Proteins phosphorylated on tyrosine as markers of human tumor cell lines

Differences between phosphotyrosine accumulation and Neu/ErbB-2 receptor expression in astrocytic proliferative processes. Implications for glial oncogenesis

BACKGROUND

Previous work has shown that enhanced growth potential of malignant astrocytomas correlates with increased expression of growth factor receptor tyrosine kinases. The functional implications of increased receptor expression were addressed by analyzing possible accumulation of phosphotyrosyl proteins in neoplastic and nonneoplastic astrocytic proliferative processes. The results were compared with the expression of Neu receptor protein (also called ErbB-2 or HER-2).

METHODS

Western immunoblots and immunocytochemistry were utilized to evaluate glioma and carcinoma cell lines, neonatal astrocytic cultures, and human brain biopsies of graded gliosis and astrocytomas. The effects of three tyrosine kinase inhibitors on 3H-thymidine uptake and cell proliferation and viability were examined in cultured glioma cells.

RESULTS

Phosphotyrosine was conspicuously elevated in all three grades of astrocytoma, but remained at low levels in nonneoplastic astrocytic proliferations. Dose-dependent decreases in DNA synthesis and proliferation of cultured glioma cells occurred after inhibition of tyrosine kinase. Neu receptor protein showed increased expression in malignant astrocytomas (including glioblastomas) and severe reactive gliosis.

CONCLUSIONS

Upregulation of tyrosyl protein phosphorylation enables differentiation of neoplastic from nonneoplastic astrocytic proliferative states. Inhibition of this phosphorylation impairs growth of cells. Increased Neu receptor protein expression can distinguish malignant from low grade astrocytomas. We speculate that genetic events leading to stably increased phosphotyrosine may be critical for neoplastic transformation of astrocytes, whereas increased receptor tyrosine kinase expression could be a factor in the aggressive growth associated with malignancy.

Chromatographic and mass spectrometric methods for the identification of phosphorylation sites in phosphoproteins

The phosphorylation sites in a model phosphoprotein, alpha s1-casein from bovine milk, have been identified by tryptic peptide mapping (Gibson and Cohen, Methods Enzymol. vol. 193, p. 480 (1990)) employing reversed-phase high performance liquid chromatography (RPHPLC)/electrospray ionization mass spectrometry (ES-MS); by infusion tandem mass spectrometry (MS/MS) and LC/MS/MS in neutral loss mode of tryptic digests of alpha s1-casein, in which the characteristic neutral loss of phosphoric acid by phosphopeptides under collision-induced dissociation (CID) conditions is exploited to highlight phosphopeptides in a tryptic digest (Covey et al., in Methods in Protein Sequence Analysis, Jörnvall et al. (Eds), Birkhäuser Verlag, Basel 1991), and by a novel method, termed LC/CID-MS, in which phosphopeptides are located in mixtures of peptides by the generation and detection of phosphate-specific fragment ions during LC/ES-MS (Huddleston et al., J. Am. Soc. Mass Spectrom. vol. 4, p. 710 (1993)). An appraisal of the efficiency, sensitivity and practicality of each of these methods in the identification of phosphorylation sites in post-translationally modified proteins is given.

Analysis of free and bound O-phosphoamino acids in urine by gas chromatography with flame photometric detection

A selective and sensitive method has been developed for the analysis of free and bound forms of O-phosphoamino acids, such as O-phosphoserine, O-phosphothreonine and O-phosphotyrosine, in urine samples by gas chromatography (GC). For free O-phosphoamino acid analysis, the urine sample was extracted with trichloroacetic acid and run through an ion exchange column. For total (free plus bound) O-phosphoamino acid analysis, the urine sample was hydrolysed in acid and base in order to release the O-phosphoamino acid from peptides or proteins. O-Phosphoamino acids in these prepared samples were converted into their N-isobutoxycarbonyl trimethyl ester derivatives and then measured by GC with flame photometric detection (FPD-GC). The calibration curve was linear in the range of 10-500 ng for each O-phosphoamino acid, and the detection limits were 30-80 pg as injection amounts. O-Phosphoamino acids in the urine samples could be selectively determined by the FPD-GC method without any influence from coexisting substances. The recoveries of O-phosphoamino acids added to urines and urine hydrolysates were 90-98% and relative standard deviations were 1.5-8.0%. By using this method, we investigated an age dependence of O-phosphoamino acid excretion in normal subjects.

Identification of O-phosphoamino acids in urine hydrolysate by gas chromatography-mass spectrometry

The occurrence of O-phosphoserine (P-Ser), O-phosphothreonine (P-Thr) and O-phosphotyrosine (P-Tyr) in the hydrolysate of human urine was demonstrated. Urine samples were separated into non-adsorbed and adsorbed fractions by DEAE-cellulose column chromatography. After acid and base hydrolyses of these fractions, the O-phosphoamino acids in the hydrolysates were converted into their N-isobutoxycarbonyl methyl ester derivatives and identified by gas chromatography-mass spectrometry. By gas chromatography with flame photometric detection, the urinary contents of P-Ser, P-Thr and P-Tyr were estimated to be 945 +/- 3, 109 +/- 3 and 2.9 +/- 0.2 ng/ml, respectively.

Effect of caffeic acid esters on carcinogen-induced mutagenicity and human colon adenocarcinoma cell growth

Propolis, a honey bee hive product, is thought to exhibit a broad spectrum of activities including antibiotic, antiviral, anti-inflammatory and tumor growth inhibition; some of the observed biological activities may be due to caffeic acid (cinnamic acid) esters that are present in propolis. In the present study we synthesized three caffeic acid esters, namely methyl caffeate (MC), phenylethyl caffeate (PEC) and phenylethyl dimethylcaffeate (PEDMC) and tested them against the 3,2'-dimethyl-4-aminobiphenyl, (DMAB, a colon and mammary carcinogen)-induced mutagenicity in Salmonella typhimurium strains TA 98 and TA 100. Also, the effect of these agents on the growth of human colon adenocarcinoma, HT-29 cells and activities of ornithine decarboxylase (ODC) and protein tyrosine kinase (PTK) was studied. Mutagenicity was induced in Salmonella typhimurium strains TA 98 and TA 100 plus S9 activation using 5 and 10 micrograms DMAB and antimutagenic activities of 0-150 microM MC, 0-60 microM PEC and 0-80 microM PEDMC were determined. The results indicate that MC, PEC and PEDMC were not mutagenic in the Salmonella tester system. DMAB-induced mutagenicity was significantly inhibited with 150 microM MC, 40-60 microM PEC and 40-80 microM PEDMC in both tester systems. Treatment of HT-29 colon adenocarcinoma cells with > 150 microM MC, 30 microM PEC and 20 microM PEDMC significantly inhibited the cell growth and syntheses of RNA, DNA and protein. ODC and PTK activities were also inhibited in HT-29 cells treated with different concentrations of MC, PEC and PEDMC. These results demonstrate that caffeic acid esters which are present in Propolis possess chemopreventive properties when tested in short-term assay systems.

Generation and use of anti-phosphotyrosine antibodies for immunoblotting

The techniques of detecting phosphotyrosine-containing proteins by AIA and purifying these proteins by affinity chromatography with anti-phosphotyrosine monoclonal antibodies are both reliable and consistent. Although not every tyrosine-phosphorylated protein can be detected and purified by using these techniques, a majority can. No doubt future studies will employ these approaches both for analyzing the specific role that tyrosine protein phosphorylation plays in regulating cell division and for investigating the properties of heretofore uncharacterized proteins that contain phosphotyrosine.

An alternative method for a fast separation of phosphotyrosine

A simple and rapid procedure is described for fully separating phosphotyrosine from phosphoserine and phosphothreonine through one-dimensional thin-layer chromatography. The migration properties of these phosphoamino acids are compared with those of CMP, UMP, ATP, ribose phosphate, and inorganic orthophosphate, considered the most frequent comigrating products derived from 32P-labeling experiments. We demonstrate that Rf values for the three phosphoamino acids differ from those displayed by the mentioned contaminating compounds. One of the most relevant advantages of this procedure is that a complete separation of phosphotyrosine can be achieved in only 90 min.

Increased tyrosine phosphorylation in ras transformed fibroblasts occurs prior to manifestation of the transformed phenotype

Murine fibroblasts transformed by ras oncogenes exhibited an increased amount of tyrosine phosphorylated proteins compared to normal cells. The pattern of phosphorylation was similar to that observed in cells chronically stimulated with EGF or PDGF, and is probably due to autocrine stimulation of receptor tyrosine kinases. NIH 3T3 cells transfected with H-ras under the control of a glucocorticoid inducible promoter were used to determine the temporal relationship among expression of p21H-ras oncoprotein, increase in tyrosine phosphorylation and appearance of the transformed morphology. Enhanced tyrosine phosphorylation was observed more than 24 hours before evidence of morphological changes. These results suggest that full transformation by ras oncogenes requires cooperation with tyrosine protein kinases.

pp60c-src activation in human colon carcinoma

We measured the in vitro protein-tyrosine kinase activity of pp60c-src from human colon carcinoma cell lines and tumors. The activity of pp60c-src from six of nine carcinoma cell lines was higher (on average, fivefold as measured by enolase phosphorylation, or eightfold as measured by autophosphorylation) than that of pp60c-src from normal colonic mucosal cells, or human or rodent fibroblasts. Similarly, the activity of pp60c-src from 13 of 21 primary colon carcinomas was five- or sevenfold higher than that of pp60c-src from normal colonic mucosa adjacent to the tumor. The increased pp60c-src activity did not result solely from an increase in the level of pp60c-src protein, suggesting the specific activity of the pp60c-src kinase is elevated in the tumor cells. pp60c-src from colon carcinoma cells and normal colonic mucosal cells was phosphorylated at similar sites. We used immunoblotting with antibodies to phosphotyrosine to identify substrates of protein-tyrosine kinases in colonic cells. Three phosphotyrosine-containing proteins were detected at significantly higher levels in most colon carcinoma cell lines than in normal colonic mucosal cells or human or rat fibroblasts. All colon carcinoma cell lines with elevated pp60c-src in vitro kinase activity, showed increased phosphorylation of proteins on tyrosine in vivo, suggesting the presence of an activated protein-tyrosine kinase(s).

p145, a protein with associated tyrosine kinase activity in a human gastric carcinoma cell line

A protein with an Mr of 145,000 (p145) was detected by antibodies to phosphotyrosine by Western blot (immunoblot) analysis. This protein was phosphorylated on tyrosine in a gastric carcinoma cell line. In cells that were metabolically labeled with 32Pi, this protein was phosphorylated on tyrosine and serine. p145 is a cysteine-rich transmembrane glycoprotein. The extracellular domain could be labeled by 125I under nonpermeating conditions and was cleaved by mild trypsin treatment of intact cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions revealed a shift of p145 mobility to an apparent Mr of 190,000. After immunoprecipitation with phosphotyrosine antibodies, p145 displayed a strong associated protein kinase activity in vitro, becoming phosphorylated on tyrosine. There was no immunological cross-reaction between p145 and known tyrosine kinases. Both in vivo and in vitro tyrosine phosphorylations were unaffected by the addition of known growth factors. However, p145 was rapidly dephosphorylated in vivo when cells were exposed to low pH, a condition that is known to dissociate ligands from their receptors. These data suggest that p145 is associated with a protein tyrosine kinase activity which, in the tumor cell line studied, is activated by an as yet unidentified factor.

Evidence for autocrine activation of a tyrosine kinase in a human gastric carcinoma cell line

Phosphotyrosine (P-Tyr) antibodies have been used to identify the phosphorylated forms of growth factor receptors and oncogene-coded tyrosine kinases. Western blot analysis of a gastric carcinoma cell line with P-Tyr antibodies revealed a tyrosine-phosphorylated protein of Mr 145,000 (P145). In addition, in vitro phosphorylation with (gamma-32P)ATP or P-Tyr immunoprecipitates of the same cells resulted in labelling of this protein on tyrosine. P145 appears to be a transmembrane glycoprotein, with features suggestive of a growth factor receptor. However, the in vivo or in vitro addition of known growth factors did not affect P145 tyrosine phosphorylation. We now report that P145 is rapidly dephosphorylated in vivo when cells are exposed to low pH, a condition known to dissociate ligands from their receptors. The addition of serum-free medium, conditioned by the gastric carcinoma cells, fully restores the tyrosine phosphorylation lost with acid treatment. These data suggest that the activity responsible for P145 phosphorylation on tyrosine, whether intrinsic to P145 itself or due to an associated kinase, is stimulated by a factor secreted by the tumor cells themselves.

p145, a protein with associated tyrosine kinase activity in a human gastric carcinoma cell line

A protein with an Mr of 145,000 (p145) was detected by antibodies to phosphotyrosine by Western blot (immunoblot) analysis. This protein was phosphorylated on tyrosine in a gastric carcinoma cell line. In cells that were metabolically labeled with 32Pi, this protein was phosphorylated on tyrosine and serine. p145 is a cysteine-rich transmembrane glycoprotein. The extracellular domain could be labeled by 125I under nonpermeating conditions and was cleaved by mild trypsin treatment of intact cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions revealed a shift of p145 mobility to an apparent Mr of 190,000. After immunoprecipitation with phosphotyrosine antibodies, p145 displayed a strong associated protein kinase activity in vitro, becoming phosphorylated on tyrosine. There was no immunological cross-reaction between p145 and known tyrosine kinases. Both in vivo and in vitro tyrosine phosphorylations were unaffected by the addition of known growth factors. However, p145 was rapidly dephosphorylated in vivo when cells were exposed to low pH, a condition that is known to dissociate ligands from their receptors. These data suggest that p145 is associated with a protein tyrosine kinase activity which, in the tumor cell line studied, is activated by an as yet unidentified factor.