Impaired intestinal NHE3 activity in the PDK1 hypomorphic mouse

Targeting PDK1 for Chemosensitization of Cancer Cells

Despite the rapid development in the field of oncology, cancer remains the second cause of mortality worldwide, with the number of new cases expected to more than double in the coming years. Chemotherapy is widely used to decelerate or stop tumour development in combination with surgery or radiation therapy when appropriate, and in many cases this improves the symptomatology of the disease. Unfortunately though, chemotherapy is not applicable to all patients and even when it is, there are many cases where a successful initial treatment period is followed by chemotherapeutic drug resistance. This is caused by a number of reasons, ranging from the genetic background of the patient (innate resistance) to the formation of tumour-initiating cells (acquired resistance). In this review, we discuss the potential role of PDK1 in the development of chemoresistance in different types of malignancy, and the design and application of potent inhibitors which can promote chemosensitization.

SGK1: The Dark Side of PI3K Signaling

Activation of the PI3K pathway is central to a variety of physiological and pathological processes. In these contexts, AKT is classically considered the de facto mediator of PI3K-dependent signaling. However, in recent years, accumulating data point to the existence of additional effectors of PI3K activity, parallel to and independent of AKT, that play critical and unique roles in mediating different developmental, homeostatic, and pathological processes. In this review, I summarize and discuss our current understanding of the function of the serine/threonine kinase SGK1 as a downstream effector of PI3K, and try to separate targets and pathways validated as uniquely SGK1-dependent from those shared with AKT.

Phosphoinositide-Dependent Protein Kinase 1 (PDK1)

Abstract. Phosphatidylinositol-3-kinase (PI3K) signaling influences susceptibility to virus infections, anoxia, obstetric complications, and cancer; which are changed in patients with schizophrenia and their first degree relatives. Therefore PI3K signaling might have impact on the pathophysiology of schizophrenia. PI3K signaling crucially involves phosphoinositide-dependent protein kinase (PDK1). Increased anxiety behavior is observed in PDK1 hypomorphic mice. Here we show enhanced prevalence of schizophrenia in carriers of the PDK1 CC genotype in human beings. Moreover, decreased parietal P300 amplitude, which is a well-studied schizophrenic endophenotype, was observed in PDK1 CC carriers. Glutamate and glutamine concentrations are increased in the frontal lobe of PDK1 dysmorphic mice and human CC individuals. Our results demonstrate that the PDK1 CC genotype is associated with increased risk to develop schizophrenia, a typical endophenotype profile observed in the disease and modified neurotransmitter concentrations in brain regions associated with the disease.

Regulation of NHE3 by lysophosphatidic acid is mediated by phosphorylation of NHE3 by RSK2

Na(+)/H(+) exchange by Na(+)/H(+) exchanger 3 (NHE3) is a major route of sodium absorption in the intestine and kidney. We have shown previously that lysophosphatidic acid (LPA), a small phospholipid produced ubiquitously by all types of cells, stimulates NHE3 via LPA5 receptor. Stimulation of NHE3 activity by LPA involves LPA5 transactivating EGF receptor (EGFR) in the apical membrane. EGFR activates proline-rich tyrosine kinase 2 (Pyk2) and ERK, both of which are necessary for NHE3 regulation. However, Pyk2 and ERK are regulated by EGFR via independent pathways and appear to converge on an unidentified intermediate that ultimately targets NHE3. The p90 ribosomal S6 kinase (RSK) family of Ser/Thr protein kinases is a known effector of EGFR and ERK. Hence, we hypothesized that RSK may be the convergent effector of Pyk2 and ERK although it is not known whether Pyk2 regulates RSK. In this study, we show that Pyk2 is necessary for the maintenance of phosphoinositide-dependent kinase 1 (PDK1) autophosphorylation, and knockdown of Pyk2 or PDK1 mitigated LPA-induced phosphorylation of RSK and stimulation of NHE3 activity. Additionally, we show that RSK2, but not RSK1, is responsible for NHE3 regulation. RSK2 interacts with NHE3 at the apical membrane domain, where it phosphorylates NHE3. Alteration of S663 of NHE3 ablated LPA-induced phosphorylation of NHE3 and stimulation of the transport activity. Our study identifies RSK2 as a new kinase that regulates NHE3 activity by direct phosphorylation.

Myosin 5b loss of function leads to defects in polarized signaling: implication for microvillus inclusion disease pathogenesis and treatment

Microvillus inclusion disease (MVID) is an autosomal recessive condition resulting in intractable secretory diarrhea in newborns due to loss-of-function mutations in myosin Vb (Myo5b). Previous work suggested that the apical recycling endosomal (ARE) compartment is the primary location for phosphoinositide-dependent protein kinase 1 (PDK1) signaling. Because the ARE is disrupted in MVID, we tested the hypothesis that polarized signaling is affected by Myo5b dysfunction. Subcellular distribution of PDK1 was analyzed in human enterocytes from MVID/control patients by immunocytochemistry. Using Myo5b knockdown (kd) in Caco-2BBe cells, we studied phosphorylated kinases downstream of PDK1, electrophysiological parameters, and net water flux. PDK1 was aberrantly localized in human MVID enterocytes and Myo5b-deficient Caco-2BBe cells. Two PDK1 target kinases were differentially affected: phosphorylated atypical protein kinase C (aPKC) increased fivefold and phosohoprotein kinase B slightly decreased compared with control. PDK1 redistributed to a soluble (cytosolic) fraction and copurified with basolateral endosomes in Myo5b kd. Myo5b kd cells showed a decrease in net water absorption that could be reverted with PDK1 inhibitors. We conclude that, in addition to altered apical expression of ion transporters, depolarization of PDK1 in MVID enterocytes may lead to aberrant activation of downstream kinases such as aPKC. The findings in this work suggest that PDK1-dependent signaling may provide a therapeutic target for treating MVID.

AKT and GSK-3 are necessary for direct ezrin binding to NHE3 as part of a C-terminal stimulatory complex: role of a novel Ser-rich NHE3 C-terminal motif in NHE3 activity and trafficking

Basal activity of the BB Na(+)/H(+) exchanger NHE3 requires multiprotein complexes that form on its C terminus. One complex stimulates basal NHE3 activity and contains ezrin and phosphoinositides as major components; how it stimulates NHE3 activity is not known. This study tested the hypothesis that ezrin dynamically associates with this complex, which sets ezrin binding. NHE3 activity was reduced by an Akti. This effect was eliminated if ezrin binding to NHE3 was inhibited by a point mutant. Recombinant AKT phosphorylated NHE3 C terminus in the domain ezrin directly binds. This domain (amino acids 475-589) is predicted to be α-helical and contains a conserved cluster of three serines (Ser(515), Ser(522), and Ser(526)). Point mutations of two of these (S515A, S515D, or S526A) reduced basal NHE3 activity and surface expression and had no Akti inhibition. S526D had NHE3 activity equal to wild type with normal Akti inhibition. Ezrin binding to NHE3 was regulated by Akt, being eliminated by Akti. NHE3-S515A and -S526D did not bind ezrin; NHE3-S515D had reduced ezrin binding; NHE3-S526D bound ezrin normally. NHE3-Ser(526) is predicted to be a GSK-3 kinase phosphorylation site. A GSK-3 inhibitor reduced basal NHE3 activity as well as ezrin-NHE3 binding, and this effect was eliminated in NHE3-S526A and -S526D mutants. The conclusions were: 1) NHE3 basal activity is regulated by a signaling complex that is controlled by sequential effects of two kinases, Akt and GSK-3, which act on a Ser cluster in the same NHE3 C-terminal domain that binds ezrin; and 2) these kinases regulate the dynamic association of ezrin with NHE3 to affect basal NHE3 activity.

OSR1-sensitive small intestinal Na+ transport

The oxidative stress responsive kinase 1 (OSR1) contributes to WNK (with no K)-dependent regulation of renal tubular salt transport, renal salt excretion, and blood pressure. Little is known, however, about a role of OSR1 in the regulation of intestinal salt transport. The present study thus explored whether OSR1 is expressed in intestinal tissue and whether small intestinal Na(+)/H(+) exchanger (NHE), small intestinal Na(+)-glucose cotransport (SGLT1), and/or colonic epithelium Na(+) channel (ENaC) differ between knockin mice carrying one allele of WNK-resistant OSR1 (osr1(+/KI)) and wild-type mice (osr1(+/+)). OSR1 protein abundance was determined by Western blotting, cytosolic pH from BCECF fluorescence, NHE activity from Na(+)-dependent realkalinization following an ammonium pulse, SGLT1 activity from glucose-induced current, and colonic ENaC activity from amiloride-sensitive transepithelial current in Ussing chamber experiments. As a result, OSR1 protein was expressed in small intestine of both osr1(+/KI) mice and osr1(+/+) mice. Daily fecal Na(+), K(+), and H(2)O excretion and jejunal SGLT1 activity were lower, whereas small intestinal NHE activity and colonic ENaC activity were higher in osr1(+/KI) mice than in osr1(+/+) mice. NHE3 inhibitor S-3226 significantly reduced NHE activity in both genotypes but did not abrogate the difference between the genotypes. Plasma osmolarity, serum antidiuretic hormone, plasma aldosterone, and plasma corticosterone concentrations were similar in both genotypes. Small intestinal NHE3 and colonic α-ENaC protein abundance were not significantly different between genotypes, but colonic phospho-β-ENaC (ser633) was significantly higher in osr1(+/KI) mice. In conclusion, OSR1 is expressed in intestine and partial WNK insensitivity of OSR1 increases intestinal NHE activity and colonic ENaC activity.

PDK1 in apical signaling endosomes participates in the rescue of the polarity complex atypical PKC by intermediate filaments in intestinal epithelia

The polarity complex atypical PKC (aPKC) is rescued from degradation on intermediate filaments by Hsp70 chaperoning. The results indicate that PDK1 participates in the rescue mechanism and is localized to apical endosomes. Inhibition of dynamin-dependent endocytosis greatly decreases the steady-state levels of aPKC and Akt in their active conformation.

Phosphorylation of the activation domain of protein kinase C (PKC) isoforms is essential to start a conformational change that results in an active catalytic domain. This activation is necessary not only for newly synthesized molecules, but also for kinase molecules that become dephosphorylated and need to be refolded and rephosphorylated. This “rescue” mechanism is responsible for the maintenance of the steady-state levels of atypical PKC (aPKC [PKCι/λ and ζ]) and is blocked in inflammation. Although there is consensus that phosphoinositide-dependent protein kinase 1 (PDK1) is the activating kinase for newly synthesized molecules, it is unclear what kinase performs that function during the rescue and where the rescue takes place. To identify the activating kinase during the rescue mechanism, we inhibited protein synthesis and analyzed the stability of the remaining aPKC pool. PDK1 knockdown and two different PDK1 inhibitors—BX-912 and a specific pseudosubstrate peptide—destabilized PKCι. PDK1 coimmunoprecipitated with PKCι in cells without protein synthesis, confirming that the interaction is direct. In addition, we showed that PDK1 aids the rescue of aPKC in in vitro rephosphorylation assays using immunodepletion and rescue with recombinant protein. Surprisingly, we found that in Caco-2 epithelial cells and intestinal crypt enterocytes PDK1 distributes to an apical membrane compartment comprising plasma membrane and apical endosomes, which, in turn, are in close contact with intermediate filaments. PDK1 comigrated with the Rab11 compartment and, to some extent, with the transferrin compartment in sucrose gradients. PDK1, pT555-aPKC, and pAkt were dependent on dynamin activity. These results highlight a novel signaling function of apical endosomes in polarized cells.

PDK1: the major transducer of PI 3-kinase actions

Most of the cellular responses to phosphatidylinositol 3-kinase activation and phosphatidylinositol 3,4,5-trisphosphate production are mediated by the activation of a group of AGC kinases comprising PKB, S6K, RSK, SGK and PKC isoforms, which play essential roles in regulating physiological processes related to cell growth, proliferation, survival and metabolism. All these growth-factor-stimulated AGC kinases possess a common upstream activator, namely PDK1, a master kinase, which, being constitutively active, is still able to phosphorylate and activate its AGC substrates in response to rises in the levels of the PtdIns(3,4,5)P(3) second messenger. In this chapter, the biochemical, structural and genetic data on the mechanism of action and physiological roles of PDK1 are reviewed, and its potential as a pharmaceutical target for the design of drugs therapeutically beneficial to treat human disease such us diabetes and cancer is discussed.

Stimulation of electrogenic intestinal dipeptide transport by the glucocorticoid dexamethasone

According to recent in vitro experiments, the peptide transporter PepT2 is stimulated by the serum- and glucocorticoid-inducible kinase SGK1. The present study explored the contribution of SGK1 to the regulation of electrogenic intestinal peptide transport. Intestinal PepT1 was expressed in Xenopus oocytes, and peptide transport was determined by dual electrode voltage clamping. Peptide transport in intestinal segments was determined utilizing Ussing chamber. Cytosolic pH (pH( i )) was determined by BCECF fluorescence and Na(+)/H(+) exchanger activity was estimated from Na(+)-dependent pH recovery (pH ( i )) following an ammonium pulse. In PepT1-expressing Xenopus oocytes, coexpression of SGK1 enhanced electrogenic peptide transport. Intestinal transport and pH( i ) of untreated mice were similar in SGK1 knockout mice (sgk1 ( -/- )) and their wild-type littermates (sgk1 ( +/+ )). Glucocorticoid treatment (4 days 10 microg/g body weight (bw)/day dexamethasone) increased peptide transport in sgk1 ( +/+ ) but not in sgk1 (-/-) mice. Irrespective of dexamethasone treatment, luminal peptide (5 mM glycyl-glycine) led to a similar early decrease of pH( i ) in sgk1 (-/-) and sgk1 (+/+) mice, but to a more profound and sustained decline of pH( i ) in sgk1 (-/-) than in sgk1 ( +/+ ) mice. In the presence and absence of glycyl-glycine, pH ( i ) was significantly enhanced by dexamethasone treatment in sgk1 ( +/+ ) mice, an effect significantly blunted in sgk1 ( -/- ) mice. During sustained exposure to glycyl-glycine, pH ( i ) was significantly larger in sgk1 (+/+) mice than in sgk1 (-/-) mice, irrespective of dexamethasone treatment. In conclusion, basal intestinal peptide transport does not require stimulation by SGK1. Glucocorticoid treatment stimulates both Na(+)/H(+) exchanger activity and peptide transport, effects partially dependent on SGK1. Moreover, chronic exposure to glycyl-glycine stimulates Na(+)/H(+) exchanger activity, an effect again involving SGK1.

IGF signaling defects as causes of growth failure and IUGR

A substantial portion of children born small for gestational age (SGA) fail to catch up height, despite normal or even elevated insulin-like growth factor (IGF1) serum levels. In most cases, the etiology of the apparent IGF1 resistance is regarded as idiopathic. However, the recent identification of human IGF1 and IGF1 receptor (IGF1R) mutations, as well as information obtained from transgenic animals, points to a strong genetic component being of pivotal importance in the development of growth retardation. These findings direct attention to molecules downstream of the IGF1R, which have both growth-promoting and, to a lesser extent, metabolic functions. Therefore, defects in these molecules are likely to participate in the etiology of human SGA.

Reduced Ca2+ entry and suicidal death of erythrocytes in PDK1 hypomorphic mice

The phosphoinositide-dependent kinase PDK1 is a key element in the phosphoinositol-3-kinase signalling pathway, which is involved in the regulation of ion channels, transporters, cell volume and cell survival. Eryptosis, the suicidal death of erythrocytes, is characterized by decrease in cell volume, cell membrane blebbing and phospholipids scrambling with phosphatidylserine exposure at the cell surface. Oxidative stress, osmotic shock or Cl- removal trigger eryptosis by activation of Ca2+-permeable cation channels and subsequent increase in cytosolic Ca2+ activity. To explore the impact of PDK1 for erythrocyte survival, eryptosis was analysed in hypomorphic mice (pdk1hm) expressing only some 25% of PDK1 and in their wild-type littermates (pdk1wt). Cell volume was estimated from forward scatter and phosphatidylserine exposure from annexin-V binding in fluorescence activated cell sorter analysis. Forward scatter was smaller in pdk1hm than in pdk1wt erythrocytes. Oxidative stress (100 microM tert-butylhydroperoxide), osmotic shock (+300 mM sucrose) and Cl- removal (replacement of Cl- with gluconate) all decreased forward scatter and increased the percentage of annexin-V-binding erythrocytes from both pdk1hm and pdk1wt mice. After treatment, the forward scatter was similar in both genotypes, but the percentage of annexin-V binding was significantly smaller in pdk1hm than in pdk1wt erythrocytes. According to Fluo-3 fluorescence, cytosolic Ca2+ activity was significantly smaller in pdk1hm than in pdk1wt erythrocytes. Treatment with Ca2+-ionophore ionomycin (1 microM) was followed by an increase in annexin-V binding to similar levels in pdk1hm and pdk1wt erythrocytes. The experiments reveal that PDK1 deficiency is associated with decreased Ca2+ entry into erythrocytes and thus with blunted eryptotic effects of oxidative stress, osmotic shock and Cl- removal.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.