Blockade of the MAP kinase pathway suppresses growth of colon tumors in vivo.

The mitogen-activated protein kinase pathway is thought to be essential in cellular growth and differentiation. Here we report the discovery of a highly potent and selective inhibitor of the upstream kinase MEK that is orally active. Tumor growth was inhibited as much as 80% in mice with colon carcinomas of both mouse and human origin after treatment with this inhibitor. Efficacy was achieved with a wide range of doses with no signs of toxicity, and correlated with a reduction in the levels of activated mitogen-activated protein kinase in excised tumors. These data indicate that MEK inhibitors represent a promising, noncytotoxic approach to the clinical management of colon cancer.

Comparative utilization of n-3 polyunsaturated fatty acids by cultured human Y-79 retinoblastoma cells.

The Y-79 retinoblastoma cell, a cultured human line derived from the retina, was utilized as a model for investigating the metabolism of n-3 polyunsaturated fatty acids in neural tissue. When cultures were incubated with 5 microM linolenic (18:3), eicosapentaenoic (20:5) or docosahexaenoic (22:6) acids, a low concentration probably representative of physiologic levels, the amount incorporated was 20:5 congruent to 18.3 greater than 22:6. Regardless of which fatty acid was provided, 65-75% of the total uptake accumulated in phosphatidylethanolamine and ethanolamine plasmalogen, suggesting that these phospholipids play an important role in n-3 polyunsaturated fatty acid metabolism. A small amount of 22:6 was converted to 20:5, which was recovered in phosphatidylinositol and phosphatidylserine. Therefore, one metabolic function of 22:6 may be to serve as an intracellular storage pool for the formation of 20:5 through retroconversion. When any of the n-3 polyunsaturates was available, the main fatty acid that accumulated in the cell phospholipids was 22:6. The extent to which 22:6 accumulated, however, depended on the particular n-3 polyunsaturated fatty acid that was available. This suggests that the 22:6 content of a neural cell, and any cellular function dependent on 22:6 content, may be regulated by changes in the type of n-3 polyunsaturate available to the nervous system.

Functional differences in insulin receptors in rat adipocyte and human placenta membranes.

Incubation of human placenta membranes with low concentrations (0.1-0.2 mM) of dithiothreitol (DTT) increased insulin binding approximately 1.4-fold, while 10 mM DTT completely inhibited insulin binding. In contrast, treatment of rat adipocyte membranes with 0.5-2.0 mM DTT increased tracer insulin binding 3- to 6-fold, while higher levels of DTT (10 mM) also fully inhibited insulin binding. Scatchard analysis of insulin binding revealed that DTT treatment of adipocyte membranes resulted in an increase in both the high and low affinity dissociation constants. Purification of adipocyte insulin receptors by wheat germ agglutinin-Sepharose chromatography, followed by insulin-agarose affinity chromatography, resulted in loss of DTT stimulation of insulin binding. Comparison of insulin receptors purified from rat adipocytes or human placenta membranes revealed no significant differences in the DTT sensitivities of insulin binding or protein kinase activities. These data suggest that the functional properties of the rat adipocyte insulin receptor are modified by its membrane environment compared to those of insulin receptors in placenta membranes or purified insulin receptors in detergent solution.

Structure-activity relationships of C-terminal endothelin hexapeptide antagonists.

The discovery of selective endothelin (ET) receptor antagonists will facilitate identification of the physiological and pathological roles for ET and its isopeptides. Structure-activity studies of the C-terminal hexapeptide of ET have been carried out to elucidate those amino acids important for receptor binding and agonist or antagonist activity. Binding studies were performed in rat heart ventricle, rabbit renal artery vascular smooth muscle cells, and rat cerebellum. In addition, many of the compounds have been evaluated functionally for their effects on endothelin-1-induced arachidonic acid release and inositol phosphate accumulation in specific cell lines. Selected compounds have been evaluated in a functional bioassay in tissue preparations specifically expressing either ETA or ETB receptors. We have previously described the structure-activity relationships in the hydrophobic C-terminal hexapeptide of ET, a region known to be highly important for receptor recognition. A mono-D-amino acid scan of the ET[16-21] revealed that substitution at His gave rise to analogs with significantly enhanced binding affinity. We have further evaluated the C-terminal region and will describe the design, synthesis, and pharmacological evaluation of several novel and potent ET peptide receptor antagonists.

Temperature dependence and effect of membrane lipid alteration on melphalan transport in L1210 murine leukemia cells.

The transport of melphalan by L1210 lymphoblastic leukemia cells was markedly temperature dependent, and the Arrhenius plot demonstrated a biphasic pattern. Modification of the lipid composition of the cell phospholipids had a significant effect on the transition temperature for melphalan transport even though the K'm and V'max were the same in both types of cells. This shift in transition temperature demonstrates that the discontinuity in the Arrhenius plot was the result of a lipid phase transition and not of an interaction of the two carrier-mediated transport processes. These data indicating differential membrane lipid effects on the transport mechanism provide further evidence of a relationship between lipids and membrane drug transport.

Suppression of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated CYP1A1 and CYP1B1 induction by 12-O-tetradecanoylphorbol-13-acetate: role of transforming growth factor beta and mitogen-activated protein kinases.

The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) enhances or suppresses the transcriptional activation of CYP1A1 by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a cell/tissue-specific manner. The basis for these effects is not known. Exposure of the immortalized human breast epithelial cell line MCF10A-Neo to TPA at the time of, or up to 12 hr prior to, the addition of TCDD strongly suppressed the transcriptional activation of CYP1A1 and CYP1B1 (IC(50) approximately 0.5 nM). A recent study (Carcinogenesis 2000;21:1303-12) demonstrated that TPA-treated MCF10A-Neo cells rapidly activate the latent transforming growth factor beta (TGFbeta) in the serum used to supplement the culture medium. The suppressive effects of TPA on CYP1A1 induction by TCDD in MCF10A-Neo cultures could be partially suppressed by: (a) co-incubation of TCDD + TPA-treated cultures with a neutralizing TGFbeta pan antibody; (b) prior removal of latent TGFbeta from the culture medium; or (c) switching cultures to serum- and growth factor-free medium immediately before the addition of TPA and TCDD. Exposure of cultures to TPA 24-48 hr prior to subsequent TPA + TCDD treatment not only inhibited the suppressive effects of TPA, but markedly enhanced CYP1A1 mRNA accumulation. TPA caused a rapid and protracted activation of extracellular signal-regulated kinases (ERKs). Pretreatment of cultures with the mitogen-activated protein kinase kinase (MEK) inhibitor PD184352 [2-(2-chloro-4-iodo-phenylamino)-N-cyclopropyl-methoxy-3,4-difluoro-benzamide] completely inhibited ERK activation by TPA. However, PD184352 did not prevent the suppressive effects of TPA on CYP1A1 activation by TCDD. These studies demonstrate that TPA initiates protein kinase C-dependent, ERK-independent processes that suppress CYP1A1 activation by TCDD in MCF10A-Neo cells. Furthermore, TGFbeta mediates a small portion of this suppressive activity.

The role of arachidonic acid in the secretion of the amyloid precursor protein (APP).

We have studied the activation of human ml-muscarinic receptors in a genetically engineered Chinese hamster ovary cell line (CHO-ml) to determine which second messenger systems affect the secretion of APP via the non-amyloidogenic route. Carbachol activation of the signaling pathways in CHO-ml cells promotes APP secretion by activation of both protein kinase C (PKC)-dependent or Ca(++)-dependent second messenger pathways. Both pathways converge to increase the enzyme activity of phospholipase A2 (PLA2), the enzyme that releases arachidonic acid from cellular stores. Directly activating PLA2 with melittin, a peptide from bee venom, or by adding arachidonic acid directly to cultured cells increases the secretion of APP. Thus, our results indicate that arachidonic acid is yet another cellular second messenger involved in regulating the metabolism of APP in addition to PKC and cytoplasmic Ca++. Moreover, activation of PLA2 appears to be an obligatory event in increasing the secretion of APP from CHO-ml cells by the various methods of activation that we have tried thus far.

Regulated expression of angiotensin II (AT2) binding sites in R3T3 cells.

R3T3 cells are a fibroblast cell line found to selectively express the AT2 subtype of angiotensin II binding sites. We have previously shown that in these cells, the AT2 sites do not appear to be coupled to G-proteins, do not modulate any of the common second messenger pathways associated with activation of angiotensin II receptors, and do not internalize bound ligand. Here we report that expression of AT2 sites in these cells are subject to modulation by a variety of conditions. In actively growing cells the expression of AT2 sites is very low, while in confluent, quiescent cells, the expression of AT2 sites is markedly increased. Addition of serum, or growth factors, to quiescent cells causes a rapid decrease in the number of cell-surface AT2 sites. Further, incubation of cells with ligands that bind to AT2 sites causes a marked increase in the number of these sites in a time and dose dependent manner indicating homologous up-regulation of expression. These results indicate that expression of AT2 sites in R3T3 cells is under sensitive and rapid control and further indicate that these cells may be an excellent model for studying the physiological regulation of expression of these sites.

Inositol phospholipid arachidonic acid metabolism in GH3 pituitary cells.

Inositol phospholipids in cultured GH3 cells, a prolactin secreting, thyrotropin-releasing hormone (TRH) sensitive rat pituitary cell line, exhibit a preferential selectivity for incorporating arachidonic acid. Fatty acid composition data show that all inositol phospholipids are enriched in stearic and arachidonic acids to a much greater degree than other cellular phospholipids. Incubation of GH3 cells with radioactive stearate, oleate, arachidonate, eicosapentaenoate or docosahexaenoate also showed that much more stearate and arachidonate were incorporated into inositol phospholipids. In short term incubations with tracer amounts of radioactive arachidonate, incorporation was initially into phosphatidylinositol (PtdIns), with phosphatidylinositol 4-phosphate (PtdIns4P), and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] being labelled at later times. During longer incubations, all of the inositol phospholipids reach equilibrium at about 10 h, and the resulting specific activities of the three fractions were similar. These findings suggest that arachidonate is incorporated initially into PtdIns and that PtdIns is then phosphorylated. There was no release of either arachidonate or eicosanoid products when GH3 cells were incubated with TRH. However, TRH stimulation of 32P-labelled GH3 cells resulted in rapid breakdown of PtdIns(4,5)P2 and PtdIns4P, with concomitant increases in [32P]phosphatidic acid and [32P]PtdIns. When the [32P]PtdIns was further analysed by argentation chromatography to separate PtdIns molecular species, it was found that tetraenoic (stearate/arachidonate) species accounted for 80% of the stimulated labelling. The selectivity for arachidonate incorporation into inositol phospholipids coupled with turnover of the arachidonate-containing molecular species suggests that inositol phospholipids containing arachidonic acid or the diacylglycerol resulting therefrom may play a vital cellular role in GH3 cells. This role may involve the operation of the PtdIns cycle itself rather than a stimulated release of arachidonate for eicosanoid formation.

Modulation of angiotensin II receptor (AT2) mRNA levels in R3T3 cells.

R3T3 cells, a mouse fibroblast cell line, express the type 2 angiotensin II receptor (AT2), but not the AT1 subtype. We previously reported that expression of AT2 sites in these cells were regulated by various conditions: 1. The number of AT2 sites increased considerably when cells were contact-inhibited; 2. Stimulation of R3T3 cells with various mitogens caused a rapid decline of AT2 binding sites; and 3. Stimulation of cells with angiotensin ligands resulted in upregulation of the AT2 sites. In this study, to determine if altered AT2 expression is under transcriptional, posttranscriptional, or translational control, we examined the level of AT2 mRNA in R3T3 cells in response to various treatments. There was a 200-fold increase in AT2 mRNA levels in quiescent cells as compared to growing cells. Results from nuclear run-on assays suggested that the differences in AT2 mRNA levels were primarily caused by changes in the rate of AT2 gene transcription. Stimulation of cells with fibroblast growth factor caused an approximate threefold reduction of AT2 mRNA levels, and also increased the rate of degradation of AT2 mRNA, which correlated with the decrease in AT2 binding activity seen under these conditions. However, whereas treatment with angiotensin ligands increased AT2 binding activity, the level of AT2 transcripts did not increase. This pattern of expression implies that regulation of AT2 receptors occurs at multiple levels, involving translational and/or posttranslational as well as transcriptional control, and further affords the cell the ability to rapidly modulate the number of AT2 binding sites in response to changing extracellular conditions.

The phosphatidylinositol response of chondrocytes in culture.

This study examined the phosphatidylinositol (PI) response in chondrocytes. The levels of 3H-inositol phosphates (IPs) were measured in guinea pig chondrocyte cultures labelled with 3H-myo-inositol and exposed to various inflammatory mediators and known agonists of the PI response, including bradykinin, carbachol, histamine, A23187, or recombinant human interleukin-1 beta. Of the mediators used to stimulate the PI response in chondrocytes, only bradykinin was effective. The bradykinin treated cells had higher levels of IPs (37-fold) than the controls. The PI response to bradykinin was dose and time dependent.

Characterization of angiotensin II (AT2) binding sites in R3T3 cells.

Binding sites for angiotensin II were found, in a line of Swiss 3T3 cells (designated as R3T3 cells), that were insensitive to Dup 753 and dithiothreitol yet were sensitive to PD 123319, making them members of the AT2 class of angiotensin II binding sites. These binding sites appeared not to be coupled to guanine nucleotide-binding proteins, and affinity labeling experiments revealed a specifically labeled protein with an apparent molecular weight of about 100,000. Treatment of cells with angiotensin II revealed no perturbation of common signaling pathways, including stimulation of phosphatidylinositol turnover, effects on levels of cAMP, tyrosine kinase activity, and release of arachidonic acid. Also, angiotensin II or PD 123319 had no effect on cell growth, mitogenesis, or hypertrophy or on mitogenesis or hypertrophy stimulated by several growth factors. These results show that the AT2 binding site is quite distinct from the AT1 site in terms of molecular weight, binding properties, and coupling to second messenger systems. Although the significance of this novel angiotensin II binding site remains obscure, the identification of cell lines selectively expressing it should greatly aid in the understanding of its regulation and function.

Depletion of arachidonic acid from GH3 cells. Effects on inositol phospholipid turnover and cellular activation.

We have adapted rat pituitary GH3 cells to grow in delipidated culture medium. In response, esterfied linoleic acid and arachidonic acid become essentially undetectable, whereas eicosa-5,8,11-trienoic acid accumulates and oleic acid increases markedly. These changes occur in all phospholipid classes, but are particularly pronounced in inositol phospholipids, where the usual stearate/arachidonate profile is replaced with oleate/eicosatrienoate (n - 9) and stearate/eicosatrienoate (n - 9). Incubation of arachidonate-depleted cells with 10 microM-arachidonic acid for only 24 h results in extensive remodelling of phospholipid fatty acids, such that close-to-normal compositions and arachidonic acid content are achieved for the inositol phospholipids. In comparison studies with arachidonic acid-depleted or -repleted cells, it was found that the arachidonate content does not affect thyrotropin-releasing-hormone (TRH)-stimulated responses measured at long time points, including [32P]Pi labelling of phosphatidylinositol and phosphatidic acid, stimulation of protein phosphorylation, and basal or TRH-stimulated prolactin release. However, transient events such as stimulated breakdown of inositol phospholipids and an initial rise in diacylglycerol are enhanced by the presence of arachidonate. These results show that arachidonic acid itself is not required for operation of the phosphatidylinositol cycle and is not an obligatory intermediate in TRH-mediated GH3 cell activation. It is possible that any structural or functional role of arachidonic acid in these processes is largely met by replacement with eicosatrienoate (n - 9). However, since arachidonate in inositol phospholipids facilitates their hydrolysis upon stimulation by TRH, arachidonic acid apparently may have a specific role in the recognition of these lipids by phospholipase C.

Amylin increases cyclic AMP formation in L6 myocytes through calcitonin gene-related peptide receptors.

The cellular function of amylin is investigated in L6 myocytes, a rat skeletal muscle cell line. Both rat amylin and human amylin-amide acutely cause a dose-dependent increase in cyclic AMP formation in L6 myocytes. 100 nM amylin stimulates intracellular cyclic AMP concentrations 12-fold, whereas human amylin-amide at this concentration causes only a 2-fold increase. Up to 10 mM human amylin has no effect on cyclic AMP levels. Rat calcitonin gene-related peptide (CGRP) is more potent than amylin, causing a 60-fold increase over basal at 1 nM, with an EC50 value of 0.2 nM. The CGRP receptor antagonist, human CGRP8-37 (hCGRP8-37), completely blocks the stimulatory effect of both rat amylin and human amylin-amide on cyclic AMP production. [125I]CGRP binds specifically to a membrane fraction prepared from L6 [125I]CGRP with a Ki of 0.9 nM, while rat amylin also displaces [125I]CGRP with a Ki of 91 nM. Specific binding of [125I]CGRP to plasma membranes of rat liver and brain is also displaced by rat amylin with Ki values of 35 nM and 37 nM, respectively. In contrast, specific binding of [125I]amylin to numerous cells and tissues, under similar conditions, can not be demonstrated. These results suggest that the cellular effects and physiological actions of amylin may be mediated through receptors for CGRP.

Butyrate effects on normal and adapted hepatoma cells: morphological response and implications for vectoral cholesterol transport.

The cell line 4IC6, adapted for growth in 6 mM sodium butyrate from Hepatoma Tissue Culture cells [R. Chalkley, and A. Shires (1985) J. Biol. Chem. 260, 7698-7704], exhibits a fourfold increase in histone acetate turnover. The 4IC6 cells were about 25 times more resistant to butyrate relative to the parental cell line as measured by cloning efficiency. This line also maintains a flatter and more extended morphology when growing in the presence of 6 mM sodium butyrate relative to the parental line. Both cell lines maintain similar intracellular butyrate levels and incorporate [1-14C]butyrate into lipids to similar extents when incubated in medium containing high levels of the fatty acid. These results show that 4IC6 cells have not attained butyrate resistance through acquiring the ability to metabolize butyrate more efficiently or in a significantly different manner when compared with the parental cell line. The membrane lipid composition was nearly identical between the two cell types. Thus the different morphologies exhibited by each cell line were not a consequence of altered membrane lipid composition. The resistant line, 4IC6, maintains about 10-fold higher cholesterol ester levels and half the level of triglycerides found in the parental line. The butyrate-resistant cells also synthesize cholesterol at about a 1.8-fold higher rate than do the parental cells. This difference in de novo synthesis is reflected by a difference of a similar factor in the amount of radioactive cholesterol the two cell lines accumulate over 12 generations. These results are discussed with respect to models for equilibration of serum lipoprotein-derived and newly synthesized cholesterol.

The marked disparity between the sizes of angiotensin type 2 receptors from different tissues is related to different degrees of N-glycosylation.

We recently described the photoaffinity labeling and partial characterization of the angiotensin type 2 (AT2) receptor from human myometrium. In the present study, specific receptors for angiotensin II (AII) were also analyzed in a murine fibroblast cell line (R3T3) and a rat pheochromocytoma cell line (PC-12). Dose-displacement experiments with PD 123319 (an AT2-selective antagonist) completely inhibited 125I-AII binding, whereas L-158,809 (an AT1-selective antagonist) had no significant effect on 125I-AII binding, thus revealing that these two cell lines express exclusively AT2 sites. High yields of covalent and selective labelling of AT2 receptors from human myometrium, R3T3 cells, and PC-12 cells were obtained with the photosensitive analogue 125I-[Sar1,Val5,p-benzoyl-Phe8]AII. Gel permeation chromatography of Triton X-100-solubilized AT2 receptors from the three different sources revealed similar Stokes' radii of about 65 A. Interestingly, upon electrophoresis under reducing conditions, marked disparities were observed between the apparent molecular masses of AT2 receptors from the three different sources. As observed previously, AT2 receptors from human myometrium showed a molecular mass of 68 +/- 4.6 kDa. AT2 receptors from PC-12 cells showed a larger molecular mass of 113 +/- 12 kDa, whereas AT2 receptors from R3T3 cells showed a molecular mass of 91 +/- 7.8 kDa. After endoglycosidase digestion with an enzyme that cleaves N-linked saccharides, the molecular masses of the denatured AT2 receptors of human myometrium, R3T3 cells, and PC-12 cells were decreased by 54%, 66%, and 73%, to 31.3 +/- 1.6 kDa, 30.9 +/- 0.7 kDa, and 30.6 +/- 0.8 kDa, respectively. Kinetic studies with AT2 receptors from human myometrium revealed a complex, multiple-step process of deglycosylation involving at least three different sites of N-linked saccharides. These results suggest that the disparity in the sizes of AT2 receptors from different sources is mostly related to different degrees of N-glycosylation. They also imply that the AT2 receptor contains at least three asparagine-linked sites of glycosylation.