Increased importin alpha protein expression in diabetic nephropathy

Elucidating the genetic relationship between ulcerative colitis and diabetic kidney disease: a bidirectional Mendelian randomization study

Introduction

Ulcerative colitis (UC) and diabetic kidney disease (DKD) are chronic disorders with multifaceted pathogenesis, posing significant challenges in clinical management. While substantial efforts have been made to investigate the individual causes of these diseases, the interplay between UC and DKD is not well understood. This study aims to elucidate the genetic association between UC and DKD through Mendelian randomization (MR) analysis, offering new insights into common biological pathways and potential clinical implications.

Methods

We conducted a bidirectional two-sample MR study utilizing data from large-scale genome-wide association studies (GWAS) for both UC and DKD. Instrumental variables (IVs) were meticulously selected according to genome-wide significance and stringent statistical criteria, ensuring robust causal inference. Various MR methodologies, including inverse variance weighting (IVW), were employed to assess the causal relationships between UC and DKD. Sensitivity analyses were also performed to validate the robustness of our findings.

Results

Our analysis revealed a significant causal relationship between genetic predisposition to UC and increased susceptibility to DKD. Specifically, individuals with a genetic susceptibility to UC exhibited a 17.3% higher risk of developing DKD. However, we found no evidence of a causal link between DKD and the risk of developing UC. Additionally, we identified shared genetic risk factors and molecular pathways linking UC and DKD, thereby highlighting potential therapeutic targets.

Discussion

This study underscores the intricate genetic interplay between UC and DKD, suggesting that individuals with UC may be at an elevated risk for developing DKD. Understanding these shared genetic pathways could facilitate the development of early detection strategies and targeted interventions for individuals at risk of DKD. Ultimately, these insights could lead to improved clinical outcomes for patients suffering from both conditions.

Kidney Proximal Tubule GLUT2-More than Meets the Eye

Tubulopathy plays a central role in the pathophysiology of diabetic kidney disease (DKD). Under diabetic conditions, the kidney proximal tubule cells (KPTCs) are exposed to an extensive amount of nutrients, most notably glucose; these nutrients deteriorate KPTCs function and promote the development and progression of DKD. Recently, the facilitative glucose transporter 2 (GLUT2) in KPTCs has emerged as a central regulator in the pathogenesis of DKD. This has been demonstrated by identifying its specific role in enhancing glucose reabsorption and glucotoxicity, and by deciphering its effect in regulating the expression of the sodium-glucose transporter 2 (SGLT2) in KPTCs. Moreover, reduction/deletion of KPTC-GLUT2 has been recently found to ameliorate DKD, raising the plausible idea of considering it as a therapeutic target against DKD. However, the underlying molecular mechanisms by which GLUT2 exerts its deleterious effects in KPTCs remain vague. Herein, we review the current findings on the proximal tubule GLUT2 biology and function under physiologic conditions, and its involvement in the pathophysiology of DKD. Furthermore, we shed new light on its cellular regulation during diabetic conditions.

HNF1A-MODY Mutations in Nuclear Localization Signal Impair HNF1A-Import Receptor KPNA6 Interactions

Mutations in hepatocyte nuclear factor (HNF)1A gene cause the most common form of Maturity-onset diabetes of the young (MODY), a monogenic subtype of diabetes mellitus. Functional characterization of mutant proteins reveals that mutations may disrupt DNA binding capacity, transactivation ability and nuclear localization of HNF1A depending on the position of the mutation. Previously identified Arg271Trp and Ser345Tyr mutations in HNF1A were found to be defective in nuclear localization. Arg271 residue resides in a region similar to classical nuclear localization signal (NLS) motif, while Ser345 does not. Importin α family members recognize NLS motifs on cargo proteins and subsequently translocate them into nucleus. Here, we first investigated the nuclear localization mechanism of wild type HNF1A protein. For this purpose, we analyzed the interaction of HNF1A with three mouse homolog importin α proteins (KPNA2, KPNA4 and KPNA6) by co-immunoprecipitation assay and molecular docking simulation. Hereby, KPNA6 was identified as the main import receptor, which is responsible for the transport of HNF1A into the nucleus. Immunolocalization studies in mouse pancreatic cells (Min6) also confirmed the co-localization of HNF1A and KPNA6 in the cytoplasm. Secondly, the interaction between KPNA6 and mutant HNF1A proteins (Arg271Trp and Ser345Tyr) was assessed. Co-immunoprecipitation studies revealed a reduced interaction compared to wild type HNF1A. Our study demonstrated for the first time that HNF1A transcription factor is recognized and transported by importin/karyopherin import family, and mutations in NLS motifs may disrupt the interaction leading to nuclear localization abnormalities and MODY phenotype.

Epigenetic Contribution to the Development and Progression of Vascular Diabetic Complications

SIGNIFICANCE

The number of people suffering from diabetes worldwide is steadily rising. Complications from diabetes, including cardiovascular and renal disease, contribute to the high morbidity and mortality associated with this disease. Recent Advances: Hyperglycemia promotes tissue damage through diverse mechanisms involving increased production of reactive oxygen species. Increased oxidative stress drives changes in chromatin structure that mediate gene expression changes leading to the upregulation of proinflammatory and profibrotic mediators. The epigenetic contribution to diabetes-induced changes in gene expression is increasingly recognized as a key factor in the development and progression of vascular diabetic complications.

CRITICAL ISSUES

The mechanisms through which stimuli from the diabetic milieu promote epigenetic changes remain poorly understood. In addition, glycemic control constitutes an important factor influencing epigenetic states in diabetes, and the phenomenon of hyperglycemic memory warrants further research.

FUTURE DIRECTIONS

Knowledge of the molecular mechanisms underlying epigenetic changes in diabetes may allow the design of novel therapeutic strategies to reduce the burden of diabetic complications. Furthermore, certain epigenetic markers are detected early during the onset of diabetes and its complications and may prove useful as biomarkers for disease risk prediction.

High Basolateral Glucose Increases Sodium-Glucose Cotransporter 2 and Reduces Sirtuin-1 in Renal Tubules through Glucose Transporter-2 Detection

Under diabetic conditions, sodium-glucose cotransporter 2 (SGLT2) for glucose uptake in proximal tubules (PTs) increases, whereas NAD⁺-dependent protein deacetylase silent mating type information regulation 2 homolog 1 (Sirtuin-1; SIRT1) for PT survival decreases. Therefore, we hypothesized that increased glucose influx by SGLT2 reduces SIRT1 expression. To test this hypothesis, db/db mice with diabetes and high-glucose (HG)-cultured porcine PT LLC-PK1 cells in a two-chamber system were treated with the SGLT2 inhibitor canagliflozin. We also examined SIRT1 and SGLT2 expression in human kidney biopsies. In db/db mice, SGLT2 expression increased with concomitant decreases in SIRT1, but was inhibited by canagliflozin. For determination of the polarity of SGLT2 and SIRT1 expression, LLC-PK1 cells were seeded into Transwell chambers (pore size, 0.4 µm; Becton Dickinson, Oxford, UK). HG medium was added to either or to both of the upper and lower chambers, which corresponded to the apical and basolateral sides of the cells, respectively. In this system, the lower chamber with HG showed increased SGLT2 and decreased SIRT1 expression. Canagliflozin reversed HG-induced SIRT1 downregulation. Gene silencing and inhibitors for glucose transporter 2 (GLUT2) blocked HG-induced SGLT2 expression upregulation. Gene silencing for the hepatic nuclear factor-1α (HNF-1α), whose nuclear translocation was enhanced by HG, blocked HG-induced SGLT2 expression upregulation. Similarly, gene silencing for importin-α1, a chaperone protein bound to GLUT2, blocked HG-induced HNF-1α nuclear translocation and SGLT2 expression upregulation. In human kidney, SIRT1 immunostaining was negatively correlated with SGLT2 immunostaining. Thus, under diabetic conditions, SIRT1 expression in PTs was downregulated by an increase in SGLT2 expression, which was stimulated by basolateral HG through activation of the GLUT2/importin-α1/HNF-1α pathway.

Nuclear importin α and its physiological importance

Importin α is recognized as a classical nuclear localization signal (cNLS) receptor which mediates nucleocytoplasmic transport. However, it rapidly accumulates in the nucleus in response to cellular stresses, including oxidative stress, causing a blockade of the classical nuclear import pathway. We set out to determine whether importin α performs roles in the nucleus after cellular exposure to stresses and discovered that it can act directly to modulate gene expression. With remarkable selectivity, importin α2 can access the promoter of Serine/threonine kinase 35 (STK35) and increase the levels of this transcript without requirement for importin β1. The nuclear accumulation of importin α occurred following exposure to stresses which decreased intracellular ATP levels and was followed by non-apoptotic cell death. Hence the gene regulatory function of nuclear importin α can direct cell fate. There are now several reports of nuclear-localized importin α proteins in diverse cellular states, including cancer. Here we discuss the physiological significance of this novel functional capacity of nuclear importin α relationship to a variety of cellular states and fates.

Identification of a selective nuclear import signal in adenosine deaminases acting on RNA

The adenosine deaminases acting on RNA (ADARs) comprise a family of RNA editing enzymes that selectively modify single codons within RNA primary transcripts with often profound impact on protein function. Little is known about the mechanisms that regulate nuclear RNA editing activity. Editing levels show cell-type specific and developmental modulation that does not strictly coincide with observed expression levels of ADARs. Here, we provide evidence for a molecular mechanism that might control nuclear import of specific ADARs and, in turn, nuclear RNA editing. We identify an in vivo ADAR3 interaction partner, importin alpha 1 (KPNA2) that specifically recognizes an arginine-rich ADAR3 sequence motif and show that it acts as a functional nuclear localization sequence. Furthermore, whereas KPNA2, but not KPNA1 or KNPA3, recognizes the ADAR3 NLS, we observe the converse binding specificity with ADAR2. Interestingly, alternative splicing of ADAR2 pre-mRNA introduces an ADAR3-like NLS that alters the interaction profile with the importins. Thus, in vivo RNA editing might be regulated, in part, through controlled subcellular localization of ADARs, which in turn is governed by the coordinated local expression of importin α proteins and ADAR protein variants.

Sugar sensing by enterocytes combines polarity, membrane bound detectors and sugar metabolism

Sugar consumption and subsequent sugar metabolism are known to regulate the expression of genes involved in intestinal sugar absorption and delivery. Here we investigate the hypothesis that sugar-sensing detectors in membranes facing the intestinal lumen or the bloodstream can also modulate intestinal sugar absorption. We used wild-type and GLUT2-null mice, to show that dietary sugars stimulate the expression of sucrase-isomaltase (SI) and L-pyruvate kinase (L-PK) by GLUT2-dependent mechanisms, whereas the expression of GLUT5 and SGLT1, did not rely on the presence of GLUT2. By providing sugar metabolites, sugar transporters, including GLUT2, fuelled a sensing pathway. In Caco2/TC7 enterocytes, we could disconnect the sensing triggered by detector from that produced by metabolism, and found that GLUT2 generated a metabolism-independent pathway to stimulate the expression of SI and L-PK. In cultured enterocytes, both apical and basolateral fructose could increase the expression of GLUT5, conversely, basolateral sugar administration could stimulate the expression of GLUT2. Finally, we located the sweet-taste receptors T1R3 and T1R2 in plasma membranes, and we measured their cognate G alpha Gustducin mRNA levels. Furthermore, we showed that a T1R3 inhibitor altered the fructose-induced expression of SGLT1, GLUT5, and L-PK. Intestinal gene expression is thus controlled by a combination of at least three sugar-signaling pathways triggered by sugar metabolites and membrane sugar receptors that, according to membrane location, determine sugar-sensing polarity. This provides a rationale for how intestine adapts sugar delivery to blood and dietary sugar provision.

Topoisomerase II binds importin alpha isoforms and exportin/CRM1 but does not shuttle between the nucleus and cytoplasm in proliferating cells

Resistance to anticancer drugs that target DNA topoisomerase II (topo II) isoforms alpha and/or beta is associated with decreased nuclear and increased cytoplasmic topo IIalpha. Earlier studies have confirmed that functional nuclear localization and export signal sequences (NLS and NES) are present in both isoforms. In this study, we show that topo II alpha and beta bind and are imported into the nucleus by importin alpha1, alpha3, and alpha5 in conjunction with importin beta. Topo IIalpha also binds exportin/CRM1 in vitro. However, wild-type topo IIalpha has only been observed in the cytoplasm of cells that are entering plateau phase growth. This suggests that topo IIalpha may shuttle between the nucleus and the cytoplasm with the equilibrium towards the nucleus in proliferating cells but towards the cytoplasm in plateau phase cells. The CRM1 inhibitor Leptomycin B increases the nuclear localization of GFP-tagged topo IIalpha with a mutant NLS, suggesting that its export is being inhibited. However, homokaryon shuttling experiments indicate that fluorescence-tagged wild-type topo II alpha and beta proteins do not shuttle in proliferating Cos-1 or HeLa cells. We conclude that topo II alpha and beta nuclear export is inhibited in proliferating cells so that these proteins do not shuttle.

In vivo analysis of importin alpha proteins reveals cellular proliferation inhibition and substrate specificity

The "classical" nuclear import pathway depends on importins alpha and beta. Humans have only one importin beta, while six alpha importins have been described. Whether or not distinct alpha importins are essential for specific import pathways in living human cells is unclear. We used RNA interference technology to specifically down-regulate the expression of ubiquitously expressed human alpha importins in HeLa cells. Down-regulation of importins alpha3, alpha5, alpha7, and beta strongly inhibited HeLa cell proliferation, while down-regulation of importins alpha1 and alpha4 had only a minor effect or no effect. Nucleoplasmin import was not prevented by down-regulation of any alpha importin, indicating that the importin alpha/beta pathway was generally not affected. In contrast, importin alpha3 or alpha5 down-regulation specifically inhibited the nuclear import of the Ran guanine nucleotide exchange factor, RCC1. Coinjection of recombinant alpha importins and RCC1 into down-regulated cells demonstrated that these transport defects were specifically caused by the limited availability of importin alpha3 in both cases. Thus, importin alpha3 is the only alpha importin responsible for the classical nuclear import of RCC1 in living cells.

A karyopherin alpha2 nuclear transport pathway is regulated by glucose in hepatic and pancreatic cells

We studied the role of the karyopherin alpha2 nuclear import carrier (also known as importin alpha2) in glucose signaling. In mhAT3F hepatoma cells, GFP-karyopherin alpha2 accumulated massively in the cytoplasm within minutes of glucose extracellular addition and returned to the nucleus after glucose removal. In contrast, GFP-karyopherin alpha1 distribution was unaffected regardless of glucose concentration. Glucose increased GFP-karyopherin alpha2 nuclear efflux by a factor 80 and its shuttling by a factor 4. These glucose-induced movements were not due to glycolytic ATP production. The mechanism involved was leptomycin B-insensitive, but phosphatase- and energy-dependent. HepG2 and COS-7 cells displayed no glucose-induced GFP-karyopherin alpha2 movements. In pancreatic MIN-6 cells, the glucose-induced movements of karyopherin alpha2 and the stimulation of glucose-induced gene transcription were simultaneously lost between passages 28 and 33. Thus, extracellular glucose regulates a nuclear transport pathway by increasing the nuclear efflux and shuttling of karyopherin alpha2 in cells in which glucose can stimulate the transcription of sugar-responsive genes.

Molecular cloning and characterization of rat karyopherin α1 gene: structure and expression

Dopamine denervation in the striata of patients with Parkinson's disease (PD) leads to changes in neural plasticity. However, the mechanisms leading to the changes are still poorly understood. In an effort to study the molecular events in the denervated striatum, we identified and cloned rat karyopherin alpha 1 (KPNA1), a member of the importin/karyopherin alpha (KPNA) family. DNA sequence analysis revealed that the full-length cDNA, encoding rat KPNA1, was 4975 bp with a short 5'-untranslated region (UTR) of 70 bp, a putative coding sequence of 1617 bp, and an unusually long 3'-UTR of 3266 bp. The gene shared a high degree of similarity with its mouse and human homologs at both cDNA and protein levels. By computational analysis of its genomic sequence, the transcription unit was shown to span a 44-kb region and consist of 13 exons varying in size from 89 (6th exon) to 3454 bp (13th exon), and 12 introns varying in size from 0.3 to 8.9 kb. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis demonstrated that KPNA1 transcript existed in various adult tissues. Both Northern blot and semi-quantitative RT-PCR analysis showed that the expression level of KPNA1 mRNA was altered in the denervated striatum post-lesion in a time-dependent manner, reaching the maximum at 2 weeks post-lesion. Our results suggest involvement of KPNA1 in the striatal responses to denervation following 6-hydroxydopamine (6-OHDA)-induced lesion.

Signaling through Cdk2, importin-alpha and NuMA is required for H2O2-induced mitosis in primary type II pneumocytes

Proliferation of alveolar type II pneumocytes, the multipotent stem cells of the alveoli, has been implicated in the development of lung adenocarcinoma. Hydrogen peroxide (H(2)O(2)), a potent promoter of signaling cascades, can mediate the transmission of many intracellular signals including those involved in cell proliferation. In this study using rat primary type II pneumocytes, we demonstrate that H(2)O(2) significantly increases mitosis through a pathway that includes cyclin-dependent kinase 2 (Cdk2); importin-alpha, a nuclear trafficking regulator; and nuclear mitotic apparatus protein (NuMA), an essential component in mitotic spindle pole formation. Upon H(2)O(2) treatment, Cdk2 is phosphorylated at position thr-160 leading to increases in importin-alpha and NuMA protein levels and resulting in a significant increase of G(2)/M phase in a roscovatine-dependent manner. Type II pneumocytes transfected with NuMA cDNA also show significant increases in G(2)/M phase, NuMA, Cdk2 thr-160 and importin-alpha expression. These effects were prevented by catalase. These results demonstrate that H(2)O(2) orchestrates a complex signaling network regulating S phase entry, nuclear trafficking and spindle pole formation through activation of Cdk2, importin-alpha, and NuMA. This pathway is essential for H(2)O(2)-induced mitosis in type II pneumocytes.