The nuts and bolts of AGC protein kinases

How moderate changes in Akt T-loop phosphorylation impact on tumorigenesis and insulin resistance

The Akt signalling pathway plays vital roles in controlling cellular responses to insulin as well as in proliferation and survival. Inhibition of Akt signalling leads to insulin resistance and type 2 diabetes, whereas hyperactivation of Akt promotes tumorigenesis. In this study, we investigate how modest changes in the activity of the Akt signalling pathway, to an extent that might be achieved by drug treatment, would impact on insulin resistance and tumorigenesis. Using insulin-resistant PDK1^K465E/K465E PH domain knock-in mice, we found that introducing the PTEN^+/− mutation to slightly stimulate Akt restored normal insulin sensitivity. Introducing the PDK1^K465E/K465E PH domain knock-in mutation into cancer-prone PTEN^+/− mice, lowered Akt activity only by about 50%, but led to a delay in tumour onset of ∼4 months in a broad range of tumours. This was also accompanied by slower growth of B cell follicular lymphomas, as monitored by magnetic resonance imaging. Our findings imply that signal transduction inhibitors that lead to a modest reduction in Akt activity would not only delay onset of tumours possessing elevated phosphoinositide 3-kinase pathway activity but would also reduce the growth rate of developed tumours.

Prioritization of driver mutations in pancreatic cancer using cancer-specific high-throughput annotation of somatic mutations (CHASM)

Over 20,000 genes were recently sequenced in a series of 24 pancreatic cancers. We applied CHASM (Cancer-specific High-throughput Annotation of Somatic Mutations) to 963 of the missense somatic missense mutations discovered in these 24 cancers. CHASM identified putative driver mutations (false discovery rate ≤0.3) in three known pancreatic cancer driver genes (P53, SMAD4, CDKN2A). An additional 15 genes with putative driver mutations include genes coding for kinases (PIK3CG, DGKA, STK33, TTK and PRKCG), for cell cycle related proteins (NEK8), and for proteins involved in cell adhesion (CMAS, PCDHB2). These and other mutations identified by CHASM point to potential "driver genes" in pancreatic cancer that should be prioritized for additional follow-up.

Characterization of PF-4708671, a novel and highly specific inhibitor of p70 ribosomal S6 kinase (S6K1)

S6K1 (p70 ribosomal S6 kinase 1) is activated by insulin and growth factors via the PI3K (phosphoinositide 3-kinase) and mTOR (mammalian target of rapamycin) signalling pathways. S6K1 regulates numerous processes, such as protein synthesis, growth, proliferation and longevity, and its inhibition has been proposed as a strategy for the treatment of cancer and insulin resistance. In the present paper we describe a novel cell-permeable inhibitor of S6K1, PF-4708671, which specifically inhibits the S6K1 isoform with a Ki of 20 nM and IC50 of 160 nM. PF-4708671 prevents the S6K1-mediated phosphorylation of S6 protein in response to IGF-1 (insulin-like growth factor 1), while having no effect upon the PMA-induced phosphorylation of substrates of the highly related RSK (p90 ribosomal S6 kinase) and MSK (mitogen- and stress-activated kinase) kinases. PF-4708671 was also found to induce phosphorylation of the T-loop and hydrophobic motif of S6K1, an effect that is dependent upon mTORC1 (mTOR complex 1). PF-4708671 is the first S6K1-specific inhibitor to be reported and will be a useful tool for delineating S6K1-specific roles downstream of mTOR.

A new tool to dissect the function of p70 S6 kinase

Developing small-molecule inhibitors that are highly selective for specific protein kinases has been and remains a serious challenge. This especially applies to members of families of related kinases with overlapping substrate specificities, such as the serine/threonine kinases of the AGC family. In this issue of the Biochemical Journal, Dario Alessi's group, in a collaboration with Pfizer, report on PF-4708671, a potent and highly selective inhibitor of S6K1 (p70 S6 kinase 1) in vitro and in cells. S6K1 is an AGC family member and a crucial effector of the mTORC1 (mammalian target of rapamycin complex 1) kinase. This is the first reported inhibitor that is highly selective for S6K1. This compound will help to understand the signalling and physiological roles of S6K1, and to dissect signalling downstream of mTORC1. S6K1 inhibitors may ultimately be useful in the treatment of diseases such as cancer where S6K1 is overexpressed, but most importantly in metabolic disease such as insulin resistance and obesity.

Roles of cAMP and cAMP-dependent protein kinase in the progression of prostate cancer: cross-talk with the androgen receptor

Prostate carcinomas are among the most frequently diagnosed and death causing cancers affecting males in the developed world. It has become clear that the molecular mechanisms that drive the differentiation of normal prostate cells towards neoplasia involve multiple signal transduction cascades that often overlap and interact. A critical mediator of cellular proliferation and differentiation in various cells (and cancers) is the cAMP-dependent protein kinase, also known as protein kinase A (PKA), and its activating secondary messenger, cAMP. PKA and cAMP have been shown to play critical roles in prostate carcinogenesis and are the subject of this review. In particular we will focus on the cross-talk between PKA/cAMP signaling and that of the androgen receptor.

Multiple implications of 3-phosphoinositide-dependent protein kinase 1 in human cancer

3-phosphoinositide-dependent protein kinase-1 (PDK1) is a central mediator of cellular signaling between phosphoinositide-3 kinase and various intracellular serine/threonine kinases, including protein kinase B, p70 ribosomal S6 kinase, serum and glucocorticoid-inducible kinase, and protein kinase C. PDK1 activates members of the AGC family of protein kinases by phosphorylating serine/threonine residues in the activation loop. Here, we review the regulatory mechanisms of PDK1 and its roles in cancer. PDK1 is activated by autophosphorylation in the activation loop and other serine residues, as well as by phosphorylation of Tyr-9 and Tyr-373/376. Src appears to recognize PDK1 following tyrosine phosphorylation. The role of heat shock protein 90 in regulating PDK1 stability and PDK1-Src complex formation are also discussed. Furthermore, we summarize the subcellular distribution of PDK1. Finally, an important role for PDK1 in cancer chemotherapy is proposed. In conclusion, a better understanding of its molecular regulatory mechanisms in various signaling pathways will help to explain how PDK1 acts as an oncogenic kinase in various cancers, and will contribute to the development of novel cancer chemotherapies.

The emerging role of the phosphatidylinositol 3-kinase/ akt/mammalian target of rapamycin signaling network in cancer stem cell biology

The cancer stem cell theory entails the existence of a hierarchically organized, rare population of cells which are responsible for tumor initiation, self-renewal/maintenance, and mutation accumulation. The cancer stem cell proposition could explain the high frequency of cancer relapse and resistance to currently available therapies. The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling pathway regulates a wide array of physiological cell functions which include differentiation, proliferation, survival, metabolism, autophagy, and motility. Dysregulated PI3K/Akt/mTOR signaling has been documented in many types of neoplasias. It is now emerging that this signaling network plays a key role in cancer stem cell biology. Interestingly, cancer stem cells displayed preferential sensitivity to pathway inhibition when compared to healthy stem cells. This observation provides the proof-of-principle that functional differences in signaling pathways between neoplastic stem cells and healthy stem cells could be identified. In this review, we present the evidence which links the signals emanating from the PI3K/Akt/mTOR cascade with the functions of cancer stem cells, both in solid and hematological tumors. We then highlight how targeting PI3K/Akt/mTOR signaling with small molecules could improve cancer patient outcome.

The human papillomavirus type 16 E6 oncoprotein activates mTORC1 signaling and increases protein synthesis

The mammalian target of rapamycin (mTOR) kinase acts as a cellular rheostat that integrates signals from a variety of cellular signal transduction pathways that sense growth factor and nutrient availability as well as intracellular energy status. It was previously reported that the human papillomavirus type 16 (HPV16) E6 oncoprotein may activate the S6 protein kinase (S6K) through binding and E6AP-mediated degradation of the mTOR inhibitor tuberous sclerosis complex 2 (TSC2) (Z. Lu, X. Hu, Y. Li, L. Zheng, Y. Zhou, H. Jiang, T. Ning, Z. Basang, C. Zhang, and Y. Ke, J. Biol. Chem. 279:35664-35670, 2004; L. Zheng, H. Ding, Z. Lu, Y. Li, Y. Pan, T. Ning, and Y. Ke, Genes Cells 13:285-294, 2008). Our results confirmed that HPV16 E6 expression causes an increase in mTORC1 activity through enhanced phosphorylation of mTOR and activation of downstream signaling pathways S6K and eukaryotic initiation factor binding protein 1 (4E-BP1). However, we did not detect a decrease in TSC2 levels in HPV16 E6-expressing cells. We discovered, however, that HPV16 E6 expression causes AKT activation through the upstream kinases PDK1 and mTORC2 under conditions of nutrient deprivation. We show that HPV16 E6 expression causes an increase in protein synthesis by enhancing translation initiation complex assembly at the 5' mRNA cap and an increase in cap-dependent translation. The increase in cap-dependent translation likely results from HPV16 E6-induced AKT/mTORC1 activation, as the assembly of the translation initiation complex and cap-dependent translation are rapamycin sensitive. Lastly, coexpression of the HPV16 E6 and E7 oncoproteins does not affect HPV16 E6-induced activation of mTORC1 and cap-dependent translation. HPV16 E6-mediated activation of mTORC1 signaling and cap-dependent translation may be a mechanism to promote viral replication under conditions of limited nutrient supply in differentiated, HPV oncoprotein-expressing proliferating cells.

Differential NDR/LATS interactions with the human MOB family reveal a negative role for human MOB2 in the regulation of human NDR kinases

MOB proteins are integral components of signaling pathways controlling important cellular processes, such as mitotic exit, centrosome duplication, apoptosis, and cell proliferation in eukaryotes. The human MOB protein family consists of six distinct members (human MOB1A [hMOB1A], -1B, -2, -3A, -3B, and -3C), with hMOB1A/B the best studied due to their putative tumor-suppressive functions through the regulation of NDR/LATS kinases. The roles of the other MOB proteins are less well defined. Accordingly, we characterized all six human MOB proteins in the context of NDR/LATS binding and their abilities to activate NDR/LATS kinases. hMOB3A/B/C proteins neither bind nor activate any of the four human NDR/LATS kinases. We found that both hMOB2 and hMOB1A bound to the N-terminal region of NDR1. However, our data suggest that the binding modes differ significantly. Our work revealed that hMOB2 competes with hMOB1A for NDR binding. hMOB2, in contrast to hMOB1A/B, is bound to unphosphorylated NDR. Moreover, RNA interference (RNAi) depletion of hMOB2 resulted in increased NDR kinase activity. Consistent with these findings, hMOB2 overexpression interfered with the functional roles of NDR in death receptor signaling and centrosome overduplication. In summary, our data indicate that hMOB2 is a negative regulator of human NDR kinases in biochemical and biological settings.

A novel pleckstrin homology domain-containing protein enhances insulin-stimulated Akt phosphorylation and GLUT4 translocation in adipocytes

Protein kinase B/Akt protein kinases control an array of diverse functions, including cell growth, survival, proliferation, and metabolism. We report here the identification of pleckstrin homology-like domain family B member 1 (PHLDB1) as an insulin-responsive protein that enhances Akt activation. PHLDB1 contains a pleckstrin homology domain, which we show binds phosphatidylinositol PI(3,4)P₂, PI(3,5)P₂, and PI(3,4,5)P₃, as well as a Forkhead-associated domain and coiled coil regions. PHLDB1 expression is increased during adipocyte differentiation, and it is abundant in many mouse tissues. Both endogenous and HA- or GFP-tagged PHLDB1 displayed a cytoplasmic disposition in unstimulated cultured adipocytes but translocated to the plasma membrane in response to insulin. Depletion of PHLDB1 by siRNA inhibited insulin stimulation of Akt phosphorylation but not tyrosine phosphorylation of IRS-1. RNAi-based silencing of PHLDB1 in cultured adipocytes also attenuated insulin-stimulated deoxyglucose transport and Myc-GLUT4-EGFP translocation to the plasma membrane, whereas knockdown of the PHLDB1 isoform PHLDB2 failed to attenuate insulin-stimulated deoxyglucose transport. Furthermore, adenovirus-mediated expression of PHLDB1 in adipocytes enhanced insulin-stimulated Akt and p70 S6 kinase phosphorylation, as well as GLUT4 translocation. These results indicate that PHLDB1 is a novel modulator of Akt protein kinase activation by insulin.

Phospholipase D stabilizes HDM2 through an mTORC2/SGK1 pathway

Phosphatidic acid (PA), the primary metabolite of the phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine, has been shown to act as a tumor promoting second messenger in many cancer cell lines. A key target of PA is the mammalian target of rapamycin (mTOR), a serine-threonine kinase that has been widely implicated in cancer cell survival signals. In agreement with its ability to relay survival signals, it has been reported that both PLD and mTOR are required for the stabilization of the p53 E3 ubiquitin ligase human double minute 2 (HDM2) protein. Thus, by stabilizing HDM2, PLD and mTOR are able to counter the pro-apoptotic signaling mediated by p53 and promote survival. mTOR exists in at least two distinct complexes-mTORC1 and mTORC2-that are both dependent on PLD-generated PA. Although PLD and its metabolite PA are clearly implicated in the transduction of survival signals to mTOR, it is not yet apparent which of the two mTOR complexes is critical for the stabilization of HDM2. We report here that the PLD/mTOR-dependent stabilization of HDM2 involves mTORC2 and the AGC family kinase serum- and glucocorticoid-inducible kinase 1 (SGK1). This study reveals that mTORC2 is a critical target of PLD-mediated survival signals and identifies SGK1 as a downstream target of mTORC2 for the stabilization of HDM2.

Phosphorylation of the Na+,K+-ATPase and the H+,K+-ATPase

Phosphorylation is a widely used, reversible means of regulating enzymatic activity. Among the important phosphorylation targets are the Na(+),K(+)- and H(+),K(+)-ATPases that pump ions against their chemical gradients to uphold ionic concentration differences over the plasma membrane. The two pumps are very homologous, and at least one of the phosphorylation sites is conserved, namely a cAMP activated protein kinase (PKA) site, which is important for regulating pumping activity, either by changing the cellular distribution of the ATPases or by directly altering the kinetic properties as supported by electrophysiological results presented here. We further review the other proposed pump phosphorylations.

Protein kinase Cgamma in colon cancer cells: expression, Thr514 phosphorylation and sensitivity to butyrate-mediated upregulation as related to the degree of differentiation

Protein kinase C (PKC) isoenzymes are expressed and activated in a cell type-specific manner, and play an essential role in tissue-specific signal transduction. The presence of butyrate at millimolar concentrations in the colon raises the question of whether it affects the expression of PKC isoenzymes in the different cell types of the colonic epithelium. We investigated the protein expression levels of PKCgamma, Thr(514)-phosphorylated PKCgamma (pPKCgamma-Thr(514)), and their subcellular distribution as affected by butyrate in a set of colon cancer cell lines. Thr(514)-phosphorylation of de novo synthesized PKCgamma is the first step in priming of the inactive PKCgamma before its release into the cytoplasm. For immunoblot analysis, we employed three antibodies, one against an unmodified sequence, mapping within 50 amino acids at its C-terminus, a second against pPKCgamma-Thr(514), and a third against pPKCgamma-pan-Thr(514). The antibody against an unmodified C-terminal peptide epitope did not recognize pPKCgamma-Thr(514), suggesting that phosphorylation at this site interferes with the binding of the antibody to the C-terminus. Marked butyrate-induced upregulation of PKCgamma occurred in HT29 cells (model for colonocyte stem cells) and HT29-derived cell lines. However, in Caco2 and IEC-18 cells (models for differentiated intestinal epithelial cells), PKCgamma was insensitive to upregulation, and present exclusively as pPKCgamma-Thr(514). Lovo and SW480 expressed higher levels of PKCgamma. In HT29 cells, butyrate-induced upregulation of the non-phosphorylated PKCgamma was observed in both the membrane and the cytosolic fraction. In Caco2 cells, the Thr(514)-phosphorylated form was present at high levels in both fractions. The presence of unphosphorylated PKCgamma in HT29 cells, and its complete absence in Caco2 cells demonstrates a cell type-dependent differential coupling of Thr(514)-phosphorylation with de novo synthesis of PKCgamma in colon cancer cells.

Conservation of the Tsc/Rheb/TORC1/S6K/S6 Signaling in Fission Yeast

The TSC/Rheb/TORC1/S6K/S6 signaling pathway plays critical roles in regulating protein synthesis and growth in eukaryotes. Our recent work using fission yeast Schizosaccharomyces pombe revealed that this signaling pathway is conserved from humans to fission yeast. In addition to target of rapamycin (TOR) homologsand tuberous sclerosis complex (TSC) homologs, fission yeast but not budding yeast, has a functional homolog of Rheb, a small G-protein acting as an activator of TOR complex 1 (TORC1). Several lines of genetic evidence suggest that the Tsc1-Tsc2 complex and Rheb act as upstream players of TORC1 in fission yeast. We have recently demonstrated that TORC1, but not TORC2, regulates phosphorylation of ribosomal protein S6 in response to nutrient availability. Candidate S6 kinase (S6K) protein has been identified. In addition, we find that rapamycin prevents a subset of TORC1 activity to regulate S6 phosphorylation in fission yeast.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.