Showing papers on "Cell growth published in 1982"

Retinoic Acid-Induced Growth Inhibition and Morphologic Differentiation of Human Neuroblastoma Cells In Vitro

Abstract: Retinoic acid (RA) induced concentration-dependent morphologic differentiation and growth inhibition in the LA-N-1 human neuroblastoma cell line. Time course studies demonstrated a significant increase in the formation of long neurites in LA-N-1 cultures within 48 hours of RA addition; maximum expression of differentiation occurred at approximately 4 days. This differentiation profile corresponded to a detectable decrease in [3H]thymidine incorporation at 48 hours and complete inhibition of cell growth after 3-4 days. The RA-induced morphologic differentiation and growth inhibition persisted despite removal of the drug. A soft agar assay system showed that RA also inhibited the ability of LA-N-1 cells to form anchorage-independent colonies and induced morphologic differentiation in colonies that did develop. These findings suggest that RA promoted the differentiation of LA-N-1 neuroblastoma cells, resulting in an altered expression of the malignant phenotype.

Tamoxifen and Metabolites in MCF7 Cells: Correlation between Binding to Estrogen Receptor and Inhibition of Cell Growth

Abstract: The binding of [3H]tamoxifen ([3H]Tam), a nonsteroidal antiestrogen, and of 4-[3H]hydroxytamoxifen ([3H]OH-Tam), a metabolite accumulated in vivo in target cell nuclei, was characterized in soluble extracts of human breast cancer MCF7 cells growing in a medium depleted in estrogens. Saturation analysis indicated a much higher affinity for OH-Tam (Kd = 0.15 nM) than for Tam (Kd = 4.8 nM). The binding of [3H]Tam and [3H]estradiol was competitive and mutually exclusive, and the binding site concentration (0.16 to 0.47 pmol/mg total protein) was similar for both ligands, strongly suggesting that antiestrogens were binding to the estrogen receptor (ER) in these cells. The ability of Tam and of some of its metabolites or derivatives to prevent the MCF7 cell growth was found to be correlated with their affinity for ER as determined by direct interaction or by binding competition with [3]estradiol on the uterine and MCF7 cytosol ER. OH-Tam was the highest-affinity compound and was 100-fold more active than Tam. The inhibitions observed were actually due to Tam and OH-Tam, respectively, since we did not detect any significant metabolism of these two labeled compounds by the MCF7 cells. N-desmethyltamoxifen, the other Tam metabolite found in high concentration in human plasma, was as effective as Tam while cis-tamoxifen appeared less effective. Compound E, which has no lateral chain, was the only tested compound having a good affinity for ER and a poor efficiency in preventing cell growth. These results support the hypothesis that antiestrogens control the growth of breast cancer by acting directly on the ER located in cancer cells.

Preferential effect of gamma interferon on the synthesis of HLA antigens and their mRNAs in human cells.

Abstract: Interferons produce a variety of biological effects on cells. They induce resistance to virus proliferation, inhibit cell growth, modify cell structure and differentiation, stimulate some immune functions and inhibit others. However, the different interferon (IFN) species may vary in their mechanism of action and, hence, in their relative efficiency for inducing each of the effect. IFN-gamma (type II) appears to show stronger immunoregulatory and growth inhibitory effects than antiviral effects, but this conclusion has been challenged in other reports. The aim of the present work is to compare the action of IFN-gamma and other (type I) interferons on the induction of (2'-5') oligo(A) synthetase which is probably part of the antiviral response and the induction of the histocompatibility HLA-A,-B,-C antigens. We have shown previously that the induction of both proteins is regulated by interferons at the mRNA level, but show here that IFN-gamma from stimulated human lymphocytes and from monkey cells transfected by cloned human IFN-gamma cDNA induced the HLA-A,-B,-C and beta 2-microglobulin mRNAs or proteins at concentrations over 100 times lower than those needed to induce the (2'-5')oligo(A) synthetase and the antiviral state. This difference was not found with IFN-alpha and -beta (type I).

Monoclonal antibody to transferrin receptor blocks transferrin binding and inhibits human tumor cell growth in vitro.

Abstract: A murine hybridoma has been obtained that produces a monoclonal antibody against the human transferrin receptor. In contrast to previously characterized monoclonal antibodies that recognize the transferrin receptor, this antibody, designated 42/6, blocks the binding of transferrin to its receptor and inhibits the growth of the human T leukemic cell line, CCRF-CEM, in vitro. Inhibition of cell growth was dose dependent, and as little as 2.5 micrograms of purified antibody per ml had a detectable effect, even though transferrin was present in the tissue culture medium in large molar excess. Cells grown in the presence of antibody for 7 days accumulated in S phase of the cell cycle. The addition of iron to antibody-treated cultures in the form of ferric complexes or ferrous sulfate did not overcome the growth inhibitory effects of the anti-transferrin-receptor antibodies. This result suggests that either transferrin is the only means by which CCRF-CEM leukemic cells can be provided with sufficient iron in vitro or that other factors in addition to iron starvation are involved in the antibody-mediated growth inhibition. The inhibition of cell growth by 42/6 monoclonal antibody suggests that monoclonal antibodies against proliferation-associated cell surface antigens, such as the transferrin receptor, may be useful pharmacological reagents to modify cell growth in vitro.

Epidermal growth factor inhibits growth of A431 human epidermoid carcinoma in serum-free cell culture.

Abstract: A medium consisting of a rich basal nutrient mixture supplemented with bovine insulin (10 micrograms/ml), human transferrin (10 micrograms/ml), human cold-insoluble globulin (5 micrograms/ml), and ethanolamine (0.5 mM) supported the growth of the A431 human epidermoid cell line in the absence of serum with a generation time equal to that of cells in serum-containing medium. Addition of epidermal growth factor (EGF) to this culture medium at concentration mitogenic for other cell types resulted in a marked inhibition of A431 cell growth. Inhibitory effects of EGF were observed at 1 ng/ml and near-maximal effects were observed at 10 ng/ml. The inhibitory effect of EGF could be reversed by the omission of EGF in subsequent medium changes and could be prevented by the addition of anti-EGF antibody to the culture medium. Inhibition of A431 cell growth by EGF also could be demonstrated in serum-containing medium.

Interactions of Rhodamine 123 with Living Cells Studied by Flow Cytometry

Abstract: The cationic fluorochrome rhodamine 123 (R123), reported to bind specifically to mitochondria of living cells, was presently investigated with respect to its uptake by a variety of cell types in various functional states and the subsequent effect of the dye on cell growth. The emission spectrum of R123 taken up by cells undergoes a 12-nm red shift, suggesting formation of a complex. Cells accumulate R123 rapidly; near maximum binding is reached after 5 to 10 min, regardless of the temperature (0-37 degrees) of incubation. There is a dose-dependent relationship between R123 concentration in the medium and the dye accumulation in the cell that covers the range of 0.1 to 10.0 and 0.1 to 5.0 microgram of R123 per ml under equilibrium and nonequilibrium conditions, respectively. Some leakage of the dye from cells occurs, following their transfer into dye-free medium. Despite the leakage, the intracellular dye can be detected after at least two cell divisions, thus indicating that: (a) the R123-labeled cells divide; (b) during division, labeled mitochondria are distributed into the daughter cells; and (c) R123 may be used as a cell tracer. Cell death often is accompanied by a transient increase in R123 fluorescence. Dead cells exhibit either uniform, strong fluorescence or show a patchy labeling pattern suggesting swollen mitochondria. With time (4 to 8 hr), dead cells lose ability to retain R123 and lyse. Uptake of R123 by living cells is increased during the transition from quiescence into the cycle, and a decrease is seen when Friend leukemia cells undergo erythroid differentiation; in all cases, changes in R123 uptake are correlated with changes in cellular RNA content. Simultaneous cell staining with R123 and ethidium or propidium provides a rapid assay of the viability of the cells and their metabolic state, i.e., as related to proliferation or motility. Pulse-labeling of cells with up to 10 microgram of R123 per ml has no significant effect on their immediate growth and cloning efficiency. In the continuous presence of R123, however, cells become specifically arrested in the G1A compartment, i.e., in early G1 phase. Detailed analysis of the cell cycle kinetics reveals that cell progression through all phases is slowed 4 hr after addition of R123. Cell exit from G1A, however, is affected as early as 2 hr following addition of R123, and with time the cells are unable to leave this compartment at all. Uncharged rhodamine dyes (rhodamine 110 and rhodamine B) do not accumulate in mitochondria and are without effect on the cell cycle. The cytostatic effect of R123 is discussed in light of the dye specificity for mitochondrial membranes and the disruption of cell energy metabolism, resulting in the inability of the cells to attain a critical content of essential components (i.e., ribosomal RNA), necessary for cell entrance into the prereplicative (G1B) compartment of G1 phase.

Estrogens stimulate cell proliferation and induce secretory proteins in a human breast cancer cell line (T47D).

Abstract: The effects of estradiol (E2) on two cloned sublines derived from the T47D human breast cancer cell line have been studied in vitro. Cell proliferation was evaluated by DNA assay, cell counts, and thymidine incorporation. The rate of synthesis of proteins released into the cell culture medium was assayed by [35S]methionine incorporation and polyacrylamide gel electrophoresis, followed by fluorography. In clone 11, which contains estrogen and progesterone receptors, estradiol (1 pM-1 nM) stimulated cell proliferation 2- to 5-fold after a lag period of 6 days. Maximal stimulation was observed with 1% or 3% fetal calf serum and without added insulin. The effect of E2 was biphasic, since the growth rate was stimulated for E2 concentrations less than 10 nM and then progressively inhibited for higher concentrations. Dexamethasone, dihydrotestosterone, progesterone, and R5020 (at 1 nM or 1 microM) did not modify cell growth. The antiestrogen Tamoxifen (1 microM) inhibited the E2-induced stimulation and decreased the growth of control cells. Estrogen also stimulated 2- to 3-fold the synthesis of approximately 60K molecular weight dalton proteins which were released into the medium. This effect was seen as early as 1 day of E2 treatment, and a plateau of stimulation was reached with 0.1 nM E2. By contrast, in clone 8, which contains low concentrations of estrogen and progesterone receptors, E2 had no effect on cell growth and stimulated slightly the synthesis of a 55K protein. These results demonstrate that E2 is able to directly stimulate the proliferation of human epithelial breast cancer cells after having stimulated the synthesis of approximately 60,000 dalton proteins released into the medium.

Synthesis, Retention, and Biological Activity of Methotrexate Polyglutamates in Cultured Human Breast Cancer Cells

Abstract: To determine the pharmacologic importance of methotrexate (MTX) polyglutamates, we examined the formation, retention, and effect of these metabolites in cultured human breast cancer cells. Two cell lines (MCF-7 and ZR-75-B) converted the drug to gamma-polyglutamate derivatives in a dose- and time-dependent reaction. After 24-h incubations with 2 muM MTX, polyglutamates of two to five amino acids in length accounted for 55.4% (51.9 nmol/g) of intracellular drug in the MCF-7 cells and 87.6% (62.4 nmol/g) of drug in ZR-75-B cells. In contrast, MDA-231 cells showed lesser accumulation of MTX, and only 32% (4.06 nmol/g) of the intracellular drug was in the form of polyglutamates, a difference that could only partially be explained by decreased ability of these cells to take up free drug from the medium. When MCF-7 and ZR-75-B cells containing polyglutamates were transferred to drug-free medium for 24 h, 22 and 51% of the total intracellular drug were, respectively, retained in each cell line. The loss of intracellular drug was primarily accounted for by disappearance of parent compound and polyglutamates containing 1-3 additional glutamyl residues. The rates of disappearance from cells decreased with increasing glutamyl chain length. All of the 4-NH(2)-10-CH(3)-PteGlu(5) and 47 and 38% of the 4-NH(2)-10-CH(3)-PteGlu(4) remained in the MCF-7 and ZR-75-B cells, respectively, and could be identified in the cytosol after 24 h in drug-free medium. The retention of MTX polyglutamates in these two cell lines in excess of dihydrofolate reductase binding capacity led to prolonged inhibition of thymidylate synthesis and loss of cell viability after removal of extracellular MTX. After 24-h incubation with 2 muM MTX and an additional 24 h in drug-free medium, [(3)H]deoxyuridine incorporation was still inhibited to 30% of control in the MCF-7 cells and 34.7% of control in ZR-75-B cells; this persistent inhibition was associated with a 30% reduction in cell numbers in each cell line during the 24-h period in drug-free medium. In contrast, [(3)H]deoxyuridine incorporation and cell growth quickly recovered to normal in the MDA-231 cells following removal of 2 muM MTX from the medium after a 24-h incubation. Prolonged inhibition of both thymidylate synthesis and cell growth was observed in this cell line in drug-free medium only after a 24-h incubation with 10 muM MTX, a condition that leads to the synthesis of 11.3 nmol/g of MTX polyglutamates. These studies demonstrate that polyglutamate formation allows a prolonged retention of drug in a noneffluxable form and prolonged inhibition of both thymidylate synthesis and cell growth following removal of extracellular drug.

Chemotaxis of Aortic Endothelial Cells in Response to Fibronectin

Abstract: We have determined the chemotactic response of bovine aortic endothelial cells to fibronectin and to endothelial cell mitogens (endothelial cell growth supplement, tumor extract), using blind-well chemotaxis chambers. Fibronectin induced a chemotactic response in bovine aortic endothelial cells; at 100 µg/ml, chemotaxis increased by 440% above that observed in negative controls ( p < 0.001). The chemotactic response plateaued with time, paralleling the disappearance of the fibronectin concentration gradient in the chambers. As further evidence that chemotaxis was measured, we observed that cell migration did not occur when cells were incubated with fibronectin in the absence of a concentration gradient. Endothelial cell mitogens increased bovine aortic endothelial cell proliferation in our experiments but did not stimulate chemotaxis above background levels. In contrast, fibronectin inhibited cell proliferation by 23%. We present a hypothetical model of the role of fibronectin in evoking endothelial cell chemotaxis during tumor neovascularization.

Characterization of antigen recognized by the monoclonal antibody (4F2): different molecular forms on human T and B lymphoblastoid cell lines.

Abstract: The monoclonal antibody 4F2 recognizes a disulfide-linked ricin-binding glycoprotein complex (Mr congruent to 125,000) composed of a sialylated heavy subunit (Mr congruent to 85,000 on T cell lines) and an unsialylated light subunit (Mr congruent to 41,000). The antigen (T85,41) recognized by 4F2 on T cell lines is structurally distinct from the antigen (B93, 41) on B cell lines. The heavy subunits, but not the light subunits, from all T cell lines examined were uniformly smaller in size than the heavy subunits from several B cell lines. This reflects differences in carbohydrate rather than protein represent in B93,41 compared with T85,41, because both heavy subunits have a common unglycosylated form (p65) and a common partially glycosylated precursor form (p68). Among non-T, non-B hematopoietic cell lines, the monocytoid line U-937 expressed an antigen that resembles B93,41, whereas the erythroleukemic line K-562 expressed an antigen more similar to T85,41. 4F2 recognizes a protein determinant on the heavy subunit (with or without N-linked glycosylation) and also the unglycosylated heavy subunit retains the ability to associate with light subunit. The light subunit itself contains no detectable N-linked carbohydrate. Unlike the transferrin receptor, synthesis of the antigen recognized by 4F2 on the promyelocytic cell line HL-60 did not diminish upon dimethylsulfoxide-induced differentiation, and thus is not tightly correlated with cell proliferation.

Relationship between in vitro growth promotion and biophysical and biochemical properties of the serum supplement.

Abstract: The normal range of many chemical, physical, and biological (hormonal) properties of fetal bovine, calf, and newborn calf sera collected commercially are presented. We attempt to relate these properties to growth promotion in vitro. Whereas no definitive conclusions could be drawn from the data, high growth hormone levels, low endotoxin levels, and low levels of bilirubin seemed to have some association with good growth promotion if the levels of the other parameters were close to the mean. Large variations from the mean of any of the parameters seemed to affect the growth properties of the serum.

Restriction point control of cell growth by a labile protein: evidence for increased stability in transformed cells.

Abstract: It has been proposed that animal cells must accumulate a labile protein(s) before they can pass the restriction (R) point in the G1 phase of the cell cycle [Rossow, P. W., Riddle, V. G. H. & Pardee, A. B. (1979) Proc. Natl. Acad. Sci. USA 76, 4446--4450]. Here, we present evidence that this R protein acquires increased stability in transformed 3T3 cells, thereby allowing these cells to continue growth under conditions that arrest untransformed cells. Low doses of cycloheximide or histidinol drastically reduced the rate at which normal 3T3 (A31) fibroblasts in early G1 could enter DNA synthesis. These drugs had less effect on entry of two tumorigenic A31 derivatives, BPA31 and SVA31, in S, although measurement of [3H]leucine incorporation showed that the inhibitors were equally effective in the three cell lines. The hypothesis is that the transformed lines are less sensitive because moderate inhibition of their R protein synthesis is compensated by lower rates of protein degradation. To test this idea, we completely inhibited cytoplasmic protein synthesis for several hours shortly before A31 and BPA31 cells had reached the R point. After removal of inhibitor, A31 cells showed delays in the onset of S that were in excess of the inhibitor pulse, consistent with decay of labile protein during the pulse. BPA31 cells showed no excess delays, suggesting a much more stable R protein. The half-life of the R protein was estimated as 2.5 hr in A31 cells, indicating that, in these cells, R protein synthesis starts at the beginning of G1. In the BPA31 cells the R protein showed no signs of decay for at least 8 hr.

Somatostatin inhibits rapid centrosomal separation and cell proliferation induced by epidermal growth factor.

Abstract: The role of endogenous growth inhibitors in the regulation of cell proliferation is unclear. Although there are numerous studies on the stimulatory effect of peptide hormones such as insulin, the somatomedins and epidermal growth factor (EGF) on cell proliferation, little is known about the existence of hormones that might exert an antiproliferative effect on cells. Somatostatin (SS), a cyclic tetradecapeptide hormone that is widely distributed in the body, exerts an inhibitory effect on numerous cellular processes. We observed that SS at concentrations of 10-8 - 10-13M, inhibited EGF-induced centrosomal separation, which recent evidence suggests is necessary for DNA synthesis in response to EGF. This SS effect was associated with inhibition of DNA synthesis and cell replication induced by EGF in gerbil fibroma and HeLa cells. Inhibition of centrosomal separation and the consequent antiproliferative effect of SS may represent a biologically significant action of this ubiquitous hormone.

The effect of epidermal growth factor (EGF) on cell proliferation of the gastrointestinal mucosa in rodents

Abstract: The effects of epidermal growth factor (EGF) on the cell production rate throughout the gastrointestinal mucosa in adult rats and mice was studied. The vincristine arrest technique combined with the micro-dissection method was used to estimate the cell production rate in the crypt.

Ornithine decarboxylase: essential in proliferation but not differentiation of human promyelocytic leukemia cells

Abstract: The ornithine decarboxylase inhibitor DL-alpha-difluoromethyl ornithine inhibited a proliferation-associated increase in ornithine decarboxylase activity in cultured human promyelocytic leukemia cells, resulting in a marked suppression of cell proliferation and subsequent cell loss. It also inhibited increases in ornithine decarboxylase activity associated with the phorbol ester-induced conversion of promyelocytic HL-60 cells to monocyte-like cells and the retinoic acid-induced conversion to granulocyte-like cells. However, the inhibition of ornithine decarboxylase activity did not prevent cellular differentiation. These results suggest that polyamine biosynthesis has a specific role in cell proliferation rather than in inducing differentiation that is not accompanied by proliferation. The data also demonstrate that cessation of proliferation in HL-60 cells is not necessarily associated with differentiation.

Coupling of proadipocyte growth arrest and differentiation. II. A cell cycle model for the physiological control of cell proliferation.

Abstract: Experimental evidence is presented that supports a cell cycle model showing that there are five distinct biological processes involved in proadipocyte differentiation. These include: (a) growth arrest at a distinct state in the G1 phase of the cell cycle; (b) nonterminal differentiation; (c) terminal differentiation; (d) loss of the differentiated phenotype; and (e) reinitiation of cell proliferation. Each of these events is shown to be regulated by specific human plasma components or other physiological factors. At two states designated GD and GD', coupling of growth arrest and differentiation is shown to occur. We propose that these mechanisms for the coupling of growth arrest and differentiation are physiologically significant and mimic the regulatory processes that control stem cell proliferation in vivo.

Suppression of Hematopoietic-Progenitor-Cell Proliferation by Ethanol and Acetaldehyde

Abstract: The effects of alcohol on bone marrow are not well understood. We measured the influence of ethanol and its metabolite, acetaldehyde, on the in vitro proliferation of hematopoietic progenitor cells from mice and human beings. Colony formation by both early and late erythroid progenitor cells was suppressed by concentrations of ethanol (0.05 to 0.2 per cent) that are easily achieved in vivo. The corresponding suppressing concentration of acetaldehyde was 0.001 per cent. In contrast, suppression of granulocyte/macrophage progenitor cells required 3.0 per cent ethanol or 0.03 per cent acetaldehyde. Spleen colony formation by pluripotent stem cells was resistant to concentrations of ethanol and acetaldehyde that suppressed in vitro colony formation of committed myeloid and erythroid progenitor cells by 50 per cent. The suppression of both myeloid and erythroid colony formation was partially reversed by supplementing the cultures with folinic acid or pyridoxine. These data provide an explanation for the preferential suppression of erythropoiesis observed clinically in ethanol abuse. They also suggest that acetaldehyde has a role in ethanol-mediated bone-marrow suppression.

Growth factors and regulation of cell growth.

Abstract: A new class of polypeptide hormones known collectively as growth factors has been identified. These polypeptides are able to stimulate DNA synthesis and mitosis of cells cultured in vitro. Growth factors have been isolated from several sources, including platelets, submaxillary glands, pituitary, brain, and medium conditioned by cells grown in vitro. Growth factors appear to behave like classic polypeptide hormones. In the cases of epidermal growth factor and nerve growth factor, specific cell membrane receptors have been studied in detail and partially purified. Platelet-derived growth factor and somatomedin-C interact synergistically and apparently, sequentially to promote cell proliferation in vitro. These studies suggest that cell proliferation is controlled by a synergistic interaction among several growth factors and, perhaps, other hormones. Several in vivo roles for growth factors and a role for tumor-cell-derived growth factors in neoplasia have been suggested.

Cholera toxin stimulation of human mammary epithelial cells in culture.

Abstract: Addition of cholera toxin to human mammary epithelial cultures derived from reduction mammoplasties and primary carcinomas greatly stimulated cell growth and increased the number of times the cells could be successfully subcultured. Other agents known to increase intracellular cAMP levels were also growth stimulatory. The increased growth potential conferred by cholera toxin enhances the usefulness of this cell culture system.

Phorbol ester induction of leukemic cell differentiation is a membrane-mediated process

Abstract: Phorbol esters are potent inducers of macrophage-like differentiation in the HL-60 promyelocytic leukemia cell line The sequence of events by which they bring about this transition is poorly understood However, it is known that phorbol esters bind to the surface membrane of HL-60 cells and to various other cells as well Our studies were directed toward determining the biologic importance of this membrane association [3H]Phorbol dibutyrate (PBu2) was specifically bound by HL-60 cells with a Kd of 23 nM and with 19 X 10(5) binding sites for [3H]PBu2 per cell There was no internalization of bound [3H]PBu2 Specific binding was fully reversible upon washing in fresh medium, and [3H]PBu2 added thereafter bound normally to its receptor Within 10 min of binding, PBu2 stimulated [14C]choline incorporation into phosphatidylcholine, with a rapid return to normal upon removal of the PBu2 Membrane-bound PBu2 progressively inhibited DNA synthesis, with 70% inhibition by 8 hr This process was interrupted if the PBu2 was removed, and little recovery of DNA synthesis occurred in previously inhibited cells Between 8 and 16 hr, PBu2 induced adherence of cells to plastic, but only in those cells in which phosphatidylcholine synthesis was stimulated, and this process was also interrupted if PBu2 was removed prior to 16 hr Similarly, nonspecific esterase, which develops after 72 hr of incubation, was induced in cells exposed to PBu2 for the initial 16 hr but not in cells exposed for 5 hr These studies demonstrate that phorbol esters exert their effects while retained at the cell surface Inhibition of cell growth and the acquisition of surface and enzymatic properties that characterize macrophages are separable events, each of which proceeds through a receptor-mediated, transmembrane process The stimulation of phosphatidylcholine synthesis appears to be a part of that process

Cell-specific differences in O6-alkylguanine DNA repair activity during continuous exposure to carcinogen

Abstract: The activity of the alkyl acceptor protein (AAP) responsible for repair of DNA containing the promutagenic lesion O6-alkylguanine was determined in hepatocytes and nonparenchymal cells (NPC) obtained from livers of control rats and rats exposed to hepatocarcinogens that primarily induce vascular or hepatocellular neoplasms. Basal levels of AAP activity were found to be 4-5 times higher in hepatocytes than in NPC. Exposure to 1,2-dimethylhydrazine or diethylnitrosamine produced a 2- to 3-fold enhancement of this activity in hepatocytes after exposure for as little as 3 days. The enhanced hepatocyte activity persisted throughout a 28-day exposure to 1,2-dimethylhydrazine. In contrast, AAP activity in NPC was decreased during the first week of exposure to 1,2-dimethylhydrazine and subsequently returned to control levels. No enhancement of AAP was apparent in the NPC. These and related data suggest that enhancement of this activity in rat hepatocytes is a response to cell proliferation. In contrast, the data clearly demonstrate that neither increased cell replication nor the presence of O6-alkylguanine was capable of enhancing AAP activity in NPC. Cellular differences in the repair of O6-alkylguanine appear to be a critical mechanism responsible for cell specificity in chemical carcinogenesis by alkylating agents.