Stimulated Raman scattering microscopy: an emerging tool for drug discovery

Optical microscopy techniques have emerged as a cornerstone of biomedical research, capable of probing the cellular functions of a vast range of substrates, whilst being minimally invasive to the cells or tissues of interest. Incorporating biological imaging into the early stages of the drug discovery process can provide invaluable information about drug activity within complex disease models. Spontaneous Raman spectroscopy has been widely used as a platform for the study of cells and their components based on chemical composition; but slow acquisition rates, poor resolution and a lack of sensitivity have hampered further development. A new generation of stimulated Raman techniques is emerging which allows the imaging of cells, tissues and organisms at faster acquisition speeds, and with greater resolution and sensitivity than previously possible. This review focuses on the development of stimulated Raman scattering (SRS), and covers the use of bioorthogonal tags to enhance sample detection, and recent applications of both spontaneous Raman and SRS as novel imaging platforms to facilitate the drug discovery process.

Spermine toxicity in BHK-21/C13 cells in the presence of bovine serum: The effect of aminoguanidine

Spermine was toxic to BHK-21/C13 cells in the presence of newborn calf serum and the toxicity, but not the metabolism of spermine, was prevented by aminoguanidine. Aminoguanidine treatment resulted in significant alterations in the polyamine profile of these cells with loss of intracellular putrescine after 4 hr of exposure. In the presence of aminoguanidine, intracellular polyamine content returned to control values at 24 hr, possibly as a result of increased uptake of exogenous spermine.

Role of Src family members in breast cancer-dependent on site of phosphorylation

Abstract #2074

Background: In vitro work implicates c-Src in breast cancer. However there is little evidence to support this in clinical specimens. Activation of Src family members are associated with phosphorylation at two different tyrosine sites Y419: the classical activation site and Y215, known to induce a 50-fold increase in activation.
 We have analysed a cohort of human breast cancers to establish if expression levels of 2 Src family members (c-Src and Lyn) are associated with survival and whether this is dependent on site of activation.
 Methods: Tissue microarrays were constructed from 895 breast cancer tumors. Median follow up was 6 years with 229 breast cancer specific deaths. Immunohistochemistry was performed using antibodies to c-Src, Lyn, pSrc419 and pSrc215 (antibodies to Y419 and Y215 will detect phosphorylation of either c-Src and Lyn). Expression was assessed by two independent scorers. All statistical calculations were performed using SPSS 15.
 Results: Membrane expression of c-Src and Lyn was rarely observed. However, cytoplasmic expression of c-Src and Lyn was frequently observed and further results presented relate to this site. High expression levels of c-Src but not Lyn were associated with HER2 positivity (p=0.001) and ER negativity (p<0.001).
 High c-Src expression was associated with a poor outcome. In direct contrast pSrc215 was associated with improved outcome on univariate and multivariate analysis.
 Antibodies to phosphorylation sites will detect both activated c-Src and Lyn (and other family members). The individual impact of phosphorylation on clinical outcome was determined by categorising expression based on overexpression of c-Src or Lyn in combination with activation at each site.
 
 Discussion: Our findings demonstrate that phosphorylation of c-Src at Y419 is associated with poor prognosis in breast cancer. In contrast phosphorylation at Y215 was independently associated with better outcome.
 Further work is ongoing to explore our hypothesis that:
 1. phosphorylation at 215 'deactivates' c-Src or desensitizes downstream pathways or,
 2. that activation of Lyn or other Src family members at 215 is responsible for good outcome.
 If confirmed, these results strongly suggest that commercial development of highly specific Src inhibitors is necessary and that these will require appropriate patient selection.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 2074.

p120-catenin is required for the collective invasion of squamous cell carcinoma cells via a phosphorylation-independent mechanism

Loss of E-cadherin-mediated cell-cell junctions has been correlated with cancer cell invasion and poor patient survival. p120-catenin has emerged as a key player in promoting E-cadherin stability and adherens junction integrity and has been proposed as a potential invasion suppressor by preventing release of cells from the constraints imposed by cadherin-mediated cell-cell adhesion. However, it has been proposed that tyrosine phosphorylation of p120 may contribute to cadherin-dependent junction disassembly during invasion. Here, we use small interfering RNA (siRNA) in A431 cells to show that knockdown of p120 promotes two-dimensional migration of cells. In contrast, p120 knockdown impairs epidermal growth factor-induced A431 invasion into three-dimensional matrix gels or in organotypic culture, whereas re-expression of siRNA-resistant p120, or a p120 isoform that cannot be phosphorylated on tyrosine, restores the collective mode of invasion employed by A431 cells in vitro. Thus, p120 promotes A431 cell invasion in a phosphorylation-independent manner. We show that the collective invasion of A431 cells depends on the presence of cadherin-mediated (P- and E-cadherin) cell-cell contacts, which are lost in cells where p120 expression is knocked down. Furthermore, membranous p120 is maintained in invasive squamous cell carcinomas in tumours suggesting that p120 may be important for the collective invasion of tumours cells in vivo.

Calpain 2 and Src dependence distinguishes mesenchymal and amoeboid modes of tumour cell invasion: a link to integrin function

Cancer cells can invade three-dimensional matrices by distinct mechanisms, recently defined by their dependence on extracellular proteases, including matrix metalloproteinases. Upon treatment with protease inhibitors, some tumour cells undergo a 'mesenchymal to amoeboid' transition that allows invasion in the absence of pericellular proteolysis and matrix degradation. We show here that in HT1080 cells, this transition is associated with weakened integrin-dependent adhesion, consistently reduced cell surface expression of the alpha2beta1 integrin collagen receptor and impaired signalling downstream, as judged by reduced autophosphorylation of focal adhesion kinase (FAK). On examining cancer cells that use defined invasion strategies, we show that distinct from mesenchymal invasion, amoeboid invasion is independent of intracellular calpain 2 proteolytic activity that is usually needed for turnover of integrin-linked adhesions during two-dimensional planar migration. Moreover, an inhibitor of Rho/ROCK signalling, which specifically impairs amoeboid-like invasion, restores cell surface expression of alpha2beta1 integrin, downstream FAK autophosphorylation and calpain 2 sensitivity--features of mesenchymal invasion. These findings link weakened integrin function to a lack of requirement for calpain 2-mediated integrin adhesion turnover during amoeboid invasion. In keeping with the need for integrin adhesion turnover, mesenchymal invasion is uniquely sensitive to Src inhibitors. Thus, the need for a major pathway that controls integrin adhesion turnover defines and distinguishes cancer cell invasion strategies.

Cell adhesion receptors, tyrosine kinases and actin modulators: a complex three-way circuitry

The interaction of cells with surrounding matrix and neighbouring cells governs many aspects of cell behaviour. Aside from transmitting signals from the external environment, adhesion receptors also receive signals from the cell interior. Here we review the interrelationship between adhesion receptors, tyrosine kinases (both growth factor receptor and non-receptor) and modulators of the actin cytoskeletal network. Deregulation of many aspects of these signalling pathways in cancer highlights the need for a better understanding of the complexities involved.

Elevated c-Src is linked to altered cell-matrix adhesion rather than proliferation in KM12C human colorectal cancer cells

Elevated expression and/or activity of c-Src, the prototype of the Src family of protein tyrosine kinases, is associated with the development of human colon cancer. However, despite the known pleiotropic effects of these kinases in promoting (a) cell growth downstream of growth factor receptors, and (b) the dynamic regulation of integrin adhesions in fibroblast model systems, their precise role in epithelial cancer cells is unknown. Here we addressed whether elevated expression and activity of cellular Src alters cell proliferation and/or cell-matrix adhesion in cancer cells from the Fidler model of colorectal metastasis. Although elevated Src correlates with ability to metastasise to the liver after intrasplenic injection, we found that this was not linked to enhanced growth, either in vitro or in vivo as sub-cutaneous tumours. However, elevated Src was associated with enhanced attachment to extracellular matrix. In addition, adhesion to fibronectin, was suppressed by agents that inhibited Src activity, while enforced elevation of Src in non-metastatic cells was sufficient to stimulate adhesion to fibronectin and enhanced assembly of adhesion complexes, without influencing cell growth. Thus, we conclude that one role of elevated Src in human colon cancer cells is to modulate integrin-dependent cell-matrix attachment and formation of adhesion structures, which may, in turn, influence cell motility and integrin-dependent cellular responses.

Coordination of cell polarization and migration by the Rho family GTPases requires Src tyrosine kinase activity

BACKGROUND

The ability of a cell to polarize and move is governed by remodeling of the cellular adhesion/cytoskeletal network that is in turn controlled by the Rho family of small GTPases. However, it is not known what signals lie downstream of Rac1 and Cdc42 during peripheral actin and adhesion remodeling that is required for directional migration.

RESULTS

We show here that individual members of the Rho family, RhoA, Rac1, and Cdc42, direct the specific intracellular targeting of c-Src tyrosine kinase to focal adhesions, lamellipodia, or filopodia, respectively, and that the adaptor function of c-Src (the combined SH3/SH2 domains coupled to green fluorescent protein) is sufficient for targeting. Furthermore, Src's catalytic activity is absolutely required at these peripheral cell-matrix attachment sites for remodeling that converts RhoA-dependent focal adhesions into smaller focal complexes along Rac1-induced lamellipodia (or Cdc42-induced filopodia). Consequently, cells in which kinase-deficient c-Src occupies peripheral adhesion sites exhibit impaired polarization toward migratory stimuli and reduced motility. Furthermore, phosphorylation of FAK, an Src adhesion substrate, is suppressed under these conditions.

CONCLUSIONS

Our findings demonstrate that individual Rho GTPases specify Src's exact peripheral localization and that Rac1- and Cdc42-induced adhesion remodeling and directed cell migration require Src activity at peripheral adhesion sites.

The protrusive phase and full development of integrin-dependent adhesions in colon epithelial cells require FAK- and ERK-mediated actin spike formation: deregulation in cancer cells

Integrins play an important role in tumour progression by influencing cellular responses and matrix-dependent adhesion. However, the regulation of matrix-dependent adhesion assembly in epithelial cells is poorly understood. We have investigated the integrin and signalling requirements of cell-matrix adhesion assembly in colon carcinoma cells after plating on fibronectin. Adhesion assembly in these, and in the adenoma cells from which they were derived, was largely dependent on alpha v beta 6 integrin and required phosphorylation of FAK on tyrosine-397. The rate of fibronectin-induced adhesion assembly and the expression of both alpha v beta 6 integrin and FAK were increased during the adenoma-to-carcinoma transition. The matrix-dependent adhesion assembly process, particularly the final stages of complex protrusion that is required for optimal cell spreading, required the activity of extracellular signal-regulated kinase (ERK). Furthermore, phosphorylated ERK was targeted to newly forming cell--matrix adhesions in the carcinoma cells but not the adenoma cells, and inhibition of FAK--tyrosine-397 phosphorylation or MEK suppressed the appearance of phosphorylated ERK at peripheral sites. In addition, inhibition of MEK--ERK activation blocked the formation of peripheral actin microspikes that were necessary for the protrusive phase of cell-matrix adhesion assembly. Thus, MEK--ERK--dependent peripheral actin re-organization is required for the full development of integrin-induced adhesions and this pathway is stimulated in an in vitro model of colon cancer progression.

The SH3 domain directs acto-myosin-dependent targeting of v-Src to focal adhesions via phosphatidylinositol 3-kinase

The v-Src oncoprotein is translocated to integrin-linked focal adhesions, where its tyrosine kinase activity induces adhesion disruption and cell transformation. We previously demonstrated that the intracellular targeting of Src is dependent on the actin cytoskeleton, under the control of the Rho family of small G proteins. However, the assembly of v-Src into focal adhesions does not require its catalytic activity or myristylation-dependent membrane association. Here, we report that the SH3 domain is essential for the assembly of focal adhesions containing the oncoprotein by mediating a switch from a microtubule-dependent, perinuclear localization to actin-associated focal adhesions; furthermore, v-Src translocation to focal adhesions requires myosin activity, at least under normal conditions when the actin cytoskeleton is being dynamically regulated. Although the SH3 domain of v-Src is also necessary for its association with focal adhesion kinase (FAK), which is often considered a likely candidate mediator of focal adhesion targeting via its carboxy-terminal targeting sequence, we show here that binding to FAK is not essential for the targeting of v-Src to focal adhesions. The p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase also associates with v-Src in an SH3-dependent manner, but in this case inhibition of PI 3-kinase activity suppressed assembly of focal adhesions containing the oncoprotein. Thus, the Src SH3 domain, which binds PI 3-kinase and which is necessary for activation of Akt downstream, is required for the actin-dependent targeting of v-Src to focal adhesions.

Adhesion-linked kinases in cancer; emphasis on src, focal adhesion kinase and PI 3-kinase

Our understanding of the complex signal transduction pathways involved in signalling within cancer cells, between cancer cells and between cancer cells and their environment has increased dramatically in recent years. Here we concentrate on three non-receptor kinases: Src, focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI 3-kinase). These form part of a complex network of intracellular signals which is thought to be important in regulating cancer cells.

E-cadherin at the cell periphery is a determinant of keratinocyte differentiation in vitro

The origin of the signal for keratinocyte differentiation is still unknown. Here, we show that Ca(2+)- and density-induced translocation of E-cadherin, but not P-cadherin, is accompanied by induction of differentiation-specific proteins in cultured keratinocytes. Antibodies that artificially cluster cell-surface E-cadherin in low extracellular Ca(2+) also induce differentiation-specific proteins, implicating E-cadherin as a determinant of keratinocyte differentiation in vitro.

The catalytic activity of the Src family kinases is required to disrupt cadherin-dependent cell-cell contacts

Despite the importance of epithelial cell contacts in determining cell behavior, we still lack a detailed understanding of the assembly and disassembly of intercellular contacts. Here we examined the role of the catalytic activity of the Src family kinases at epithelial cell contacts in vitro. Like E- and P-cadherin, Ca(2+) treatment of normal and tumor-derived human keratinocytes resulted in c-Yes (and c-Src and Fyn), as well as their putative substrate p120(CTN), being recruited to cell-cell contacts. A tyrosine kinase inhibitor with selectivity against the Src family kinases, PD162531, and a dominant-inhibitory c-Src protein that interferes with the catalytic function of the endogenous Src kinases induced cell-cell contact and E-cadherin redistribution, even in low Ca(2+), which does not normally support stable cell-cell adhesion. Time-lapse microscopy demonstrated that Src kinase inhibition induced stabilization of transiently formed intercellular contacts in low Ca(2+). Furthermore, a combination of E- and P-cadherin-specific antibodies suppressed cell-cell contact, indicating cadherin involvement. As a consequence of contact stabilization, normal cells were unable to dissociate from an epithelial sheet formed at high density and repair a wound in vitro, although individual cells were still motile. Thus, cadherin-dependent contacts can be stabilized both by high Ca(2+) and by inhibiting Src activity in low (0.03 mM) Ca(2+) in vitro.

Increased dosage and amplification of the focal adhesion kinase gene in human cancer cells

Focal adhesion kinase (pp125FAK) is present at sites of cell/extracellular matrix adhesion and has been implicated in the control of cell behaviour. In particular, as a key component of integrin-stimulated signal transduction pathways, pp125FAK is involved in cellular processes such as spreading, motility, growth and survival. In addition, a number of reports have indicated that pp125FAK may be up-regulated in human tumour cells of diverse origin, and consequently, a role has been proposed for pp125FAK in the development of invasive cancers. However, to date the mechanisms that lead to elevated pp125FAK expression in tumour cells have not been determined. Here we used in situ hybridization to confirm chromosome 8q as the genomic location of the human fak gene and report that elevation of pp125FAK protein in cell lines derived from invasive squamous cell carcinomas is accompanied by gains in copy number of the fak gene in all cases examined. In addition, we observed increased fak copy number in frozen sections of squamous cell carcinomas. Furthermore, increased dosage of the fak gene was also observed in many cell lines derived from human tumours of lung, breast and colon, including two cell lines Calu3 and HT29, in which fak was amplified. In addition, in an in vitro model for human colon cancer progression there was a copy number gain of the fak gene during conversion from adenoma to carcinoma, which was associated with increased pp125FAK protein expression. Thus, we show for the first time that many cell lines derived from invasive epithelial tumours have increased dosage of the fak gene, which may contribute to the elevated protein expression commonly observed. Although other genes near the fak locus are co-amplified or increased in copy number, including the proto-oncogene c-myc, the biological properties of pp125FAK in controlling the growth, survival and invasiveness of tumour cells, suggest that it may contribute to the selection pressure for maintaining increased dosage of the region of chromosome 8q that encodes these genes.

Geldanamycin-induced cytotoxicity in human colon-cancer cell lines: evidence against the involvement of c-Src or DT-diaphorase

We investigated two of the major proposed modes of action of the benzoquinoid ansamycin geldanamycin using a pair of human colon-carcinoma cell lines, BE and HT29. One potential mechanism of action in colorectal cancer is the inhibition of c-Src kinase activity, since this proto-oncogene is hyperexpressed in human large-bowel tumours. Our results show that despite the 9-fold higher level of c-Src kinase activity found in HT29 cells, there was only a 1.4-fold difference in cytotoxicity as compared with BE cells, the latter being the most sensitive. Moreover, even at concentrations of geldanamycin that resulted in cell kill of 80% or more after a 24-h period of exposure, there was no effect on c-Src kinase activity in HT29 cells, although c-Src protein was depleted at supralethal levels of exposure. We also investigated the metabolism of the quinone moiety of geldanamycin by DT-diaphorase, an enzyme that activates certain quinone antibiotics such as mitomycin C and is hyperexpressed in colorectal cancer cells. Geldanamycin was shown to be a substrate for DT-diaphorase present in HT29 cells. However, the lack of a major differential in cytotoxicity between HT29 and BE cells indicates that this is unlikely to be pharmacologically significant, since the former contains high levels of enzyme activity, whereas BE cells have no significant activity due to a point mutation in the DT-diaphorase (NQO1) gene. Although reduction of geldanamycin was also catalysed by non-DT-diaphorase reductases in HT29 and BE cells, providing the potential for free radical induction, this is unlikely to be significant since studies previously reported by us elsewhere showed that cells exposed to geldanamycin exhibited no evidence of DNA damage. Thus, as far as the mode of action of geldanamycin in human colon-carcinoma cells is concerned, the present results rule out two major possibilities, namely, the involvement of c-Src tyrosine kinase inhibition and DT-diaphorase metabolism.

Inhibition of phospholipid signalling and proliferation of Swiss 3T3 cells by the wortmannin analogue demethoxyviridin

Growth factors and certain oncogenes activate a range of phospholipid-mediated signal transduction pathways resulting in cell proliferation. Demethoxyviridin (DMV), a structural analogue of wortmannin and recently reported as a potent inhibitor of phosphoinositide-3-kinase, inhibited bombesin plus insulin-stimulated increase in cell number in Swiss 3T3 cells, a model of cell proliferation. The drug produced cytostatic effects at concentrations below 1 microM and cytotoxic effects at 10 microM. In intact Swiss 3T3 cells DMV inhibited insulin-stimulated PI 3- and 4-kinases and bombesin-stimulated phospholipases C, D and A2 in the nanomolar range. DMV also inhibited bombesin-stimulated tyrosine phosphorylation of a range of proteins at nM concentrations. This study shows that DMV inhibited multiple stimulated signalling pathways which lead to increased Swiss 3T3 cell proliferation. A stable analogue of DMV may have chemotherapeutic potential.

A role for epidermal growth factor receptor, c-Src and focal adhesion kinase in an in vitro model for the progression of colon cancer

We have examined the function of the epidermal growth factor (EGF) receptor, c-Src and focal adhesion kinase (FAK) in the progression of colon cancer using an in vitro progression model. A non-tumorigenic cell line was derived from a premalignant colonic adenoma (PC/AA) from which a clonogenic variant was established (AA/C1). Following sequential treatment with sodium butyrate and the carcinogen N-methyl-N'-nitro-N-nitro-soguanidine an anchorage-independent line was isolated which, with time in culture, became tumorigenic when injected into athymic nude mice (AA/C1/SB10). We have shown that both EGF receptor and FAK protein levels were elevated in the carcinoma cells as compared to the adenoma cells, while the expression and activity of c-Src were unaltered during the adenoma to carcinoma transition. EGF induced the movement of the carcinoma cells into a reconstituted basement membrane which was not seen with the premalignant adenoma cells. This increased motility was accompanied by an EGF-induced increase in c-Src kinase activity, relocalisation of c-Src to the cell periphery and phosphorylation of FAK in the carcinoma cells but not in the adenoma cells. This suggests that c-Src plays a role in the biological behaviour of colonic carcinoma cells induced by migratory factors such as EGF, perhaps acting in conjunction with FAK to regulate focal adhesion turnover and tumour cell motility. Furthermore, although c-Src has been implicated in colonic tumour progression, we demonstrate here that in the adenoma to carcinoma in vitro model c-Src is not the driving force for this progression but co-operates with other molecules in carcinoma development.

Translocation of Src kinase to the cell periphery is mediated by the actin cytoskeleton under the control of the Rho family of small G proteins

We have isolated Swiss 3T3 subclones that are resistant to the mitogenic and morphological transforming effects of v-Src as a consequence of aberrant translocation of the oncoprotein under low serum conditions. In chicken embryo and NIH 3T3 fibroblasts under similar conditions, v-Src rapidly translocates from the perinuclear region to the focal adhesions upon activation of the tyrosine kinase, resulting in downstream activation of activator protein-1 and mitogen-activated protein kinase, which are required for the mitogenic and transforming activity of the oncoprotein. Since serum deprivation induces cytoskeletal disorganization in Swiss 3T3, we examined whether regulators of the cytoskeleton play a role in the translocation of v-Src, and also c-Src, in response to biological stimuli. Actin stress fibers and translocation of active v-Src to focal adhesions in quiescent Swiss 3T3 cells were restored by microinjection of activated Rho A and by serum. Double labeling with anti-Src and phalloidin demonstrated that v-Src localized along the reformed actin filaments in a pattern that would be consistent with trafficking in complexes along the stress fibers to focal adhesions. Furthermore, treatment with the actin-disrupting drug cytochalasin D, but not the microtubule-disrupting drug nocodazole, prevented v-Src translocation. In addition to v-Src, we observed that PDGF-induced, Rac-mediated membrane ruffling was accompanied by translocation of c-Src from the cytoplasm to the plasma membrane, an effect that was also blocked by cytochalasin D. Thus, we conclude that translocation of Src from its site of synthesis to its site of action at the cell membrane requires an intact cytoskeletal network and that the small G proteins of the Rho family may specify the peripheral localization in focal adhesions or along the membrane, mediated by their effects on the cytoskeleton.

In vivo pharmacology and anti-tumour evaluation of the tyrphostin tyrosine kinase inhibitor RG13022

Amplification and increased expression of many growth factor receptors, including the epidermal growth factor receptor (EGFR), has been observed in human tumours. One therapeutic strategy for overcoming EGF autocrine control of tumour growth is inhibition of EGFR protein tyrosine kinase (PTK). A series of low molecular weight molecules have been identified which inhibit the EGFR PTK in vitro and demonstrate antiproliferative activity against human cancer cell lines with high expression of EGFR. A significant growth delay in squamous cancer xenografts has been reported for one of these compounds, the tyrphostin RG13022. Based on these encouraging results, we sought to confirm the activity of RG13022 in vivo and relate the effects to the in vivo plasma disposition. RG13022 and three additional peaks were detected by HPLC following intraperitoneal administration of 20 mg kg-1 RG13022 in MF1 nu/nu mice. RG13022 demonstrated rapid biexponential elimination from plasma with a terminal half-life of 50.4 min. RG13022 plasma concentrations were less than 1 microM by 20 min post injection. A primary product was identified as the geometrical isomer (E)-RG13022. Both RG13022 and its geometrical isomer inhibited DNA synthesis in HN5 cells after a 24 h in vitro incubation (IC50 = 11 microM and 38 microM respectively). Neither RG13022 nor its geometrical isomer displayed significant cytotoxicity. RG13022 had no influence on the growth of HN5 tumours when administered chronically, starting either on the day of tumour inoculation or after establishment of tumour xenografts. The rapid in vivo elimination of RG13022 has potential significance to the development of this and other related tyrphostin tyrosine kinase inhibitors, as plasma concentrations fell below that required for in vitro activity by 20 min post injection. The lack of in vivo tumour growth delay suggests that a more optimal administration schedule for RG13022 would include more frequent injections or continuous administration. An improved formulation for RG13022 is therefore required before further development of this or other similar protein tyrosine kinase inhibitors can be made. Alternative strategies should also be sought which display longer lasting in vivo exposures.

Synthesis and antiproliferative activity of tyrphostins containing quinoline moieties

Tyrphostins are a series of benzylidenemalononitrile derivatives synthesized by condensing aromatic aldehydes with malononitrile derivatives. The use of heteroaromatic aldehydes in this process has received little attention. Accordingly, 27 tyrphostins containing a 2-, 3- or 4-substituted quinoline moiety were synthesized, of which 21 are novel compounds Compounds containing the 2-aminoethene-1, 1-dinitrile moiety in each series were the most potent inhibitors of the EGF receptor kinase in a cell-free enzyme assay (compounds 2, 11 and 20), having IC50 values of 1.7, 27.0 and 4.7 microM respectively. For each R group substitution the order of potency was 2-quinolines > 4-quinolines > 3-quinolines. Compounds 2, 11 and 20 were unable to inhibit the epidermal growth factor (EGF) receptor autophosphorylation in intact cells; however, they were able to inhibit the EGF-dependent phosphorylation of a 50 kDa protein. These three compounds were able to inhibit EGF-dependent proliferation in a fibroblast cell more efficiently than serum-stimulated proliferation, suggesting that their mechanism of action may be linked to the EGF receptor signalling pathway. Compound 2 exhibited a degree of cell line selectivity in the US National Cancer Institute in vitro human tumour cell line panel. The majority of non-small cell lung cancer lines were relatively resistant to compound 2, while most of the colon, CNS, melanoma and renal lines were relatively sensitive. Further work is required to elucidate the mechanism of action of this interesting group of substituted-quinoline compounds and to determine whether for compounds 2, 11 and 20 this is related to inhibition of EGF receptor function.

Synthesis and biological evaluation of a series of tyrphostins containing nitrothiophene moieties as possible epidermal growth factor receptor tyrosine kinase inhibitors

A series of 36 nitrothiophene tyrphostins were synthesized, 32 of which were novel structures. Their ability to inhibit the epidermal growth factor (EGF) receptor tyrosine kinase was assessed in a cell-free assay. Compounds containing a dinitrile, 2-aminoethene-1, 1-dinitrile or a thioamide group were good inhibitors of the receptor tyrosine kinase. Although anti-proliferative and cytotoxic activity was seen, no evidence of inhibition of EGF receptor autophosphorylation in intact cells was observed. The compounds showed no preferential inhibition of EGF-dependent proliferation of fibroblasts transfected with the EGF receptor. Furthermore, in a panel of squamous cell carcinoma cell lines with varying levels of EGF receptor expression, there was no selective cell kill of lines with the highest EGF receptor expression. The 2-nitro-5-substituted-thiophenes and the 2-nitro-3-substituted-thiophenes showed reduction potentials falling within the range likely to be reduced by cellular reducing agents, while the 2-nitro-4-substituted-thiophenes and 4-nitro-2-substituted-thiophenes did not. Compounds from the 2-nitro-5-substituted-thiophene series were shown to induce DNA damage, while no evidence of DNA damage was demonstrated with compounds from the 2-nitro-4-substituted-thiophene series. The 2-nitro-5-substituted-thiophene compound 4 showed significant tumour-type selectivity in the US National Cancer Institute human tumour cell line panel. The leukaemia cell lines were particularly sensitive to the compound, as were the majority of the colon cancer, melanoma and breast cancer cell lines, while the central nervous system-derived lines and the non-small cell lung cancer lines were particularly resistant. Further work is required to determine the precise mechanisms involved in these effects.

Cell-cycle arrest and p53 accumulation induced by geldanamycin in human ovarian tumour cells

We have analysed the cell-cycle arrests and cytotoxicity of the A2780 human ovarian cell line in response to geldanamycin, a benzoquinoid ansamycin that can inhibit tyrosine kinases. Geldanamycin causes a dose-dependent G2 arrest and reversible inhibition of entry into the S phase in A2780 cells. After a 3-h exposure to 0.1 microM geldanamycin, the cells show an increase in accumulation of p53 protein that is maximal at 24 h after drug exposure. Increased p53 levels can be induced in cells by DNA-damaging agents; however, using alkaline elution and sister chromatid exchange assays we detect no DNA damage induced by geldanamycin. Using dominant negative mutant TP53 transfectants of A2780 we have analysed the possible dependence of geldanamycin-induced cell-cycle arrests on the presence of functional p53. We observe no difference in cell-cycle arrests in mutant p53 transfectants known to have the p53-DNA damage-response pathway inactivated as compared with vector-alone controls. Similarly, we observe no difference in clonogenic resistance to the cytotoxicity of geldanamycin in these cells. These results suggest that geldanamycin can induce increased p53 protein by a mechanism not involving DNA damage. Furthermore, the cell-cycle arrests and cytotoxic effects of geldanamycin in these cells are not mediated by p53-dependent pathways.

Alterations in EGF-dependent proliferative and phosphorylation events in squamous cell carcinoma cell lines by a tyrosine kinase inhibitor

The epidermal growth factor (EGF) receptor is overexpressed in squamous cell carcinoma and the EGF receptor has been proposed as a potential target for new therapeutic agents in this tumour type. We have utilized a tyrphostintype inhibitor of the EGF receptor tyrosine kinase domain (RG50864) to study EGF-dependent proliferation and phosphorylation in two human squamous cell carcinoma cell lines. There were selected on the basis that whereas both cell lines have a large number of EGF receptors, one is growth inhibited by EGF (A431) while the proliferation of the other cell line (B2A4) is stimulated by EGF. EGF induced receptor autophosphorylation in each of the two cell lines; however, the level of phosphorylation was greater in the A431 cells than in the B2A4 cells. The pattern of proteins phosphorylated in response to EGF was different in the two squamous cell lines. RG50864 antagonized the EGF-dependent proliferation of B2A4 cells, but was unable to reverse the inhibitory effect of EGF on A431 cell growth. RG50864 partially inhibited EGF receptor autophosphorylation in both cell lines and completely inhibited the EGF-dependent phosphorylation of other cellular proteins, one of which co-migrated with MAP2kinase in both cell lines. Moreover, different dose-response relationships for the inhibition of phosphorylation of various proteins were observed in A431 versus B2A4 cells. As a substrate competitive inhibitor of the EGF receptor tyrosine kinase, the primary mode of action of RG50864 may be to prevent the association and/or phosphorylation of multiple specific substrates of the receptor in a fashion which may be cell line dependent. The precise relationship of these phosphorylation events to tyrphostin sensitivity remains to be established.

Synthesis and antiproliferative activity of tyrphostins containing heteroaromatic moieties

A series of benzylidenemalononitrile derivatives previously synthesized by condensing aromatic aldehydes with malononitrile derivatives are known as tyrphostins. In this study, 32 tyrphostins were synthesized, 19 of which are novel compounds. Both hydroxylated derivatives and compounds containing heteroaromatic moieties were prepared. We have confirmed and extended the observation that the tyrphostins displayed an enhancement in their ability to inhibit the epidermal growth factor (EGF) receptor tyrosine kinase domain as the number of hydroxyl groups on the aromatic portion was increased. IC50 values of 1-5 microM were readily achieved. Some inhibitory activity was seen with the heteroaromatic structures, with two compounds exhibiting IC50 values of 56 and 77 microM. However, these derivatives were poor inhibitors of the EGF receptor tyrosine kinase activity as compared to the hydroxylated derivatives. The ability of the 32 tyrphostins synthesized in the present study to inhibit proliferation of a human breast adenocarcinoma cell line (MCF-7) was determined using [3H]thymidine incorporation as a measure of DNA synthesis. Some of the compounds containing pyridine, imidazole or thiophene portions displayed antiproliferative activity comparable to that of tyrphostins prepared from 3,4,5-trihydroxybenzaldehyde. The lack of inhibitory effect of these heteroaromatic compounds on the EGF receptor tyrosine kinase activity suggests that their antiproliferative activity is not related to inhibition of EGF receptor function. As the growth of the MCF-7 cell line is governed by other factors, such as the insulin-like growth factors (IGFs) and oestradiol, it is also still to be established whether the antiproliferative activity of the hydroxylated tyrphostins is directly related to inhibition of the EGF receptor tyrosine kinase activity.

In vitro antitumour activity of the novel imidazoisoquinoline SDZ 62-434

The novel imidazoisoquinoline SDZ 62-434, originally identified as a platelet-activating factor (PAF) antagonist, has antiproliferative activity in a range of cell lines from human solid and haematological malignancies. Using an MTT cytotoxicity assay, IC50 values of 5 microM - 111 microM were observed following a 24 h exposure. Similar results were obtained using a clonogenic assay. The HT29 colon adenocarcinoma was particularly sensitive while the MCF-7 breast carcinoma was the most resistant in our panel. Only a 2-3 fold cross-resistance was seen in the doxorubicin and cisplatin resistant variants of the A2780 ovarian carcinoma; the drug did not modulate sensitivity to doxorubicin in either parent or resistant lines. No cross-resistance to SDZ 62-434 was seen in a doxorubicin-resistant MCF-7 variant. Cytotoxicity was not due to non-specific membrane lysis. The potent PAF antagonist WEB 2086 did not modulate SDZ 62-434 cytotoxicity, indicating no role for PAF receptors. Precursor incorporation studies in A2780 cells showed that DNA synthesis was inhibited more effectively than protein synthesis while RNA synthesis was unaffected. SDZ 62-434 inhibited both bombesin and platelet-derived growth factor-induced DNA synthesis in quiescent Swiss 3T3 cells. This suggests a possible role for SDZ 62-434 as an inhibitor of signal transduction in cancer cells.

Tyrosine kinase inhibitors

Tyrosine kinases are associated with the cytoplasmic domains of growth factor receptors as well as oncoproteins and many have the potential to cause transformation if mutated or hyperexpressed. Tyrosine kinases therefore represent an excellent target for the development of cancer drugs. A large number of inhibitors have now been identified and many show promising cytostatic activity, particularly using in vitro models. Some in vivo activity has been reported. Progress with various structural classes is reviewed. It is not clear whether specific or broad spectrum tyrosine kinase inhibitors should be developed as potential anticancer drugs. It does seem likely, however, that tyrosine inhibitors will enter clinical trial in cancer patients.

Mechanisms of spermine toxicity in baby-hamster kidney (BHK) cells. The role of amine oxidases and oxidative stress

Spermine was toxic to BHK-21/C13 cells in the absence of any extracellular metabolism of the amine. Inhibition of copper-containing amine oxidases with aminoguanidine partially prevented the response, whereas inhibition of polyamine oxidase with MDL-72,527 exacerbated the effect. Oxidation by an intracellular copper-containing amine oxidase may be involved in the toxicity of spermine, whereas the polyamine-interconversion pathway appears to play a cytoprotective role. There was no evidence for spermine imposing a state of oxidative stress within the cells. Inhibition of catalase and glutathione reductase did not alter the cytotoxicity of spermine, and there was no excretion of oxidized glutathione into the extracellular medium. The results suggest that spermine itself can exert a toxic effect directly on the cells.

Spermine toxicity and glutathione depletion in BHK-21/C13 cells

Spermine, a polycationic amine, produced a dose-dependent inhibition of BHK-21/C13 cell growth. This response was not due to the extracellular metabolism of spermine by an amine oxidase found in bovine serum, as the toxicity was observed when the cells were grown in medium supplemented with horse serum. Three indices were used to monitor cell growth, cell number, protein content and [3H]thymidine incorporation into DNA. Spermine (2mM) caused significant reductions in all three measurements after a 6-8 hr exposure. The amine was rapidly taken up into the cells reaching levels 15-16-fold greater than in control cells within 2 hr. There was a rapid loss of intracellular reduced glutathione following exposure to toxic concentrations of spermine, which occurred before any effect on cell growth. Three methods for the determination of intracellular glutathione content were compared in this system. The effect on both cell growth and glutathione was reversible following removal of spermine from the extracellular medium. The possible mechanisms involved in this toxic response are discussed with particular reference to the depletion in intracellular reduced glutathione.