PI3K and Erk MAPK mediate ErbB signaling in Xenopus gastrulation

ErbB signaling regulates cell adhesion and movements during Xenopus gastrulation, but the downstream pathways involved have not been elucidated. In this study, we show that phosphatidylinositol-3 kinase (PI3K) and Erk mitogen-activated protein kinase (MAPK) mediate ErbB signaling to regulate gastrulation. Both PI3K and MAPK function sequentially in mesoderm specification and movements, and ErbB signaling is important only for the late phase activation of these pathways to control cell behaviors. Activation of either PI3K or Erk MAPK rescues gastrulation defects in ErbB4 morphant embryos, and restores convergent extension in the trunk mesoderm as well as coherent cell migration in the head mesoderm. The two signals preferentially regulate different aspects of cell behaviors, with PI3K more efficient in rescuing cell adhesion and spreading and MAPK more effective in stimulating the formation of filopodia. PI3K and MAPK also weakly activate each other, and together they modulate gastrulation movements. Our results reveal that PI3K and Erk MAPK, which have previously been considered as mesodermal inducing signals, also act downstream of ErbB signaling to participate in regulation of gastrulation morphogenesis.     

Ras signals principally via Erk in G1 but cooperates with PI3K/Akt for Cyclin D induction and S-phase entry

Numerous studies exploring oncogenic Ras or manipulating physiological Ras signalling have established an irrefutable role for Ras as driver of cell cycle progression. Despite this wealth of information the precise signalling timeline and effectors engaged by Ras, particularly during G1, remain obscure as approaches for Ras inhibition are slow-acting and ill-suited for charting discrete Ras signalling episodes along the cell cycle. We have developed an approach based on the inducible recruitment of a Ras-GAP that enforces endogenous Ras inhibition within minutes. Applying this strategy to inhibit Ras stepwise in synchronous cell populations revealed that Ras signaling was required well into G1 for Cyclin D induction, pocket protein phosphorylation and S-phase entry, irrespective of whether cells emerged from quiescence or G2/M. Unexpectedly, Erk, and not PI3K/Akt or Ral was activated by Ras at mid-G1, albeit PI3K/Akt signalling was a necessary companion of Ras/Erk for sustaining cyclin-D levels and G1/S transition. Our findings chart mitogenic signaling by endogenous Ras during G1 and identify limited effector engagement restricted to Raf/MEK/Erk as a cogent distinction from oncogenic Ras signalling.     

Cannabinoids down-regulate PI3K/Akt and Erk signalling pathways and activate proapoptotic function of Bad protein

Cannabinoids were shown to induce apoptosis of glioma cells in vitro and tumor regression in vivo, but mechanisms of their antiproliferative action remain elusive. In the present studies, C6 cells were exposed to a synthetic cannabinoid, WIN 55,212-2, which produced down-regulation of the Akt and Erk signalling pathways prior to appearance of any sign of apoptosis. We hypothesized that cannabinoid-induced cell death may be mediated by a Bcl-2 family member--Bad, whose function is hampered by these kinases due to control of its phosphorylation state. Using Western blot analysis, we found that levels of phosphorylated Bad, but not total Bad protein, decreased under exposure to WIN 55,212-2. WIN 55,212-2 treatment further resulted in mitochondrial depolarization and activation of caspase cascade. Thus, we suggest that the increase of proapoptotic Bad activity is an important link between the inhibition of survival pathways and an onset of execution phase of cannabinoid-induced glioma cell death.     

PI3K activates negative and positive signals to regulate TRB3 expression in hepatic cells

TRB3 is a pseudokinase whose expression is regulated during stress response and changing of nutrient status. TRB3 negatively regulates Akt activation and noticeably, TRB3 expression is induced by insulin. Here, we sought to determine the dynamic relationship between TRB3 expression and Akt activation. We find that insulin induces TRB3 expression in cell type dependent manner such that in hepatic cells and adipocytes but not Beta cells and muscle cells. In Fao hepatoma cells, induction of TRB3 expression by insulin restrains Akt activation and renders Akt refractory to further activation. In addition, we have also analyzed the roles of PI3K and its downstream kinases Akt and atypical PKC in TRB3 expression. Induction of TRB3 expression by insulin requires PI3K. However, inactivation of Akt enhances TRB3 expression whereas inhibition of PKCzeta expression impairs TRB3 expression induced by insulin. Our data demonstrated that PI3K conveys both negative and positive signals to TRB3 expression. We suggest that insulin-induced TRB3 expression functions as an indicator how multiple insulin-induced signal transduction pathways are balanced.     

Gab1 transduces PI3K-mediated erythropoietin signals to the Erk pathway and regulates erythropoietin-dependent proliferation and survival of erythroid cells

In this study, we examined the biological functions of Gab1 in erythropoietin receptor (EPOR)-mediated signaling in vivo. Knockdown of Gab1 by the introduction of the Gab1 siRNA expression vector into F-36P human erythroleukemia (F-36P-Gab1-siRNA) cells resulted in a reduction of cell proliferation and survival in response to EPO. EPO-induced activation of Erk1/2 but not of Akt was significantly suppressed in F-36P-Gab1-siRNA cells compared with mock-transfected F-36P cells. The co-immunoprecipitation experiments revealed an EPO-enhanced association of Gab1 with the Grb2–SOS1 complex and SHP-2 in F-36P cells. A selective inhibitor of phosphatidylinositol 3-kinase (PI3K) LY294002 and short interfering RNA (siRNA) duplexes targeting the p85 regulatory subunit of PI3K (p85-siRNA) independently suppressed tyrosine phosphorylation of Gab1; its association with Grb2, SHP-2 and p85; and the activation of Erk in EPO-treated F-36P cells. LY294002 inhibited EPO-induced tyrosine phosphorylation of Gab1 and its association with Grb2 in human primary EPO-sensitive erythroid cells. The co-immunoprecipitation experiments using the Jak inhibitor AG490 or siRNA duplexes targeting Jak2 and in vitro binding experiments demonstrated that Jak2 regulated Gab1-mediated Erk activation through tyrosine phosphorylation of Gab1. Taken together, these results suggest that Gab1 couples PI3K-mediated EPO signals with the Ras/Erk pathway and that Gab1 plays an important role in EPOR-mediated signal transduction involved in the proliferation and survival of erythroid cells.     

Trypanosoma cruzi extracellular amastigotes selectively trigger the PI3K/Akt and Erk pathways during HeLa cell invasion

Cell invasion by Trypanosoma cruzi extracellular amastigotes involves different signaling pathways to induce phagocytosis-like mechanisms. Previous works indicated that PI3K/Akt, Src and Erk might be involved in EA invasion; however, participation of these molecules in this process remains elusive. Here, we observed that EA activated Akt, Erk but not Src. Interference of EA invasion with specific inhibitors corroborated this observation. Our results show that EA is capable of selectively triggering complex signaling pathways. Activation of PI3K/Akt and Erk, kinases related to actin cytoskeleton rearrangement and phagocytosis, reinforces the idea that T. cruzi EA subverts the phagocytic machinery during invasion.     

Modulation of mitochondrial apoptosis by PI3K inhibitors

Most anticancer therapies exert their action by triggering programmed cell death (apoptosis) in cancer cells. The mitochondrial pathway of apoptosis is initiated by mitochondrial outer membrane permeabilization, leading to the release of apoptogenic factors such as cytochrome c or Smac from the mitochondrial intermembrane space into the cytosol. Mitochondrial outer membrane permeabilization is tightly controlled, for example by pro- and anti-apoptotic proteins of the Bcl-2 family. Recent evidence indicates that inhibition of the PI3K/Akt/mTOR pathway by small-molecule PI3K inhibitors primes cancer cells to mitochondrial apoptosis by tipping the balance towards pro-apoptotic Bcl-2 proteins, resulting in increased mitochondrial outer membrane permeabilization. Thus, mitochondrial apoptotic events play an important role in PI3K inhibitor-mediated sensitization for apoptosis.     

PI3K enters beta-testing

Phosphoinositide-3-OH kinases (PI3K) are critical regulators of cell metabolism, growth, and survival. In a recent publication in Nature, Jia et al. (2008) identify specific functions of the p110beta isoform of PI3K in glucose metabolism, cellular proliferation, and tumorigenesis.     

Asymmetric PI3K Activity in Lymphocytes Organized by a PI3K-Mediated Polarity Pathway

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Oncogenic PIK3CA-driven mammary tumors frequently recur via PI3K pathway-dependent and PI3K pathway-independent mechanisms

PIK3CA gain-of-function mutations are a common oncogenic event in human malignancy, making phosphatidylinositol 3-kinase (PI3K) a target for cancer therapy. Despite the promise of targeted therapy, resistance often develops, leading to treatment failure. To elucidate mechanisms of resistance to PI3K-targeted therapy, we constructed a mouse model of breast cancer conditionally expressing human PIK3CA(H1047R). Notably, most PIK3CA(H1047R)-driven mammary tumors recurred after PIK3CA(H1047R) inactivation. Genomic analyses of recurrent tumors revealed multiple lesions, including focal amplification of Met or Myc (also known as c-Met and c-Myc, respectively). Whereas Met amplification led to tumor survival dependent on activation of endogenous PI3K, tumors with Myc amplification became independent of the PI3K pathway. Functional analyses showed that Myc contributed to oncogene independence and resistance to PI3K inhibition. Notably, PIK3CA mutations and c-MYC elevation co-occur in a substantial fraction of human breast tumors. Together, these data suggest that c-MYC elevation represents a potential mechanism by which tumors develop resistance to current PI3K-targeted therapies.     

C2-ceramide enhances sorafenib-induced caspase-dependent apoptosis via PI3K/AKT/mTOR and Erk signaling pathways in HCC cells

Applied Microbiology and Biotechnology - Sorafenib as an effective multikinase inhibitor has been approved for the clinical treatment against advanced hepatocellular carcinoma (HCC). HCC treatment...     

Regulation of actomyosin contractility by PI3K in sensory axons

Phosphatidylinositol 3-kinase (PI3K) activity is known to be required for the extension of embryonic sensory axons. Inhibition of PI3K has also been shown to mediate axon retraction and growth cone collapse in response to semaphorin 3A. However, the effects of inhibiting PI3K on the neuronal cytoskeleton are not well characterized. We have previously reported that semaphorin 3A-induced axon retraction involves activation of myosin II, the formation of an intra-axonal F-actin bundle cytoskeleton, and blocks the formation of F-actin patches that serve as precursors to filopodial formation in axons. We now report that inhibition of PI3K results in activation of myosin II in axons. Inhibition of myosin II activity, or its upstream regulatory kinase RhoA-kinase, blocked axon retraction induced by inhibition of PI3K. In addition, inhibition of PI3K also induced intra-axonal F-actin bundles, which likely serve as a substratum for myosin II-based force generation during axon retraction. In axons, filopodia are formed from axonal F-actin patch precursors. Analysis of axonal F-actin patch formation in eYFP-actin expressing neurons revealed that inhibition of PI3K blocked formation of axonal F-actin patches, and thus filopodial formation. These data provide insights into the regulation of the neuronal cytoskeleton by PI3K and are consistent with the notion that decreased levels of PI3K activity mediate axon retraction and growth cone collapse in response to semaphorin 3A.     

Pik3ip1 Modulates Cardiac Hypertrophy by Inhibiting PI3K Pathway

Cardiac hypertrophy is an adaptive response to various physiological and pathological stimuli. Phosphoinositide-3 kinase (PI3K) is a highly conserved lipid kinase involved in physiological cardiac hypertrophy (PHH). PI3K interacting protein1 (Pik3ip1) shares homology with the p85 regulatory subunit of PI3K and is known to interact with the p110 catalytic subunit of PI3K, leading to attenuation of PI3K activity in liver and immune cells. However, the role of Pik3ip1 in the heart remains unknown. In the present study, the effects of Pik3ip1 on cardiac hypertrophy were examined. We found that the expression level of Pik3ip1 was markedly higher in cardiomyocytes than in fibroblasts. The interaction of Pik3ip1 with the p110a subunit of PI3K in the heart was identified by immunoprecipitation using neonatal rat cardiomyocytes (NRCM). Approximately 35% knockdown of Pik3ip1 was sufficient to induce myocardial hypertrophy. Pik3ip1 deficiency was shown to lead to activation of PI3K/protein kinase B (AKT)/ mammalian target of rapamycin (mTOR) signaling pathway, increasing protein synthesis and cell size. However, adenovirus-mediated overexpression of Pik3ip1 attenuated PI3K-mediated cardiac hypertrophy. Pik3ip1 was upregulated by PHH due to swimming training, but not by pathological cardiac hypertrophy (PAH) due to pressure-overload, suggesting that Pik3ip1 plays a compensatory negative role for PHH. Collectively, our results elucidate the mechanisms for the roles of Pik3ip1 in PI3K/AKT signaling pathway.