Ciliary ARL13B inhibits developmental kidney cystogenesis in mouse

ARL13B is a small GTPase enriched in cilia. Deletion of Arl13b in mouse kidney results in renal cysts and an associated absence of primary cilia. Similarly, ablation of cilia leads to kidney cysts. To investigate whether ARL13B functions from within cilia to direct kidney development, we examined kidneys of mice expressing an engineered cilia-excluded ARL13B variant, ARL13BV358A. These mice retained renal cilia and developed cystic kidneys. Because ARL13B functions as a guanine nucleotide exchange factor (GEF) for ARL3, we examined kidneys of mice expressing an ARL13B variant that lacks ARL3 GEF activity, ARL13BR79Q. We found normal kidney development with no evidence of cysts in these mice. Taken together, our results show that ARL13B functions within cilia to inhibit renal cystogenesis during mouse development, and that this function does not depend on its role as a GEF for ARL3.

Ciliary ARL13B inhibits developmental kidney cystogenesis in mouse

ARL13B is a small GTPase enriched in cilia. Deletion of Arl13b in mouse kidney results in renal cysts and an associated absence of primary cilia. Similarly, ablation of cilia leads to kidney cysts. To investigate whether ARL13B functions from within cilia to direct kidney development, we examined kidneys of mice expressing an engineered cilia-excluded ARL13B variant, ARL13BV358A. These mice retained renal cilia and developed cystic kidneys. Because ARL13B functions as a guanine nucleotide exchange factor (GEF) for ARL3, we examined kidneys of mice expressing an ARL13B variant that lacks ARL3 GEF activity, ARL13BR79Q. We found normal kidney development with no evidence of cysts in these mice. Taken together, our results show that ARL13B functions within cilia to inhibit renal cystogenesis during mouse development, and that this function does not depend on its role as a GEF for ARL3.

Unmet Clinical Need: Developing Prognostic Biomarkers and Precision Medicine to Forecast Early Tumor Relapse, Detect Chemo-Resistance and Improve Overall Survival in High-Risk Breast Cancer

Chemo-resistant breast cancer is a major barrier to curative treatment for a significant number of women with breast cancer. Neoadjuvant chemotherapy (NACT) is standard first- line treatment for most women diagnosed with high-risk TNBC, HER2+, and locally advanced ER+ breast cancer. Current clinical prognostic tools evaluate four clinicopathological factors: Tumor size, LN status, pathological stage, and tumor molecular subtype. However, many similarly treated patients with identical residual cancer burden (RCB) following NACT experience distinctly different tumor relapse rates, clinical outcomes and survival. This problem is particularly apparent for incomplete responders with a high-risk RCB classification following NACT. Therefore, there is a pressing need to identify new prognostic and predictive biomarkers, and develop novel curative therapies to augment current standard of care (SOC) treatment regimens to save more lives. Here, we will discuss these unmet needs and clinical challenges that stand in the way of precision medicine and personalized cancer therapy.

Abstract 584: Conquering undruggable oncogenic K-RAS-driven incurable metastatic cancer, and delivering precision medicine at neoadjuvant settings

SIAH is a new and potent anti-K-RAS drug target in human cancer. The oncogenic EGFR/HER2/K-RAS pathway activation is pivotal in driving uncontrolled tumor growth and systemic metastasis. Thus, counteracting ERBB/K-RAS hyperactivation in attempt to reverse malignant transformation and inhibit latent tumor growth is an important area for new therapy development against late-stage and metastatic cancer. Guided by Drosophila studies, we found that SIAH (Seven-In-Absentia Homologue) is the most downstream “gatekeeper” required for proper K-RAS signaling. Based on its extraordinarily evolutionary conservation, SIAH E3 ligase is well positioned to serve as an ideal drug target for developing new anti-K-RAS and anticancer therapy. We have shown that anti-SIAH-based therapy is indeed effective in inhibiting tumorigenesis and metastasis of pancreatic, lung and breast cancer cells in xenograft models. Importantly, we have shown that anti-SIAH-based anti-K-RAS strategy is effective against well-established, super-large and late-stage pancreatic and triple-negative breast tumors in a xenograft model in vivo. Through these studies, we have successfully identified a new oncogenic K-RAS "vulnerability," SIAH, in high-grade metastatic cancer. We aim to design and develop potent SIAH inhibitor, and translate these findings to the clinic to benefit more cancer patients with therapy-refractory, relapsed and metastatic diseases in the future. SIAH is a therapy-responsive and prognostic biomarker in human cancer: SIAH expression can be used to monitor tumor responses, and identify resistant tumor clones post-NST. SIAH and EGFR outperform ER, PR, HER2 and Ki67 as two robust, sensitive and prognostic biomarkers to predict survival in breast cancer patients with lymph node metastases. The prognostic power of SIAH and EGFR, alone or in combination, is comparable to the clinical gold standards of clinical predictors (LN positivity, mammary tumor size, grade, stage and molecular subtypes in combination), and imaging-guided technology. A marked reduction in SIAH/EGFR expression post-NST would indicate effective therapy and increased survival, while persistent high SIAH/EGFR expression post-NST would indicate ineffective therapy and decreased survival. The therapy-induced changes in SIAH expression are prognostic in quantifying effective/ineffective therapies, differentiating partial responders, identifying resistant tumor clones, and predicting remission/relapse in breast cancer at neoadjuvant settings. The identification of therapy-responsive and prognostic biomarkers is of paramount importance to stratify patients and guide therapies in clinical oncology and personalized medicine. By validating the RAS/SIAH pathway-centered prognostic biomarkers, we hope to guide standard therapies and improve patient survival in the future.

Citation Format: Amy H. Tang, Robert E. Van Sciver, Elizaveta Svyatova, Kevin Kanda, Michael P. Lee, Caroline Dasom Lee, Lauren L. Siewertsz Van Reesema, Alex C, Lafever, Amber L. Collier, Apoorva S. Iyer, L.D. Britt, Janet S. Winston, Cynthia A. Allen, David Z. Chang, Gloria M. Petersen, Richard A. Hoefer. Conquering undruggable oncogenic K-RAS-driven incurable metastatic cancer, and delivering precision medicine at neoadjuvant settings [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 584.

Phylogenetic analysis of the SINA/SIAH ubiquitin E3 ligase family in Metazoa

Background

The RAS signaling pathway is a pivotal developmental pathway that controls many fundamental biological processes including cell proliferation, differentiation, movement and apoptosis. Drosophila Seven-IN-Absentia (SINA) is a ubiquitin E3 ligase that is the most downstream signaling “gatekeeper” whose biological activity is essential for proper RAS signal transduction. Vertebrate SINA homologs (SIAHs) share a high degree of amino acid identity with that of Drosophila SINA. SINA/SIAH is the most conserved signaling component in the canonical EGFR/RAS/RAF/MAPK signal transduction pathway.

Results

Vertebrate SIAH1, 2, and 3 are the three orthologs to invertebrate SINA protein. SINA and SIAH1 orthologs are found in all major taxa of metazoans. These proteins have four conserved functional domains, known as RING (Really Interesting New Gene), SZF (SIAH-type zinc finger), SBS (substrate binding site) and DIMER (Dimerization). In addition to the siah1 gene, most vertebrates encode two additional siah genes (siah2 and siah3) in their genomes. Vertebrate SIAH2 has a highly divergent and extended N-terminal sequence, while its RING, SZF, SBS and DIMER domains maintain high amino acid identity/similarity to that of SIAH1. But unlike vertebrate SIAH1 and SIAH2, SIAH3 lacks a functional RING domain, suggesting that SIAH3 may be an inactive E3 ligase. The SIAH3 subtree exhibits a high degree of amino acid divergence when compared to the SIAH1 and SIAH2 subtrees. We find that SIAH1 and SIAH2 are expressed in all human epithelial cell lines examined thus far, while SIAH3 is only expressed in a limited subset of cancer cell lines.

Conclusion

Through phylogenetic analyses of metazoan SINA and SIAH E3 ligases, we identified many invariant and divergent amino acid residues, as well as the evolutionarily conserved functional motifs in this medically relevant gene family. Our phylomedicinal study of this unique metazoan SINA/SIAH protein family has provided invaluable evolution-based support towards future effort to design logical, potent, and durable anti-SIAH-based anticancer strategies against oncogenic K-RAS-driven metastatic human cancers. Thus, this method of evolutionary study should be of interest in cancer biology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-1024-x) contains supplementary material, which is available to authorized users.

Abstract 4824: The “gatekeeper” function of Drosophila Seven-IN-Absentia (SINA) E3 ligase and its human homologs, SIAH1 and SIAH2, is highly conserved for proper RAS signal transduction in Drosophila development

Seven-IN-Absentia (SINA) is an evolutionarily conserved E3 ubiquitin ligase that is the most downstream signaling module identified in the RAS signal transduction cascade. Underscoring the importance of SINA is its high evolutionary conservation with over 83% amino acid identity shared between Drosophila SINA and its human SINA homologs (SIAHs). As a major signaling “gatekeeper” in the RAS pathway, we have shown that SIAH is required for oncogenic K-RAS-driven tumorigenesis and metastasis in human pancreatic, lung and breast cancer. Since SIAHs appear to be the ideal drug target to inhibit “undruggable” K-RAS activation, it is important to precisely characterize the activity, regulation, and substrate targeting mechanism(s) of this highly conserved family of SINA/SIAH E3 ligases. By deploying the elegant and well-established Drosophila development system, we are able to study RAS activation and SINA function under normal physiological conditions. In the developing Drosophila eye, photoreceptor cells are recruited sequentially and acquire their distinctive cell fates through a series of local inductive events. The 800x cell arrays allowed us to dissect the role of SINA/SIAH downstream of RAS activation in photoreceptor cell development. To delineate SINA function, we performed an F1 modifier screen using ethyl methanesulfonate (EMS) and X-ray radiation, isolating 28 novel sina mutant alleles. These mutant alleles exhibit much stronger mutant phenotypes than those of the previously published sina2 and sina3 alleles, suggesting that the sina2 and sina3 alleles are hypomorphic alleles. Sequencing analysis of these sinamutant alleles reveals the functional roles of mutated residues and protein domains. To define SINA/SIAH functional conservation, we have generated a complete panel of transgenic fly models that express either wild-type (WT) or dominant negative (DN) SINA/SIAH. The corresponding UAS-sina/siahGOF/LOF phenotypes have been characterized using sev-, GMR-, dpp- and salivary gland-GAL4 drivers to elucidate the developmental outcomes of altered SINA/SIAH expression upon RAS activation. Ectopic expression of sinaWT/DN/siahWT/DN in neurons resulted in dramatic changes in neuronal cell fate in the developing eye and notum, causing PNS neurodegenerative phenotypes. Our results show that the biological functions of fly SINA and human SIAH1/SIAH2 are evolutionarily conserved and functionally interchangeable. Mechanistic insights and regulatory principles learned from Drosophila can be directly applied to cancer biology to develop and validate next-generation anti-SIAH-based anti-K-RAS and anticancer therapy in the future.

Citation Format: Robert E. Van Sciver, Yajun Cao, Atique U. Ahmed, Amy H. Tang. The “gatekeeper” function of Drosophila Seven-IN-Absentia (SINA) E3 ligase and its human homologs, SIAH1 and SIAH2, is highly conserved for proper RAS signal transduction in Drosophila development [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4824. doi:10.1158/1538-7445.AM2017-4824

SIAH and EGFR, Two RAS Pathway Biomarkers, are Highly Prognostic in Locally Advanced and Metastatic Breast Cancer

BACKGROUND

Metastatic breast cancer exhibits diverse and rapidly evolving intra- and inter-tumor heterogeneity. Patients with similar clinical presentations often display distinct tumor responses to standard of care (SOC) therapies. Genome landscape studies indicate that EGFR/HER2/RAS "pathway" activation is highly prevalent in malignant breast cancers. The identification of therapy-responsive and prognostic biomarkers is paramount important to stratify patients and guide therapies in clinical oncology and personalized medicine.

METHODS

In this study, we analyzed matched pairs of tumor specimens collected from 182 patients who received neoadjuvant systemic therapies (NST). Statistical analyses were conducted to determine whether EGFR/HER2/RAS pathway biomarkers and clinicopathological predictors, alone and in combination, are prognostic in breast cancer.

FINDINGS

SIAH and EGFR outperform ER, PR, HER2 and Ki67 as two logical, sensitive and prognostic biomarkers in metastatic breast cancer. We found that increased SIAH and EGFR expression correlated with advanced pathological stage and aggressive molecular subtypes. Both SIAH expression post-NST and NST-induced changes in EGFR expression in invasive mammary tumors are associated with tumor regression and increased survival, whereas ER, PR, and HER2 were not. These results suggest that SIAH and EGFR are two prognostic biomarkers in breast cancer with lymph node metastases.

INTERPRETATION

The discovery of incorporating tumor heterogeneity-independent and growth-sensitive RAS pathway biomarkers, SIAH and EGFR, whose altered expression can be used to estimate therapeutic efficacy, detect emergence of resistant clones, forecast tumor regression, differentiate among partial responders, and predict patient survival in the neoadjuvant setting, has a clear clinical implication in personalizing breast cancer therapy.

FUNDING

This work was supported by the Dorothy G. Hoefer Foundation for Breast Cancer Research (A.H. Tang); Center for Innovative Technology (CIT)-Commonwealth Research Commercialization Fund (CRCF) (MF14S-009-LS to A.H. Tang), and National Cancer Institute (CA140550 to A.H. Tang).

Abstract 518: Loss of Caveolin-1 in Mouse, Rat and Human Proximal Tubule Cells Increases GRK4 Activity and Decreases D1-Like Receptor Cell Surface Levels

Normal dopaminergic signaling in the renal proximal tubule under conditions of sodium loading leads to natriuresis without the need to increase blood pressure. Low caveolin-1 (CAV1) expression in the kidney is associated with dopaminergic signaling defects and salt sensitive hypertension in both mice and rats. We and others have shown that SNPs in human CAV1 are associated with hypertension. We hypothesize that low CAV1 expression causes increased GRK4 kinase activity due to lack of physical interaction and inhibition of GRK4; thus overactive GRK4 would inactivate the dopamine-1 receptor (D 1 R). We have previously shown that CAV1 reduction, knockdown or knockout in mouse, rat and human renal proximal tubule cells (RPTC) leads to decreased dopamine D 1 -like receptor binding at the proximal tubule cell surface and that re-expression of CAV1 alpha isoform is necessary to rescue this dopaminergic defect. Here we extend those studies by showing that defects in D 1 R cell surface expression due to low CAV1 expression can also be rescued by inhibiting GRK4 in human RPTC using GRK4 siRNA or by introducing heparin (1 μmol/L, 24 hrs) in mouse and rat RPTC. GRK4 siRNA lowered GRK4 expression by greater than 80% in all four human RPTC cell lines and partially restored fluorescent D 1 -like receptor antagonist bodipy630 SKF83566 cell surface binding (scrambled siRNA 66,231±7,286 RFU, GRK4 siRNA 96,206±11,035 RFU, P<0.05, N=28). In RPTC from CAV1 knockout mice, electroporation of heparin restored bodipy630 SKF83566 cell surface binding (vehicle 21,346±3,219 RFU, heparin 58,380±8,032 RFU, P<0.05, N=7). In rat SHR RPTC, CAV1 expression is very low, and the stable transfection of CAV1 partially restored their D 1 -like receptor surface expression. Electroporation of heparin in these CAV1 transfected cells (SHR CAV1) restored bodipy630 SKF83566 cell surface binding back to levels found in WKY RPTC (SHR CAV1 35,241±4,280 RFU, SHR CAV1 heparin 54,218±6,564 RFU, WKY 56,821± 3,546 RFU, P<0.05, N=7). In summary, we show that dopamine D 1 -like receptor defects caused by loss of CAV1 are likely due to increased GRK4 activity and can be restored by either increasing CAV1 expression or decreasing GRK4 expression.

The cooperative roles of the dopamine receptors, D1R and D5R, on the regulation of renal sodium transport

Determining the individual roles of the two dopamine D₁-like receptors (D₁R and D₅R) on sodium transport in the human renal proximal tubule has been complicated by their structural and functional similarity. Here we used a novel D₅R-selective antagonist (LE-PM436) and D₁R or D₅R-specific gene silencing to determine second messenger coupling pathways and heterologous receptor interaction between the two receptors. D₁R and D₅R co-localized in renal proximal tubule cells and physically interact, as determined by co-immunoprecipitation and FRET microscopy. Stimulation of renal proximal tubule cells with fenoldopam (D₁R/D₅R agonist) led to both adenylyl cyclase and phospholipase C (PLC) activation using real-time FRET biosensors ICUE3 and CYPHR, respectively. Fenoldopam increased cAMP accumulation and PLC activity and inhibited both NHE3 and NaKATPase activities. LE-PM436 and D₅R siRNA blocked the fenoldopam-stimulated PLC pathway but not cAMP accumulation, while D₁R siRNA blocked both fenoldopam-stimulated cAMP accumulation and PLC signaling. Either D₁R or D₅R siRNA, or LE-PM436 blocked the fenoldopam dependent inhibition of sodium transport. Further studies using the cAMP-selective D₁R/D₅R agonist SKF83822 and PLC-selective D₁R/D₅R agonist SKF83959 confirmed the cooperative influence of the two pathways on sodium transport. Thus, D₁R and D₅R interact in the inhibition of NHE3 and NaKATPase activity, the D₁R primarily by cAMP, while the D₁R/D₅R heteromer modulates the D₁R effect through a PLC pathway.

Abstract 626: Caveolin-1 (CAV1) Levels Determine the Cell Surface Expression of the Dopamine D1-like Receptors

In human renal proximal tubule cells, the dopamine-1 receptor (D1R) increases cAMP production and negatively regulates sodium transport. Defects in D1R signaling have been associated with SNPs in G-protein coupled receptor kinase type 4 (GRK4), which hyperphosphorylates and decreases D1R function, leading to sodium retention. We have previously shown that knockdown of CAV1 in proximal tubule cells impairs sodium transport. In this report we show that CAV1 reduction, knockdown or knockout leads to decreased dopamine D1-like receptor binding at the proximal tubule cell surface. Dopamine D1-like brush border and cell surface binding was measured using Bodipy-FL conjugated SKF83566 . WT and Caveolin 1 knockout (CAV1KO) kidney cortex brush border membrane isolation was performed by magnetic lotus tetragonolobus agglutinin affinity purification and fluorescent SKF83566 binding by microplate fluorometry. CAV1KO had a 35.8±3.3% decrease in binding vs WT (p<0.01, n=6 per group, WT level 18,492 RFU above background). In hTERT immortalized human renal proximal tubule cell lines, lentiviral shRNAi CAV1 stable knockdown cell lines had a 76.4±2.5%, p<0.001 n=4, reduction in CAV1 expression by western blot. Cell surface D1-like receptor binding was reduced by 34.0±1.3% (p<0.001, n=4) in the CAV1 knockdown cell lines (scrambled control levels 117193 RFU). We previously have shown that CAV1 expression is dramatically reduced in the hypertensive SHR proximal tubule cell line vs the normotensive WKY proximal tubule cell line. The SHR cell line had a 57.5±1.2% (p<0.001, n=7) reduction in D1-like receptor binding (WKY 116114 RFU). Re-expression of rat CAV1 alpha or CAV1 beta isoform in the SHR cell line by stable lentiviral infection increased binding by 51.6±2.1%, and 63.3±1.6% respectively (p<0.01, n=14). Sodium transport was decreased by CAV1 alpha and CAV1 beta expression in SHR cell lines. The average number of domes per 10x objective fields of view was 9.5±1.6 in SHR cell, 4.5±0.3 in SHR CAV1alpha (p<0.05, n=4 vs SHR) and in SHR CAV1beta 5.0±0.3 (p<0.05, n=4 vs SHR). In mouse, rat, and human renal proximal tubule cells, CAV1 is necessary for dopamine D1-like receptor cell surface localization and for normal dopaminergic inhibition of sodium transport.

Abstract 79: Caveolin-1 Knockout Mice Have Salt-Sensitive Hypertension and Dopamine-1 Receptor Defects in Renal Cortex and Isolated Renal Proximal Tubule Cells

Dopamine-1 receptors (D1R) are necessary for kidney proximal tubule-dependent natriuresis and maintenance of normal blood pressure, especially under high salt conditions. G-protein coupled receptor kinase 4 (GRK4) is a negative regulator of D1R function and single nucleotide polymorphisms in GRK4 have been associated with both hypertension and salt sensitivity in humans. Caveolin-1 (CAV1) directly binds to GRK4 and decreases kinase activity. We hypothesized that CAV1 knockout mice (CAV1KO) would have increased GRK4 kinase activity due to lack of physical interaction and inhibition of GRK4; thus overactive GRK4 would inactivate the D1R. Mean arterial blood pressure (MAP, in mmHg ±SEM) measured over 5 days was not significantly different for Wild-type mice (WT, 128.9±4.2 mmHg, n=4) vs CAV1KO (129.5±3.5 mmHg, n=4) on normal chow (0.3% sodium). However, on a 4% high sodium diet, the MAP of CAV1KO mice increased in just 2 days by 20.1±4.2 mmHg (p<0.05 vs either Day 0 CAV1KO or Day 2 WT, n=4). The CAV1KO MAP increased by 25.9±6.6 mmHg by day 7 (p<0.05 vs either Day 0 CAV1KO or Day 7 WT, n=4). Hyperphosphorylation and inactivation of the D1R in renal cortex was examined by looking at phospho-serine D1R by immuno-precipitation and Western dot blotting. A 92.5% ± 18.8 SEM increase in phospho-D1R was found in the CAV1KO renal cortex (n=4, p<0.01 vs WT; 14,574/7570 RFU). Cortical slices were made and incubated for 30 minutes with fenoldopam (FEN, 10 μM) with or without LE300 (D1R-like antagonist, 10 μM) or vehicle (VEH). Cyclic AMP was measured by TR-FRET (Lance, Perkin Elmer). FEN significantly increased cAMP 5.6 fold ± 1.2 SEM (n=4, p<0.01 vs VEH; 7.84/1.4 pmole/mg protein) in WT but not in CAV1KO slices, and this effect was completely blocked by LE300. Primary mouse CAV1KO and WT renal proximal tubule cell lines were established and monensin (sodium ionophore, 5 μM, 30 minutes)-induced plasma membrane D1R recruitment increased as measured by confocal microscopy in WT (30.4% ± 7.4 SEM, n=11, p<0.01 vs VEH; 8990/6894 RFU) but not in CAV1KO proximal tubule cells. In summary, CAV1 is necessary in high salt conditions for maintaining normal blood pressure in mice and for preserving normal D1R function in kidney cortex and in mouse renal proximal tubule cells.

A novel role for c-Myc in G protein-coupled receptor kinase 4 (GRK4) transcriptional regulation in human kidney proximal tubule cells

The G protein-coupled receptor kinase 4 (GRK4) negatively regulates the dopaminergic system by desensitizing the dopamine-1-receptor. The expressional control of GRK4 has not been reported, but here we show that the transcription factor c-Myc binds to the promoter of GRK4 and positively regulates GRK4 protein expression in human renal proximal tubule cells (RPTCs). Addition of phorbol esters to RPTCs not only increased c-Myc binding to the GRK4 promoter but also increased both phospho-c-Myc and GRK4 expression. The phorbol ester-mediated increase in GRK4 expression was completely blocked by the c-Myc inhibitor, 10074-G5, indicating that GRK4 is downstream of phospho-c-Myc. The autocrine production of angiotensin II (Ang II) in RPTCs increased the phosphorylation and activation of c-Myc and subsequently GRK4 expression. 3-Amino-4-thio-butyl sulfonate, an inhibitor of aminopeptidase A, increased RPTC secretion of Ang II. 3-Amino-4-thio-butyl sulfonate or Ang II increased the expression of both phospho-c-Myc and GRK4, which was blocked by 10074-G5. Blockade of the Ang II type 1 receptor with losartan decreased phospho-c-Myc and GRK4 expression. Both inhibition of c-Myc activity and blockade of Ang II type 1 receptor restored the coupling of dopamine-1-receptor to adenylyl cyclase stimulation in uncoupled RPTCs, whereas phorbol esters or Ang II caused the uncoupling of normally coupled RPTCs. We suggest that the Ang II type 1 receptor impairs dopamine-1-receptor function via c-Myc activation of GRK4. This novel pathway may be involved in the increase in blood pressure in hypertension that is mediated by increased activity of the renin-angiotensin system and decreased activity of the renal dopaminergic system.

A linear relationship between the ex-vivo sodium mediated expression of two sodium regulatory pathways as a surrogate marker of salt sensitivity of blood pressure in exfoliated human renal proximal tubule cells: the virtual renal biopsy

BACKGROUND

Salt sensitivity (SS) of blood pressure (BP) affects 25% of adults, shares comorbidity with hypertension, and has no convenient diagnostic test. We tested the hypothesis that urine-derived exfoliated renal proximal tubule cells (RPTCs) could diagnose the degree of an individual's SS of BP.

METHODS

Subjects were selected who had their SS of BP determined 5 y prior to this study (salt-sensitive: ≥7 mm Hg increase in mean arterial pressure (MAP) following transition from a random weekly diet of low (10 mmol/day) to high (300 mmol/day) sodium (Na(+)) intake, N=4; inverse salt-sensitive (ISS): ≥7 mm Hg increase in MAP transitioning from a high to low Na(+) diet, N=3, and salt-resistant (SR): <7 mm Hg change in MAP transitioned on either diet, N=5). RPTC responses to 2 independent Na(+) transport pathways were measured.

RESULTS

There was a negative correlation between the degree of SS and dopamine-1 receptor (D1R) plasma membrane recruitment (y=-0.0107x+0.68 relative fluorescent units (RFU), R(2)=0.88, N=12, P<0.0001) and angiotensin II-stimulated intracellular Ca(++) (y=-0.0016x+0.0336, R(2)=0.7112, P<0.001, N=10) concentration over baseline.

CONCLUSIONS

Isolating RPTCs from urine provides a personalized cell-based diagnostic test of SS index that offers advantages over a 2-week controlled diet with respect to cost and patient compliance. Furthermore, the linear relationship between the change in MAP and response to 2 Na(+) regulatory pathways suggests that an individual's RPTC response to intracellular Na(+) is personalized and predictive.

Abstract 616: Caveolin-1 (CAV1) inhibits G Protein Coupled Receptor Kinase Type 4 (GRK4) kinase activity

In human renal proximal tubule cells, the dopamine-1 receptor (D1R) has been shown to increase cAMP production and negatively regulate sodium transport. Defects in D1R signaling have been associated with SNPs in G-protein-coupled receptor kinase type 4 (GRK4) which hyperphosphorylates and decreases D1R function, and the D1R uncoupling phenotype is fully reverted by reducing GRK4 expression with siRNA. We have also shown that CAV1 co-immunoprecipitates with GRK4, and that the normal D1R inhibition of the sodium transporter, NaKATPase alpha, can be blocked using siRNA to CAV1. We therefore hypothesized that CAV1 directly physically interacts with GRK4 and inhibits its kinase activity. Using purified human GRK4 and CAV1 proteins and an in-vitro peptide homogeneous kinase assay, we investigated the effects of caveolin-1 (CAV1) on the kinase activity of GRK4. By overexpressing and isolating a tandem affinity purified protein (TAP tagged) functional version of human CAV1 isolated from a human renal proximal tubule cell line, we were able to ensure that all of the post-translational modifications of the protein were present. Utilizing a peptide Ser/Thr kinase activity assay, we demonstrated that with an excess of CAV1, the kinase activity of GRK4 is inhibited by 37.8±1.3% (n=4, p<0.05). Using this same assay, we showed that purified human calcium calmodulin 1 (CALM1), a known inhibitor of GRK4 when using a complete cell lysate, does not significantly decrease the kinase activity of purified GRK4 using a CALM1 concentration over 100 times the published Kd value. This suggests that there is a missing co-factor or factors required for CALM1 to inhibit GRK4. In summary, we report for the first time that CAV1 inhibits the kinase activity of GRK4, and that a known inhibitor of GRK4, CALM1, does not directly inhibit GRK4 kinase activity using purified proteins in an in-vitro kinase activity assay.

Abstract 495: A Novel Test for Low Salt Sensitivity: Angiotensin type-II Receptor Recruitment After Dopamine-1 Receptor Stimulation in Urine-Derived Renal Proximal Tubule Cells

Salt-sensitivity of blood pressure is an inappropriate increase in blood pressure following high salt intake. Subjects in our clinical study were typed according to their salt-sensitivity status into 3 categories: High-Salt-Sensitive (HSS; ≥ 7 mmHg increase in mean arterial pressure (MAP) on a high salt diet of 300 mEq of sodium, 17% prevalence), Low-Salt-Sensitive (LSS:, who paradoxically showed a ≥ 7 mmHg increase in MAP on a low salt diet of 10 mEq of sodium, 11% prevalence), and Salt-Resistant (SR, individuals who showed no significant increase in blood pressure on either diet, 72% prevalence). We previously demonstrated that LSS subjects show increased recruitment of the natriuretic dopamine-1 receptor (D1R) to the plasma membrane following a salt stimulation as compared to HSS subjects. Stimulation of the D1R in RPTC with fenoldopam (dopaminergic agonist) results in recruitment of the natriuretic angiotensin type-2 receptor (AT2R) to the cell surface. We hypothesized that LSS individuals may also demonstrate an enhanced AT2R RPTC membrane recruitment compared to HSS individuals when challenged with fenoldopam. In order to gain access to fresh RPTC from each subject, we isolated exfoliated RPTC from randomly voided urine from SR, LSS, and HSS subjects from our clinical study. We measured three subjects from each category with a minimum of three voids for each subject. We counted individual cells as independent events using both the confocal microscope (n=245) and the flow cytometer (n=5344). We found an inverse correlation between AT2R recruitment and the degree of salt-sensitivity of blood pressure. Fenoldopam stimulated AT2R recruitment as measured by confocal microscopy (y = -0.0047x + 0.4966, R2 = 0.2488, P<0.0001) and flow cytometry (y =-0.057x + 1.5645, R2=0.2912, P=0.0185). Flow cytometry provided a more sensitive diagnostic for LSS than HSS subjects. AT2R recruitment was more predictive of LSS than HSS. AT2R recruitment may be used as a rapid method to test for LSS individuals who need to be identified and encouraged to increase their sodium intake in order to avoid paradoxical hypertension.

Abstract 293: c-Myc Inactivation by PP2A is Necessary for Dopaminergic Activity in the Proximal Tubule

We have previously shown that c-Myc regulates the expression of G protein-coupled receptor kinase 4 (GRK4). The inhibition of c-Myc can revert a dopamine-1 receptor (D 1 R) uncoupled renal proximal tubule cell (uRPTC) to normal (nRPTC). The c-Myc inhibitor, 10074-G5 (30 μM, 3 hr), restored coupling of D 1 R mediated adenylyl cyclase stimulation in uRPTCs, causing a significant increase in cAMP levels (16.21% + 0.97; p<0.01; n=47) in response to the dopamine D 1 R/D 5 R agonist, fenoldopam (FEN), than when compared to cells pre-treated with DMSO vehicle (VEH). We have also shown that FEN leads to PP2A activation. Therefore we tested the hypothesis that an inhibitory role of PP2A on c-Myc may play a role in regulating dopaminergic function. PMA (100 nM) was added to either nRPTC or uRPTC for 3hrs. In nRPTC, the total c-Myc from immunoprecipitation increased by 138.7% + 4.1%; p<0.01; n=3 with respect to VEH, while in uRPTC the total c-Myc increased by 214.5% + 11.3%; p<0.01; n=3. The PP2A/Myc association decreased by 46.0% + 1.16%; p<0.05; n=3 with respect VEH in nRPTC and decreased in uRPTC by 46.9% + 8.71%; p<0.05; n=3. These data suggest that c-Myc is activated only when PP2A binding is low. Addition of FEN (3 hrs) decreased the total c-Myc in nRPTC by 22.6% + 9.1%; p=0.06; n=3 but not in uRPTC. In nRPTC, the PP2A/Myc association increased by 56.6% + 15.1%; p <0.05; n=3 when compared to VEH, but showed no response in uRPTC. Addition of natriuretic peptide angiotensin-III (Ang III, 10 nM) decreased the expression of c-Myc by 29.4% + 16.8%; p<0.05; n=3, but showed no response in uRPTC. The PP2A/Myc binding increased by 109.3% + 30.1%; p<0.05; n=3 in nRPTC, with no increase in uRPTC. Here we show for the first time in nRPTC that both FEN and Ang III decrease total c-Myc while increasing PP2A/Myc co-immunoprecipitation, leading to inactivation of c-Myc. In uRPTC, no significant change in the total c-Myc or the association of PP2A/Myc was detected in response to FEN or Ang III. This suggests that both a lack of expressional control of c-Myc, in addition to a lack of inhibition of c-Myc by PP2A in uRPTC, may have a role in the etiology of D 1 R uncoupling and human renal proximal tubule-related hypertension. Therefore, c-Myc inhibitors may provide a new approach for treating hypertension.

Abstract 32: A Rapid Method for Clinical Diagnosis of Salt Sensitivity of Blood Pressure

Salt-sensitivity of blood pressure (BP) is a cardiovascular risk that affects 25% of the world’s population due to its resulting hypertension, although independent of BP. Salt-sensitivity is detected with a two week controlled diet, which is difficult to administer in the clinical setting. We therefore developed a rapid method of diagnosis based on exfoliated renal proximal tubule cells (RPTC) in urine. Subjects were divided into 3 salt sensitivity index categories: High-Salt-Sensitive (HSS; ≥ 7 mmHg increase in mean arterial pressure (MAP) on a high salt diet of 300 mEq of sodium, 17%prevalence), Low-Salt-Sensitive (LSS; ≥ 7 mmHg increase in MAP on a low salt diet of 10 mEq of sodium, 11% prevalence) and Salt Resistant (SR; ≤ 7 mmHg increase in MAP on both high and low salt diets, 72% prevalence) (Carey et al., in review). Three individuals were analyzed in each category on a minimum of 3 separate occasions. Cells were isolated from urine using centrifugation and measured for dopamine-1 receptor (D1R) plasma membrane recruitment using fluorescently-labeled antibodies under a confocal microscope as well as in a flow cytometer. Confocal microscopy analysis (total of 100 RPTCs for the 9 subjects) showed a negative correlation between salt-sensitivity index and D1R surface recruitment in RPTCs in their response to salt stimulation (y = -0.0073x + 0.5248, p = 0.0159). Flow cytometry analysis (total of 4938 RPTCs for the 9 subjects) also demonstrated a negative correlation between salt-induced D1R recruitment and salt-sensitivity (y = -2.547x + 239.97, p < 0.0001). Flow cytometry analysis showed a greater degree of separation amongst the subjects than confocal microscopy analysis, and would allow for a rapid diagnostic use of exfoliated renal cells in urine. Cryopreserved RPTCs (viability = 57.16% ± 9.15%, n = 12) compare favorably with cell viability from freshly voided urine cells and were still capable of eliciting intracellular sodium-mediated D1R recruitment. Cryopreservation thus enables batch collection, transport and processing of specimens between sites. We expect these procedures to provide a novel and convenient method of diagnosing the salt-sensitivity index in humans.

Inhibition of renal caveolin-1 reduces natriuresis and produces hypertension in sodium-loaded rats

Renal dopamine receptor function and ion transport inhibition are impaired in essential hypertension. We recently reported that caveolin-1 (CAV1) and lipid rafts are necessary for normal D(1)-like receptor-dependent internalization of Na-K-ATPase in human proximal tubule cells. We now hypothesize that CAV1 is necessary for the regulation of urine sodium (Na(+)) excretion (U(Na)V) and mean arterial blood pressure (MAP) in vivo. Acute renal interstitial (RI) infusion into Sprague-Dawley rats of 1 μg·kg⁻¹·min⁻¹ fenoldopam (FEN; D(1)-like receptor agonist) caused a 0.46 ± 0.15-μmol/min increase in U(Na)V (over baseline of 0.29 ± 0.04 μmol/min; P < 0.01). This increase was seen in Na(+)-loaded rats, but not in those under a normal-sodium load. Coinfusion with β-methyl cyclodextrin (βMCD; lipid raft disrupter, 200 μg·kg⁻¹·min⁻¹) completely blocked this FEN-induced natriuresis (P < 0.001). Long-term (3 day) lipid raft disruption via continuous RI infusion of 80 μg·kg⁻¹·min⁻¹ βMCD decreased renal cortical CAV1 expression (47.3 ± 6.4%; P < 0.01) and increased MAP (32.4 ± 6.6 mmHg; P < 0.001) compared with vehicle-infused animals. To determine whether the MAP rise was due to a CAV1-dependent lipid raft-mediated disruption, Na(+)-loaded rats were given a bolus RI infusion of CAV1 siRNA. Two days postinfusion, cortical CAV1 expression was decreased by 73.6 ± 8.2% (P < 0.001) and the animals showed an increase in MAP by 17.4 ± 2.9 mmHg (P < 0.01) compared with animals receiving scrambled control siRNA. In summary, acute kidney-specific lipid raft disruption decreases CAV1 expression and blocks D(1)-like receptor-induced natriuresis. Furthermore, chronic disruption of lipid rafts or CAV1 protein expression in the kidney induces hypertension.