EGFR plays a pivotal role in the regulation of polyamine-dependent apoptosis in intestinal epithelial cells

Biochemical Basis of Skin Disease Mal de Meleda: SLURP-1 Mutants Differently Affect Keratinocyte Proliferation and Apoptosis

Mal de Meleda is an autosomal recessive palmoplantar keratoderma associated with mutations in a gene encoding SLURP-1. SLURP-1 controls growth, differentiation, and apoptosis of keratinocytes by interaction with α7-type nicotinic acetylcholine receptors. SLURP-1 has a three-finger structure with a β-structural core (head) and three prolonged loops (fingers). To determine the role of SLURP-1 mutations, we produced 22 mutant variants of the protein, including those involved in Mal de Meleda pathogenesis. All mutants except R71H, R71P, T52A, R96P, and L98P were produced in the folded form. SLURP-1 reduces the growth of Het-1A keratinocytes; thus, we studied the influence of the mutations on its antiproliferative activity. Mutations in loops I and III led to the protein inactivation, whereas most mutations in loop II increased SLURP-1 antiproliferative activity. Alanine substitutions of R96 and L98 residues located in the protein head resulted in the appearance of additional pro-apoptotic activity. Our results agree with the diversity of Mal de Meleda phenotypes. Using obtained functional data, the SLURP-1/α7 type nicotinic acetylcholine receptor complex was modeled in silico. Our study provides functional and structural information about the role of the SLURP-1 mutations in Mal de Meleda pathogenesis and predicts SLURP-1 variants, which could drive the disease.

Dsg2 via Src-mediated transactivation shapes EGFR signaling towards cell adhesion

Rapidly renewing epithelial tissues such as the intestinal epithelium require precise tuning of intercellular adhesion and proliferation to preserve barrier integrity. Here, we provide evidence that desmoglein 2 (Dsg2), an adhesion molecule of desmosomes, controls cell adhesion and proliferation via epidermal growth factor receptor (EGFR) signaling. Dsg2 is required for EGFR localization at intercellular junctions as well as for Src-mediated EGFR activation. Src binds to EGFR and is required for localization of EGFR and Dsg2 to cell-cell contacts. EGFR is critical for cell adhesion and barrier recovery. In line with this, Dsg2-deficient enterocytes display impaired barrier properties and increased cell proliferation. Mechanistically, Dsg2 directly interacts with EGFR and undergoes heterotypic-binding events on the surface of living enterocytes via its extracellular domain as revealed by atomic force microscopy. Thus, our study reveals a new mechanism by which Dsg2 via Src shapes EGFR function towards cell adhesion.

Transactivation of Epidermal Growth Factor Receptor by G Protein-Coupled Receptors: Recent Progress, Challenges and Future Research

Both G protein-coupled receptors (GPCRs) and receptor-tyrosine kinases (RTKs) regulate large signaling networks, control multiple cell functions and are implicated in many diseases including various cancers. Both of them are also the top therapeutic targets for disease treatment. The discovery of the cross-talk between GPCRs and RTKs connects these two vast signaling networks and complicates the already complicated signaling networks that regulate cell signaling and function. In this review, we focus on the transactivation of epidermal growth factor receptor (EGFR), a subfamily of RTKs, by GPCRs. Since the first report of EGFR transactivation by GPCR, significant progress has been made including the elucidation of the mechanisms underlying the transactivation. Here, we first provide a basic picture for GPCR, EGFR and EGFR transactivation by GPCR. We then discuss the progress made in the last five years and finally provided our view of the future challenge and future researches needed to overcome these challenges.

Atractylodes macrocephala Koidz stimulates intestinal epithelial cell migration through a polyamine dependent mechanism

ETHNOPHARMACOLOGICAL RELEVANCE

Atractylodes macrocephala Koidz (AMK), a valuable traditional Chinese herbal medicine, has been widely used in clinical practice for treating patients with disorders of the digestive system. AMK has shown noteworthy promoting effect on improving gastrointestinal function and immunity, which might represent a promising candidate for the treatment of intestinal mucosa injury. The aim of this study was to investigate the efficacy of AMK on intestinal mucosal restitution and the underlying mechanisms via intestinal epithelial (IEC-6) cell migration model.

MATERIALS AND METHODS

A cell migration model of IEC-6 cells was induced by a single-edge razor blade along the diameter of the cell layers in six-well polystyrene plates. After wounding, the cells were grown in control cultures and in cultures containing spermidine (5μM, SPD, reference drug), alpha-difluoromethylornithine (2.5mM, DFMO, polyamine inhibitor), AMK (50, 100, and 200mg/L), DFMO plus SPD and DFMO plus AMK for 12h. The polyamines content was detected by high-performance liquid chromatography (HPLC) with pre-column derivatization. The Rho mRNAs expression levels were assessed by Q-RT-PCR. The Rho and non-muscle myosin II proteins expression levels were analyzed by Western blot. The formation and distribution of non-muscle myosin II stress fibers were monitored with immunostaining techniques using specific antibodies and observed by confocal microscopy. Cell migration assay was carried out using inverted microscope and the Image-Pro Plus software. All of these indexes were used to evaluate the effectiveness of AMK.

RESULTS

(1) Treatment with AMK caused significant increases in cellular polyamines content and Rho mRNAs and proteins expression levels, as compared to control group. Furthermore, AMK exposure increased non-muscle myosin II protein expression levels and formation of non-muscle myosin II stress fibers, and resulted in an acceleration of cell migration in IEC-6 cells. (2) Depletion of cellular polyamines by DFMO resulted in a decrease of cellular polyamines levels, Rho mRNAs and proteins expression, non-muscle myosin II protein formation and distribution, thereby inhibiting IEC-6 cell migration. AMK not only reversed the inhibitory effects of DFMO on the polyamines content, Rho mRNAs and proteins expression, non-muscle myosin II protein formation and distribution, but also restored cell migration to control levels.

CONCLUSIONS

The results obtained from this study revealed that AMK significantly stimulates the migration of IEC-6 cells through a polyamine dependent mechanism, which could accelerate the healing of intestinal injury. These findings suggest the potential value of AMK in curing intestinal diseases characterized by injury and ineffective repair of the intestinal mucosa in clinical practice.

Cellular functions regulated by phosphorylation of EGFR on Tyr845

The Src gene product (Src) and the epidermal growth factor receptor (EGFR) are prototypes of oncogene products and function primarily as a cytoplasmic non-receptor tyrosine kinase and a transmembrane receptor tyrosine kinase, respectively. The identification of Src and EGFR, and the subsequent extensive investigations of these proteins have long provided cutting edge research in cancer and other molecular and cellular biological studies. In 1995, we reported that the human epidermoid carcinoma cells, A431, contain a small fraction of Src and EGFR in which these two kinase were in physical association with each other, and that Src phosphorylates EGFR on tyrosine 845 (Y845) in the Src-EGFR complex. Y845 of EGFR is located in the activation segment of the kinase domain, where many protein kinases contain kinase-activating autophosphorylation sites (e.g., cAMP-dependent protein kinase, Src family kinases, transmembrane receptor type tyrosine kinases) or trans-phosphorylation sites (e.g., cyclin-dependent protein kinase, mitogen-activated protein kinase, Akt protein kinase). A number of studies have demonstrated that Y845 phosphorylation serves an important role in cancer as well as normal cells. Here we compile the experimental facts involving Src phosphorylation of EGFR on Y845, by which cell proliferation, cell cycle control, mitochondrial regulation of cell metabolism, gamete activation and other cellular functions are regulated. We also discuss the physiological relevance, as well as structural insights of the Y845 phosphorylation.

Spermine, a molecular switch regulating EGFR, integrin β3, Src, and FAK scaffolding

Intracellular polyamine levels are highly regulated by the activity of ornithine decarboxylase (ODC), which catalyzes the first rate-limiting reaction in polyamine biosynthesis, producing putrescine, which is subsequently converted to spermidine and spermine. We have shown that polyamines regulate proliferation, migration, and apoptosis in intestinal epithelial cells. Polyamines regulate key signaling events at the level of the EGFR and Src. However, the precise mechanism of action of polyamines is unknown. In the present study, we demonstrate that ODC localizes in lamellipodia and in adhesion plaques during cell spreading. Spermine regulates EGF-induced migration by modulating the interaction of the EGFR with Src. The EGFR interacted with integrin β3, Src, and focal adhesion kinase (FAK). Active Src (pY418-Src) localized with FAK during spreading and migration. Spermine prevented EGF-induced binding of the EGFR with integrin β3, Src, and FAK. Activation of Src and FAK was necessary for EGF-induced migration in HEK293 cells. EGFR-mediated Src activation in live HEK293 cells using a FRET based Src reporter showed that polyamine depletion significantly increased Src kinase activity. In vitro binding studies showed that spermine directly binds Src, and preferentially interacts with the SH2 domain of Src. The physical interaction between Src and the EGFR was severely attenuated by spermine. Therefore, spermine acts as a molecular switch in regulating EGFR-Src coupling both physically and functionally. Upon activation of the EGFR, integrin β3, FAK and Src are recruited to EGFR leading to the trans-activation of both the EGFR and Src and to the Src-mediated phosphorylation of FAK. The activation of FAK induced Rho-GTPases and subsequently migration. This is the first study to define mechanistically how polyamines modulate Src function at the molecular level.

Mouse models to investigate the function of spermine

Many functions have been ascribed to polyamines, but there has been no clear identification of a unique role for spermine. The Gy mouse has a deletion of part of the X chromosome that includes the SMS gene encoding spermine synthase. Tissues from male Gy mice have no spermine but increased spermidine. They have multiple abnormalities including a tendency to sudden death, small size, circling behavior and other neurological symptoms, sterility and deafness. These changes are reversed by breeding with mice expressing a spermine synthase transgene. Detailed studies of hearing in Gy mice show that the absence of spermine synthase leads to loss of the endocochlear potential. Since this potential requires the cochlear lateral wall-specific Kir4.1 channel, regulation by spermine of transport via these channels appears to be an essential function. A similar spermine-related defect in the functioning of cardiac Kir channels could account for arrhythmias leading to sudden death. The effect of the absence of spermine on glutamate receptor ion channels in the brain may account for the neurological symptoms and could contribute to the lack of fertility and normal growth but more direct effects on gene expression are also possible. Advantages and limitations of the Gy model are discussed.

Mdm2 inhibition induces apoptosis in p53 deficient human colon cancer cells by activating p73- and E2F1-mediated expression of PUMA and Siva-1

Camptothecin (CPT) and Nutlin-3 caused apoptosis by increasing p53 protein and its activation in intestinal epithelial cells (IEC-6). We studied the effectiveness of these inducers on apoptosis in human colon cancer cells (Caco2) lacking p53 expression. CPT failed to activate caspase-3 and cause apoptosis in these cells. The absence of p53 expression, higher basal Bcl-xL and lower Bax proteins prevented CPT-induced apoptosis. However, the Mdm2 antagonist Nutlin-3 induced apoptosis in a dose dependent manner by activating caspases-9 and -3. Nutlin-3 prevented the activation of AKT via PTEN-mediated inhibition of the PI3K pathway. Nutlin-3 increased the phosphorylation of retinoblastoma protein causing E2F1 release leading to induction of Siva-1. Nutlin-3-mediated degradation of Mdm2 caused the accumulation of p73, which induced the expression of p53 up-regulated modulator of apoptosis (PUMA). E2F1 and p73 knockdown decreased the expression of Siva and PUMA, respectively and abolished Nutlin-3-induced caspase-3 activation. Cycloheximide (CHX) inhibited Nutlin-3-induced Siva, Noxa, and PUMA expression and inhibited apoptosis in IEC-6 and Caco2 cells. These results indicate that translation of mRNAs induced by Nutlin-3 is critical for apoptosis. In summary, apoptosis in Caco2 cells lacking functional p53 occurred following the disruption of Mdm2 binding with p73 and Rb leading to the expression of pro-apoptotic proteins, PUMA, Noxa, and Siva-1.

Inhibition of Mdm2 sensitizes human retinal pigment epithelial cells to apoptosis

PURPOSE

Because recent studies indicate that blocking the interaction between p53 and Mdm2 results in the nongenotoxic activation of p53, the authors sought to investigate whether the inhibition of p53-Mdm2 binding activates p53 and sensitizes human retinal epithelial cells to apoptosis.

METHODS

Apoptosis was evaluated by the activation of caspases and DNA fragmentation assays. The Mdm2 antagonist Nutlin-3 was used to dissociate p53 from Mdm2 and, thus, to increase p53 activity. Knockdown of p53 expression was accomplished by using p53 siRNA.

RESULTS

ARPE-19 and primary RPE cells expressed high levels of the antiapoptotic proteins Bcl-2 and Bcl-xL. Exposure of these cells to camptothecin (CPT) or TNF-α/ cycloheximide (CHX) failed to induce apoptosis. In contrast, treatment with the Mdm2 antagonist Nutlin-3 in the absence of CPT or TNF-α/CHX increased apoptosis. Activation of p53 in response to Nutlin-3 also increased levels of Noxa, p53-upregulated modulator of apoptosis (PUMA), and Siva-1, decreased expression of Bcl-2 and Bcl-xL, and simultaneously increased caspases-9 and -3 activities and DNA fragmentation. Knockdown of p53 decreased the basal expression of p21Cip1 and Bcl-2, inhibited the Nutlin-3-induced upregulation of Siva-1 and PUMA expression, and consequently inhibited caspase-3 activation.

CONCLUSIONS

These results indicate that the normally available pool of intracellular p53 is predominantly engaged in the regulation of cell cycle checkpoints by p21Cip1 and does not trigger apoptosis in response to DNA-damaging agents. However, the blockage of p53 binding to Mdm2 frees a pool of p53 that is sufficient, even in the absence of DNA-damaging agents, to increase the expression of proapoptotic targets and to override the resistance of RPE cells to apoptosis.

Polyamine-dependent activation of Rac1 is stimulated by focal adhesion-mediated Tiam1 activation

Integrin receptors cluster on the cell surface and bind to extra cellular matrix (ECM) proteins triggering the formation of focal contacts and the activation of various signal transduction pathways that affect the morphology, motility, gene expression and survival of adherent cells. Polyamine depletion prevents the increase in autophosphorylation of focal adhesion kinase (FAK) and Src during attachment. Rac activity also shows a steady decline, and its upstream guanine nucleotide exchange factor (GEF), Tiam1 also shows a reduction in total protein level when cells are depleted of polyamines. When Tiam1 and Rac1 interaction was inhibited by NSC-23766, there was not only a decrease in Rac1 activity as expected but also a decrease in FAK auto-phosphorylation. Inhibition of Src activity by PP2 also reduced FAK autophosphorylation, which implies that Src modulates FAK autophosphorylation. From the data obtained in this study we conclude that FAK and Src are rapidly activated upon fibronectin mediated signaling leading to Tiam1-mediated Rac1 activation and that intracellular polyamines influence the signaling strength by modulating interaction of Src with Tiam1 using focal adhesion kinase as a scaffolding site.

Post-transcriptional regulation of MEK-1 by polyamines through the RNA-binding protein HuR modulating intestinal epithelial apoptosis

MEK-1 [MAPK (mitogen-activated protein kinase) kinase-1] is an important signal transducing enzyme that is implicated in many aspects of cellular functions. In the present paper, we report that cellular polyamines regulate MEK-1 expression at the post-transcriptional level through the RNA-binding protein HuR (Hu-antigen R) in IECs (intestinal epithelial cells). Decreasing the levels of cellular polyamines by inhibiting ODC (ornithine decarboxylase) stabilized MEK-1 mRNA and promoted its translation through enhancement of the interaction between HuR and the 3'-untranslated region of MEK-1 mRNA, whereas increasing polyamine levels by ectopic ODC overexpression destabilized the MEK-1 transcript and repressed its translation by reducing the abundance of HuR-MEK-1 mRNA complex; neither intervention changed MEK-1 gene transcription via its promoter. HuR silencing rendered the MEK-1 mRNA unstable and inhibited its translation, thus preventing increases in MEK-1 mRNA and protein in polyamine-deficient cells. Conversely, HuR overexpression increased MEK-1 mRNA stability and promoted its translation. Inhibition of MEK-1 expression by MEK-1 silencing or HuR silencing prevented the increased resistance of polyamine-deficient cells to apoptosis. Moreover, HuR overexpression did not protect against apoptosis if MEK-1 expression was silenced. These results indicate that polyamines destabilize the MEK-1 mRNA and repress its translation by inhibiting the association between HuR and the MEK-1 transcript. Our findings indicate that MEK-1 is a key effector of the HuR-elicited anti-apoptotic programme in IECs.

Mammalian polyamine metabolism and function

Polyamines are ubiquitous small basic molecules that play multiple essential roles in mammalian physiology. Their cellular content is highly regulated and there is convincing evidence that altered metabolism is involvement in many disease states. Drugs altering polyamine levels may therefore have a variety of important targets. This review will summarize the current state of understanding of polyamine metabolism and function, the regulation of polyamine content, and heritable pathological conditions that may be derived from altered polyamine metabolism.

Elevated polyamines induce c-MYC overexpression by perturbing quadruplex-WC duplex equilibrium

The biological role of quadruplexes and polyamines has been independently associated with cancer. However, quadruplex-polyamine mediated transcriptional regulation remain unaddressed. Herein, using c-MYC quadruplex model, we have attempted to understand quadruplex–polyamine interaction and its role in transcriptional regulation. We initially employed biophysical approach involving CD, UV and FRET to understand the role of polyamines (spermidine and spermine) on conformation, stability, molecular recognition of quadruplex and to investigate the effect of polyamines on quadruplex–Watson Crick duplex transition. Our study demonstrates that polyamines affect the c-MYC quadruplex conformation, perturb its recognition properties and delays duplex formation. The relative free energy difference (ΔΔG°) between the duplex and quadruplex structure indicate that polyamines stabilize and favor c-MYC quadruplex over duplex. Further, we investigated the influence of polyamine mediated perturbation of this equilibrium on c-MYC expression. Our results suggest that polyamines induce structural transition of c-MYC quadruplex to a transcriptionally active motif with distinctive molecular recognition property, which drives c-MYC expression. These findings may allow exploiting quadruplex–polyamines interaction for developing antiproliferative strategies to combat aberrant gene expression.