Showing papers on "Carcinogenesis published in 1987"

Detection of high incidence of K- ras oncogenes during human colon tumorigenesis

Abstract: RNAse A mismatch cleavage analysis of 66 primary human colon tumors reveals a high incidence of K-ras genes with mutations at position 12. No apparent correlation was found between the presence of mutant oncogenes and the degree of invasiveness of the tumours but evidence for ras mutational activation in premalignant tissue was obtained.

The inheritance of epigenetic defects.

Abstract: Evidence from many sources shows that the control of gene expression in higher organisms is related to the methylation of cytosine in DNA, and that the pattern of methylation is inherited. Loss of methylation, which can result from DNA damage, will lead to heritable abnormalities in gene expression, and these may be important in oncogenesis and aging. Transformed permanent lines often lose gene activity through de novo methylation. It is proposed that epigenetic defects in germline cells due to loss of methylation can be repaired by recombination at meiosis but that some are transmitted to offspring.

Structural evidence for the authenticity of the human retinoblastoma gene.

Abstract: The retinoblastoma (Rb) gene is the prototype for a class of recessive human cancer genes in which loss of activity of both normal alleles is thought to be associated with tumorigenesis. Sixteen of 40 retinoblastomas examined with a complementary DNA probe shown to be the Rb gene had identifiable structural changes of the Rb gene including in some cases homozygous internal deletions with corresponding truncated transcripts. An osteosarcoma also had a homozygous internal deletion with a truncated transcript. In addition, possible hot spots for deletion were identified within the Rb genomic locus. Among those tumors with no identifiable structural changes there was either absence of an Rb transcript or abnormal expression of the Rb transcript. Comparison of the structural changes in the tumor cells and fibroblasts of certain patients provided support for Knudson's two-hit hypothesis for the development of retinoblastoma at the molecular level. The ability to detect germline structural deletions in fibroblasts from some patients with bilateral retinoblastoma also indicates that the isolated gene is useful for diagnostic purposes.

Increased expression of the putative growth factor receptor p185HER2 causes transformation and tumorigenesis of NIH 3T3 cells

Abstract: The HER2 gene encodes a cell-surface glycoprotein with extensive homology to the epidermal growth factor receptor. Recently it was found to be amplified in about 30% of primary human breast malignancies. In experiments designed to assess the role of the HER2 gene in oncogenesis, we found that overexpression of unaltered HER2 coding sequences in NIH 3T3 cells resulted in cellular transformation and tumorigenesis.

Overexpression of the EGF receptor-related proto-oncogene erbB-2 in human mammary tumor cell lines by different molecular mechanisms

Abstract: Amplification of the erbB/EGF receptor and a structurally related gene, designated erbB-2, have previously been detected in a variety of human tumors In a series of human mammary tumor cell lines, analysis of transcripts of these genes revealed elevated levels of one or the other in more than 60% of tumors analyzed Eight cell lines demonstrated erbB-2 mRNA levels ranging from 4- to 128-fold above those of normal controls erbB-2 expression was evaluated in comparison to the expression level of actin observed in these cell lines There was no evidence of an aberrantly sized erbB-2 transcript in any of these lines Immunoblot analysis indicated elevation in levels of the 185-kd product of the erbB-2 gene expressed by these cells In four lines erbB-2 gene amplification in the absence of an apparent gene rearrangement was demonstrated In a representative cell line of this type, SK-BR-3, the amplified erbB-2 gene copies were located in an aberrant chromosomal location Four additional cell lines, which demonstrated 4- to 8-fold overexpression of erbB-2 mRNA, did not exhibit gene amplification In a representative cell line of this type ZR-75-1, an apparently normal chromosomal location was found for the erbB-2 gene Our findings indicate that overexpression of the erbB-2 gene in mammary tumor cell lines is frequent and associated with different genetic abnormalities

Tumor cell instability, diversification, and progression to the metastatic phenotype: from oncogene to oncofetal expression

Abstract: It is proposed that tumor cell instability and the expression of cellular diversification mechanisms ensure that malignant neoplasms contain heterogeneous, phenotypically diverse tumor cell subpopulations In such potentially unstable cellular mixtures of tumor cell phenotypes, some malignant cells may ultimately evolve with the most favorable properties for their progression to metastatic cells Rates of cellular phenotypic instability and phenotypic diversification as well as their underlying causes appear to vary greatly among different tumor cells, and they are probably modulated by further genetic and chromosome changes and more frequently by intra- and extracellular epigenetic events that also differ, depending on the nature of the tumor cells and their cellular and microenvironmental interactions Diversified malignant cells are characterized by quantitative and perhaps a few qualitative differences in gene expression, which may explain their abilities to undergo rapid changes in phenotypic properties As tumor diversification and selection proceed uniquely in vivo, highly malignant cell subpopulations may eventually become dominant and gradually and independently lose their cellular and microenvironmental responsiveness Tumor cell diversification mechanisms may be similar or identical to normal developmentally regulated diversification mechanisms that are used during embryonic and postembryonic cell diversification and development

Loss of heterozygosity of chromosome 3p markers in small-cell lung cancer

Abstract: Specific chromosomal deletions sometimes associated with tumours such as retinoblastoma (chromosome 13q14)1 and Wilm's tumour (chromosome 11p13)2 have led to the hypothesis that recessive genes may be involved in tumorigenesis3. This hypothesis is supported by demonstration of allele loss specific for these regions using polymorphic DNA markers4–9 and by the isolation of a complementary DNA clone for the retinoblastoma gene10. A cytogenetic deletion in chromosome 3 (p14–p23) was reported in small-cell lung cancer (SCLC) by Whang-Peng et al11,12. At least one homologue of chromosome 3 was affected in the majority of SCLC tumours; however, the multiple chromosomal changes seen presented the possibility that chromosome 3 was rearranged, not deleted. We used polymorphic DNA probes for chromosome 3p and compared tumour and constitutional genotypes of nine SCLC patients. Our data show loss of alleles of chromosome 3p markers in tumour DNA of all nine patients supporting the hypothesis that this region contributes to tumorigenesis in SCLC.

The Approaching Era of the Tumor Suppressor Genes

Abstract: Genes that can inhibit the expression of the tumorigenic phenotype have been detected by the fusion of normal and malignant cells, the phenotypic reversion of in vitro transformants, the induction of terminal differentiation of malignant cell lineages, the loss of "recessive cancer genes," the discovery of regulatory sequences in the immediate vicinity of certain oncogenes, and the inhibition of tumor growth by normal cell products Such tumor suppressor genes will probably turn out to be as, if not more, diversified as the oncogenes Consideration of both kinds of genes may reveal common or interrelated functional properties

Deletions of a DNA sequence in retinoblastomas and mesenchymal tumors: organization of the sequence and its encoded protein.

Abstract: Retinoblastoma is a childhood tumor that can arise because of mutant alleles acquired as somatic or germinal mutations. The mutant allele can be carried in the germ line. The mutations creating these alleles act by inactivating copies of a recessive oncogene located within band q14 of chromosome 13 and termed the RB1 locus. We have reported isolation of a cDNA fragment that recognizes chromosomal sequences possessing many of the attributes of the retinoblastoma gene associated with the RB1 locus. We now report that this segment is additionally the target of somatic mutations in mesenchymal tumors among patients having no apparent predisposition to retinoblastoma and no previous evidence of retinoblastoma. These tumors provide additional evidence that the cloned sequences are representative of a gene that is a frequent target of inactivation during tumorigenesis. Sequence analysis of this cDNA provides little insight into its normal functional role.

A novel early growth response gene rapidly induced by fibroblast, epithelial cell and lymphocyte mitogens.

Abstract: Mitogens evoke many alterations in gene expression in eukaryotic cells. Genes which are activated rapidly and transiently, that are evolutionarily conserved and whose induction is shared by diverse cell types when exposed to different growth stimuli are likely to be of critical importance in transducing mitogenic signals and regulating cellular proliferation. c-myc and c-fos are the only known genes fulfilling these criteria. We report on the molecular cloning of a novel early growth response (egr) gene which also satisfies these conditions. In response to serum, its 3.7 kb mRNA is induced dramatically in mouse fibroblasts reaching a peak level at about 30 minutes that is ten times higher than the maximal value attained by c-fos mRNA. This transcript is induced by the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate and is "superinduced" by serum and cycloheximide together. Importantly, the gene is highly induced by different mitogens in a wide array of cell types: insulin stimulated rat hepatoma cells, adenosine diphosphate treated monkey kidney epithelial cells, and phytohemagglutinin stimulated human peripheral blood lymphocytes. Given the many properties that this gene shares with c-myc and c-fos, it may play a key role in the control of cell growth and perhaps in oncogenesis.

Molecular genetic approach to human meningioma: loss of genes on chromosome 22

Abstract: A molecular genetic approach employing polymorphic DNA markers has been used to investigate the role of chromosomal aberrations in meningioma, one of the most common tumors of the human nervous system. Comparison of the alleles detected by DNA markers in tumor DNA versus DNA from normal tissue revealed chromosomal alterations present in primary surgical specimens. In agreement with cytogenetic studies of cultured meningiomas, the most frequent alteration detected was loss of heterozygosity on chromosome 22. Forty of 51 patients were constitutionally heterozygous for at least one chromosome 22 DNA marker. Seventeen of the 40 constitutionally heterozygotic patients (43%) displayed hemizygosity for the corresponding marker in their meningioma tumor tissues. Loss of heterozygosity was also detected at a significantly lower frequency for markers on several other autosomes. In view of the striking association between acoustic neuroma and meningioma in bilateral acoustic neurofibromatosis and the discovery that acoustic neuromas display specific loss of genes on chromosome 22, we propose that a common mechanism involving chromosome 22 is operative in the development of both tumor types. Fine-structure mapping to reveal partial deletions in meningiomas may provide the means to clone and characterize a gene (or genes) of importance for tumorigenesis in this and possibly other clinically associated tumors of the human nervous system.

Activated Oncogenes in B6C3F1 Mouse Liver Tumors: Implications for Risk Assessment

Abstract: The validity of mouse liver tumor end points in assessing the potential hazards of chemical exposure to humans is a controversial but important issue, since liver neoplasia in mice is the most frequent tumor target tissue end point in 2-year carcinogenicity studies The ability to distinguish between promotion of background tumors versus a genotoxic mechanism of tumor initiation by chemical treatment would aid in the interpretation of rodent carcinogenesis data Activated oncogenes in chemically induced and spontaneously occurring mouse liver tumors were examined and compared as one approach to determine the mechanism by which chemical treatment caused an increased incidence of mouse liver tumors Data suggest that furan and furfural caused an increased incidence in mouse liver tumors at least in part by induction of novel weakly activating point mutations in ras genes even though both chemicals did not induce mutations in Salmonella assays In addition to ras oncogenes, two activated raf genes and four non-ras transforming genes were detected The B6C3F1 mouse liver may thus provide a sensitive assay system to detect various classes of proto-oncogenes that are susceptible to activation by carcinogenic insult As illustrated with mouse liver tumors, analysis of activated oncogenes in spontaneously occurring and chemically induced rodent tumors will provide information at a molecular level to aid in the use of rodent carcinogenesis data for risk assessment

Synthesis and secretion of platelet-derived growth factor by human breast cancer cell lines.

Abstract: We report that human breast cancer cells secrete a growth factor that is biologically and immunologically similar to platelet-derived growth factor (PDGF). Serum-free medium conditioned by estrogen-independent MDA-MB-231 or estrogen-dependent MCF-7 cells contains a mitogenic or "competence" activity that is capable of inducing incorporation of [3H]thymidine into quiescent Swiss 3T3 cells in the presence of platelet-poor plasma. In addition, the conditioned medium contains an activity that competes with 125I-labeled PDGF for binding to PDGF receptors on normal human fibroblasts. The secretion of PDGF-like activity by the hormone-responsive cell line MCF-7 is stimulated by 17 beta-estradiol. Like authentic PDGF, the PDGF-like activity produced by breast cancer cells is stable after acid and heat treatment (95 degrees C) and inhibited by reducing agents. The mitogenic activity comigrates with a material of approximately equal to 30 kDa on NaDodSO4/polyacrylamide gels. Immunoprecipitation with PDGF antiserum of proteins from metabolically labeled cell lysates and conditioned medium followed by analysis on nonreducing NaDodSO4/polyacrylamide gels identified proteins of 30 and 34 kDa. Upon reduction, the 30- and 34-kDa bands were converted to 15- and 16-kDa bands suggesting that the immunoprecipitated proteins were made up of two disulfide-linked polypeptides similar to PDGF. Hybridization studies with cDNA probes for the A chain of PDGF and the B chain of PDGF/SIS identified transcripts for both PDGF chains in the MCF-7 and MDA-MB-231 cells. The data summarized above provide conclusive evidence for the synthesis and hormonally regulated secretion of a PDGF-like mitogen by breast carcinoma cells. Production of a PDGF-like growth factor by breast cancer cell lines may be important in mediating paracrine stimulation of tumor growth.

Ha-ras oncogene expression directed by a milk protein gene promoter: tissue specificity, hormonal regulation, and tumor induction in transgenic mice.

Abstract: The activated human Ha-ras oncogene was subjected to the control of the promoter region of the murine whey acidic protein (Wap) gene, which is expressed in mammary epithelial cells in response to lactogenic hormones. The Wap-ras gene was stably introduced into the mouse germ line of five transgenic mice (one male and four females). Wap-ras expression was observed in the mammary glands of lactating females in two lines derived from female founders. The tissue-directed and hormone-dependent Wap expression was conferred on the Ha-ras oncogene. The signals governing Wap expression are located within 2.5 kilobases of 5' flanking sequence. The other two lines derived from female founders did not express the chimeric gene. In the line derived from the male founder, the Wap-ras gene is integrated into the Y chromosome. Expression was found in the salivary gland of male animals only. After a long latency, Wap-ras-expressing mice developed tumors. The tumors arose in tissues expressing Wap-ras--i.e., mammary or salivary glands. Compared to the corresponding nonmalignant tissues, Wap-ras expression was enhanced in the tumors.

Estrogen-induced lysosomal proteases secreted by breast cancer cells: a role in carcinogenesis?

Abstract: In an attempt to understand the mechanism by which estrogens stimulate cell proliferation and mammary carcinogenesis, metastatic human breast cancer cell lines (MCF7, ZR75-1) were found to secrete a 52,000 dalton (52K) protein under estrogen stimulation. Following its purification to homogeneity, the 52K protein was identified as a secreted procathepsin-D-like aspartyl protease bearing mannose-6-phosphate signals. This precursor displays an in vitro autocrine mitogenic activity on estrogen-deprived MCF7 cells and is able to degrade basement membrane and proteoglycans following its autoactivation. The total protease (52K + 48K and 34K) was detected and assayed by monoclonal antibodies and was found to be highly concentrated in proliferative and cystic mastopathies. In breast cancer, its cytosolic concentration appears to be correlated more to tumor invasiveness than to hormone responsiveness. The mRNA of the 52K protease accumulates rapidly following estradiol treatment, as was shown by Northern blot analysis with cloned cDNA. The 52K cathepsin-D-like protease is the first example of a lysosomal protease induced by estrogens in cancer cells. Results obtained using different approaches suggest that two cysteinyl cathepsins are also related to cell transformation and invasiveness. It has been proposed that cathepsin-B is involved in breast cancer and metastatic melanoma, and its regulation by estrogen has been shown in the rat uterus. Cathepsin-L corresponds to the major excreted protein (MEP) whose synthesis and secretion are markedly increased by transformation of NIH 3T3 cells with Ki ras and are regulated by several growth factors. In addition to secreted autocrine growth factors and to other proteases (plasminogen activator, collagenase), lysosomal cathepsins may therefore play an important role in the process of tumor growth and invasion as long as their precursor is secreted abundantly.

Regulation of bcl-2 proto-oncogene expression during normal human lymphocyte proliferation.

Abstract: The bcl-2 and c-myc proto-oncogenes are brought into juxtaposition with the immunoglobulin heavy chain locus in particular B-cell lymphomas, resulting in high levels of constitutive accumulation of their messenger RNAs. Precisely how the products of the bcl-2 and c-myc genes contribute to tumorigenesis is unknown, but observations that c-myc expression is rapidly induced in nonneoplastic lymphocytes upon stimulation of proliferation raise the possibility that this proto-oncogene is involved in the control of normal cellular growth. In addition to c-myc, the bcl-2 proto-oncogene also was expressed in normal human B and T lymphocytes after stimulation with appropriate mitogens. Comparison of the regulation of the expression of these proto-oncogenes demonstrated marked differences and provided evidence that, in contrast to c-myc, levels of bcl-2 messenger RNA are regulated primarily through transcriptional mechanisms.

Carcinogen-induced mdr overexpression is associated with xenobiotic resistance in rat preneoplastic liver nodules and hepatocellular carcinomas

Abstract: We have previously reported the isolation of a human breast cancer cell line resistant to doxorubicin (adriamycin; AdrR MCF-7 cells) that has also developed the phenotype of multidrug resistance (MDR). MDR in this cell line is associated with increased expression of mdr (P glycoprotein) gene sequences. The development of MDR in AdrR MCF-7 cells is also associated with changes in the expression of several phase I and phase II drug-detoxifying enzymes. These changes are remarkably similar to those associated with development of xenobiotic resistance in rat hyperplastic liver nodules, a well-studied model system of chemical carcinogenesis. Using an mdr-encoded cDNA sequence isolated from AdrR MCF-7 cells, we have examined the expression of mdr sequences in rat livers under a variety of experimental conditions. The expression of mdr increased 3-fold in regenerating liver. It was also elevated (3- to 12-fold) in several different samples of rat hyperplastic nodules and in four of five hepatomas that developed in this system. This suggests that overexpression of mdr, a gene previously associated with resistance to antineoplastic agents, may also be involved in the development of resistance to xenobiotics in rat hyperplastic nodules. In addition, although the acute administration of 2-acetylaminofluorene induced an 8-fold increase in hepatic mdr-encoded RNA, performance of a partial hepatectomy either before or after administration of 2-acetylaminofluorene resulted in a greater than 80-fold increase in mdr gene expression over that in normal untreated livers. This represents an important in vivo model system in which to study the acute regulation of this drug resistance gene.

Rodent fibroblast tumours expressing human myc and ras genes: growth, metastasis and endogenous oncogene expression.

Abstract: The effects of expression of human c-myc and both mutated (T24) and normal forms of human Ha-ras-1 were studied in an aneuploid rat fibroblast line (208F). Mutated T24 Ha-ras was also studied in a near-diploid cell derived from early passage Chinese hamster lung fibroblasts (CHL). In contrast to the parental fibroblasts, cells expressing any of the human oncogenes engendered rapidly growing tumours in immune-suppressed animals. Blood- and lymph-borne metastases were observed from both ras- and myc-expressing cells. In general ras-expressing cells were more aggressive than those expressing myc. In the 208F background, expression of c-myc was associated with an incidence of mitosis similar to that in tumours expressing T24 Ha-ras, but incidence of single cell death by apoptosis was higher. Quantitatively, expression of human oncogene mRNA was constant during growth in vivo, and similar to that sometimes observed in human neoplasms. Of 9 endogenous proto-oncogenes, 7 showed no change in expression from the parental fibroblasts, but c-abl and c-fos were strongly expressed in all cells expressing human ras or myc. Thus these tumorigenic cells, although transfected with single human oncogenes, all expressed oncogenes with both nuclear- and membrane-associated products.

Oncogenesis of the lens in transgenic mice.

Abstract: Neoplastic tumors of the ocular lens of vertebrates do not naturally occur. Transgenic mice carrying a hybrid gene comprising the murine alpha A-crystallin promoter (-366 to +46) fused to the coding sequence of the SV40 T antigens developed lens tumors, which obliterated the eye cavity and even invaded neighboring tissue, thus establishing that the lens is not refractive to oncogenesis. Large-T antigen was detected early in lens development; it elicited morphological changes and specifically interfered with differentiation of lens fiber cells. Both alpha- and beta-crystallins persisted in many of the lens tumor cells, while gamma-crystallin was selectively reduced. Accessibility, characteristic morphology, and defined protein markers make this transparent epithelial eye tissue a potentially useful system for testing tumorigenicity of oncogenes and for studying malignant transformation from its inception until death of the animal.

The raf oncogene is associated with a radiation-resistant human laryngeal cancer.

Abstract: In order to identify the genetic factors associated with the radiation-resistant human laryngeal carcinoma cell line (SQ-20B), tumor cell DNA was transfected into NIH/3T3 cells. A high incidence (six out of six) of raf sequences was found in transfected NIH/3T3 clones and the tumorigenic potential of SQ-20B DNA could be linked to genomic fragments that represent most of the kinase domain of human c-raf-1. An apparently unaltered 3.5-kilobase pair (kb) human c-raf transcript was identified in SQ-20B cells but was not observed in the transfected NIH/3T3 cell clones. Two new transcripts (4.2 kb and 2.6 kb) were found in tumorigenic clones; the large transcript was missing in a very poorly tumorigenic clone. Cytogenetic analysis indicated that the normal autosomes of chromosome 3 were absent in SQ-20B karyotypes and had formed apparently stable marker chromosomes. Unlike the recipient NIH/3T3 cell line, 30 percent of the transformed clone-1 metaphases had minute and double-minute chromosomes representative of amplified DNA sequences. The frequency of the c-raf-1 identification by NIH/3T3 transfection of SQ-20B DNA suggests the presence of some genetic abnormality within this locus.

Retroviruses as Carcinogens and Pathogens: Expectations and Reality

Abstract: Retroviruses (without transforming genes) are thought to cause leukemias and other cancers in animals and humans because they were originally isolated from those diseases and because experimental infections of newborns may induce leukemias with probabilities of 0 to 90% According to this hypothesis viral cancers should be contagious, polyclonal, and preventable by immunization However, retroviruses are rather widespread in healthy animals and humans where they typically cause latent infections and antiviral immunity The leukemia risk of such infections is less than 01% and thus about as low as that of virus-free controls Indeed retroviruses are not sufficient to initiate transformation because of the low percentage of symptomatic virus carriers and the complete lack of transforming function in vitro; because of the striking discrepancies between the long latent periods of 05 to 10 years for carcinogenesis and the short eclipse of days to weeks for virus replication and direct pathogenic and immunogenic effects; because there is no gene with a late transforming function, since all genes are essential for replication; because host genes, which do not inhibit virus, inhibit tumorigenesis up to 100% if intact and determine the nature of the tumor if defective; and above all because of the monoclonal origin of viral leukemias, defined by viral integration sites that are different in each tumor On these bases the probability that a virus-infected cell will become transformed is estimated to be about 10(-11) The viruses are also not necessary to maintain transformation, since many animal and all bovine and human tumors do not express viral antigens or RNA or contain only incomplete proviruses Thus as carcinogens retroviruses do not necessarily fulfill Koch's first postulate and do not or only very rarely (10(-11)) fulfill the third Therefore it has been proposed that retroviruses transform inefficiently by activating latent cellular oncogenes by for example provirus integration This predicts diploid tumors with great diversity, because integration sites are different in each tumor However, the uniformity of different viral and even nonviral tumors of the same lineage, their common susceptibility to the same tumor resistance genes, and transformation-specific chromosome abnormalities shared with non-viral tumors each argue for cellular transforming genes Indeed clonal chromosome abnormalities are the only known transformation-specific determinants of viral tumors Since tumors originate with these abnormalities, these or associated events, rather than preexisting viruses, must initiate transformation(ABSTRACT TRUNCATED AT 250 WORDS)

Altered Levels of Protein Kinase C and Ca2+-dependent Protein Kinases in Human Colon Carcinomas

Abstract: Protein kinase C (PKC) is a Ca2+- and phospholipid-dependent protein kinase which is implicated in tumor promotion, since it has been demonstrated to be a high affinity receptor for tumor promoters such as 12- O -tetradecanoylphorbol-13-acetate. Colon carcinogenesis appears to proceed through distinct stages of initiation and promotion. The present studies show that PKC and calcium-dependent protein kinase specific activities are reduced in human colon carcinomas when compared to their normal adjacent colon mucosa. There were significantly higher Ca2+-dependent protein kinase and PKC specific activities observed in both the cytosolic and particulate fractions of the normal mucosa relative to the corresponding values obtained with the carcinoma fractions. The average specific activity ratios were 5.1 (normal cytosolic/carcinoma cytosolic) and 3.7 (normal particulate/carcinoma particulate) for PKC. PKC activity was reduced in the carcinoma tissues with respect to both protein and tissue weight. The percentage of Ca2+-dependent protein kinase and PKC activities that were present in the particulate fraction of each of the samples varied considerably among tissues, and in general there was no systematic difference between the carcinoma and normal mucosa samples. However, in the carcinoma samples that contained an extensive admixture of benign adenomatous tissue, the particulate fractions consistently contained greater than 60% of the total Ca2+-dependent protein kinase and PKC activities. The present studies indicate that colon carcinogenesis is associated with alterations in cellular levels of protein kinase activities.