SGK1/Nedd4-2 signaling pathway regulates the activity of human organic anion transporters 3

Serum and glucocorticoid-regulated kinase 1 (SGK1) as an emerging therapeutic target for cardiac diseases

Cardiac diseases encompass a wide range of conditions that affect the structure and function of the heart. These conditions are a leading cause of morbidity and mortality worldwide. The serum- and glucocorticoid-inducible kinase 1 (SGK1) is a serine/threonine kinase that plays a significant role in various cellular processes, including cell survival and stress response. Alterations in SGK1 activity can have significant impacts on health and disease. Multiple research findings have indicated that SGK1 is associated with heart disease due to its involvement in cardiac hypertrophy and fibrosis. This article reviews different signaling pathways associated with SGK1 activity in various heart conditions, including the SGK1/NF-κB and PI3K/SGK1 pathways.

Inhibition the ubiquitination of ENaC and Na,K-ATPase with erythropoietin promotes alveolar fluid clearance in sepsis-induced acute respiratory distress syndrome

Sepsis-induced acute respiratory distress syndrome (ARDS) causes significant fatalities worldwide and lacks pharmacological intervention. Alveolar fluid clearance (AFC) plays a pivotal role in the remission of ARDS and is markedly impaired in the pathogenesis of ARDS. Here, we demonstrated that erythropoietin could effectively ameliorate lung injury manifestations and lethality, restore lung function and promote AFC in a rat model of lipopolysaccharide (LPS)-induced ARDS. Moreover, it was proven that EPO-induced restoration of AFC occurs through triggering the total protein expression of ENaC and Na,K-ATPase channels, enhancing their protein abundance in the membrane, and suppressing their ubiquitination for degeneration. Mechanistically, the data indicated the possible involvement of EPOR/JAK2/STAT3/SGK1/Nedd4-2 signaling in this process, and the pharmacological inhibition of the pathway markedly eliminated the stimulating effects of EPO on ENaC and Na,K-ATPase, and subsequently reversed the augmentation of AFC by EPO. Consistently, in vitro studies of alveolar epithelial cells paralleled with that EPO upregulated the expression of ENaC and Na,K-ATPase, and patch-clamp studies further demonstrated that EPO substantially strengthened sodium ion currents. Collectively, EPO could effectively promote AFC by improving ENaC and Na,K-ATPase protein expression and abundance in the membrane, dependent on inhibition of ENaC and Na,K-ATPase ubiquitination, and resulting in diminishing LPS-associated lung injuries.

Protein Kinases and Cross-talk between Post-translational Modifications in the Regulation of Drug Transporters

Drug transporters are modulators for drug absorption, distribution, and excretion. Key drug transporters including P-glycoprotein and breast cancer resistance protein of the ABC superfamily; organic anion transporting polypeptide 1B1 and 1B3, organic anion transporter 1 and 3, and organic cation transporter 2, as well as multidrug and toxin extrusion 1 and 2 of the SLC superfamily have been recommended by regulatory agencies to be investigated and evaluated in drug-drug interaction (DDI) studies due to their important roles in determining the efficacy, toxicity and DDI of various drugs. Drug transporters are subjected to multiple levels of control and post-translational modifications (PTMs) provide rapid and versatile ways of regulation. Under pathologic and/or pharmacological conditions, PTMs may be altered in the cellular system, leading to functional changes of transporter proteins. Phosphorylation is by far the most actively investigated form of PTMs in the regulation of transporters. Further, studies in recent years also found that protein kinases coordinate with other PTMs for the dynamic control of these membrane proteins. Here we summarized the regulation of major drug transporters by protein kinases and their cross-talking with other PTMs that may generate a complex regulatory network for fine-tuning the function of these important drug processing modulators. SIGNIFICANCE STATEMENT: Kinases regulate drug transporters in versatile manners; Kinase regulation cross-talks with other PTMs, forming a complex network for transporter regulation; Pathological and/or pharmacological conditions may alter PTMs and affect transporter function with different molecular mechanisms.

Acetylation, Phosphorylation, Ubiquitination (Oh My!): Following Post-Translational Modifications on the Ubiquitin Road

Ubiquitination is controlled by a series of E1, E2, and E3 enzymes that can ligate ubiquitin to cellular proteins and dictate the turnover of a substrate and the outcome of signalling events such as DNA damage repair and cell cycle. This process is complex due to the combinatorial power of ~35 E2 and ~1000 E3 enzymes involved and the multiple lysine residues on ubiquitin that can be used to assemble polyubiquitin chains. Recently, mass spectrometric methods have identified that most enzymes in the ubiquitination cascade can be further modified through acetylation or phosphorylation under particular cellular conditions and altered modifications have been noted in different cancers and neurodegenerative diseases. This review provides a cohesive summary of ubiquitination, acetylation, and phosphorylation sites in ubiquitin, the human E1 enzyme UBA1, all E2 enzymes, and some representative E3 enzymes. The potential impacts these post-translational modifications might have on each protein function are highlighted, as well as the observations from human disease.

Mechanistic insights into the role of serum-glucocorticoid kinase 1 in diabetic nephropathy: A systematic review

Aberrant expression of serum-glucocorticoid kinase 1 (SGK1) contributes to the pathogenesis of multiple disorders, including diabetes, hypertension, obesity, fibrosis, and metabolic syndrome. SGK1 variant is expressed in the presence of insulin and several growth factors, eventually modulating various ion channels, carrier proteins, and transcription factors. SGK1 also regulates the enzymatic activity of Na⁺ K⁺ ATPase, glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, and phosphomannose mutase impacting cell cycle regulation, neuroexcitation, and apoptosis. Ample evidence supports the crucial role of aberrant SGK1 expression in hyperglycemia-mediated secondary organ damage. Diabetic nephropathy (DN), a dreadful microvascular complication of diabetes, is the leading cause of end-stage renal failures with high morbidity and mortality rate. The complex pathogenesis of DN encompasses several influencing factors, including transcriptional factors, inflammatory markers, cytokines, epigenetic modulators, and abnormal enzymatic activities. SGK1 plays a pivotal role by controlling various physiological functions associated with the occurrence and progression of DN; therefore, targeting SGK1 may favorably influence the clinical outcome in patients with DN. This review aimed to provide mechanistic insights into SGK1 regulated DN pathogenesis and summarize the evidence supporting the therapeutic potential of SGK1 inhibition and its consequences on human health.

Dexamethasone enhances the lung metastasis of breast cancer via a PI3K-SGK1-CTGF pathway

Dexamethasone (Dex), as a pretreatment agent, is widely used to attenuate the side effects of chemotherapy in breast cancer treatment. However, whether and how Dex affects breast cancer metastasis remain to be furtherly understood. In this study, we established several mouse breast cancer metastatic models to study the effect of Dex in vitro and in vivo. Transwell, Western Blot and RNA interference were applied to study the molecular mechanism of Dex in promoting breast cancer cell migration. Meanwhile, the effect of Dex on lung metastasis of breast cancer in Dex combined with PTX chemotherapy was discussed. Our results confirmed that Dex could promote breast cancer cell metastasis both in vitro and in vivo. Mechanistic studies revealed that this pro-metastatic effect of Dex was mediated by the GR-PI3K-SGK1-CTGF pathway in tumor cells. Ligation of Dex and glucocorticoid receptor (GR) on tumor cells activated the PI3K signaling pathway and upregulated serum glucocorticoid-inducible kinase 1 (SGK1) expression, and then increased the expression of connective tissue growth factor (CTGF) through Nedd4l-Smad2. Moreover, Dex was the leading factor for lung metastasis in a standard regimen for breast cancer treatment with paclitaxel and Dex. Importantly, targeting SGK1 with the inhibitor GSK650394 remarkably reduced lung metastasis in this regimen. Our present data provide new insights into Dex-induced breast cancer metastasis and indicate that SGK1 could be a candidate target for the treatment of breast cancer metastasis.

Oral Proteasomal Inhibitors Ixazomib, Oprozomib, and Delanzomib Upregulate the Function of Organic Anion Transporter 3 (OAT3): Implications in OAT3-Mediated Drug-Drug Interactions

Organic anion transporter 3 (OAT3) is mainly expressed at the basolateral membrane of kidney proximal tubules, and is involved in the renal elimination of various kinds of important drugs, potentially affecting drug efficacy or toxicity. Our laboratory previously reported that ubiquitin modification of OAT3 triggers the endocytosis of OAT3 from the plasma membrane to intracellular endosomes, followed by degradation. Oral anticancer drugs ixazomib, oprozomib, and delanzomib, as proteasomal inhibitors, target the ubiquitin-proteasome system in clinics. Therefore, this study investigated the effects of ixazomib, oprozomib, and delanzomib on the expression and transport activity of OAT3 and elucidated the underlying mechanisms. We showed that all three drugs significantly increased the accumulation of ubiquitinated OAT3, which was consistent with decreased intracellular 20S proteasomal activity; stimulated OAT3-mediated transport of estrone sulfate and p-aminohippuric acid; and increased OAT3 surface expression. The enhanced transport activity and OAT3 expression following drug treatment resulted from an increase in maximum transport velocity of OAT3 without altering the substrate binding affinity, and from a decreased OAT3 degradation. Together, our study discovered a novel role of anticancer agents ixazomib, oprozomib, and delanzomib in upregulating OAT3 function, unveiled the proteasome as a promising target for OAT3 regulation, and provided implication of OAT3-mediated drug-drug interactions, which should be warned against during combination therapies with proteasome inhibitor drugs.

Regulation of organic anion transporters: Role in physiology, pathophysiology, and drug elimination

The members of the organic anion transporter (OAT) family are mainly expressed in kidney, liver, placenta, intestine, and brain. These transporters play important roles in the disposition of clinical drugs, pesticides, signaling molecules, heavy metal conjugates, components of phytomedicines, and toxins, and therefore critical for maintaining systemic homeostasis. Alterations in the expression and function of OATs contribute to the intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs, and to many pathophysiological conditions. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. This review will present an update on the recent advance in understanding the cellular and molecular mechanisms underlying the regulation of renal OATs, emphasizing on the post-translational modification (PTM), the crosstalk among these PTMs, and the remote sensing and signaling network of OATs. Such knowledge will provide significant insights into the roles of these transporters in health and disease.

Expression signature of miRNAs and the potential role of miR-195-5p in high-glucose-treated rat cardiomyocytes

MicroRNAs are endogenous small noncoding RNAs that posttranscriptionally regulate the expression of target genes and play crucial roles in diverse physiopathologic processes. In the current study, we examined the microRNA (miRNA) expression profile of high-glucose-treated neonatal rat cardiomyocytes and the potential mechanisms. Differentially expressed miRNAs were analyzed by a miRNA microarray and validated by a quantitative real-time polymerase chain reaction in high-glucose-treated rat cardiomyocytes. Based on the results of our previous study and the bioinformatics prediction, we identified miR-195-5p/SGK1/Nedd4-2/hERG as the top-ranked signal pathway in diabetes cell model in vitro. In summary, our present study provides novel insights into the regulatory mechanism of miR-195-5p/SGK1/Nedd4-2/hERG in rat cardiomyocytes under high-glucose stress, which may provide a novel idea for the development of diagnostic and therapeutic strategies for diabetic cardiomyopathy in the future.

Activation of Protein Kinase A Stimulates SUMOylation, Expression, and Transport Activity of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad variety of anionic drugs from kidney, thus avoiding their possible toxicity in the body. We earlier established that activation of protein kinase C (PKC) enhances OAT3 ubiquitination, which promotes OAT3 internalization from the cell plasma membrane to intracellular endosomes and consequent degradation. As a result, OAT3 expression and transport activity are reduced. In the current study, we discovered that protein kinase A (PKA) had an opposite effect to PKC on the regulation of OAT3. We showed that activation of PKA by Bt2-cAMP stimulated OAT3 transport activity, which was largely caused by an enhanced plasma membrane expression of the transporter, kinetically reflected as an augmented maximal transport velocity V_max without notable alteration in substrate-binding affinity K_m. Additionally, we showed that PKA activation accelerated the rate of OAT3 recycling from intracellular compartments to the plasma membrane and decelerated the rate of OAT3 degradation. We further showed that OAT3 is subjected to post-translational modification by SUMO-2 and SUMO-3 not by SUMO-1. PKA activation enhanced OAT3 SUMOylation, which was accompanied by a reduced OAT3 ubiquitination. Finally, insulin-like growth factor 1 significantly stimulated OAT3 transport activity and SUMOylation through PKA signaling pathway. In conclusion, this is the first demonstration that PKA stimulated OAT3 expression and transport activity by altering the trafficking kinetics of OAT3 possibly through the crosstalk between SUMOylation and ubiquitination. Our studies are consistent with a remote sensing and signaling model for transporters (Wu et al. in Mol Pharmacol. 79(5):795-805, 2011).

The mechanistic links between insulin and human organic anion transporter 4

Human organic anion transporter 4 (hOAT4) belongs to a class of organic anion transporters that exert critical function in the secretion, absorption, and distribution of numerous drugs in the body, such as anti-viral drugs, anti-cancer therapeutics, antibiotics, antihypertensive medicine, and anti-inflammatory drugs. hOAT4 is richly existent in the kidney and placenta. We previously established that serum- and glucocorticoid-inducible kinases (sgk) stimulate hOAT4 expression and transport activity by abrogating the inhibitory effect of a ubiquitin ligase Nedd4-2. Insulin is one of the upstream signaling molecules for sgk. We therefore investigated the effect of insulin on hOAT4 function. We showed that insulin stimulated hOAT4 expression and transport activity, and the action of insulin was abolished in cells overexpressing Nedd4-2-specific siRNA to knockdown the endogenous Nedd4-2. We further showed that insulin phosphorylated serine 327 on Nedd4-2 and weakened the interaction between hOAT4 and Nedd4-2. Interestingly, in cells overexpressing sgk2, the stimulatory effect of insulin on hOAT4 was diminished. In addition, the stimulatory effect of insulin on hOAT4 was blocked by wortmannin and buparlisib, two PI3K inhibitors. In conclusion, our study demonstrated that insulin stimulates hOAT4 expression and transport activity by abrogating the inhibition effect of Nedd4-2 on the transporter. Moreover, insulin regulates hOAT4 by competing with sgk2 rather than through sgk2.

AG490, a JAK2-specific inhibitor, downregulates the expression and activity of organic anion transporter-3

Human organic anion transporter-3 (hOAT3) is richly expressed in the kidney, where it plays critical roles in the secretion of clinically important drugs, including anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. In the current study, we examined the role of AG490, a specific inhibitor of the Janus tyrosine kinase 2 (JAK2), in hOAT3 transport activity in the kidney COS-7 cells. AG490 induced a time- and concentration-dependent inhibition of hOAT3-mediated uptake of estrone sulfate, a prototypical substrate for the transporter. The inhibitory effect of AG490 correlated with a reduced expression of hOAT3 at the cell surface. Our lab previously demonstrated that Nedd4-2, a ubiquitin ligase, down regulates OAT expression and transport activity by enhancing OAT ubiquitination, which leads to an internalization of OAT from cell surface to intracellular compartments and subsequent degradation. In the current study, we showed that treatment of hOAT3-expressing cells with AG490 resulted in an enhanced hOAT3 ubiquitination and degradation, which was accompanied by a strengthened association of Nedd4-2 with hOAT3 and a reduction in Nedd4-2 phosphorylation. SiRNA knockdown of endogenous Nedd4-2 abrogated the effects of AG490 on hOAT3. In summary, our study demonstrated that AG490 regulates hOAT3 expression and transport activity through the modulation of Nedd4-2.

The activity of organic anion transporter-3: Role of dexamethasone

Human organic anion transporter-3 (hOAT3) is richly expressed in the kidney, where it plays critical roles in the secretion, from the blood to urine, of clinically important drugs, such as anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. In the current study, we examined the role of dexamethasone in hOAT3 transport activity in the kidney HEK293 cells. Cis-inhibition study showed that dexamethasone exhibited a concentration-dependent inhibition of hOAT3-mediated uptake of estrone sulfate, a prototypical substrate for the transporter, with IC₅₀ value of 49.91 μM. Dixon plot analysis revealed that inhibition by dexamethasone was competitive with a Ki = 47.08 μM. In contrast to the cis-inhibition effect of dexamethasone, prolonged incubation (6 h) of hOAT3-expressing cells with dexamethasone resulted in an upregulation of hOAT3 expression and transport activity, kinetically revealed as an increase in the maximum transport velocity V_max without meaningful alteration in substrate-binding affinity K_m. Such upregulation was abrogated by GSK650394, a specific inhibitor for serum- and glucocorticoid-inducible kinases (sgk). Dexamethasone also enhanced sgk1 phosphorylation. Our study demonstrated that dexamethasone exhibits dual effects on hOAT3: it is a competitive inhibitor for hOAT3-mediated transport, and interestingly, when entering the cells, it stimulates hOAT3 expression and transport activity through sgk1.