Inhibitors of phosphatidylinositol signalling as antiproliferative agents

Effects of Different Salinity Stress on the Transcriptomic Responses of Freshwater Crayfish (Procambarus clarkii, Girard, 1852)

Salinization of freshwater ecosystems is a pressing global issue. Changes in salinity can exert severe pressure on aquatic animals and jeopardize their survival. Procambarus clarkii is a valuable freshwater aquaculture species that exhibits some degree of salinity tolerance, making it an excellent research model for freshwater aquaculture species facing salinity stress. In the present study, crayfish were exposed to acute low salt (6 ppt) and high salt (18 ppt) conditions. The organisms were continuously monitored at 6, 24, and 72 h using RNA-Seq to investigate the mechanisms of salt stress resistance. Transcriptome analysis revealed that the crayfish responded to salinity stress with numerous differentially expressed genes, and most of different expression genes was observed in high salinity group for 24h. GO and KEGG enrichment analyses indicated that metabolic pathways were the primary response pathways in crayfish under salinity stress. This suggests that crayfish may use metabolic pathways to compensate for energy loss caused by osmotic stress. Furthermore, gene expression analysis revealed the differential expression of immune and antioxidant-related pathway genes under salinity stress, implying that salinity stress induces immune disorders in crayfish. More genes related to cell proliferation, differentiation, and apoptosis, such as the Foxo, Wnt, Hippo, and Notch signaling pathways, responded to high-salinity stress. This suggests that regulating the cellular replication cycle and accelerating apoptosis may be necessary for crayfish to cope with high-salinity stress. Additionally, we identified 36 solute carrier family (SLC) genes related to ion transport, depicting possible ion exchange mechanisms in crayfish under salinity stress. These findings aimed to establish a foundation for understanding crustacean responses to salinity stress and their osmoregulatory mechanisms.

ADP ribosylation factor 6 (ARF6) promotes acrosomal exocytosis by modulating lipid turnover and Rab3A activation

Regulated secretion is a central issue for the specific function of many cells; for instance, mammalian sperm acrosomal exocytosis is essential for egg fertilization. ARF6 (ADP-ribosylation factor 6) is a small GTPase implicated in exocytosis, but its downstream effectors remain elusive in this process. We combined biochemical, functional, and microscopy-based methods to show that ARF6 is present in human sperm, localizes to the acrosomal region, and is required for calcium and diacylglycerol-induced exocytosis. Results from pulldown assays show that ARF6 exchanges GDP for GTP in sperm challenged with different exocytic stimuli. Myristoylated and guanosine 5'-3-O-(thio)triphosphate (GTPγS)-loaded ARF6 (active form) added to permeabilized sperm induces acrosome exocytosis even in the absence of extracellular calcium. We explore the ARF6 signaling cascade that promotes secretion. We demonstrate that ARF6 stimulates a sperm phospholipase D activity to produce phosphatidic acid and boosts the synthesis of phosphatidylinositol 4,5-bisphosphate. We present direct evidence showing that active ARF6 increases phospholipase C activity, causing phosphatidylinositol 4,5-bisphosphate hydrolysis and inositol 1,4,5-trisphosphate-dependent intra-acrosomal calcium release. We show that active ARF6 increases the exchange of GDP for GTP on Rab3A, a prerequisite for secretion. We propose that exocytic stimuli activate ARF6, which is required for acrosomal calcium efflux and the assembly of the membrane fusion machinery. This report highlights the physiological importance of ARF6 as a key factor for human sperm exocytosis and fertilization.

Role of melatonin on electromagnetic radiation-induced oxidative stress and Ca2+ signaling molecular pathways in breast cancer

AIMS

Exposure to electromagnetic radiation (EMR) may increase breast cancer risk by inducing oxidative stress and suppressing the production of melatonin. Aim of the present review is to discuss the mechanisms and risk factors of EMR and oxidative stress-induced breast cancer, to summarize the controlled studies evaluating measures for prevention, and to conclude with evidence-based strategies for prevention.

MATERIALS

Review of the relevant literature and results from our recent basic studies, as well as critical analyses of published systematic reviews were obtained from the Pubmed and the Science Citation Index.

RESULTS

It has been proposed that chronic exposure to EMR may increase the risk of breast cancer by suppressing the production of melatonin; this suppression may affect the development of breast cancer either by increasing levels of circulation of estrogen or through over production of free oxygen radicals. Most epidemiological studies have also indicated overall effect of EMR exposure in premenopausal women, particularly for estrogen receptor positive breast tumors. Enhanced voltage-dependent Ca(2+) current and impaired inhibitory G-protein function, and derangement of intracellular organelles with a Ca(2+) buffering effect, such as endoplasmic reticulum and mitochondria have been also shown to contribute to disturbed Ca(2+) signaling in breast cancer.

CONCLUSION

Melatonin may modulate breast cancer through modulation of enhanced oxidative stress and Ca(2+) influx in cell lines. However, there is not enough evidence on increased risk of breast cancer related to EMR exposure.

Diacylglycerol stimulates acrosomal exocytosis by feeding into a PKC- and PLD1-dependent positive loop that continuously supplies phosphatidylinositol 4,5-bisphosphate

Acrosomal exocytosis involves a massive fusion between the outer acrosomal and the plasma membranes of the spermatozoon triggered by stimuli that open calcium channels at the plasma membrane. Diacylglycerol has been implicated in the activation of these calcium channels. Here we report that this lipid promotes the efflux of intraacrosomal calcium and triggers exocytosis in permeabilized human sperm, implying that diacylglycerol activates events downstream of the opening of plasma membrane channels. Furthermore, we show that calcium and diacylglycerol converge in a signaling pathway leading to the production of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Addition of diacylglycerol promotes the PKC-dependent activation of PLD1. Rescue experiments adding phosphatidic acid or PIP(2) and direct measurement of lipid production suggest that both PKC and PLD1 promote PIP(2) synthesis. Inhibition of different steps of the pathway was reverted by adenophostin, an agonist of IP(3)-sensitive calcium channels, indicating that PIP(2) is necessary to keep these channels opened. However, phosphatidic acid, PIP(2), or adenophostin could not trigger exocytosis by themselves, indicating that diacylglycerol must also activate another factor. We found that diacylglycerol and phorbol ester stimulate the accumulation of the GTP-bound form of Rab3A. Together our results indicate that diacylglycerol promotes acrosomal exocytosis by i) maintaining high levels of IP(3) - an effect that depends on a positive feedback loop leading to the production of PIP(2) - and ii) stimulating the activation of Rab3A, which in turn initiates a cascade of protein interactions leading to the assembly of SNARE complexes and membrane fusion.

Inositol 1,4,5-trisphosphate-induced Ca2+ signalling is involved in estradiol-induced breast cancer epithelial cell growth

Background

Ca²⁺ is a ubiquitous messenger that has been shown to be responsible for controlling numerous cellular processes including cell growth and cell death. Whereas the involvement of IP₃-induced Ca²⁺ signalling (IICS) in the physiological activity of numerous cell types is well documented, the role of IICS in cancer cells is still largely unknown. Our purpose was to characterize the role of IICS in the control of growth of the estrogen-dependent human breast cancer epithelial cell line MCF-7 and its potential regulation by 17β-estradiol (E₂).

Results

Our results show that the IP₃ receptor (IP₃R) inhibitors caffeine, 2-APB and xestospongin C (XeC) inhibited the growth of MCF-7 stimulated by 5% foetal calf serum or 10 nM E₂. Furthermore, Ca²⁺ imaging experiments showed that serum and E₂ were able to trigger, in a Ca²⁺-free medium, an elevation of internal Ca²⁺ in a 2-APB and XeC-sensitive manner. Moreover, the phospholipase C (PLC) inhibitor U-73122 was able to prevent intracellular Ca²⁺ elevation in response to serum, whereas the inactive analogue U-73343 was ineffective. Western-blotting experiments revealed that the 3 types of IP₃Rs are expressed in MCF-7 cells and that a 48 hours treatment with 10 nM E₂ elevated IP₃R3 protein expression level in an ICI-182,780 (a specific estrogen receptor antagonist)-dependent manner. Furthermore, IP₃R3 silencing by the use of specific small interfering RNA was responsible for a drastic modification of the temporal feature of IICS, independently of a modification of the sensitivity of the Ca²⁺ release process and acted to counteract the proliferative effect of 10 nM E₂.

Conclusions

Altogether, our results are in favour of a role of IICS in MCF-7 cell growth, and we hypothesize that the regulation of IP₃R3 expression by E₂ is involved in this effect.

Sphingosine 1-phosphate and sphingosine kinase are involved in a novel signaling pathway leading to acrosomal exocytosis

Regulated secretion is a central issue for the specific function of many cells; for instance, mammalian sperm acrosomal exocytosis is essential for egg fertilization. Sphingosine 1-phosphate is a bioactive sphingolipid that regulates crucial physiological processes. Here we report that this lipid triggers acrosomal exocytosis in human sperm by a mechanism involving a G(i)-coupled receptor. Real-time imaging showed a remarkable increase of cytosolic calcium upon activation with sphingosine 1-phosphate and pharmacological experiments indicate that the process requires extracellular calcium influx through voltage and store-operated calcium channels and efflux from intracellular stores through inositol 1,4,5-trisphosphate-sensitive calcium channels. Sphingosine 1-phosphate-induced exocytosis requires phospholipase C and protein kinase C activation. We investigated possible sources of the lipid. Western blot indicates that sphingosine kinase 1 is present in spermatozoa. Indirect immunofluorescence showed that phorbol ester, a potent protein kinase C activator that can also trigger acrosomal exocytosis, redistributes sphingosine kinase 1 to the acrosomal region. Functional assays showed that phorbol ester-induced exocytosis depends on the activation of sphingosine kinase 1. Furthermore, incorporation of (32)P to sphingosine demonstrates that cells treated with the phorbol ester increase their sphingosine kinase activity that yields sphingosine 1-phosphate. We present here the first evidence indicating that human spermatozoa produce sphingosine 1-phosphate when challenged with an exocytic stimulus. These observations point to a new role of sphingosine 1-phosphate in a signaling cascade that facilitates acrosome reaction providing some clues about novel lipid molecules involved in exocytosis.

Wortmannin-C20 conjugates generate wortmannin

We report on C20-6-(N-methylamino)hexanoic conjugates of wortmannin featuring a tertiary enamine attached to the C20 that inhibit phosphoinositol-3-OH kinase (PI3K) by producing wortmannin (Wm) through an intramolecular attack. The generation of Wm by these conjugates permits the design of Wm based PI3K inhibitors that need not fit into the ATP pocket of PI3K, including Wm conjugates of BSA, IgG, or beads. Wm generating WmC20-N(Me)-hexanoate conjugates offer an approach to the design of targeted or slow release forms of Wm which may inhibit PI3K in tissues more selectively than the parent Wm, a compound which has desirable anti-inflammatory and anti-proliferative activities but which also has a variety of toxic effects.

Vasoactive intestinal peptide (VIP) induces c-fos expression in LNCaP prostate cancer cells through a mechanism that involves Ca2+ signalling. Implications in angiogenesis and neuroendocrine differentiation

The effect of vasoactive intestinal peptide (VIP) on intracellular Ca(2+) levels and its relationship with the expression of c-fos and vascular endothelial growth factor (VEGF) as well as with neuroendocrine (NE) differentiation were investigated in human prostate LNCaP cells. VIP induced the expression of c-fos mRNA as studied by reverse transcription polymerase chain reaction (RT-PCR). It was accompanied by VIP stimulation of c-fos protein synthesis, as measured by Western blot analysis. VIP enhanced intracellular Ca(2+) levels as evaluated using the calcium probe fura-2. VIP regulation of c-fos expression depended on [Ca(2+)](i) concentration since the intracellular calcium chelator BAPTA/AM decreased c-fos expression (both mRNA and protein) to basal levels. As shown by means of real-time RT-PCR, VIP stimulated VEGF mRNA expression: the effect was inhibited by 40% in the presence of curcumin (an inhibitor of AP-1 binding), and it was dependent on Ca(2+) since BAPTA/AM inhibited this VIP action by 43%. Similar observations were made on the effects of BAPTA/AM and curcumin on VIP stimulation of VEGF protein expression. Simultaneous treatment of cells with the protein kinase A inhibitor H89 and BAPTA/AM completely blocked this VIP effect, whereas each agent alone led only to a partial inhibition. In addition, the calcium chelator blocked by 37% the ability of VIP to induce NE cell differentiation as estimated by the observation of neurite development. These features support a VIP signalling pathway that could be mediated through both cAMP and [Ca(2+)](i) increase in prostate LNCaP cancer cells. Moreover, our data suggest the implication of c-Fos on the induction of the main angiogenic factor VEGF since the promoter region of the VEGF gene possesses AP-1 (i.e., c-Fos/c-Jun heterodimer) response elements. This feature represents a link between the nuclear oncogene c-fos, angiogenesis and NE differentiation by means of an initiating signal upon VIP receptors.

Sensitivity of oocyte-expressed epithelial Na+ channel to glibenclamide

The effect of glibenclamide on heterologously expressed amiloride-sensitive sodium channels (ENaCs) was investigated in Xenopus oocytes. The ENaC is a heteromer and consists of alpha-, beta- and gamma-subunits and the alpha- and beta-subunits have previously been shown to confer sensitivity to glibenclamide. We coexpressed either colonic rat alpha- (ralpha) or guinea-pig alpha-subunit (gpalpha) with Xenopus betagamma-subunits. The gpalphaxbetagamma was significantly stimulated by glibenclamide (100 microM) (184+/-15%), whereas the ralpha-combination was slightly down-regulated by the sulfonylurea (79+/-4%). The stimulating effect did not interfere with Na(+)-self-inhibition resulting from intracellular accumulation of Na(+)-ions. We exchanged cytosolic termini between both orthologs but the gpalpha-chimera with the termini from rat retained sensitivity to glibenclamide. The effect of glibenclamide on Xenopus ENaC (xENaC) was inhibited by ADP-beta-S but not by ATP-gamma-S, when applied intracellularly. Intracellular loading with Na(+)-ions after inhibition of Na(+)/K(+)-ATPases with ouabain prevented an up-regulation of ENaC activity by glibenclamide. Pretreatment of oocytes expressing xENaC with edelfosine (ET-18-OCH(3)) slightly reduced stimulation of I(ami) (118+/-12%; control: 132+/-9%) while phosphatidylinositol-4,5-biphosphate (PIP(2)) significantly reduced the effect of glibenclamide to 101+/-3%.

Phosphoinositide 3-kinase inhibition in cancer treatment

Over the past ten years, our knowledge of the integral role that the phospho-inositide 3-kinases (PI3Ks) and their 3'-phosphorylated lipid products (3'-phosphorylated phosphoinositides; 3P-PIs) play in the mediation of signal transduction, cytoskeletal rearrangements and membrane trafficking has expanded considerably. They are now known to be involved in the regulation of cell growth, differentiation, mobility, proliferation and survival and hence they have become a potential target for the control of the growth and spread of cancer cells. More recently, the correlation of the multiplicity of isomers (both catalytic and regulatory) within the different classes of the PI3Ks with their functional relevance has become possible. This, combined with our further understanding of the protein recognition patterns for their different 3P-PIs and the newly-described pathways in the control of the levels of these by dephosphorylation, has provided new aspects and areas for interference in these multiple PI3K signalling pathways. However, in the search for effective, non-toxic, drugs for use in the treatment of cancers, these individual targets for PI3K inhibition need to be further correlated with the specific in vivo effects on cell survival, invasivity and metastatic potential. Here, the range of PI3K inhibition targets are discussed in the light of recent experimental findings, with a view to the exploitation of their specificities in new approaches to effective cancer treatments based on PI3K activity inhibition.

Involvement of phospholipid signal transduction pathways in morphine tolerance in mice

1. Opioid tolerance involves an alteration in the activity of intracellular kinases such as cyclic AMP-dependent protein kinase (PKA). Drugs that inhibit PKA reverse morphine antinociceptive tolerance. The hypothesis was tested that phospholipid pathways are also altered in morphine tolerance. Inhibitors of the phosphatidylinositol and phosphatidylcholine pathways were injected i.c.v. in an attempt to acutely reverse morphine antinociceptive tolerance. 2. Seventy-two hours after implantation of placebo or 75 mg morphine pellets, mice injected i.c.v. with inhibitor drug were challenged with morphine s.c. for generation of dose-response curves in the tail-flick test. Placebo pellet-implanted mice received doses of inhibitor drug having no effect on morphine's potency, in order to test for tolerance reversal in morphine pellet-implanted mice. Injection of the phosphatidylinositol-specific phospholipase C inhibitor ET-18-OCH3 significantly reversed tolerance, indicating a potential role for inositol 1,4,5-trisphosphate (IP3) and protein kinase C (PKC) in tolerance. Alternatively, phosphatidylcholine-specific phospholipase C increases the production of diacylglycerol and activation of PKC, without concomitant production of IP3. D609, an inhibitor of phosphatidylserine-specific phospholipase C, also reversed tolerance. Heparin is an IP3 receptor antagonist. Injection of low molecular weight heparin also reversed tolerance. PKC was also examined with three structurally dissimilar inhibitors. Bisindolylmaleimide I, Go-7874, and sangivamycin significantly reversed tolerance. 3. Chronic opioid exposure leads to changes in phospholipid metabolism that have a direct role in maintaining a state of tolerance. Evidence is accumulating that opioid tolerance disrupts the homeostatic balance of several important signal transduction pathways.

Granulocyte-macrophage colony-stimulating factor-activated signaling pathways in human neutrophils. Involvement of Jak2 in the stimulation of phosphatidylinositol 3-kinase

Granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates many of the biological activities of human neutrophils. The signaling pathways via which these effects are mediated are not fully understood. We have shown previously that GM-CSF treatment of human neutrophils activates the Janus kinase/signal transducers and activators of transcription (Jak/STAT) pathway and, more specifically, Jak2, STAT3, and STAT5B in neutrophils. GM-CSF also stimulates the activity of the phosphatidylinositol 3-kinase (PI3-kinase) in a tyrosine kinase-dependent manner. Here we report that pretreating the cells with a Jak2 inhibitor (AG-490) abolishes tyrosine phosphorylation of the p85 subunit of PI3-kinase induced by GM-CSF. Furthermore, p85 was found to associate with Jak2, but not with Lyn, in stimulated cells in situ and with its autophosphorylated form in vitro; however, Jak2 did not bind to either of the two Src homology 2 (SH2) domains of the p85 subunit of PI3-kinase. Although STAT5B bound to the carboxyl-terminal SH2 domain of p85, it was absent from the complex containing PI3-kinase and Jak2. These results suggest that stimulation of the activity of PI3-kinase induced by GM-CSF is mediated by Jak2 and that the association between Jak2 and p85 depends on an adaptor protein yet to be identified.

Substitution of 15(S)hydroxyeicosatetraenoic acid in phosphatidylinositol alters the growth of liver epithelial cells

We investigated the substitution of 15(S)-hydroxyeicosatetraenoic acid (15(S)HETE) in phospholipid signaling pathways and its consequences on the growth of non-transformed (NT-) and spontaneously transformed (T-) rat liver epithelia cells (RLEC). 15(S)HETE was selectively incorporated into the sn-2 position of phosphatidylinositol (PI) and at a higher rate into T-RLEC. RLEC rapidly mobilized the resulting 15(S)HETE-containing PI (15(S)HETE-PI) and produced 1-acyl,2-[1(S)HETE]-glycerol. Although total diacylglycerol levels were similar in both cell types, the ratio 1-acyl,2-[15(S)HETE]-glycerol / 15(S)HETE-PI was higher in NT-RLEC, suggesting a lower mobilization of 15(S)HETE-PI in T-RLEC. Using rat brain protein kinase C, 1-stearoyl,2-[15(S)HETE]-glycerol was as potent an in vitro protein kinase C activator as 1-stearoyl,2-arachidonoyl-glycerol. Finally, selective substitution of 15(S)HETE in PI altered DNA synthesis in T-RLEC: whereas low concentrations of 15(S)HETE (1 nM and 10 nM) in these cells were mitogenic, higher concentrations resulted in a 30% inhibition of DNA synthesis.

Calcium signaling in prostate cancer cells: evidence for multiple receptors and enhanced sensitivity to bombesin/GRP

BACKGROUND

Cellular calcium is an important second messenger for growth regulation. We sought to identify potentially important receptors on prostate tumor cells by screening over 20 agonists for their ability to increase intracellular free calcium ([Ca2+]i) in several human prostate tumor cell lines.

METHODS

Intracellular calcium mobilization was detected using fura-2.

RESULTS

We found bombesin, GRP, ATP/UTP, lysophosphatidic acid, thrombin, endothelin, histamine, and bradykinin increased [Ca2+]i in the advanced tumor cell lines DU-145, PC3, and PPC-1. Bombesin failed to elevate [Ca2+]i in an immortalized human prostate cell line. Rank-order of potency studies suggested the presence of P2U nucleotide receptors for ATP/UTP on prostate epithelial cells. Potency studies also revealed GRP > > bombesin > > neuromedin B at elevating [Ca2+]i in responding tumor cells.

CONCLUSIONS

These findings indicate that androgen independent prostate tumor cell lines express multiple receptors capable of elevating intracellular calcium, and suggest that GRP receptors may be selectively expressed and/or coupled to calcium signaling during prostate tumor progression. Calcium sensitive cellular events may therefore contribute to the progression of prostate cancer.

Granulocyte-Macrophage Colony-Stimulating Factor–Activated Signaling Pathways in Human Neutrophils. I. Tyrosine Phosphorylation-Dependent Stimulation of Phosphatidylinositol 3-Kinase and Inhibition by Phorbol Esters

Phosphatidylinositol 3-kinase (PI3-kinase) is a cytosolic enzyme that plays key roles in mediating signaling through many receptors. The heterodimeric form of PI3-kinase is made up of a regulatory subunit, p85, and a catalytic subunit, p110. Although granulocyte-macrophage colony-stimulating factor (GM-CSF) has been shown to activate PI3-kinase, the mechanisms by which this activation is mediated and regulated are incompletely understood. Here we show that treatment of human neutrophils with GM-CSF induced both time- and concentration-dependent increases in the level of tyrosine phosphorylation of p85. The ability of GM-CSF to activate PI3-kinase was abolished by pretreating the cells with erbstatin, a tyrosine kinase inhibitor. The simultaneous treatment of the cells with GM-CSF and phorbol esters such as phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDBu) significantly inhibited both the tyrosine phosphorylation of p85 and the activation of PI3-kinase. The inhibitory effects of phorbol esters were not induced by their inactive analogues and they were selective to the stimulation of tyrosine phosphorylation of p85 since phorbol esters did not alter the enhancement of the pattern of tyrosine phosphorylation of other cellular proteins, including that of Jak2 induced by GM-CSF. However, PMA significantly inhibited the in situ tyrosine phosphorylation and the activation of lyn observed in response to GM-CSF. The results suggest that the activation of PI3-kinase by GM-CSF is mediated by the tyrosine phosphorylation of p85 and that this activation is downregulated by PKC possibly via the inhibition of lyn.

Differential effects of constitutively active phosphatidylinositol 3-kinase on glucose transport, glycogen synthase activity, and DNA synthesis in 3T3-L1 adipocytes

Phosphatidylinositol 3-kinase (PI3K) activation is necessary for many insulin-induced metabolic and mitogenic responses. However, it is unclear whether PI3K activation is sufficient for any of these effects. To address this question we increased PI3K activity in differentiated 3T3-L1 adipocytes by adenovirus-mediated expression of both the inter-SH2 region of the regulatory p85 subunit of PI3K (iSH2) and the catalytic p110 alpha subunit (p110). Coexpression resulted in PI3K activity that exceeded insulin-stimulated activity by two- to fivefold in cytosol, total membranes, and the low density microsome (LDM) fraction, the site of greatest insulin stimulation. While insulin increased glucose transport 15-fold, coexpression of iSH2-p110 increased transport (5.2-) +/- 0.7-fold with a parallel increase in GLUT4 translocation to the plasma membrane. Constitutive activation of PI3K had no effect on maximally insulin-stimulated glucose transport. Neither basal nor insulin-stimulated activity of glycogen synthase or mitogen-activated protein kinase was altered by iSH2-p110 coexpression. DNA synthesis was increased twofold by insulin in control 3T3-L1 adipocytes transduced with beta-galactosidase-encoding recombinant adenovirus, while iSH2-p110 coexpression increased DNA synthesis fivefold. These data indicate that (i) increased PI3K activity is sufficient to activate some but not all metabolic responses to insulin, (ii) activation of PI3K to levels exceeding the effect of insulin in adipocyte LDM results in only a partial stimulation of glucose transport, and (iii) increased PI3K activity in the absence of growth factor or oncoprotein stimulation is a potent stimulus of DNA synthesis.

Calcium signalling and cell proliferation

The orderly sequence of events that constitutes the cell cycle is carefully regulated. A part of this regulation depends upon the ubiquitous calcium signalling system. Many growth factors utilize the messenger inositol trisphosphate (InsP3) to set up prolonged calcium signals, often organized in an oscillatory pattern. These repetitive calcium spikes require both the entry of external calcium and its release from internal stores. One function of this calcium signal is to activate the immediate early genes responsible for inducing resting cells (G0) to re-enter the cell cycle. It may also promote the initiation of DNA synthesis at the G1/S transition. Finally, calcium contributes to the completion of the cell cycle by stimulating events at mitosis. The role of calcium in cell proliferation is highlighted by the increasing number of anticancer therapies and immunosuppressant drugs directed towards this calcium signalling pathway.

Analogs of alkyllysophospholipids: chemistry, effects on the molecular level and their consequences for normal and malignant cells

In the search for new approaches to cancer therapy, the first alkyllysophospholipid (ALP) analogs were designed and studied about two decades ago, either as potential immunomodulators or as antimetabolites of phospholipid metabolism. In the meantime, it has been demonstrated that they really act in this way. However, their special importance is based on the fact that, in addition, they interfere with key events of signal transduction, such as hormone (or cytokine)-receptor binding or processing, protein kinase C or phospholipase C function and phosphatidylinositol and calcium metabolism. There are no strict structural requirements for their activity. Differences in the cellular uptake or the state of cellular differentiation seem to be mainly responsible for higher or lower sensitivities of cells towards ALP analogs. Consequences of the molecular effects mentioned on the cellular level are cytostasis, induction of differentiation (while in contrast the effects of known inducers of differentiation such as 12-O-tetradecanoylphorbol-13-acetate are inhibited, probably as a consequence of protein kinase C inhibition) and loss of invasive properties. Already in sublytic concentrations, alterations in the membrane structure were observed, and lysis may begin at concentrations not much higher than those causing the other effects described. Few ALP analogs have already entered clinical studies or are in clinical use. ALP analogs are the only antineoplastic agents that do not act directly on the formation and function of the cellular replication machinery. Therefore, their effects are independent of the proliferative state of the target cells. Because of their interference with cellular regulatory events, including those failing in cancer cells, ALP analogs, beyond their clinical importance, are interesting model compounds for the development of new, more selective drugs for cancer therapy.

Alternatively spliced juxtamembrane domain of a tyrosine kinase receptor is a multifunctional regulatory site. Deletion alters cellular tyrosine phosphorylation pattern and facilitates binding of phosphatidylinositol-3-OH kinase to the hepatocyte growth factor receptor

The hepatocyte growth factor (HGF) receptor is a tyrosine kinase receptor that mediates signal transduction upon ligand stimulation. This receptor is present in mouse tissues as two major isoforms differing by a 47-amino acid segment in the juxtamembrane domain, an alternatively spliced cytoplasmic region adjacent to the transmembrane domain of the receptor. We report here that the juxtamembrane domain of the receptor is involved in the regulation of downstream signal transduction. The two receptor isoforms were transiently expressed in COS-7 cells. Both exogenous receptors underwent autophosphorylation and subsequently stimulated a set of protein tyrosine phosphorylations that were not present in control cells. Comparisons of phosphotyrosine profiles of transfected cell lysates induced by receptor isoforms demonstrated that at least three phosphorylated proteins of approximately 62, approximately 35, and approximately 30 kDa were differentially induced by the receptor isoforms, suggesting that the juxtamembrane domain of a kinase receptor can play a role in selective signal transduction. Furthermore, the p85 subunit of phosphatidylinositol-3-OH kinase (PI3 kinase) co-precipitated with the small isoform of the HGF receptor, and this association was dramatically inhibited by treatment with 12-O-tetradecanoylphorbol-13-acetate. Since removal of the juxtamembrane domain facilitates the binding of p85 to the receptor, it is likely that the juxtamembrane region plays a role in negative regulation of the binding of PI3 kinase to the HGF receptor. Our study establishes novel molecular sequelae of alternative splicing of an intracellular domain of the HGF receptor.