Action In Carcinogenesis Research Articles

CtBP: A global regulator of balancing acts and homeostases.

The classical role of C-terminal binding protein (CtBP) is that of a global corepressor. However, its exact mechanism of repression is not known. In this review, we elucidate the repression motif used by CtBP. Further, we provide other unifying features of its mechanism of action. For example, in the presence of a high NADH/NAD+ ratio in the cell, causing a low glycolytic condition, the NADH-bound dimeric form of CtBP causes global repression, maintaining balances and homeostases of many cellular processes, under the cell surveillance of p53 and NFkB. In contrast, in the presence of a low NADH/NAD+ ratio, causing a high glycolytic condition, the NADH-free monomeric form of CtBP blocks p53 function and NFkB-mediated transcription. Further, a low NADH/NAD+ ratio upsets the homeostases and balances in the absence of the cell surveillances of p53 and NFkB, causing global instability, the dominant outcome of CtBP's action in carcinogenesis, in cells in a high glycolytic state.

Incongruent Biology of Redistributed Chemokine Action in Carcinogenesis

The promotional antigenicity profiles exhibited by proliferating tumor cells allows for a permissive microenvironment as projected by immature DC. It is within redefined milieu of chemokine reactivity that redistribution of antigenic stimuli allow for projected modulations of the integral immune systems as proposed by the redefined ch emokine reactivity. It is the simple juxtaposition of multiple range of antigenicity that perforce allows for immune tolerance and permissiveness to emerge. The recharacterization of such antigenicity is recharacterization of inherent component pathway reactivity as indeed projected chemokine redistribution within profiles of the immune response to proliferating tumor cell beds.

The Role of Beta HPV Types and HPV-Associated Inflammatory Processes in Cutaneous Squamous Cell Carcinoma.

Cutaneous squamous cell carcinoma (cSCC) is a common form of skin cancer with a complex but not fully understood pathogenesis. Recent research suggests the role of beta human papillomavirus (HPV) types and HPV-associated inflammatory processes in cSCC development. Beta HPV types are components of the normal flora; however, under the influence of certain cofactors, the virus may trigger a malignant process. Dysregulation of the immune system (chronic inflammation and immunosuppression), environmental factors (ultraviolet radiation), and genetic factors are the most important cofactors involved in beta HPV-related carcinogenesis. In addition, the oncoproteins E6 and E7 of beta HPV types differ biochemically from their counterparts in the structure of alpha HPV types, resulting in different mechanisms of action in carcinogenesis. The aim of our manuscript is to present an updated point of view on the involvement of beta HPV types in cSCC pathogenesis.

Systemic biological study for identification of miR-299-5p target genes in cancer

Background and aimMicroRNAs have an essential role in cancer development and progression. They have high potential for using as diagnostic biomarkers and also for therapeutic purposes. MiR-299-5p is frequently dysregulated in variety of cancers. However, there is no precise information about its mechanism of action. So, in this study the key target genes of miR-299-5p were identified through a systems biology approach to perceive the miR-299-5p mechanism of action in carcinogenesis. MethodsTCGA Pan-Cancer (PANCAN) cohort data was used for miR-229-5p Survival analysis. MiRwalk database was used to predict the target genes of miR-299-5p. The co-predicted targets using three or more different software were chosen. Further analysis was carried out using DAVID database for functional annotation and KEGG pathway analysis. Subsequently, STRING online tool and Cytoscape software were used for protein-protein interaction (PPI) network analysis and module selection. ResultsMiR-229-5p can significantly predict overall survival of cancer patients. Among 5475 predicted targets for miR-229-5p, 1583 of them were co-predicted by three or more software. The co-predicted targets were mainly enriched in biological processes such as the regulation of the single-organism cellular processes. Four important pathways including Focal adhesion, P53 signaling pathway, renal cell carcinoma, and O-glycan biosynthesis were identified. Eight significant modules with 13 essential hub genes, including RAP1A, XIAP, BIRC3, CCND2, PAK2, PXN, PPP2CB, PPP1R12A, CDKN1A, CYCS, SERPINE1, SIAH1, and HIF1A were recognized after network analysis. ConclusionIn this study we screened many false positive target genes predicted for miR-299-5. Finally, 13 essential hub genes were introduced. These genes potentially are potent cancer biomarkers and suitable therapeutic targets in targeted cancer therapies.

Oncogene RNA helicase DDX6 promotes the process of c-Myc expression in gastric cancer cells.

Human DEAD-box RNA helicase gene DDX6 was cloned from B-cell lymphoma cell line RC-K8. Previously, we reported that DDX6 acts as oncogene in several cancers such as colorectal cancer and hepatocellular carcinoma. However, the detailed mechanism of DDX6 action in carcinogenesis is largely unknown. In this study, we examined the functions of DDX6 in clinical gastric cancer (GC) samples and GC cells. DDX6 protein expression levels of cancer samples were higher than those of the adjacent normal tissues in 25 clinical GC samples (median value: 1.4 times higher). Also, the results of an RNA immunoprecipitation-assay (RIP-assay) showed that DDX6 associated with c-Myc mRNA. Moreover, enforced overexpression of DDX6 promoted both mRNA and protein expression of c-Myc in GC cells. On the other hand, the gene silencing of DDX6 induced growth suppression through down-regulation of c-Myc in GC cells grown in either two or three dimensions. Furthermore, c-Myc mRNA expression levels of cancer samples were higher than those of the adjacent normal tissues in DDX6 up-regulated-GC clinical samples. Our findings in this study suggested that DDX6 acted as oncogene in GC cells through promotion of c-Myc expression by association with the mRNA of c-Myc.

2-Nitroanisole-induced oxidative DNA damage in Salmonella typhimurium and in rat urinary bladder cells

2-Nitroanisole (2-NA) is used in the manufacturing of azo dyes and causes cancer, mainly in the urinary bladder. Previous in vivo genotoxic data seems to be insufficient to explain the mechanism through which 2-NA induces carcinogenesis, and several bladder carcinogens were reported to induce oxidative DNA damage. Thus, we examined the potential induction of oxidative DNA damage by 2-NA using bacterial strain YG3008, a mutMST-deficient derivative of strain TA100. Consequently, strain YG3008, when compared with strain TA100, was found to be more sensitive to 2-NA, indicating oxidative DNA damage in bacterial cells. For further investigation, we performed the comet assay using the urinary bladder and liver of rats, with and without human 8-oxoguanine DNA-glycosylase 1 (hOGG1), to confirm the potential of 2-NA for inducing oxidative DNA damage. Simultaneously, we conducted a micronucleus test using bone marrow from rats to assess the genotoxicity of 2-NA in vivo. 2-NA was administered orally to male Fischer 344 rats for 3 consecutive days. The rats were divided into 6 treatment groups: 3 groups treated with 2-NA at doses of 125, 250, and 500mg/kg; a group treated with the combination of 2-NA and glutathione-SH (GSH); a negative control group; and a positive control group. The comet assay without hOGG1 detected no DNA damage in the liver or urinary bladder, and the micronucleus test did not show clastogenic effects in bone marrow cells. However, the comet assay with hOGG1 was positive in the urinary bladder samples, indicating the induction of oxidative DNA damage in the urinary bladder for the group treated with 2-NA at 500mg/kg. Moreover, an antioxidant of GSH significantly reduced oxidative DNA damage caused by 2-NA. These results indicate that oxidative DNA damage is a possible mode of action for carcinogenesis in the urinary bladder of rats treated with 2-NA.

Abstract A17: Physical activity and epigenetic age among normative aging study participants

Abstract Low levels of physical activity are associated with increased risk and mortality of common cancers including breast and colon, with strong evidence for increasing risk with age. Epigenetic age is a recently developed concept using DNA methylation measurements to describe biological age at the level of human tissues, cells, and organs. Epigenetic age represents deviation from expected DNA methylation patterns associated with chronological age (defined as Δ_age), and is suggested as a possible biological mechanism driving observed associations between physical activity and cancer risk. However, no studies have evaluated associations between physical activity level and Δ_age. The Normative Aging Study (NAS) is a cohort study of healthy aging conducted by the Veterans Affair Administration. Among NAS participants (n=656), we conducted multivariate linear regression to prospectively evaluate associations between physical activity (MET-hours per week) and Δ_age across two follow-up visits. The visits were on average 3.6 years apart, with a range of 2-8 years. Δ_age was estimated using Horvath’s DNA methylation online calculator and based upon DNA methylation patterns at the 71 cytosine-phosphate-guanine (CpGs) loci used in Hannum’s model. The calculator accounts for cell proportion of white blood cells. We adjusted models for chronological age, body mass index, education, smoking, alcohol consumption, and prevalent cancer status. Overall, we observed statistically significant associations between high physical activity levels and lower Δ_age, or younger epigenetic age. Among individuals reporting the highest level of physical activity (≥ 30 MET-hours per week), Δ_age was 1.44 (95% CI: 0.43 to 2.45) and 2.20 (95% CI: 1.04 to 3.36) years lower compared to those reporting low activity (<12 MET-hours per week) at the first and second visit, respectively. Furthermore, compared to low physical activity, high activity at the first visit was associated with 2.46 (95% CI: 1.25 to 3.67) years lower Δ_age at the second visit, and 3.72 (95% CI: 1.08 to 6.36) months/calendar year lower Δ_age rate of change, or decelerated epigenetic aging over time. In summary, the results of the current study demonstrate that physical activity influences DNA methylation patterns at the CpG loci associated with epigenetic age. Future studies are necessary to characterize further the relationship between physical activity and epigenetic age, especially as a potential biological mechanism of action in carcinogenesis. However, epigenetic age represents a unique set of markers with the potential to identify high-risk populations and, in turn, develop interventions to mitigate risk of cancer. Citation Format: Elizabeth A. Hibler, Yinan Zheng, Lei Liu, Wei Zhang, Frank J. Penedo, Siobhan M. Phillips, David E. Conroy, Joel Schwartz, Pantel Vokonas, Andrea Baccarelli, Qi Dai, Lifang Hou. Physical activity and epigenetic age among normative aging study participants. [abstract]. In: Proceedings of the AACR Special Conference: Developmental Biology and Cancer; Nov 30-Dec 3, 2015; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(4_Suppl):Abstract nr A17.

Long non-coding antisense RNA KRT7-AS is activated in gastric cancers and supports cancer cell progression by increasing KRT7 expression.

Alterations in long non-coding RNAs (lncRNAs) are associated with human carcinogenesis. One group of lncRNAs, which are antisense in orientation to coding mRNAs (ASs), have been recently described in cancers but are poorly understood. We sought to identify ASs involved in human gastric cancer (GC) and to elucidate their mechanisms of action in carcinogenesis. We performed massively parallel RNA sequencing in GCs and matched normal tissues, as well as in GC-derived and normal gastric epithelial cell lines. One AS, designated KRT7-AS, was selected due to its marked upregulation and concordant expression with its cognate sense counterpart, KRT7, in GC tissues and cell lines. KRT7-AS formed an RNA-RNA hybrid with KRT7 and controlled KRT7 expression at both the mRNA and the post-transcriptional levels. Moreover, forced overexpression of the KRT7-overlapping region (OL) of KRT7-AS (but not its non-KRT7-overlapping portions) increased keratin 7 protein levels in cells. Finally, forced overexpression of full-length (FL) KRT7-AS or OL KRT7-AS (but not its non-KRT7-overlapping regions) promoted GC cell proliferation and migration. We conclude that lncRNA KRT7-AS promotes GC, at least in part, by increasing KRT7 expression.

Is copper chelation an effective anti-angiogenic strategy for cancer treatment?

Angiogenesis and the acquisition of an angiogenic phenotype is important for cancer cell proliferation. Copper in an essential trace element that participates in many enzymatic complexes like the cytochrome c, superoxide dismutase and lysyl oxidase and it is involved in processes, like embryogenesis, growth, angiogenesis and carcinogenesis. In particular, its involvement in carcinogenesis was described for the first time in oral submucous fibrosis, where fibroblasts produce large amounts of collagen in the presence of copper. Copper’s action in carcinogenesis is two-fold: (1) it participates in reactions with an increased redox potential that result in the production of oxidative products and oxidative stress. Through this mechanism, copper may cause DNA mutations in the nucleus and mitochondria or alterations to membrane phospholipids, (2) it participates in angiogenesis even in the absence of angiogenic molecules, as it was reported for the first time in rabbit cornea models with copolymer pellets charged with PGE1.Copper chelation regimens like penicillamine and tetrathiomolybdate are being described in the literature as having anti-angiogenic, anti-fibrotic and anti-inflammatory actions. Animal models of brain cancer that evaluated the anti-angiogenic properties of copper, have proven evidence of the reduction of tumor’s microvascular supply, tumor volume and vascular permeability after plasma copper levels reduction. Interestingly, plasma copper levels reduction was shown to suppress micrometastases generation in mice models of breast cancer.We hypothesize that copper chelation therapy: increases oxidative stress in cancer cells to a level that does not allow survival because of the reduction of anti-oxidative enzymes production. It may also result in inhibition of angiogenesis and of the initiation of the angiogenic switch, because copper normally enhances endothelial cell migration and proliferation, improves binding of growth factors to endothelial cells and enhances the expression of angiogenic molecules. Copper chelation may also reduce extracellular matrix degradation and cancer spread, through reduction of MMP-9 production and probably of other collagenases and may inhibit propagation of micrometastases. However, copper chelation therapy may enhance angiogenesis through reduction of thrombospondin-1, that results into an increase in VEGF–VEGFR2 complexes and a high level of active MMP-9.These hypotheses help in understanding of the anti-angiogenic action of copper chelation therapies and of the complex network of interactions between copper and other molecules involved in angiogenesis. It may also stimulate further research regarding differences in copper metabolism, the effects of anti-copper regimens on organs, the development of resistance, and their possible angiogenic action through thrombospondin expression reduction.

Molecular mechanism of thyroid hormone action in carcinogenesis

Thyroid hormones, 3,5,3'5'-tetraiodothyronine (thyroxine) and 3,5,3'-triiodothyronine (T3) contribute to the regulation of key cellular processes, including proliferation, differentiation, apoptosis and metabolism. Thus, deregulation of thyroid hormone pathway can result in serious disturbances of cellular physiology such as those observed in tumoral transformation. The intra- and extracellular concentrations of thyroid hormones (TH) are regulated by three types of iodothyronine deiodinases (DIO1, DIO2, and DIO3). These enzymes catalyze two types of reactions: deiodination in position 5' of iodothyronines (catalyzed by DIO1 and DIO2), resulting in activation of thyroid hormones, and deiodination in position 5 of iodothyronines (catalyzed by DIO1 and DIO3), which results in inactivation of thyroid hormones. Changes in expression and activity of iodothyronine deiodinases result in modulations of thyroid hormone concentrations and thus influence cellular processes. The actions of thyroid hormone are mediated by thyroid hormone receptors (TRs) which influence intracellular processes via modulation of expression of target genes. In this process, known as genomic mechanism of TH action, thyroid hormone receptors act as transcription factors whose activity is modulated by the presence of ligand, T3. TH can also influence cellular processes via non-genomic pathways in which TH modulate activity of membrane or intracellular receptors and activate different intracellular signaling pathways, involving protein kinases, for instance PI3K, and Akt/PKB. In the process of carcinogenesis the signaling pathway of thyroid hormones is disturbed. This is illustrated by observations of mutations and deregulated expression of thyroid hormone receptors, altered expression and activity of iodothyronine deiodinases as well as decreased intracellular concentrations of T3 in tumours. Whether these changes play a causative role in the process of carcinogenesis is currently a matter of discussion. In vitro studies in cancer cell lines and mouse models showed that increased expression of DIO3 together with enhanced proteasomal degradation of DIO2 in basal cell carcinoma stimulates tumoral proliferation. Moreover, thyroid hormone receptor beta-1 was shown to act as a powerful suppressor of tumour invasiveness and metastasis in mouse models of breast cancer and hepatocarcinoma. These studies suggest that local tissue hypothyroidism may rather support tumoral transformation and even initiate enhanced cancerous proliferation. In contrast, studies on the effects of systemic hypothyroidism and cancer risk show opposite or conflicting results. For instance, hypothyroidism was associated with increased risk of hepatocellular carcinoma, and cancers of lung and prostate. In case of breast cancer, different studies reported that hypothyroidism was linked to reduced risk of the disease in humans while when studied in mice, hypothyroidism reduced tumour growth but enhanced metastasis. Finally, studies performed on cell line and mouse xenografts of thyroid and kidney cancers showed that inhibition of integrin-mediated thyroid hormone signaling can effectively result in inhibition of tumour growth. All these studies show that the role of thyroid hormones in carcinogenic process is complex and presumably depends on type of cancer and extra- or intracellular effects of TH signaling. The results showing benefits from the use of TH pathway genes in cancer treatment and diagnosis suggest that further and more in depth studies are needed.

The Role of TGFβ Signaling in Squamous Cell Cancer: Lessons from Mouse Models.

TGFβ1 is a member of a large growth factor family including activins/inhibins and bone morphogenic proteins (BMPs) that have a potent growth regulatory and immunomodulatory functions in normal skin homeostasis, regulation of epidermal stem cells, extracellular matrix production, angiogenesis, and inflammation. TGFβ signaling is tightly regulated in normal tissues and becomes deregulated during cancer development in cutaneous SCC and many other solid tumors. Because of these diverse biological processes regulated by TGFβ1, this cytokine and its signaling pathway appear to function at multiple points during carcinogenesis with distinct effects. The mouse skin carcinogenesis model has been a useful tool to dissect the function of this pathway in cancer pathogenesis, with transgenic and null mice as well as small molecule inhibitors to alter the function of the TGFβ1 pathway and assess the effects on cancer development. This paper will review data on changes in TGFβ1 signaling in human SCC primarily HNSCC and cutaneous SCC and different mouse models that have been generated to investigate the relevance of these changes to cancer. A better understanding of the mechanisms underlying the duality of TGFβ1 action in carcinogenesis will inform potential use of this signaling pathway for targeted therapies.

In vitro (?-Glucosidase and ?-Amylase Inhibition) and in vivo Antidiabetic Property of Phytic Acid (IP6) in Streptozotocin-Nicotinamide-Induced Type 2 Diabetes Mellitus (NIDDM) in Rats

Phytic acid, inositol hexaphosphate (IP6) a natural plant constituent and antioxidant exhibits protective action in carcinogenesis, Alzheimer's, hypercholesterolemia, diabetes and inflammations when taken in diet. The aim of this study is to evaluate effect of phytic acid in streptozotocin (STZ)-nicotinamide-induced type 2 diabetes in rats. The STZ-nicotinamide induced diabetic rats were orally treated with vehicle (2%w/v Tween 80), glimepiride (2.5 mg/kg) and IP6 (650 mg/kg) for 28 days. The blood glucose level, body weight, glycosylated haemoglobin (HbA1C), lipid profile, lipid peroxidation, antioxidant status (liver and small intestine) was measured and compared with control. Xanthine dehydrogenase (XDH) and xanthine (XO) activity was measured in small intestine of diabetic rats. In vitro inhibition of carbohydrate digestive enzymes (α-glucosidase and α-amylase) was also determined. IP6, significantly (P<0.01) reduced glucose level, HbA1C, lipid profile and lipid peroxidation, and increased body weight, high density lipoprotein level and antioxidant status in liver and small intestine. Decrease in XO and increase in XDH activity was observed in treatment groups compared to diabetic control. Dose dependent inhibition of α-glucosidase and (α-amylase activity was observed for phytic acid when compared to standard drug acarbose. These results clearly indicate that IP6 possess promising in vitro and in vivo antidiabetic activity.

Chemoprevention of breast cancer by targeting cyclooxygenase-2 and peroxisome proliferator-activated receptor-γ (Review)

Cyclooxygenase-2 (COX-2) and peroxisome proliferator-activated receptor-gamma (PPARgamma) have emerged as candidate molecules that hold great promise for cancer chemoprevention. COX-2 increased expression and PPARgamma inactivation occur during mammary gland carcinogenesis. COX-2 and PPARgamma may contribute to breast cancer induction either directly or via their effects on factors known to influence tumor development, e.g., nuclear factor-kappaB and vascular endothelial growth factor. Inhibition of COX-2 or activation of PPARgamma prevents mammary carcinomas in experimental animals with little toxicity. Combinational treatment with COX-2 inhibitor and PPARgamma agonists may produce synergistic anti-tumorigenic effects without significant toxicity and, therefore, be an effective strategy to prevent human breast cancer. Establishing a relationship between COX-2 and PPARgamma in this malignancy may provide the basis for a novel chemopreventive strategy based on the modulation of both molecules simultaneously. This review evaluates experimental and epidemiological findings suggesting a possible role of COX-2 and PPARgamma in the development of human breast cancer and presents evidence substantiating their coordinated action in carcinogenesis and finally develops a rationale for the simultaneous targeting of both molecules as a potentially effective strategy to prevent breast malignancy.

Hormones and hormone antagonists: mechanisms of action in carcinogenesis of endometrial and breast cancer.

Proliferation of breast and endometrial cells is under the control of ovarian steroid hormones (SHs) such as oestrogen and progesterone. They mediate diverse physiological functions via interaction with nuclear-localised steroid hormone receptors (HRs). The SH receptor complex modifies the expression of SH-regulated genes by binding to conserved binding sites in their promoter region or through cross-talk with other transcription factors. In non-malignant tissues, HRs are in balance with other factors regulating proliferation, differentiation and apoptosis. While dysfunction of the regulatory mechanisms is a part of malignant transformation, functional SH receptors can promote growth of SH-responsive tumours. Therefore, anti-hormones that block the interaction of steroid hormones with the SH receptor are useful tools for the treatment of SH-responsive carcinomas. However, a portion of ER-positive breast cancers and most endometrial cancers do not respond to anti-oestrogens and continued treatment results in hormone resistance, mostly without loss of the ER. This review focuses on the mechanisms of action of hormones and anti-hormones in breast and endometrial carcinomas.

Mammary gland development and tumorigenesis in estrogen receptor knockout mice.

Estrogens are important for the development of the mammary gland and strongly associated with oncogenesis in this tissue. The biological effects of estrogens are mediated through the estrogen receptor (ER), a member of the nuclear receptor superfamily. The estrogen/ER signaling pathway plays a central role in mammary gland development, regulating the expression and activity of other growth factors and their receptors. The generation of the ER knockout (ERKO) mouse has made it possible to directly understand the contribution of ER in mammary development and has provided an unique opportunity to study estrogen action in carcinogenesis. A mammary oncogene (Wnt-1) was introduced into the ERKO background to determine if the absence of the ER would affect the development of tumors induced by oncogenic stimulation. The development, hyperplasia, and tumorigenesis in mammary glands from the ERKO/Wnt-1 mouse line are described. These studies provide the impetus to evaluate the effect of other oncogenes in mammary tumorigenesis in the absence of estrogen/ER signaling.

Protective mechanisms of dietary fibers in nutritional carcinogenesis.

Fibers in foods are complex carbohydrates. There are several types of fiber, but, for the purpose of mechanistic insight into their mode of protective action in carcinogenesis, classification into two broad types, soluble and insoluble fibers, is warranted. Soluble fibers are present in fruits, vegetables, and certain grains like oats. This type of fiber undergoes metabolism in the small intestine and especially in the large intestine through bacterial enzymes, converting it to products that increase stool size only moderately. But, they have appreciable effects in modifying the metabolism of colon carcinogens like azoxymethane to yield detoxified products and, thus, reducing colon carcinogenesis. In contrast, insoluble fibers present in sizeable amounts in bran cereals, like wheat or rice, are not significantly metabolized by enzymes in the intestinal flora. Such fibers increase stool size substantially through several mechanisms, including higher water retention. The larger bulk dilutes carcinogens, especially tumor promoters such as secondary bile acids, resulting in lower risk of colon cancer in animals and in humans. Evidence in animal models and in humans also indicates that fiber may lower the risk of breast cancer, possibly via an endocrine mechanism. Based on these concepts, increased intake of total fiber, but especially of wheat bran cereal fiber, to yield a daily stool in adults of about 200 grams can significantly reduce the risk of colon cancer and, to a lesser but definite extent, of breast cancer. Thus, adequate fiber intake from cereals, fruits, and vegetables can help prevent important types of human cancer.

Alterations induced by phenobarbital, a liver tumor promoter, in hepatocyte receptors for insulin and glucagon and glycogen metabolism.

Exposure of rats to phenobarbital (PB), a tumor promoter in the two-stage hepatocarcinogenesis model, in their diet (0.06%) induces alterations in insulin receptors in the hepatocytes. There is a decrease both in the number of receptors and the dissociation constant (Kd) when compared with animals fed control laboratory diet. The number of insulin receptors/cell and the Kd were respectively: 183,000 +/- 19,000 and 15.3 +/- 2.5 nM for controls; 47,000 +/- 5000 and 2.8 +/- 0.3 nM for PB. The glycogen synthesis in response to insulin was found to be unresponsive in the hepatocytes from rats exposed to PB. Glucagon receptors on hepatocytes, however, were unaltered in animals treated with PB or fed a choline-deficient (CD) diet and the glucagon-stimulated glycogenolytic responses were also comparable to the controls. There is, therefore, a selective alteration in the hepatocyte surface membrane receptors. Both PB and CD have been shown to reduce the hepatic cell membrane receptors for epidermal growth factor, indicating that the two different tumor promoters alter peptide receptors with endogenous protein kinase activities. This similar though selective effect of the tumor promoters on cell surface receptors may be of significance in their action in carcinogenesis by having an effect on the alteration of regulation of cell growth and metabolism.