Monica Palmada

Publications (44)197.46 Total impact

• Article: Up-Regulation of Amino Acid Transporter SLC6A19 Activity and Surface Protein Abundance by PKB/Akt and PIKfyve.

  Evgenii Bogatikov, Carlos Munoz, Tatsiana Pakladok, Ioana Alesutan, Manzar Shojaiefard, Guiscard Seebohm, Michael Föller, Monica Palmada, Christoph Böhmer, Stefan Bröer, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: Background: The amino acid transporter B0AT1 (SLC6A19) accomplishes concentrative cellular uptake of neutral amino acids. SLC6A19 is stimulated by serum- & glucocorticoid-inducible kinase (SGK) isoforms. SGKs are related to PKB/Akt isoforms, which also stimulate several amino acid transporters. PKB/Akt modulates glucose transport in part by phosphorylating and thus activating phosphatidylinositol-3-phosphate-5-kinase (PIKfyve), which fosters carrier protein insertion into the cell membrane. The present study explored whether PKB/Akt and/or PIKfyve stimulate SLC6A19. Methods: SLC6A19 was expressed in Xenopus oocytes with or without wild-type PKB/Akt or inactive (T308A/S473A)PKB/Akt without or with additional expression of wild-type PIKfyve or PKB/Akt-resistant (S318A)PIKfyve. Electrogenic amino acid transport was determined by dual electrode voltage clamping. Results: In SLC6A19-expressing oocytes but not in water-injected oocytes, the addition of the neutral amino acid L-leucine (2 mM) to the bath generated a current (I(le)), which was significantly increased following coexpression of PKB/Akt, but not by coexpression of (T308A/S473A)PKB/Akt. The effect of PKB/Akt was augmented by additional coexpression of PIKfyve but not of (S318A)PIKfyve. Coexpression of PKB/Akt enhanced the maximal transport rate without significantly modifying the affinity of the carrier. The decline of I(le) following inhibition of carrier insertion by brefeldin A (5 µM) was similar in the absence and presence of PKB/Akt indicating that PKB/Akt stimulated carrier insertion into rather than inhibiting carrier retrieval from the cell membrane. Conclusion: PKB/Akt up-regulates SLC6A19 activity, which may foster amino acid uptake into PKB/Akt-expressing epithelial and tumor cells.
  Cellular Physiology and Biochemistry 12/2012; 30(6):1538-1546. · 2.86 Impact Factor
• Article: The serum and glucocorticoid inducible kinases SGK1-3 stimulate the neutral amino acid transporter SLC6A19.

  Christoph Böhmer, Mentor Sopjani, Fabian Klaus, Ricco Lindner, Jörg Laufer, Shankarganesh Jeyaraj, Florian Lang, Monica Palmada
  [show abstract] [hide abstract]
  ABSTRACT: The neutral amino acid transporter SLC6A19 (B(0)AT1) plays a decisive role in transport of neutral amino acids in the kidney and intestine. Recently, mutations in SLC6A19 were identified that result in severe neutral aminoaciduria known as Hartnup disorder. SLC6A19 expression and function is controlled by the brush-border angiotensin-converting enzyme 2 (ACE2). Beyond that the mechanisms regulating SLC6A19 function are unknown. The SLC6A19 sequence contains a conserved putative phosphorylation site for the serum and glucocorticoid inducible kinase isoforms SGK1-3, kinases known to regulate a variety of channels and transporters. The present study explored the role of SGK1-3 in the regulation of SLC6A19. As shown by two-electrode voltage clamp in the Xenopus oocyte expression system, leucine-induced currents in SLC6A19 expressing oocytes were activated by the protein kinases SGK1-3. The putative phosphorylation site on the transporter is not essential for SLC6A19 regulation by the kinases. As determined by quantitative immunoassay and electrophysiology, the kinases increase SLC6A19 currents by increasing the cell surface expression of the protein without altering the affinity of the carrier. Following inhibition of carrier insertion into the cell membrane by treatment with brefeldin A (BFA), the leucine-induced current declined significantly slower in Xenopus oocytes expressing SLC6A19 together with SGK1 than in oocytes expressing SLC6A19 alone, a finding pointing to SGK-mediated transporter stabilization in the plasma membrane. Coexpression of ACE2 markedly increased leucine-induced currents in SLC6A19 expressing oocytes that were further enhanced by SGK1-3 kinases. In conclusion, SGK isoforms are novel potent stimulators of SLC6A19 and may thus participate in the regulation of neutral amino acid transport in vivo.
  Cellular Physiology and Biochemistry 01/2010; 25(6):723-32. · 2.86 Impact Factor
• Article: Downregulation of angiogenin transcript levels and inhibition of colonic carcinoma by gum arabic (Acacia senegal).

  Omaima Nasir, Kan Wang, Michael Föller, Madhuri Bhandaru, Diana Sandulache, Ferruh Artunc, Teresa F Ackermann, Ammar Ebrahim, Monica Palmada, Karin Klingel, Amal M Saeed, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: Gum Arabic (GA), a nutrient from dried exudate of Acacia senegal, is widely used as emulsifier and stabilizer. It stimulates sodium and water absorption in diarrhea. This study explored the effects of GA in colonic tissue. Mice were treated with GA (10% wt/vol) in drinking water and gene array was performed. As GA modified several tumor-relevant genes, chemical cancerogenesis (intraperitoneal injection of 20 mg/kg 1,2-dimethylhydrazine followed by 3 cycles of 3% dextrane sodium sulphate in drinking water) was induced with or without GA treatment. Within 4 days, GA treatment decreased the colonic transcript levels of the angiogenetic factors angiogenin 1, angiogenin 3, and angiogenin 4 by 78 +/- 18%, 88 +/- 15%, and 92 +/- 13%, respectively (n = 5 each), and of further genes including CD38 antigen, aquaporin4, interleukin18, vav-3-oncogene, gamma(+)-amino acid transporter, sulfatase1, ubiquitinD, and chemokine ligand5. According to Western blotting, GA treatment similarly decreased angiogenin protein expression, and according to immunohistochemistry, it decreased ss-catenin expression. Chemical cancerogenesis resulted in multiple colonic tumors within 12 wk. GA treatment (10% wt/wt) in drinking water significantly decreased the number of tumors by 70%. The observations disclose a powerful anticarcinogenic effect of GA. The nutrient could thus be used for the prophylaxis against colon carcinoma particularly in individuals at enhanced risk.
  Nutrition and Cancer 01/2010; 62(6):802-10. · 2.78 Impact Factor
• Article: Regulation of the glutamate transporter EAAT4 by PIKfyve.

  Ioana S Alesutan, Oana N Ureche, Joerg Laufer, Fabian Klaus, Agathe Zürn, Ricco Lindner, Nathalie Strutz-Seebohm, Jeremy M Tavaré, Christoph Boehmer, Monica Palmada, Undine E Lang, Guiscard Seebohm, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: The excitatory amino-acid transporter EAAT4 (SLC1A6), a Na(+),glutamate cotransporter expressed mainly in Purkinje cells, serves to clear glutamate from the synaptic cleft. EAAT4 activity is stimulated by the serum and glucocorticoid inducible kinase SGK1. SGK1-dependent regulation of the Na(+),glucose transporter SGLT1 (SLC5A1) and the creatine transporter CreaT (SLC6A8) has recently been shown to involve the mammalian phosphatidylinositol-3-phosphate-5-kinase PIKfyve (PIP5K3). The present experiments thus explored whether SGK1-dependent EAAT4-regulation similarly involves PIKfyve. In Xenopus oocytes expressing EAAT4, but not in water injected oocytes, glutamate induced a current which was significantly enhanced by coexpression of PIKfyve and SGK1. The glutamate induced current in Xenopus oocytes coexpressing EAAT4 and both, PIKfyve and SGK1, was significantly larger than the current in Xenopus oocytes expressing EAAT4 together with either kinase alone. Coexpression of the inactive SGK1 mutant (K127N)SGK1 did not significantly alter glutamate induced current in EAAT4-expressing Xenopus oocytes and abolished the stimulation of glutamate induced current by coexpression of PIKfyve. The stimulating effect of PIKfyve was abrogated by replacement of the serine with alanine in the SGK consensus sequence ((S318A)PIKfyve). Furthermore, coexpression of (S318A)PIKfyve significantly blunted the stimulating effect of SGK1 on EAAT4 activity. The observations disclose that PIKfyve indeed participates in the regulation of EAAT4.
  Cellular Physiology and Biochemistry 01/2010; 25(2-3):187-94. · 2.86 Impact Factor
• Article: The C-terminal PDZ-binding motif in the Kv1.5 potassium channel governs its modulation by the Na+/H+ exchanger regulatory factor 2.

  Jörg Laufer, Christoph Boehmer, Sankarganesh Jeyaraj, Martin Knuwer, Fabian Klaus, Ricco Lindner, Monica Palmada, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: Kv1.5 belongs to the family of voltage-gated potassium (Kv) channels and contains a N- and a C-terminal PDZ-binding motif that might be recognized by PDZ domains on the scaffold proteins NHERF1 and NHERF2. Expression studies in Xenopus oocytes demonstrated that NHERF1 and NHERF2 activate Kv1.5, an effect requiring the C-terminal PDZ-binding motif on Kv1.5. NHERF2 enhances Kv1.5 activity and cell surface expression as determined by electrophysiology and immunoassays. NHERF2 elevates Kv1.5 abundance at the plasma membrane by decreasing channel internalization as proven by Brefeldin A experiments. Kv1.5 is stimulated by the serum and glucocorticoid inducible kinase SGK1, a kinase known to interact with the second PDZ domain of NHERF2. This study aims to identify if SGK1 and NHERF2 synergize to increase Kv1.5 currents. Expression of NHERF2 potentiated SGK1-mediated Kv1.5 activation, which was significantly attenuated by deletion of the second PDZ domain in NHERF2. Specificity of observed effects was verified by evaluating the influence of NHERFs on Kv1.3, a known SGK1 target that contains an internal PDZ binding motif. In summary, our results suggest that NHERFs might participate in the regulation of electrical excitability in part by controlling Kv1.5 surface abundance and by clustering signal transduction molecules to the channel.
  Cellular Physiology and Biochemistry 02/2009; 23(1-3):25-36. · 2.86 Impact Factor
• Article: Regulation of the glutamate transporter EAAT2 by PIKfyve.

  Eva-Maria Gehring, Agathe Zurn, Fabian Klaus, Joerg Laufer, Mentor Sopjani, Ricco Lindner, Nathalie Strutz-Seebohm, Jeremy M Tavaré, Christoph Boehmer, Monica Palmada, Undine E Lang, Guiscard Seebohm, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: The Na(+),glutamate cotransporter EAAT2 is expressed in astrocytes and clears glutamate from the synaptic cleft. EAAT2 dependent currrent is stimulated by the serum and glucocorticoid inducible kinase SGK1. Phosphorylation targets of SGK1 include the human phosphatidylinositol-3-phosphate-5-kinase PIKfyve (PIP5K3). Nothing is known, however, on the role of PIKfyve in the regulation of EAAT2. The present experiments thus explored, whether PIKfyve expression modifies EAAT2 dependent currrent and protein abundance in the cell membrane. In Xenopus oocytes expressing EAAT2 but not in water injected oocytes application of glutamate (2 mM) induced an inward current (I(glu)). Coexpression of either, SGK1 or PIKfyve, significantly enhanced I(glu) in EAAT2 expressing oocytes. I(glu) was significantly higher in Xenopus oocytes coexpressing EAAT2, SGK1 and PIKfyve than in Xenopus oocytes expressing EAAT2 and either, SGK1 or PIKfyve, alone. Additional coexpression of the inactive mutant of the serum and glucocorticoid inducible kinase (K127N)SGK1 did not significantly alter I(glu) in EAAT2 expressing oocytes and significantly decreased I(glu) in oocytes coexpressing EAAT2 together with PIKfyve. The stimulating effect of PIKfyve on I(glu) was abrogated by replacement of the serine in the SGK consensus sequence by alanine ((S318A)PIKfyve). Furthermore, additional coexpression of (S318A)PIKfyve virtually abolished I(glu) in Xenopus oocytes coexpressing SGK1 and EAAT2. Confocal microscopy reveals that PIKfyve enhances the EAAT2 protein abundance in the cell membrane. The observations disclose that PIKfyve indeed participates in the regulation of EAAT2.
  Cellular Physiology and Biochemistry 01/2009; 24(5-6):361-8. · 2.86 Impact Factor
• Article: Modulation of the voltage-gated potassium channel Kv1.5 by the SGK1 protein kinase involves inhibition of channel ubiquitination.

  Christoph Boehmer, Jörg Laufer, Sankarganesh Jeyaraj, Fabian Klaus, Ricco Lindner, Florian Lang, Monica Palmada
  [show abstract] [hide abstract]
  ABSTRACT: The serum and glucocorticoid inducible kinase SGK1 is involved in dexamethasone-induced inhibition of insulin secretion by increasing voltage-gated potassium channel (Kv) activity. SGK1 upregulates the Kv1.5 channel but the precise mechanism underlying the SGK1 dependent regulation of Kv1.5 has not been defined yet. The present study explored the signal transduction processes involved. Expression studies in Xenopus oocytes revealed that SGK1 promotes channel activity by interfering with the Nedd4-2 ubiquitination pathway, irrespective of the presence of putative SGK1 phosphorylation sites on Kv1.5. Expression of the ubiquitin ligase Nedd4-2 declined Kv1.5 currents by ubiquitinating and thereby reducing Kv1.5 plasma membrane expression. Increasing concentrations of SGK1 gradually compensated the inhibiting effect of Nedd4-2 on Kv1.5. Enhanced Kv1.5 surface abundance by SGK1 reflects decreased channel internalization as indicated by Brefeldin A experiments. In conclusion, Kv1.5 upregulation by SGK1 involves inhibition of channel ubiquitination by Nedd4-2 that leads to Kv1.5 stabilization in the plasma membrane. Our results suggest that the kinase might participate in the regulation of insulin secretion in part by controlling Kv1.5 surface abundance.
  Cellular Physiology and Biochemistry 02/2008; 22(5-6):591-600. · 2.86 Impact Factor
• Article: The peptide transporter PEPT2 is targeted by the protein kinase SGK1 and the scaffold protein NHERF2.

  Christoph Boehmer, Monica Palmada, Fabian Klaus, Sankarganesh Jeyaraj, Ricco Lindner, Jorg Laufer, Hannelore Daniel, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: PEPT1 and PEPT2 are members of the family of proton-dependent oligopeptide transporters that mediate electrogenic uphill transport of small peptides and peptidomimetics into a variety of cells. Kinetic properties and substrate recognition sites of those transporters have been well defined previously. Little is known, however, about regulation of those transporters. Both PEPT isoforms contain putative phosphorylation sites for the serum and glucocorticoid inducible kinase SGK1 and a C-terminal PDZ binding motif that might be recognized by PDZ domains of the Na(+)/H(+) exchanger regulatory factors NHERF1 and NHERF2. Thus, the present study attempted to clarify the role of SGK1 and NHERFs in the modulation of PEPT isoforms. Expression studies in Xenopus oocytes with subsequent electrophysiology and immunoassays revealed that SGK1 and NHERF2, but not the NHERF1 isoform specifically enhance PEPT2 function and surface abundance. The kinase is effective through phosphorylation of (185)Ser within the SGK1 consensus site, since disruption of this site prevented transporter modulation by the kinase. NHERF2 failed to regulate the C-terminal deletion mutant (PEPT2DeltaC) indicating that the C-terminal PDZ-binding motif in PEPT2 governs transport modulation by NHERF2. Coexpression of NHE3 stimulates PEPT2 activity to a similar extent as coexpression of NHERF2. Dynasore experiments demonstrated that SGK1 and NHERF2 activate PEPT2 by stabilizing the transporter at the cell surface. In conclusion, the present results reveal two novel PEPT2 posttranslational modulators, SGK1 and NHERF2, which might regulate transport of oligopeptides and peptidomimetic drugs.
  Cellular Physiology and Biochemistry 02/2008; 22(5-6):705-14. · 2.86 Impact Factor
• Article: EAAT4 phosphorylation at the SGK1 consensus site is required for transport modulation by the kinase.

  Jeyaganesh Rajamanickam, Monica Palmada, Florian Lang, Christoph Boehmer
  [show abstract] [hide abstract]
  ABSTRACT: EAAT4 (SLC1A6) is a Purkinje-Cell-specific post-synaptic excitatory amino acid transporter that plays a major role in clearing synaptic glutamate. EAAT4 abundance and function is known to be modulated by the serum and glucocorticoid inducible kinase (SGK) 1 but the precise mechanism of kinase action has not been defined yet. The present work aims to identify the molecular mechanism of EAAT4 modulation by the kinase. The EAAT4 sequence bears two putative SGK1 consensus sites (at Thr40 and Thr504) at the amino and carboxy terminus that are conserved among species. Expression studies in Xenopus oocytes demonstrated that EAAT4-mediated [(3)H] glutamate uptake and cell surface abundance are enhanced by co-expression of SGK1. Disruption of the SGK1 phosphorylation site at threonine 40 ((T40A)EAAT4) or of both phosphorylation sites ((T40AT504A)EAAT4) abrogated the effect of SGK1 on transporter function and expression. SGK1 modulates several transport proteins via inhibition of the ubiquitin ligase Nedd4-2. Co-expression of Nedd4-2 inhibited wild-type EAAT4 but not the (T40AT504A)EAAT4 mutant. Besides, RNA interference-mediated reduction of endogenous Nedd4-2 (xNedd4-2) expression increased the activity of the transporter. In conclusion, maximal glutamate transport modulation by SGK1 is accomplished by direct EAAT4 stimulation and to a lesser extent by inhibition of intrinsic Nedd4-2.
  Journal of Neurochemistry 09/2007; 102(3):858-66. · 4.06 Impact Factor
• Chapter: Kinases, Cell Volume, and the Regulation of Chloride Channels

  Florian Lang, Albrecht Lepple-Wienhues, Ildicko Szabo, Erich Gulbins, Monica Palmada, Sabine Wallisch, Christoph Böhmer, Karin Klingel, Reinhard Kandolf
  07/2007: pages 73-81;
• Article: Role of SGK1 kinase in regulating glucose transport via glucose transporter GLUT4.

  Sankarganesh Jeyaraj, Christoph Boehmer, Florian Lang, Monica Palmada
  [show abstract] [hide abstract]
  ABSTRACT: Insulin stimulates glucose transport into muscle and fat cells by enhancing GLUT4 abundance in the plasma membrane through activation of phosphatidylinositol 3-kinase (PI3K). Protein kinase B (PKB) and PKCzeta are known PI3K downstream targets in the regulation of GLUT4. The serum- and glucocorticoid-inducible kinase SGK1 is similarly activated by insulin and capable to regulate cell surface expression of several metabolite transporters. In this study, we evaluated the putative role of SGK1 in the modulation of GLUT4. Coexpression of the kinase along with GLUT4 in Xenopus oocytes stimulated glucose transport. The enhanced GLUT4 activity was paralleled by increased transporter abundance in the plasma membrane. Disruption of the SGK1 phosphorylation site on GLUT4 ((S274A)GLUT4) abrogated the stimulating effect of SGK1. In summary, SGK1 promotes glucose transporter membrane abundance via GLUT4 phosphorylation at Ser274. Thus, SGK1 may contribute to the insulin and GLUT4-dependent regulation of cellular glucose uptake.
  Biochemical and Biophysical Research Communications 06/2007; 356(3):629-35. · 2.48 Impact Factor
• Article: Resistance of mice lacking the serum- and glucocorticoid-inducible kinase SGK1 against salt-sensitive hypertension induced by a high-fat diet.

  Dan Yang Huang, Krishna M Boini, Hartmut Osswald, Björn Friedrich, Ferruh Artunc, Susanne Ullrich, Jeyaganesh Rajamanickam, Monica Palmada, Peer Wulff, Dietmar Kuhl, Volker Vallon, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: Mineralocorticoids enhance expression and insulin stimulates activity of the serum- and glucocorticoid-inducible kinase SGK1, which activates the renal epithelial Na+)channel (ENaC). Under a salt-deficient diet, SGK1 knockout mice (sgk1-/-) excrete significantly more NaCl than their wild-type littermates (sgk1+/+) and become hypotensive. The present experiments explored whether SGK1 participates in the hypertensive effects of a high-fat diet and high-salt intake. Renal SGK1 protein abundance of sgk1+/+ mice was significantly elevated after a high-fat diet. Under a control diet, fluid intake, blood pressure, urinary flow rate, and urinary Na+, K+, and Cl- excretion were similar in sgk1-/- and sgk1+/+ mice. Under a standard diet, high salt (1% NaCl in the drinking water for 25 days) increased fluid intake, urinary flow rate, and urinary Na+, K+, and Cl- excretion similarly in sgk1-/- and sgk1+/+ mice without significantly altering blood pressure. A high-fat diet alone (17 wk) did not significantly alter fluid intake, urinary flow rate, urinary Na+, K+, or Cl- excretion, or plasma aldosterone levels but increased plasma insulin, total cholesterol, triglyceride concentrations, and systolic blood pressure to the same extent in both genotypes. Additional salt intake (1% NaCl in the drinking water for 25 days) on top of a high-fat diet did not affect hyperinsulinemia or hyperlipidemia but increased fluid intake, urinary flow rate, and urinary NaCl excretion significantly more in sgk1-/- than in sgk1+/+ mice. Furthermore, in animals receiving a high-fat diet, additional salt intake increased blood pressure only in sgk1+/+ mice (to 132 +/- 3 mmHg) but not in sgk1-/- mice (120 +/- 4 mmHg). Thus lack of SGK1 protects against the hypertensive effects of a combined high-fat/high-salt diet.
  American journal of physiology. Renal physiology 01/2007; 291(6):F1264-73. · 3.68 Impact Factor
• Article: Impaired intestinal NHE3 activity in the PDK1 hypomorphic mouse.

  Ciprian Sandu, Ferruh Artunc, Monica Palmada, Rexhep Rexhepaj, Florian Grahammer, Azeemudeen Hussain, Chris Yun, Dario R Alessi, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: In vitro experiments have demonstrated the stimulating effect of serum- and glucocorticoid-inducible kinase (SGK)1 on the activity of the Na+/H+ exchanger (NHE3). SGK1 requires activation by phosphoinositide-dependent kinase (PDK)1, which may thus similarly play a role in the regulation of NHE3-dependent epithelial electrolyte transport. The present study was performed to explore the role of PDK1 in the regulation of NHE3 activity. Because mice completely lacking functional PDK1 are not viable, hypomorphic mice expressing approximately 20% of PDK1 (pdk1(hm)) were compared with their wild-type littermates (pdk1(wt)). NHE3 activity in the intestine and PDK1-overexpressing HEK-293 cells was estimated by utilizing 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein fluorescence for the determination of intracellular pH. NHE activity was reflected by the Na+-dependent pH recovery from an ammonium prepulse (DeltapH(NHE)). The pH changes after an ammonium pulse allowed the calculation of cellular buffer capacity, which was not significantly different between pdk1(hm) and pdk1(wt) mice. DeltapH(NHE) was in pdk1(hm) mice, only 30 +/- 6% of the value obtained in pdk1(wt) mice. Conversely, DeltapH(NHE) was 32 +/- 7% larger in PDK1-overexpressing HEK-293 cells than in HEK-293 cells expressing the empty vector. The difference between pdk1(hm) and pdk1(wt) mice and between PDK1-overexpressing and empty vector-transfected HEK cells, respectively, was completely abolished in the presence of the NHE3 inhibitor S3226 (10 microM). In conclusion, defective PDK1 expression leads to significant impairment of NHE3 activity in the intestine, pointing to a role of PDK1-dependent signaling in the regulation of NHE-mediated electrolyte transport.
  AJP Gastrointestinal and Liver Physiology 12/2006; 291(5):G868-76. · 3.43 Impact Factor
• Article: (Patho)physiological significance of the serum- and glucocorticoid-inducible kinase isoforms.

  Florian Lang, Christoph Böhmer, Monica Palmada, Guiscard Seebohm, Nathalie Strutz-Seebohm, Volker Vallon
  [show abstract] [hide abstract]
  ABSTRACT: The serum- and glucocorticoid-inducible kinase-1 (SGK1) is ubiquitously expressed and under genomic control by cell stress (including cell shrinkage) and hormones (including gluco- and mineralocorticoids). Similar to its isoforms SGK2 and SGK3, SGK1 is activated by insulin and growth factors via phosphatidylinositol 3-kinase and the 3-phosphoinositide-dependent kinase PDK1. SGKs activate ion channels (e.g., ENaC, TRPV5, ROMK, Kv1.3, KCNE1/KCNQ1, GluR1, GluR6), carriers (e.g., NHE3, GLUT1, SGLT1, EAAT1-5), and the Na+-K+-ATPase. They regulate the activity of enzymes (e.g., glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, phosphomannose mutase-2) and transcription factors (e.g., forkhead transcription factor FKHRL1, beta-catenin, nuclear factor kappaB). SGKs participate in the regulation of transport, hormone release, neuroexcitability, cell proliferation, and apoptosis. SGK1 contributes to Na+ retention and K+ elimination of the kidney, mineralocorticoid stimulation of salt appetite, glucocorticoid stimulation of intestinal Na+/H+ exchanger and nutrient transport, insulin-dependent salt sensitivity of blood pressure and salt sensitivity of peripheral glucose uptake, memory consolidation, and cardiac repolarization. A common ( approximately 5% prevalence) SGK1 gene variant is associated with increased blood pressure and body weight. SGK1 may thus contribute to metabolic syndrome. SGK1 may further participate in tumor growth, neurodegeneration, fibrosing disease, and the sequelae of ischemia. SGK3 is required for adequate hair growth and maintenance of intestinal nutrient transport and influences locomotive behavior. In conclusion, the SGKs cover a wide variety of physiological functions and may play an active role in a multitude of pathophysiological conditions. There is little doubt that further targets will be identified that are modulated by the SGK isoforms and that further SGK-dependent in vivo physiological functions and pathophysiological conditions will be defined.
  Physiological Reviews 11/2006; 86(4):1151-78. · 26.87 Impact Factor
• Article: Serum- and glucocorticoid-inducible kinase 1 mediates salt sensitivity of glucose tolerance.

  Krishna M Boini, Anita M Hennige, Dan Yang Huang, Björn Friedrich, Monica Palmada, Christoph Boehmer, Florian Grahammer, Ferruh Artunc, Susanne Ullrich, Diana Avram, Hartmut Osswald, Peer Wulff, Dietmar Kuhl, Volker Vallon, Hans-Ulrich Häring, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: Excess salt intake decreases peripheral glucose uptake, thus impairing glucose tolerance. Stimulation of cellular glucose uptake involves phosphatidylinositide-3-kinase (PI-3K)-dependent activation of protein kinase B/Akt. A further kinase downstream of PI-3K is serum- and glucocorticoid-inducible kinase (SGK)1, which is upregulated by mineralocorticoids and, thus, downregulated by salt intake. To explore the role of SGK1 in salt-dependent glucose uptake, SGK1 knockout mice (sgk1(-/-)) and their wild-type littermates (sgk1(+/+)) were allowed free access to either tap water (control) or 1% saline (high salt). According to Western blotting, high salt decreased and deoxycorticosterone acetate (DOCA; 35 mg/kg body wt) increased SGK1 protein abundance in skeletal muscle and fat tissue of sgk1(+/+) mice. Intraperitoneal injection of glucose (3 g/kg body wt) into sgk1(+/+) mice transiently increased plasma glucose concentration approaching significantly higher values ([glucose]p,max) in high salt (281 +/- 39 mg/dl) than in control (164 +/- 23 mg/dl) animals. DOCA did not significantly modify [glucose]p,max in control sgk1(+/+) mice but significantly decreased [glucose]p,max in high-salt sgk1(+/+) mice, an effect reversed by spironolactone (50 mg/kg body wt). [Glucose]p,max was in sgk1(-/-) mice insensitive to high salt and significantly higher than in control sgk1(+/+) mice. Uptake of 2-deoxy-d-[1,2-(3)H]glucose into skeletal muscle and fat tissue was significantly smaller in sgk1(-/-) mice than in sgk1(+/+) mice and decreased by high salt in sgk1(+/+) mice. Transfection of HEK-293 cells with active (S422D)SGK1, but not inactive (K127N)SGK, stimulated phloretin-sensitive glucose uptake. In conclusion, high salt decreases SGK1-dependent cellular glucose uptake. SGK1 thus participates in the link between salt intake and glucose tolerance.
  Diabetes 08/2006; 55(7):2059-66. · 8.29 Impact Factor
• Article: Renal Ca2+ handling in sgk1 knockout mice.

  Diana Sandulache, Florian Grahammer, Ferruh Artunc, Guido Henke, Azeemudeen Hussain, Omaima Nasir, Andreas Mack, Björn Friedrich, Volker Vallon, Peer Wulff, Dietmar Kuhl, Monica Palmada, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: Coexpression studies in Xenopus oocytes revealed the ability of the serum- and glucocorticoid-inducible kinase 1 (SGK1) to stimulate the renal epithelial Ca(2+) channel TRPV5. SGK1 increases the abundance of the channel protein in the plasma membrane, an effect requiring the participation of the Na(+)/H(+) exchanger regulating factor 2 (NHERF2). The present study was performed to explore the role of SGK1 in the regulation of renal Ca(2+) handling in vivo. To this end, TRPV5, calbindin D-28K abundance, and renal Ca(2+) excretion were analyzed in gene-targeted mice lacking functional SGK1 (sgk1( -/- )) and their age- and sex-matched littermates (sgk1( +/+ )). Immunohistochemistry revealed lower abundance of TRPV5 and calbindin D-28K protein in sgk1( -/- ) mice than in sgk1( +/+ ) mice, both fed with control diet. Feeding the mice a Ca(2+)-deficient diet marked ly increased TRPV5 protein abundance in both genotypes. Renal Ca(2+) excretion under control diet was significantly lower in sgk1 ( -/- ) than in sgk1( +/+ ) mice. The Ca(2+)-deficient diet decreased renal excretion of Ca(2+) to the same levels in both phenotypes. Furosemide increased fractional Ca(2+) excretion and dissipated the difference between phenotypes. We conclude that lack of SGK1 may lead to decrease in TRPV5 abundance in connecting tubules but does not abrogate TRPV5 regulation. The decrease in abundance of TRPV5 in connecting tubules of sgk1( -/- ) mice is presumably compensated for by enhanced Ca(2+) reabsorption in upstream nephron segments such as the loop of Henle, which may indirectly result from impaired SGK1-dependent Na(+) reabsorption in the aldosterone-sensitive distal part of the nephron, salt loss, and enhanced Na(+) (and Ca(2+)) reabsorption in those upstream nephron segments.
  Pflügers Archiv - European Journal of Physiology 08/2006; 452(4):444-52. · 4.46 Impact Factor
• Article: Post-translational regulation of EAAT2 function by co-expressed ubiquitin ligase Nedd4-2 is impacted by SGK kinases.

  Christoph Boehmer, Monica Palmada, Jeyaganesh Rajamanickam, Roman Schniepp, Susan Amara, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: The human excitatory amino acid transporter (EAAT)2 is the major glutamate carrier in the mammalian CNS. Defective expression of the transporter results in neuroexcitotoxicity that may contribute to neuronal disorders such as amyotrophic lateral sclerosis (ALS). The serum and glucocorticoid inducible kinase (SGK) 1 is expressed in the brain and is known to interact with the ubiquitin ligase Nedd4-2 to modulate membrane transporters and ion channels. The present study aimed to investigate whether SGK isoforms and the related kinase, protein kinase B (PKB), regulate EAAT2. Expression studies in Xenopus oocytes demonstrated that glutamate-induced inward current (IGLU) was stimulated by co-expression of SGK1, SGK2, SGK3 or PKB. IGLU is virtually abolished by Nedd4-2, an effect abrogated by additional co-expression of either kinase. The kinases diminish the effect through Nedd4-2 phosphorylation without altering Nedd4-2 protein abundance. SGKs increase the transporter maximal velocity without significantly affecting substrate affinity. Similar to glutamate-induced currents, [3H] glutamate uptake and cell surface abundance of the transporter were increased by the SGK isoforms and down-regulated by the ubiquitin ligase Nedd4-2. In conclusion, all three SGK isoforms and PKB increase EAAT2 activity and plasma membrane expression and thus, may participate in the regulation of neuroexcitability.
  Journal of Neurochemistry 06/2006; 97(4):911-21. · 4.06 Impact Factor
• Article: SGK1 kinase upregulates GLUT1 activity and plasma membrane expression.

  Monica Palmada, Christoph Boehmer, Ahmad Akel, Jeyaganesh Rajamanickam, Sankarganesh Jeyaraj, Konrad Keller, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: Phosphatidylinositol 3-kinase (PI3 kinase) inhibition disrupts the ability of insulin to stimulate GLUT1 and GLUT4 translocation into the cell membrane and thus glucose transport. The effect on GLUT4 but not on GLUT1 is mediated by activation of protein kinase B (PKB). The serum- and glucocorticoid-inducible kinase SGK1, a further kinase downstream of PI3 kinase, regulates several transporters by enhancing their plasma membrane abundance. GLUT1 contains a consensus site ((95)Ser) for phosphorylation by SGK1. Thus, the present study investigated whether GLUT1 is regulated by the kinase. Tracer-flux studies in Xenopus oocytes and HEK-293 cells demonstrated that GLUT1 transport is enhanced by constitutively active (S422D)SGK1. The effect requires the kinase catalytical activity since the inactive mutant (K127N)SGK1 failed to modulate GLUT1. GLUT1 stimulation by (S422D)SGK1 is not due to de novo protein synthesis but rather to an increase of the transporter's abundance in the plasma membrane. Kinetic analysis revealed that SGK1 enhances maximal transport rate without altering GLUT1 substrate affinity. These observations suggest that SGK1 regulates GLUT1 and may contribute to or account for the PI3 kinase-dependent but PKB-independent stimulation of GLUT1 by insulin.
  Diabetes 03/2006; 55(2):421-7. · 8.29 Impact Factor
• Article: Upregulation of HERG channels by the serum and glucocorticoid inducible kinase isoform SGK3.

  Gottlieb Maier, Monica Palmada, Jeyaganesh Rajamanickam, Ekaterina Shumilina, Christoph Böhmer, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: Human ether-a-go-go (HERG) channels participate in the repolarization of the cardiac action potential. Loss of function mutations of HERG lead to delayed cardiac repolarization reflected by prolonged QT interval. HERG channels are regulated through a signaling cascade involving phosphatidylinositol 3 (PI3) kinase. Downstream targets of PI3 kinase include the serum and glucocorticoid inducible kinase (SGK) and protein kinase B (PKB) isoforms. The present study has been performed to explore whether SGK1 and SGK3 participate in the regulation of HERG channel activity. HERG was expressed in Xenopus oocytes with or without additional expression of SGK1 or SGK3. Chemiluminescence was employed to determine HERG plasma membrane protein abundance. Coexpression of SGK3 but not of SGK1 in Xenopus oocytes resulted in an increase of steady state current (I(HERG)) and enhanced cell membrane protein abundance without affecting gating kinetics of the channel. Replacement of serine by alanine at the two SGK consensus sites decreased I(HERG) but neither mutation abolished the stimulating effect of SGK3. In conclusion, SGK3 participates in the regulation of HERG by increasing HERG protein abundance in the plasma membrane and may thus modify the duration of the cardiac action potential.
  Cellular Physiology and Biochemistry 02/2006; 18(4-5):177-86. · 2.86 Impact Factor
• Article: Decreased intestinal glucose transport in the sgk3-knockout mouse.

  Ciprian Sandu, Rexhep Rexhepaj, Florian Grahammer, James A McCormick, Guido Henke, Monica Palmada, Srinivas Nammi, Undine Lang, Marco Metzger, Lothar Just, Thomas Skutella, Kevin Dawson, Jian Wang, David Pearce, Florian Lang
  [show abstract] [hide abstract]
  ABSTRACT: Xenopus oocyte coexpression experiments revealed the capacity of the serum- and glucocorticoid-inducible kinase isoform 3 (SGK3) to up-regulate a variety of transport systems including the sodium-dependent glucose transporter SGLT1. The present study explored the functional significance of SGK3-dependent regulation of intestinal transport. To this end, experiments were performed in gene targeted mice lacking functional sgk3 (sgk3(-/-)) and their wild type littermates (sgk3(+/+)). Oral food intake and fecal dry weight were significantly larger in sgk3(-/-) than in sgk3(+/+) mice. Glucose-induced current (I(g)) in Ussing chamber as a measure of Na(+) coupled glucose transport was significantly smaller in sgk3(-/-) than in sgk3(+/+) mouse jejunal segments. Fasting plasma glucose concentrations were significantly lower in sgk3(-/-) than in sgk3(+/+) mice. Intestinal electrogenic transport of phenylalanine, cysteine, glutamine and proline were not significantly different between sgk3(-/-) and sgk3(+/+) mice. In conclusion, SGK3 is required for adequate intestinal Na(+) coupled glucose transport and impaired glucose absorption may contribute to delayed growth and decreased plasma glucose concentrations of SGK3 deficient mice. The hypoglycemia might lead to enhanced food intake to compensate for impaired intestinal absorption.
  Pflügers Archiv - European Journal of Physiology 01/2006; 451(3):437-44. · 4.46 Impact Factor