiratory infections. Some manifestations of precocious aging may be attributed to both primary and secondary effects of FOP. At the level of lesion formation in FOP, soft tissue injury resulting in hypoxia, cell damage, and inflammation may lead to the accumulation of senescent cells as in aged tissue. Production of Activin A, platelet-derived growth factor, metalloproteinases, interleukin 6, and other inflammatory cytokines as part of the senescence-associated secretory phenotype could conceivably mediate the initial signaling cascade that results in the intense fibroproliferative response as well as the tissue-resident stem cell reprogramming leading up to ectopic endochondral bone formation. Consideration of FOP as a segmental progeroid syndrome offers a unique perspective into potential mechanisms of normal aging and may also provide insight for identification of new targets for therapeutic interventions in FOP.

LncRNA MALAT1 modified progression of clear cell kidney carcinoma (KIRC) by regulation of miR-194-5p/ACVR2B signaling

This investigation was purposed to extrapolate whether and how lncRNA MALAT1, miR-194-5p, and ACVR2B altered development of clear cell kidney carcinoma (KIRC). We totally gathered 318 pairs of KIRC tissues and adjacent normal tissues, and also purchased human KIRC cell lines and normal human proximal tubular epithelial cell line. Besides, si-MALAT1, pcDNA-MALAT1, miR-194-5p mimic, miR-194-5p inhibitor, and negative control (NC) were, respectively, transfected into KIRC cells. The viability, proliferation, and apoptosis of the cells were determined with CCK-8 assay, colony formation assay, and flow cytometry. Dual-luciferase reporter gene assay was implemented to validate the targeted relationships between MALAT1 and miR-194-5p, as well as between miR-194-5p and ACVR2B. The results showed that highly expressed MALAT1, ACVR2B, and lowly expressed miR-194-5p were associated with larger tumor size (≥4 cm), advanced TNM stage and poor prognosis of KIRC patients, when, respectively, compared with lowly expressed MALAT1, ACVR2B, and highly expressed miR-194-5p (P < 0.05). Transfection of pcDNA-MALAT1, miR-194-5p inhibitor, and pcDNA-ACVR2B conferred the KIRC cells with promoted viability and proliferation, as well as reduced apoptosis (P < 0.05). Treatment of rats with pcDNA-MALAT1, miR-194-5p inhibitor, or pcDNA-ACVR2B also contributed to larger tumor size growing in them (P < 0.05). Moreover, MALAT1 could directly target miR-194-5p to suppress its expression, and ACVR2B was the targeted molecule of miR-194-5p (P < 0.05). Finally, ACVR2B could reverse the effects exerted by miR-194-5p on viability, proliferation, and apoptosis of KIRC cells (P < 0.05). In conclusion, LncRNA MALAT1/miR-194-5p/ACVR2B signaling was regarded as a candidate pathway for modulating KIRC progression.

Human Epicardial Adipose Tissue Activin A Expression Predicts Occurrence of Postoperative Atrial Fibrillation in Patients Receiving Cardiac Surgery

BACKGROUND

Activin A secreted by epicardial adipose tissue (EAT) plays a major role in the progress of atrial fibrosis. However, the potential of Activin A in predicting the occurrence of postoperative atrial fibrillation (POAF) has yet to be elucidated. We aimed to investigate the predicable value of Activin A expression in EAT on POAF.

METHODS

A total of 89 patients receiving cardiac surgery without atrial fibrillation (AF) history were enrolled in this study, including 49 patients with valvular heart disease (VHD) and 40 patients with non-valvular heart disease (NVHD). Activin A expression in EAT was determined by quantitative polymerase chain reaction (qPCR), while the thickness of EAT (EATT) was estimated by echocardiography. New onset POAF before discharge was documented.

RESULTS

Eventually 32 patients (36.0%) developed POAF, including 20 patients with VHD (40.8%) and 12 patients with NVHD (30.0%). Activin A expression was higher in POAF than sinus rhythm (SR) patients, whether for VHD or NVHD group (All p<0.001). In general, Activin A expression predicted the occurrence of POAF with a sensitivity of 65.6% and specificity of 91.2% (AUC: 0.795; 95%CI: 0.693-0.897, p<0.001). Subgroup analysis showed that EATT was not significant for the VHD group in predicting POAF (p=0.07), while Activin A expression demonstrated a sensitivity of 60.0% and specificity of 89.7% (AUC: 0.745; 95%CI: 0.601-0.889, p<0.001). Multivariate regression analysis showed that Activin A expression in EAT was an independent risk factor for POAF (OR: OR=1.067, 95%CI:1.002-1.136, p=0.042).

CONCLUSIONS

Activin A expression in EAT is an independent risk factor for POAF, which can be used for prediction of POAF, especially for patients with VHD.

Identification of Urinary Activin A as a Novel Biomarker Reflecting the Severity of Acute Kidney Injury

Acute kidney injury (AKI) is a common but complex condition that is associated with increased morbidity and mortality. In the present study, we examined whether urinary activin A, a member of the TGF-beta superfamily, is present in mice with ischemia-reperfusion injury and in humans with AKI, as well as its potential as a biomarker for AKI. Expression of activin A was markedly increased in ischemic mouse kidneys. In situ hybridization demonstrated that activin mRNA was expressed in tubular cells of ischemic kidneys but not of normal kidneys. Immunoreactive activin A, which was absent in normal kidneys, was detected in the cytoplasm of proximal tubular cells in ischemic kidneys. Activin A was undetectable in the urine of normal mice. In contrast, activin A was significantly increased in the urine of ischemic mice at 3 h after reperfusion. Urinary activin A levels increased according to the period of ischemia. In humans, urinary activin A was almost undetectable in healthy volunteers and in patients with pre-renal AKI, but was significantly increased in patients with renal AKI. There was no significant correlation between urinary activin A and serum activin A. Collectively, urinary activin A might be a useful biomarker reflecting the severity of AKI.

Involvement of infiltrating macrophage-derived activin A in the progression of renal damage in MRL-lpr mice

Lupus nephritis is a life-threatening complication of systemic lupus erythematosus (SLE). Various growth factors, cytokines, and chemokines are implicated in the development of SLE. However, the pathophysiological processes involved in the development of lupus nephritis still remain unclear. In this study, we examined the involvement of activin A, a member of the transforming growth factor β (TGF-β) superfamily, in the progression of renal damage in lupus-prone MRL- lpr mice. Activin A was not expressed in the kidneys of control MRL-MpJ mice but was detectable in perivascular infiltrating cluster of differentiation 68 (CD68)-positive cells in the kidneys of MRL- lpr mice. Urinary activin A, which was also absent in MRL-MpJ mice, was detectable in MRL- lpr mice from 16 wk onward. Urinary activin A levels were significantly correlated with the number of perivascular inflammatory cell layers, the number of crescentic glomeruli, and the percentage of Elastica van Gieson (EVG)-positive fibrotic areas, but not with urinary protein levels or serum activin A. When activin action was blocked in vivo by the intraperitoneal administration of an activin antagonist, follistatin, the number of crescentic glomeruli, percentage of EVG-positive fibrotic areas, CD68-positive cell infiltration, and proteinuria were significantly reduced in a dose-dependent manner. These data suggest that infiltrating macrophage-derived activin A is involved in the progression of renal damage in MRL- lpr mice.

Ligand trap for the activin type IIA receptor protects against vascular disease and renal fibrosis in mice with chronic kidney disease

The causes of cardiovascular mortality associated with chronic kidney disease (CKD) are partly attributed to the CKD-mineral bone disorder (CKD-MBD). The causes of the early CKD-MBD are not well known. Our discovery of Wnt (portmanteau of wingless and int) inhibitors, especially Dickkopf 1, produced during renal repair as participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical. In the search for such factors, we studied the effects of activin receptor type IIA (ActRIIA) signaling by using a ligand trap for the receptor, RAP-011 (a soluble extracellular domain of ActRIIA fused to a murine IgG-Fc fragment). In a mouse model of CKD that stimulated atherosclerotic calcification, RAP-011 significantly increased aortic ActRIIA signaling assessed by the levels of phosphorylated Smad2/3. Furthermore, RAP-011 treatment significantly reversed CKD-induced vascular smooth muscle dedifferentiation as assessed by smooth muscle 22α levels, osteoblastic transition, and neointimal plaque calcification. In the diseased kidneys, RAP-011 significantly stimulated αklotho levels and it inhibited ActRIIA signaling and decreased renal fibrosis and proteinuria. RAP-011 treatment significantly decreased both renal and circulating Dickkopf 1 levels, showing that Wnt activation was downstream of ActRIIA. Thus, ActRIIA signaling in CKD contributes to the CKD-MBD and renal fibrosis. ActRIIA signaling may be a potential therapeutic target in CKD.

Activin A expression in esophageal carcinoma and its association with tumor aggressiveness and differentiation

The aim of the present study was to investigate the expression of activin A in esophageal carcinoma and its association with tumor differentiation and metastasis. A total of 57 esophageal carcinoma patients and 36 controls were included in the current study. The mRNA and protein expression levels of activin A in esophageal tumors or normal esophageal tissues were determined by reverse transcription-quantitative polymerase chain reaction and immunohistochemical analysis. In addition, the association of activin A expression with esophageal carcinoma differentiation, metastasis and recurrence postoperatively was analyzed. The mRNA and protein expression levels of activin A in esophageal carcinoma were significantly higher compared with those in normal esophageal tissues (P<0.05). The expression of activin A was higher in poorly-/moderately-differentiated esophageal tumor tissues compared with that of well-differentiated or control tissues (P<0.05). Furthermore, the expression of activin A in poorly-differentiated esophageal tumor tissues was higher compared with that of moderately-differentiated tissues (P<0.05). A positive correlation was also observed between differentiation degree and activin A expression. The expression of activin A was higher in patients with lymph node metastasis compared with those without metastasis (P<0.05). The cumulative survival rate of patients with a high expression of activin A at 1, 2 and 3 years postoperatively was significantly decreased compared with that of patients with a lower expression of activin A (P<0.05); by contrast, the cumulative recurrence rate was significantly higher in patients with a lower activin A expression (P<0.05). In conclusion, abnormal expression of activin A was detected in esophageal tumor tissues, which was correlated with the tumor differentiation, metastasis, survival and recurrence. In conclusion, activin A may be used as an auxiliary index in the diagnosis and prognosis of clinical esophageal carcinoma.

Regenerative medicine for the kidney: renotropic factors, renal stem/progenitor cells, and stem cell therapy

The kidney has the capacity for regeneration and repair after a variety of insults. Over the past few decades, factors that promote repair of the injured kidney have been extensively investigated. By using kidney injury animal models, the role of intrinsic and extrinsic growth factors, transcription factors, and extracellular matrix in this process has been examined. The identification of renal stem cells in the adult kidney as well as in the embryonic kidney is an active area of research. Cell populations expressing putative stem cell markers or possessing stem cell properties have been found in the tubules, interstitium, and glomeruli of the normal kidney. Cell therapies with bone marrow-derived hematopoietic stem cells, mesenchymal stem cells, endothelial progenitor cells, and amniotic fluid-derived stem cells have been highly effective for the treatment of acute or chronic renal failure in animals. Embryonic stem cells and induced pluripotent stem cells are also utilized for the construction of artificial kidneys or renal components. In this review, we highlight the advances in regenerative medicine for the kidney from the perspective of renotropic factors, renal stem/progenitor cells, and stem cell therapies and discuss the issues to be solved to realize regenerative therapy for kidney diseases in humans.

Follistatin, an activin antagonist, ameliorates renal interstitial fibrosis in a rat model of unilateral ureteral obstruction

Activin, a member of the TGF-β superfamily, regulates cell growth and differentiation in various cell types. Activin A acts as a negative regulator of renal development as well as tubular regeneration after renal injury. However, it remains unknown whether activin A is involved in renal fibrosis. To clarify this issue, we utilized a rat model of unilateral ureteral obstruction (UUO). The expression of activin A was significantly increased in the UUO kidneys compared to that in contralateral kidneys. Activin A was detected in glomerular mesangial cells and interstitial fibroblasts in normal kidneys. In UUO kidneys, activin A was abundantly expressed by interstitial α-SMA-positive myofibroblasts. Administration of recombinant follistatin, an activin antagonist, reduced the fibrotic area in the UUO kidneys. The number of proliferating cells in the interstitium, but not in the tubules, was significantly lower in the follistatin-treated kidneys. Expression of α-SMA, deposition of type I collagen and fibronectin, and CD68-positive macrophage infiltration were significantly suppressed in the follistatin-treated kidneys. These data suggest that activin A produced by interstitial fibroblasts acts as a potent profibrotic factor during renal fibrosis. Blockade of activin A action may be a novel approach for the prevention of renal fibrosis progression.

Activin A induction of erythroid differentiation sensitizes K562 chronic myeloid leukemia cells to a subtoxic concentration of imatinib

Chronic myeloid leukemia (CML) is a hematopoietic stem/progenitor cell disorder in which Bcr-Abl oncoprotein inhibits cell differentiation. Differentiation induction is considered an alternative strategy for treating CML. Activin A, a member of the transforming growth factor-β superfamily, induces erythroid differentiation of CML cells through the p38 MAPK pathway. In this study, treatment of the K562 CML stem/progenitor cell line with activin A followed by a subtoxic concentration of the Bcr-Abl inhibitor imatinib strongly induced growth inhibition and apoptosis compared with simultaneous treatment with activin A and imatinib. Imatinib-induced growth inhibition and apoptosis following activin A pretreatment were dose- and time-dependent. Imatinib-induced growth inhibition and apoptosis were also dependent on the pretreatment dose of activin A. More than 90% of the activin A-induced increases in glycophorin A-positive cells were sensitive to imatinib. However, only some of original glycophorin A-positive cells in the activin A treatment group were sensitive to imatinib. Sequential treatment with activin A and imatinib decreased Bcr-Abl, procaspase-3, Mcl-1, and Bcl-xL and also induced cleavage of procaspase-3/poly(ADP-ribose)polymerase. The reduction of erythroid differentiation in p38 MAPK dominant-negative mutants or by short hairpin RNA knockdown of p38 MAPK decreased the growth inhibition and apoptosis mediated by sequential treatment with activin A and imatinib. Furthermore, the same inhibition level of multidrug resistance 1 expression was observed in cells treated with activin A alone, treated sequentially with activin A and imatinib, or treated simultaneously with activin A and imatinib. The p38 MAPK inhibitor SB-203580 can restore activin A-inhibited multidrug resistance 1 expression. Taken together, our results suggest that a subtoxic concentration of imatinib could exhibit strong cytotoxicity against erythroid-differentiated K562 CML cells.

Tumor suppressor FLCN inhibits tumorigenesis of a FLCN-null renal cancer cell line and regulates expression of key molecules in TGF-beta signaling

Background

Germline mutations in the FLCN gene are responsible for the development of fibrofolliculomas, lung cysts and renal neoplasia in Birt-Hogg-Dube' (BHD) syndrome. The encoded protein folliculin (FLCN) is conserved across species but contains no classic motifs or domains and its function remains unknown. Somatic mutations or loss of heterozygosity in the remaining wild type copy of the FLCN gene have been found in renal tumors from BHD patients suggesting that FLCN is a classic tumor suppressor gene.

Results

To examine the tumor suppressor function of FLCN, wild-type or mutant FLCN (H255R) was stably expressed in a FLCN-null renal tumor cell line, UOK257, derived from a BHD patient. When these cells were injected into nude mice, tumor development was inversely dependent upon the level of wild-type FLCN expression. We identified genes that were differentially expressed in the cell lines with or without wild-type FLCN, many of which are involved in TGF-β signaling, including TGF-β2 (TGFB2), inhibin β A chain (INHBA), thrombospondin 1 (THBS1), gremlin (GREM1), and SMAD3. In support of the in vitro data, TGFB2, INHBA, THBS1 and SMAD3 expression levels were significantly lower in BHD-associated renal tumors compared with normal kidney tissue. Although receptor mediated SMAD phosphorylation was not affected, basal and maximal TGF-β-induced levels of TGFB2, INHBA and SMAD7 were dramatically reduced in FLCN-null cells compared with FLCN-restored cells. Secreted TGF-β2 and activin A (homo-dimer of INHBA) protein levels were also lower in FLCN-null cells compared with FLCN-restored cells. Consistent with a growth suppressive function, activin A (but not TGF-β2) completely suppressed anchorage-independent growth of FLCN-null UOK257 cells.

Conclusions

Our data demonstrate a role for FLCN in the regulation of key molecules in TGF-β signaling and confirm deregulation of their expression in BHD-associated renal tumors. Thus, deregulation of genes involved in TGF-β signaling by FLCN inactivation is likely to be an important step for tumorigenesis in BHD syndrome.

Peroxisome proliferator-activated receptor gamma down-regulates follistatin in intestinal epithelial cells through SP1

Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) down-regulates the expression of follistatin mRNA in intestinal epithelial cells in vivo. The mechanism of PPARgamma-mediated down-regulation of follistatin was investigated using non-transformed, rat intestinal epithelial cells (RIE-1). RIE cells expressed activin A, the activin receptors ActRI and ActRII, and the follistatin-315 mRNA. RIE-1 cells responded to endogenous activin A, and this response was antagonized by follistatin, as evidenced by changes in cell growth and regulation of an activin-responsive reporter. Using RIE-1 cells, we show that activation of PPARgamma by rosiglitazone reduced follistatin mRNA levels in a dose- and concentration-dependent manner. Down-regulation of follistatin by rosiglitazone required the DNA binding domain of PPARgamma and was dependent upon dimerization with the retinoid X receptor. Inhibition of follistatin expression by rosiglitazone was not associated with decreased follistatin mRNA stability, suggesting that regulation may be at the promoter level. Analysis of the follistatin promoter revealed consensus binding sites for AP-1, AP-2, and Sp1. Targeting the AP-1 pathway with SP600125, an inhibitor of JNK, and TAM67, a dominant negative c-Jun, had no effect on PPARgamma-mediated down-regulation of follistatin. However, the follistatin promoter was dramatically regulated by Sp1, and this regulation was inhibited by PPARgamma expression. Knockdown of Sp1 expression relieved repression of follistatin levels by rosiglitazone. Moreover, PPARgamma was found to interact with Sp1 and repress its transcriptional activation function. Collectively, our data indicate that repression of Sp1 transcriptional activity by PPARgamma is the underlying mechanism responsible for PPARgamma-mediated regulation of follistatin expression.