PDK1 recruitment to the SHPS-1 signaling complex enhances insulin-like growth factor-i-stimulated AKT activation and vascular smooth muscle cell survival

Downregulation of CD147 induces malignant melanoma cell apoptosis via the regulation of IGFBP2 expression

Cluster of differentiation (CD)147, as a transmembrane glycoprotein, is highly expressed in a variety of tumors. Accumulating evidence has demonstrated that CD147 serves critical roles in tumor cell death and survival; however, the underlying mechanism requires further investigation. In the present study, it was revealed that CD147 knockdown significantly increased melanoma cell apoptosis. In addition, downregulation of CD147 reversed the malignant phenotype of melanoma, as demonstrated by the induction of tumor cell apoptosis in a xenograft mouse model. In addition, a human apoptosis antibody array was performed and 9 differentially expressed apoptosis-related proteins associated with CD147 were identified, including insulin-like growth factor-binding protein 2 (IGFBP2). Additionally, CD147 knockdown was observed to significantly decreased IGFBP2 expression at the mRNA and protein levels in melanoma cells. Providing that IGFBP2 is a downstream molecule in the phosphatase and tensin homolog (PTEN)/phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) signaling pathway, the effects of CD147 on this particular pathway were investigated. Interestingly, the expression of phosphorylated (p)-AKT and p‑mechanistic target of rapamycin was attenuated, whereas PTEN was markedly upregulated in CD147-underexpressing melanoma cells. Furthermore, application of a PI3K‑specific inhibitor also decreased IGFBP2 expression. Importantly, IGFBP2 was highly expressed in clinical tissues of melanoma compared with the control group, and its expression exhibited a positive association with CD147. The present study revealed that CD147 served a critical role in mediating the apoptosis of melanoma cells via IGFBP2 and the PTEN/PI3K/AKT signaling pathway. IGFBP2 and CD147 were observed to be overexpressed in clinical melanoma tissues; IGFBP2 was shown to be positively associated with CD147 expression, suggesting that CD147 may be considered as a potential therapeutic target for chemotherapy or prevention for in melanoma.

PRR14 is a novel activator of the PI3K pathway promoting lung carcinogenesis

Chromosomal focal amplifications often cause an increase in gene copy number, contributing to the pathogenesis of cancer. PRR14 overexpression is associated with recurrent locus amplification in lung cancer, and it correlates with a poor prognosis. We show that increased PRR14 expression promoted and reduced PRR14 expression impeded lung cancer cell proliferation. Interestingly, PRR14 cells were markedly enlarged in size and exhibited an elevated activity of the PI3-kinase/Akt/mTOR pathway, which was associated with a heightened sensitivity to the inhibitors of PI3K and mammalian target of rapamycin (mTOR). Biochemical analysis revealed that PRR14, as a proline-rich protein, binds to the Src homology 3 (SH3) domains of GRB2 resulting in PI3K activation. Significantly, two cancer patient-derived PRR14 mutants displayed considerably augmented GRB2-binding and an enhanced ability of promoting cell proliferation. Together with the in vivo data demonstrating a strong tumor-promoting activity of PRR14 and the mutants, our work uncovered this proline-rich protein as a novel activator of the PI3K pathway that promoted tumorigenesis in lung cancer.

Non-canonical Activation of Akt in Serum-Stimulated Fibroblasts, Revealed by Comparative Modeling of Pathway Dynamics

The dynamic behaviors of signaling pathways can provide clues to pathway mechanisms. In cancer cells, excessive phosphorylation and activation of the Akt pathway is responsible for cell survival advantages. In normal cells, serum stimulation causes brief peaks of extremely high Akt phosphorylation before reaching a moderate steady-state. Previous modeling assumed this peak and decline behavior (i.e., “overshoot”) was due to receptor internalization. In this work, we modeled the dynamics of the overshoot as a tool for gaining insight into Akt pathway function. We built an ordinary differential equation (ODE) model describing pathway activation immediately upstream of Akt phosphorylation at Thr³⁰⁸ (Aktp³⁰⁸). The model was fit to experimental measurements of Aktp³⁰⁸, total Akt, and phosphatidylinositol (3,4,5)-trisphosphate (PIP3), from mouse embryonic fibroblasts with serum stimulation. The canonical Akt activation model (the null hypothesis) was unable to recapitulate the observed delay between the peak of PIP3 (at 2 minutes), and the peak of Aktp³⁰⁸ (at 30–60 minutes). From this we conclude that the peak and decline behavior of Aktp³⁰⁸ is not caused by PIP3 dynamics. Models for alternative hypotheses were constructed by allowing an arbitrary dynamic curve to perturb each of 5 steps of the pathway. All 5 of the alternative models could reproduce the observed delay. To distinguish among the alternatives, simulations suggested which species and timepoints would show strong differences. Time-series experiments with membrane fractionation and PI3K inhibition were performed, and incompatible hypotheses were excluded. We conclude that the peak and decline behavior of Aktp³⁰⁸ is caused by a non-canonical effect that retains Akt at the membrane, and not by receptor internalization. Furthermore, we provide a novel spline-based method for simulating the network implications of an unknown effect, and we demonstrate a process of hypothesis management for guiding efficient experiments.

Influential pathways of cell signalling (such as PI3K/Akt) are routinely communicated using simple textbook-like diagrams that show only the most widely-accepted steps of the pathway. At the same time, there are countless other molecular influences relevant to each pathway, documented in the published literature, and more are being published every week. It should perhaps come as little surprise that during a routine observation of the Akt activation pathway, a simulation of the canonical model was mathematically incompatible with our observed dynamics. To progress beyond the standard, simplified model without testing an unreasonable number of molecular candidates individually, we employed computational modeling to analyze the dynamics of pathway activation. We asked when and where a non-canonical deviation could occur, relative to the canonical pathway. We used the timing of downstream activation to solve for the possible times of upstream initiation. By categorizing unknown effects by their dynamics, we were able to prune away implausible hypotheses using an efficient number of in vitro experiments. At the end we had a single plausible explanation for non-canonical Akt activation in our cells, and we confirmed experimentally that Akt is retained at the membrane after PIP3 is no longer present.

Hyperglycemia stimulates p62/PKCζ interaction, which mediates NF-κB activation, increased Nox4 expression, and inflammatory cytokine activation in vascular smooth muscle

Hyperglycemia leads to vascular smooth muscle cell (VSMC) dedifferentiation and enhances responses to IGF-I. Prior studies showed that hyperglycemia stimulated NADPH oxidase 4 (Nox4) synthesis, and IGF-I facilitated its recruitment to a signaling complex where it oxidized src, leading to AKT and MAPK activation. To determine the mechanism that led to these changes, we analyzed the roles of p62 (sequestrosome1) and PKCζ. Hyperglycemia induced a 4.9 ± 1.0-fold increase in p62/PKCζ association, and disruption of PKCζ/p62 using a peptide inhibitor or p62 knockdown reduced PKCζ activation (78 ± 6%). 3-Phosphoinoside-dependent protein kinase 1 was also recruited to the p62 complex and directly phosphorylated PKCζ, leading to its activation (3.1 ± 0.4-fold). Subsequently, activated PKCζ phosphorylated p65 rel, which led to increased Nox4 synthesis. Studies in diabetic mice confirmed these findings (6.0 ± 0.4-fold increase in p62/PKCζ) and their disruption of attenuated Nox4 synthesis (76 ± 9% reduction). PKCζ/p62 activation stimulated inflammatory cytokine production and enhanced IGF-I-stimulated VSMC proliferation. These results define the molecular mechanism by which PKCζ is activated in response to hyperglycemia and suggest that this could be a mechanism by which other stimuli such as cytokines or metabolic stress function to stimulate NF-κB activation, thereby altering VSMC sensitivity to IGF-I.

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.

Tanshinone IIA Prevents Rat Basilar Artery Smooth Muscle Cells Proliferation by Inactivation of PDK1 During the Development of Hypertension

Basilar vascular smooth muscle cells (BASMCs) hyperplasia is a prominent feature of cerebrovascular remodeling and stroke during the development of hypertension. Tanshinone IIA (Tan) has been reported to exhibit a protective effect against the pathological features of hypertension. Previous studies have shown that phosphoinostitide-3 kinase (PI3K)/3'-phosphoinostitide dependent kinase (PDK1)/AKT pathway is involved in the regulation of proliferation of various cell types. Therefore, there may be a crosstalk between Tan antihypertension processes and PI3K/PDK1/AKT proliferative effect in BASMCs. To test this hypothesis, we used a 2-kidney, 2-clip hypertension model to examine the effect of Tan on PI3K/PDK1/AKT pathway by cellular, molecular, and biochemical approaches. Our results revealed that the abundance of PDK1 in plasma was paralleled with an increase in blood pressure and the cross-sectional area of basilar artery in hypertensive rats. Tan decreased blood pressure and hypertension-induced PDK1 phosphorylation but produced no effect on the phosphorylation of PI3K. Moreover, Tan attenuated endothelin 1 induced the activation of PDK1/AKT pathway in rat BASMCs. Tan could inhibit cell cycle transition by regulating the expression of cyclin D1 and p27, in turn, prevent proliferation of BASMCs. Our study provides a novel mechanism by which Tan prevents cerebrovascular cell proliferation during hypertension, and thus Tan may be a potential therapeutic agent for cerebrovascular remodeling and stroke.

Activated α2-macroglobulin binding to cell surface GRP78 induces T-loop phosphorylation of Akt1 by PDK1 in association with Raptor

PDK1 phosphorylates multiple substrates including Akt by PIP3-dependent mechanisms. In this report we provide evidence that in prostate cancer cells stimulated with activated α2-macroglobulin (α2M*) PDK1 phosphorylates Akt in the T-loop at Thr(308) by using Raptor in the mTORC1 complex as a scaffold protein. First we demonstrate that PDK1, Raptor, and mTOR co-immunoprecipitate. Silencing the expression, not only of PDK1, but also Raptor by RNAi nearly abolished Akt phosphorylation at Akt(Thr308) in Raptor-immunoprecipitates of α2M*-stimulated prostate cancer cells. Immunodepleting Raptor or PDK from cell lysates of cells treated with α2M* drastically reduced Akt phosphorylation at Thr(308), which was recovered by adding the supernatant of Raptor- or PDK1-depleted cell lysates, respectively. Studies of insulin binding to its receptor on prostate cancer cells yielded similar results. We thus demonstrate that phosphorylating the T-loop Akt residue Thr(308) by PDK1 requires Raptor of the mTORC1 complex as a platform or scaffold protein.

Receptor tyrosine kinase (RTK) signalling in the control of neural stem and progenitor cell (NSPC) development

Important developmental responses are elicited in neural stem and progenitor cells (NSPC) by activation of the receptor tyrosine kinases (RTK), including the fibroblast growth factor receptors, epidermal growth factor receptor, platelet-derived growth factor receptors and insulin-like growth factor receptor (IGF1R). Signalling through these RTK is necessary and sufficient for driving a number of developmental processes in the central nervous system. Within each of the four RTK families discussed here, receptors are activated by sets of ligands that do not cross-activate receptors of the other three families, and therefore, their activation can be independently regulated by ligand availability. These RTK pathways converge on a conserved core of signalling molecules, but differences between the receptors in utilisation of signalling molecules and molecular adaptors for intracellular signal propagation become increasingly apparent. Intracellular inhibitors of RTK signalling are widely involved in the regulation of developmental signalling in NSPC and often determine developmental outcomes of RTK activation. In addition, cellular responses of NSPC to the activation of a given RTK may be significantly modulated by signal strength. Cellular propensity to respond also plays a role in developmental outcomes of RTK signalling. In combination, these mechanisms regulate the balance between NSPC maintenance and differentiation during development and in adulthood. Attribution of particular developmental responses of NSPC to specific pathways of RTK signalling becomes increasingly elucidated. Co-activation of several RTK in developing NSPC is common, and analysis of co-operation between their signalling pathways may advance knowledge of RTK role in NSPC development.

SHPS-1 deficiency induces robust neuroprotection against experimental stroke by attenuating oxidative stress

Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1), also known as Signal-regulatory protein alpha (SIRPα) or SIRPA is a transmembrane protein that is predominantly expressed in neurons, dendritic cells, and macrophages. This study was conducted to investigate the role of SHPS-1 in the oxidative stress and brain damage induced by acute focal cerebral ischemia. Wild-type (WT) and SHPS-1 mutant (MT) mice were subjected to middle cerebral artery occlusion (60 min) followed by reperfusion. SHPS-1 MT mice had significantly reduced infarct volumes and improved neurological function after brain ischemia. In addition, neural injury and oxidative stress were inhibited in SHPS-1 MT mice. The mRNA and protein levels of the antioxidant genes nuclear factor-E2-related factor 2 (Nrf2) and heme oxygenase 1 were up-regulated in SHPS-1 MT mice. The SHPS-1 mutation suppressed the phosphorylation of SHP-1 and SHP-2 and increased the phosphorylation of Akt and GSK3β. These results provide the first demonstration that SHPS-1 plays an important role in the oxidative stress and brain injury induced by acute cerebral ischemia. The activation of Akt signaling and the up-regulation of Nrf2 and heme oxygenase 1 likely account for the protective effects that were observed in the SHPS-1 MT mice.

Induced PDK1 kinase activity suppresses apoptosis in intestinal epithelial cells by activating Akt signaling following polyamine depletion

Apoptosis plays a critical role in the maintenance of gut mucosal homeostasis and is highly regulated by numerous factors including polyamines. Decreasing cellular polyamines promotes the resistance of intestinal epithelial cells (IECs) to apoptosis by increasing Akt kinase activity, but the exact mechanisms by which polyamine depletion activates Akt remain unknown. 3-phosphoinositide-dependent protein kinase-1 (PDK1), functions as a downstream of phosphatidylinositol-3 kinase (PI3K) and upstream of Akt and serves as a major regulator of Akt activity. The current study determined if polyamines regulate Akt activity by altering PDK1. Studies were conducted in IEC-6 cells, derived from rat small intestinal crypts. Depletion of cellular polyamines induced PDK1 phosphorylation and increased its kinase activity, which were prevented by exogenous polyamine putrescine. Induced PDK1 activation following polyamine depletion was associated with an increase in phosphorylated Akt (pAkt) and Akt kinase activity. In contrast, polyamine depletion did not alter levels of total PDK1 and Akt proteins. PDK1 silencing in polyamine-deficient cells not only prevented the induced Akt activation but also blocked the increased resistance to apoptosis. These results indicate that polyamine depletion enhanced Akt phosphorylation by increasing PDK1 kinase activity, thereby protecting IECs against apoptosis.

Insulin-like growth factor-binding protein-2 is required for osteoclast differentiation

Global deletion of the Igfbp2 gene results in the suppression of bone turnover. To investigate the role of insulin-like growth factor-binding protein-2 (IGFBP-2) in regulating osteoclast differentiation, we cultured Igfbp2(-/-) bone marrow cells and found a reduction in the number of osteoclasts and impaired resorption. Addition of full-length IGFBP-2 restored osteoclast differentiation, fusion, and resorption. To determine the molecular domains of IGFBP-2 that were required for this effect to be manifest, Igfbp2(-/-) bone marrow cells were transfected with constructs in which the heparin-binding (HBD) or the IGF-binding domains of IGFBP-2 were mutated. We found that both domains were necessary for osteoclastogenesis because expression of the mutated forms of either domain failed to support the formation of functionally mature osteoclasts. To discern the mechanism by which IGFBP-2 regulates osteoclast formation, PTEN abundance and phosphorylation status as well as AKT responsiveness to IGF-I were analyzed. Igfbp2(-/-) cells had elevated levels of PTEN and phospho-PTEN compared with controls. Expression of wild-type IGFBP-2 reduced the level of PTEN to that of wild-type cells. Cells expressing the IGF-binding mutant showed suppression of PTEN and phospho-PTEN equivalent to the wild-type protein, whereas those expressing the IGFBP-2 HBD mutant showed no PTEN suppression. When the ability of IGF-I to stimulate AKT activation, measured by Thr³⁰⁸ and Ser⁴⁷³ phosphorylation, was analyzed, stimulation of Ser⁴⁷³ in response to IGF-I in preosteoclasts required the presence of intact IGFBP-2. This effect was duplicated by the addition of a CK2 inhibitor that prevents the phosphorylation of PTEN. In contrast, in fully differentiated osteoclasts, stimulation of Thr³⁰⁸ phosphorylation required the presence of intact IGFBP-2. We conclude that IGFBP-2 is an important regulator of osteoclastogenesis and that both the heparin- and the IGF-binding domains of IGFBP-2 are essential for the formation of fully differentiated and functional osteoclasts.

Suppression of AMPK activation via S485 phosphorylation by IGF-I during hyperglycemia is mediated by AKT activation in vascular smooth muscle cells

As a metabolic sensor, the serine/threonine protein kinase AMP-activated protein kinase (AMPK) promotes the adaptation of cells to signals arising from nutrients, hormones, and growth factors. The ability of IGF-I to stimulate protein synthesis is suppressed by AMPK, therefore, these studies were undertaken to determine whether IGF-I modulates AMPK activity. IGF-I dose-dependently suppressed phosphorylation of AMPK T172, and it stimulated AMPK S485 phosphorylation in vascular smooth muscle cells (VSMC). To determine whether stimulation of AMPK S485 phosphorylation was mediating this response, VSMC were transduced with a mutant AMPKα (AMPK S485A). Expression of this altered form inhibited the ability of IGF-I to suppress AMPK T172 activation, which resulted in inhibition of IGF-I-stimulated phosphorylation of P70S6 kinase. In contrast, expression of an AMPK S485D mutant resulted in constitutive suppression of AMPK activity and was associated with increased IGF-I-stimulated P70S6K phosphorylation and protein synthesis. The addition of a specific AKT inhibitor or expression of an AKT1 short hairpin RNA inhibited AMPK S485 phosphorylation, and it attenuated the IGF-I-induced decrease in AMPK T172 phosphorylation. Exposure to high glucose concentrations suppressed AMPK activity and stimulated S485 phosphorylation, and IGF-I stimulated a further increase in S485 phosphorylation and AMPK T172 suppression. We conclude that AMPK S485 phosphorylation negatively regulates AMPK activity by modulating the T172 phosphorylation response to high glucose and IGF-I. IGF-I stimulates S485 phosphorylation through AKT1. The results suggest that AMPK plays an inhibitory role in modulating IGF-I-stimulated protein synthesis and that IGF-I must down-regulate AMPK activity to induce an optimal anabolic response.