Specific function of phosphoinositide 3-kinase beta in the control of DNA replication

Structural basis for activation and inhibition of class I phosphoinositide 3-kinases

Phosphoinositide 3-kinases (PI3Ks) are implicated in a broad spectrum of cellular activities, such as growth, proliferation, differentiation, migration, and metabolism. Activation of class I PI3Ks by mutation or overexpression correlates with the development and maintenance of various human cancers. These PI3Ks are heterodimers, and the activity of the catalytic subunits is tightly controlled by the associated regulatory subunits. Although the same p85 regulatory subunits associate with all class IA PI3Ks, the functional outcome depends on the isotype of the catalytic subunit. New PI3K partners that affect the signaling by the PI3K heterodimers have been uncovered, including phosphate and tensin homolog (PTEN), cyclic adenosine monophosphate-dependent protein kinase (PKA), and nonstructural protein 1. Interactions with PI3K regulators modulate the intrinsic membrane affinity and either the rate of phosphoryl transfer or product release. Crystal structures for the class I and class III PI3Ks in complexes with associated regulators and inhibitors have contributed to developing isoform-specific inhibitors and have shed light on the numerous regulatory mechanisms controlling PI3K activation and inhibition.

Elevated PI3K signaling drives multiple breast cancer subtypes

Most human breast tumors have mutations that elevate signaling through a key metabolic pathway that is induced by insulin and a number of growth factors. This pathway serves to activate an enzyme known as phosphatidylinositol 3' kinase (PI3K) as well as to regulate proteins that signal in response to lipid products of PI3K. The specific mutations that activate this pathway in breast cancer can occur in genes coding for tyrosine kinase receptors, adaptor proteins linked to PI3K, catalytic and regulatory subunits of PI3K, serine/threonine kinases that function downstream of PI3K, and also phosphatidylinositol 3' phosphatase tumor suppressors that function to antagonize this pathway. While each genetic change results in net elevation of PI3K pathway signaling, and all major breast cancer subtypes show pathway activation, the specific mutation(s) involved in any one tumor may play an important role in defining tumor subtype, prognosis and even sensitivity to therapy. Here, we describe mouse models of breast cancer with elevated PI3K signaling, and how they may be used to guide development of novel therapeutics.

Structure of lipid kinase p110β/p85β elucidates an unusual SH2-domain-mediated inhibitory mechanism

Phosphoinositide 3-kinases (PI3Ks) are essential for cell growth, migration, and survival. The structure of a p110β/p85β complex identifies an inhibitory function for the C-terminal SH2 domain (cSH2) of the p85 regulatory subunit. Mutagenesis of a cSH2 contact residue activates downstream signaling in cells. This inhibitory contact ties up the C-terminal region of the p110β catalytic subunit, which is essential for lipid kinase activity. In vitro, p110β basal activity is tightly restrained by contacts with three p85 domains: the cSH2, nSH2, and iSH2. RTK phosphopeptides relieve inhibition by nSH2 and cSH2 using completely different mechanisms. The binding site for the RTK's pYXXM motif is exposed on the cSH2, requiring an extended RTK motif to reach and disrupt the inhibitory contact with p110β. This contrasts with the nSH2 where the pY-binding site itself forms the inhibitory contact. This establishes an unusual mechanism by which p85 SH2 domains contribute to RTK signaling specificities.

Nuclear but not cytosolic phosphoinositide 3-kinase beta has an essential function in cell survival

Class I(A) phosphoinositide 3-kinases (PI3Ks) are heterodimeric enzymes composed of a p85 regulatory and a p110 catalytic subunit that induce the formation of 3-polyphosphoinositides, which mediate cell survival, division, and migration. There are two ubiquitous PI3K isoforms p110α and p110β that have nonredundant functions in embryonic development and cell division. However, whereas p110α concentrates in the cytoplasm, p110β localizes to the nucleus and modulates nuclear processes such as DNA replication and repair. At present, the structural features that determine p110β nuclear localization remain unknown. We describe here that association with the p85β regulatory subunit controls p110β nuclear localization. We identified a nuclear localization signal (NLS) in p110β C2 domain that mediates its nuclear entry, as well as a nuclear export sequence (NES) in p85β. Deletion of p110β induced apoptosis, and complementation with the cytoplasmic C2-NLS p110β mutant was unable to restore cell survival. These studies show that p110β NLS and p85β NES regulate p85β/p110β nuclear localization, supporting the idea that nuclear, but not cytoplasmic, p110β controls cell survival.

Stepwise loading of yeast clamp revealed by ensemble and single-molecule studies

In ensemble and single-molecule experiments using the yeast proliferating cell nuclear antigen (PCNA, clamp) and replication factor C (RFC, clamp loader), we have examined the assembly of the RFC·PCNA·DNA complex and its progression to holoenzyme upon addition of polymerase δ (polδ). We obtained data that indicate (i) PCNA loading on DNA proceeds through multiple conformational intermediates and is successful after several failed attempts; (ii) RFC does not act catalytically on a primed 45-mer templated fork; (iii) the RFC·PCNA·DNA complex formed in the presence of ATP is derived from at least two kinetically distinguishable species; (iv) these species disassemble through either unloading of RFC·PCNA from DNA or dissociation of PCNA into its component subunits; and (v) in the presence of polδ only one species converts to the RFC·PCNA·DNA·polδ holoenzyme. These findings redefine and deepen our understanding of the clamp-loading process and reveal that it is surprisingly one of trial and error to arrive at a heuristic solution.

The p110α and p110β isoforms of class I phosphatidylinositol 3-kinase are involved in toll-like receptor 5 signaling in epithelial cells

BACKGROUND

Bacterial flagellin triggers inflammation in mammalian cells via Toll-like receptor (TLR) 5. Release of the chemokine IL-8 in response to flagellin involves NF-κB, p38 MAP kinase, and phosphatidylinositol 3-kinase (PI3K). However, PI3K has been reported to be either pro- or anti-inflammatory in different model systems. We hypothesized that this could be due to different activities of the p110α and β isoforms of PI3K.

RESULTS

PI3K and Akt were rapidly activated in Caco-2 colon carcinoma cells by flagellin. Using a plasmid-based shRNA delivery system and novel p110 isoform-specific inhibitors, we found that flagellin-induced IL-8 production was dependent on both p110α and p110β. However in the mouse, inhibition of p110β but not p110α reduced the increase of serum IL-6 levels induced by intraperitoneal injection of flagellin.

CONCLUSIONS

These data demonstrate that the p110α and β isoforms of class IA PI3K are both required for the proinflammatory response to flagellin.

Spi-1/PU.1 oncogene accelerates DNA replication fork elongation and promotes genetic instability in the absence of DNA breakage

The multistage process of cancer formation is driven by the progressive acquisition of somatic mutations. Replication stress creates genomic instability in mammals. Using a well-defined multistep leukemia model driven by Spi-1/PU.1 overexpression in the mouse and Spi-1/PU.1-overexpressing human leukemic cells, we investigated the relationship between DNA replication and cancer progression. Here, using DNA molecular combing and flow cytometry methods, we show that Spi-1 increases the speed of replication by acting specifically on elongation rather than enhancing origin firing. This shortens the S-phase duration. Combining data from Spi-1 knockdown in murine cells with Spi-1 overexpression in human cells, we provide evidence that inappropriate Spi-1 expression is directly responsible for the replication alteration observed. Importantly, the acceleration of replication progression coincides with an increase in the frequency of genomic mutations without inducing DNA breakage. Thus, we propose that the hitherto unsuspected role for spi-1 oncogene in promoting replication elongation and genomic mutation promotes blastic progression during leukemic development.

Purinergic receptor-mediated rapid depletion of nuclear phosphorylated Akt depends on pleckstrin homology domain leucine-rich repeat phosphatase, calcineurin, protein phosphatase 2A, and PTEN phosphatases

Akt is an important oncoprotein, and data suggest a critical role for nuclear Akt in cancer development. We have previously described a rapid (3-5 min) and P2X7-dependent depletion of nuclear phosphorylated Akt (pAkt) and effects on downstream targets, and here we studied mechanisms behind the pAkt depletion. We show that cholesterol-lowering drugs, statins, or extracellular ATP, induced a complex and coordinated response in insulin-stimulated A549 cells leading to depletion of nuclear pAkt. It involved protein/lipid phosphatases PTEN, pleckstrin homology domain leucine-rich repeat phosphatase (PHLPP1 and -2), protein phosphatase 2A (PP2A), and calcineurin. We employed immunocytology, immunoprecipitation, and proximity ligation assay techniques and show that PHLPP and calcineurin translocated to the nucleus and formed complexes with Akt within 3 min. Also PTEN translocated to the nucleus and then co-localized with pAkt close to the nuclear membrane. An inhibitor of the scaffolding immunophilin FK506-binding protein 51 (FKBP51) and calcineurin, FK506, prevented depletion of nuclear pAkt. Furthermore, okadaic acid, an inhibitor of PP2A, prevented the nuclear pAkt depletion. Chemical inhibition and siRNA indicated that PHLPP, PP2A, and PTEN were required for a robust depletion of nuclear pAkt, and in prostate cancer cells lacking PTEN, transfection of PTEN restored the statin-induced pAkt depletion. The activation of protein and lipid phosphatases was paralleled by a rapid proliferating cell nuclear antigen (PCNA) translocation to the nucleus, a PCNA-p21(cip1) complex formation, and cyclin D1 degradation. We conclude that these effects reflect a signaling pathway for rapid depletion of pAkt that may stop the cell cycle.

A constitutively activated form of the p110beta isoform of PI3-kinase induces prostatic intraepithelial neoplasia in mice

Recent work has shown that ablation of p110beta, but not p110alpha, markedly impairs tumorigenesis driven by loss of phosphatase and tensin homolog (PTEN) in the mouse prostate. Other laboratories have reported complementary data in human prostate tumor lines, suggesting that p110beta activation is necessary for tumorigenesis driven by PTEN loss. Given the multiple functions of PTEN, we wondered if p110beta activation also is sufficient for tumorigenesis. Here, we report that transgenic expression of a constitutively activated p110beta allele in the prostate drives prostate intraepithelial neoplasia formation. The resulting lesions are similar to, but are clearly distinct from, the ones arising from PTEN loss or Akt activation. Array analyses of transcription in multiple murine prostate tumor models featuring PI3K/AKT pathway activation allowed construction of a pathway signature that may be useful in predicting the prognosis of human prostate tumors.

The emerging mechanisms of isoform-specific PI3K signalling

Phosphoinositide 3-kinases (PI3Ks) function early in intracellular signal transduction pathways and affect many biological functions. A further level of complexity derives from the existence of eight PI3K isoforms, which are divided into class I, class II and class III PI3Ks. PI3K signalling has been implicated in metabolic control, immunity, angiogenesis and cardiovascular homeostasis, and is one of the most frequently deregulated pathways in cancer. PI3K inhibitors have recently entered clinical trials in oncology. A better understanding of how the different PI3K isoforms are regulated and control signalling could uncover their roles in pathology and reveal in which disease contexts their blockade could be most beneficial.

Nuclear phosphoinositide 3-kinase beta controls double-strand break DNA repair

Class I phosphoinositide 3-kinases are enzymes that generate 3-poly-phosphoinositides at the cell membrane following transmembrane receptor stimulation. Expression of the phosphoinositide 3-kinase beta (PI3Kbeta) isoform, but not its activity, is essential for early embryonic development. Nonetheless, the specific function of PI3Kbeta in the cell remains elusive. Double-strand breaks (DSB) are among the most deleterious lesions for genomic integrity; their repair is required for development. We show that PI3Kbeta is necessary for DSB sensing, as PI3Kbeta regulates binding of the Nbs1 sensor protein to damaged DNA. Indeed, Nbs1 did not bind to DSB in PI3Kbeta-deficient cells, which showed a general defect in subsequent ATM and ATR activation, resulting in genomic instability. Inhibition of PI3Kbeta also retarded the DNA repair but the defect was less marked than that induced by PI3Kbeta deletion, supporting a kinase-independent function for PI3Kbeta in DNA repair. These results point at class I PI3Kbeta as a critical sensor of genomic integrity.

Essential role of the p110beta subunit of phosphoinositide 3-OH kinase in male fertility

Phosphoinositide 3-kinases (PI3K) are key molecular players in male fertility. However, the specific roles of different p110 PI3K catalytic subunits within the spermatogenic lineage have not been characterized so far. Herein, we report that male mice expressing a catalytically inactive p110beta develop testicular hypotrophy and impaired spermatogenesis, leading to a phenotype of oligo-azoospermia and defective fertility. The examination of testes from p110beta-defective tubules demonstrates a widespread loss in spermatogenic cells, due to defective proliferation and survival of pre- and postmeiotic cells. In particular, p110beta is crucially needed in c-Kit-mediated spermatogonial expansion, as c-Kit-positive cells are lost in the adult testis and activation of Akt by SCF is blocked by a p110beta inhibitor. These data establish that activation of the p110beta PI3K isoform by c-Kit is required during spermatogenesis, thus opening the way to new treatments for c-Kit positive testicular cancers.

More than just kinases: the scaffolding function of PI3K

Recently, it has been reported that some members of the PI3K family might have a "double identity"; in other words, PI3K have been found to act not only as classical kinases, but also as scaffolding proteins. Until now, the use of knockout mice has been considered sufficient to model the effects of PI3K inhibition and to predict the outcome of anti-PI3K pharmacological treatments by observing the resulting phenotypes. These studies supported the view that PI3K may represent promising pharmacological targets for cancer and inflammation. However, in selected cases, different experimental strategies of gene targeting of the same locus have resulted in distinct phenotypes. This demonstrates that "knocking-out" a gene is not necessarily equivalent to "knocking-in" an inactivating point mutation (Vanhaesebroeck et al. in Cell 118:274-276, 2004). Specifically, knockout and kinase-dead models have led to the discovery that PI3Kγ and β may act independently of their kinase activity, likely as adaptor proteins.

PI3K p110 alpha and p110 beta have differential effects on Akt activation and protection against oxidative stress-induced apoptosis in myoblasts

Catalytic subunits of PI3K play a critical role in growth factor signaling and survival by phosphorylating inositol lipids. We found that PI3K Class IA p110α and p110β have distinct functions in myoblasts. Inhibition of p110α reduced IGF-I-stimulated Akt activity and prevented IGF-I-mediated survival in H₂O₂-treated cells; in contrast, siRNA knockdown of p110β increased IGF-I-stimulated Akt activity. However, inhibition of p110β catalytic activity did not increase IGF-I-stimulated Akt activity, suggesting a role for p110β protein interactions rather than decreased generation of phosphoinositides in this effect. Increased Akt activity in p110β-deficient myoblasts was associated with diminished ERK activation as well as ERK-dependent IRS-1 636/639 phosphorylation, findings we show to be independent of p110β catalytic function, but associated with IGF-IR endocytosis. We also report that IGF-I protects myoblasts from H₂O₂-induced apoptosis through a mechanism that requires p110α, but may be independent of Akt or ERK under conditions of Akt and ERK inhibition. These observations suggest that both p110α and p110β are essential for growth and metabolism in myoblasts. Overall, our results provide new evidence for the roles of p110 isoforms in promoting cellular proliferation and homeostasis, IGF-IR internalization, and in opposing apoptosis.

Effects of PI3K catalytic subunit and Akt isoform deficiency on mTOR and p70S6K activation in myoblasts

The PI3K/Akt/mTOR signaling pathway is critical for cellular growth and survival in skeletal muscle, and is activated in response to growth factors such as insulin-like growth factor-I (IGF-I). We found that in C2C12 myoblasts, deficiency of PI3K p110 catalytic subunits or Akt isoforms had distinct effects on phosphorylation of mTOR and p70S6K. siRNA-mediated knockdown of PI3K p110alpha, p110beta, and simultaneous knockdown of p110alpha and p110beta resulted in increased basal and IGF-I-stimulated phosphorylation of mTOR S2448 and p70S6K T389; however, phosphorylation of S6 was reduced in p110beta-deficient cells, possibly due to reductions in total S6 protein. We found that IGF-I-stimulated Akt1 activity was enhanced in Akt2- or Akt3-deficient cells, and that knockdown of individual Akt isoforms increased mTOR/p70S6K activation in an isoform-specific fashion. Conversely, levels of IGF-I-stimulated p70S6K phosphorylation in cells simultaneously deficient in both Akt1 and Akt3 were increased beyond those seen with loss of any single Akt isoform, suggesting an alternate, Akt-independent mechanism that activates mTOR/p70S6K. Our results collectively suggest that mTOR/p70S6K is activated in a PI3K/Akt-dependent manner, but that in the absence of p110alpha or Akt, alternate pathway(s) may mediate activation of mTOR/p70S6K in C2C12 myoblasts.