NF-κB addiction and its role in cancer: 'one size does not fit all'.

Activation of nuclear factor (NF)-κB, one of the most investigated transcription factors, has been found to control multiple cellular processes in cancer including inflammation, transformation, proliferation, angiogenesis, invasion, metastasis, chemoresistance and radioresistance. NF-κB is constitutively active in most tumor cells, and its suppression inhibits the growth of tumor cells, leading to the concept of 'NF-κB addiction' in cancer cells. Why NF-κB is constitutively and persistently active in cancer cells is not fully understood, but multiple mechanisms have been delineated including agents that activate NF-κB (such as viruses, viral proteins, bacteria and cytokines), signaling intermediates (such as mutant receptors, overexpression of kinases, mutant oncoproteins, degradation of IκBα, histone deacetylase, overexpression of transglutaminase and iNOS) and cross talk between NF-κB and other transcription factors (such as STAT3, HIF-1α, AP1, SP, p53, PPARγ, ß-catenin, AR, GR and ER). As NF-κB is 'pre-active' in cancer cells through unrelated mechanisms, classic inhibitors of NF-κB (for example, bortezomib) are unlikely to mediate their anticancer effects through suppression of NF-κB. This review discusses multiple mechanisms of NF-κB activation and their regulation by multitargeted agents in contrast to monotargeted agents, thus 'one size does not fit all' cancers.

Epidermal growth factor (EGF) activates nuclear factor-kappaB through IkappaBalpha kinase-independent but EGF receptor-kinase dependent tyrosine 42 phosphorylation of IkappaBalpha.

Overexpression of epidermal growth factor (EGF) receptor and constitutive activation of nuclear factor-kappaB (NF-kappaB) are frequently encountered in tumor cells. Although EGF has been shown to induce NF-kappaB activation, the mechanism is poorly understood. EGF activated NF-kappaB DNA binding, induced NF-kappaB reporter activity and the expression of antiapoptotic and cell-proliferative gene products. Interestingly, non-small cell lung adenocarcinoma cell lines (HCC827 and H3255), which exhibit EGFR amplification, showed ligand-independent activation of NF-kappaB. Unlike tumor-necrosis factor (TNF), however, EGF failed to induce IkappaBalpha phosphorylation and ubiquitination and the activation of IkappaBalpha kinase (IKK). Although DN-IKKbeta inhibited TNF-induced NF-kappaB activity, DN-IKKbeta had no effect on EGF-induced NF-kappaB activation, suggesting that EGF-induced NF-kappaB activation is IKK independent. Using dominant-negative plasmids, we also demonstrated the role of TRADD, TRAF2, NIK and Ras in EGF-induced NF-kappaB activation. By using specific antibodies and IkappaBalpha plasmid, which is mutated at tyrosine 42 to phenylalanine, we show that EGF induced the tyrosine phosphorylation of IkappaBalpha at residue 42. Furthermore, EGF receptor kinase inhibitor blocked IkappaBalpha phosphorylation and consequent NF-kappaB activation. Overall, our results indicate that tyrosine phosphorylation of IkappaBalpha at residue 42 is critical for EGF-induced NF-kappaB activation pathway.

Evidence that TNF-TNFR1-TRADD-TRAF2-RIP-TAK1-IKK pathway mediates constitutive NF-kappaB activation and proliferation in human head and neck squamous cell carcinoma.

Constitutively activated nuclear factor-kappaB (NF-kappaB) has been associated with a variety of aggressive tumor types, including head and neck squamous cell carcinoma (HNSCC); however, the mechanism of its activation is not fully understood. Therefore, we investigated the molecular pathway that mediates constitutive activation of NF-kappaB in a series of HNSCC cell lines. We confirmed that NF-kappaB was constitutively active in all HNSCC cell lines (FaDu, LICR-LON-HN5 and SCC4) examined as indicated by DNA binding, immunocytochemical localization of p65, by NF-kappaB-dependent reporter gene expression and its inhibition by dominant-negative (DN)-inhibitory subunit of NF-kappaB (IkappaBalpha), the natural inhibitor of NF-kappaB. Constitutive NF-kappaB activation in HNSCC was found to be due to constitutive activation of IkappaBalpha kinase (IKK); and this correlated with constitutive expression of phosphorylated forms of IkappaBalpha and p65 proteins. All HNSCC showed the expression of p50, p52, p100 and receptor-interacting protein; all linked with NF-kappaB activation. The expression of constitutively active NF-kappaB in HNSCC is mediated through the tumor necrosis factor (TNF) signaling pathway, as NF-kappaB reporter activity was inhibited by DN-TNF receptor-associated death domain (TRADD), DN-TNF receptor-associated factor (TRAF)2, DN-receptor-interacting protein (RIP), DN-transforming growth factor-beta-activated kinase 1 (TAK1), DN-kappa-Ras, DN-AKT and DN-IKK but not by DN-TRAF5 or DN-TRAF6. Constitutive NF-kappaB activation was also associated with the autocrine expression of TNF, TNF receptors and receptor-activator of NF-kappaB and its ligand in HNSCC cells but not interleukin (IL)-1beta. All HNSCC cell lines expressed IL-6, a NF-kappaB-regulated gene product. Furthermore, treatment of HNSCC cells with anti-TNF antibody downregulated constitutively active NF-kappaB, and this was associated with inhibition of IL-6 expression and cell proliferation. Our results clearly demonstrate that constitutive activation of NF-kappaB is mediated through the TRADD-TRAF2-RIP-TAK1-IKK pathway, making TNF a novel target in the treatment of head and neck cancer.

Anethole blocks both early and late cellular responses transduced by tumor necrosis factor: effect on NF-kappaB, AP-1, JNK, MAPKK and apoptosis.

Anethole, a chief constituent of anise, camphor, and fennel, has been shown to block both inflammation and carcinogenesis, but just how these effects are mediated is not known. One possibility is TNF-mediated signaling, which has also been associated with both inflammation and carcinogenesis. In the present report we show that anethole is a potent inhibitor of TNF-induced NF-kappaB activation (an early response) as monitored by electrophoretic mobility shift assay, IkappaBalpha phosphorylation and degradation, and NF-kappaB reporter gene expression. Suppression of IkappaBalpha phosphorylation and NF-kappaB reporter gene expression induced by TRAF2 and NIK, suggests that anethole acts on IkappaBalpha kinase. Anethole also blocked the NF-kappaB activation induced by a variety of other inflammatory agents. Besides NF-kappaB, anethole also suppressed TNF-induced activation of the transcription factor AP-1, c-jun N-terminal kinase and MAPK-kinase. In addition, anethole abrogated TNF-induced apoptosis as measured by both caspase activation and cell viability. The anethole analogues eugenol and isoeugenol also blocked TNF signaling. Anethole suppressed TNF-induced both lipid peroxidation and ROI generation. Overall, our results demonstrate that anethole inhibits TNF-induced cellular responses, which may explain its role in suppression of inflammation and carcinogenesis. Oncogene (2000).

Photo-oxidative stress down-modulates the activity of nuclear factor-kappaB via involvement of caspase-1, leading to apoptosis of photoreceptor cells.

The mechanisms of photoreceptor cell death via apoptosis, in retinal dystrophies, are largely not understood. In the present report we show that visible light exposure of mouse cultured 661W photoreceptor cells at 4.5 milliwatt/cm2 caused a significant increase in oxidative damage of 661W cells, leading to apoptosis of these cells. These cells show constitutive expression of nuclear factor-kappaB (NF-kappaB), and light exposure of photoreceptor cells results in lowering of NF-kappaB levels in both the nuclear and cytosolic fractions in a time-dependent manner. Immunoblot analysis of IkappaBalpha and p50, and p65 (RelA) subunits of NF-kappaB, suggested that photo-oxidative stress results in their depletion. Immunocytochemical studies using antibody to RelA subunit of NF-kappaB further revealed the presence of this subunit constitutively both in the nucleus and cytoplasm of the 661W cells. Upon exposure to photo-oxidative stress, a depletion of the cytoplasmic and nuclear RelA subunit was observed. The depletion of NF-kappaB appears to be mediated through involvement of caspase-1. Furthermore, transfection of these cells with a dominant negative mutant IkappaBalpha greatly enhanced the kinetics of down modulation of NF-kappaB, resulting in a faster photo-oxidative stress-induced apoptosis. Taken together, these studies show that the presence of NF-kappaB RelA subunit in the nucleus is essential for protection of photoreceptor cells against apoptosis mediated by an oxidative pathway.

Protein tyrosine kinase inhibitors block tumor necrosis factor-induced activation of nuclear factor-kappaB, degradation of IkappaBalpha, nuclear translocation of p65, and subsequent gene expression.

Several inflammatory effects of tumor necrosis factor (TNF) are known to be mediated through activation of a nuclear transcription factor NF-kappaB, but how TNF activates NF-kappaB is incompletely understood. In the present report, we examined the role of protein tyrosine kinases (PTK) in TNF-mediated NF-kappaB activation by using genistein and erbstatin, two potent inhibitors of PTK. The treatment of human myeloid U-937 cells with either inhibitor completely suppressed the TNF-induced NF-kappaB activation in a dose- and time-dependent manner. Suppression correlated with PTK activity, since among the structural analogues of genistein, only an active inhibitor of PTK, quercetin blocked TNF-induced NF-kappaB activation and not daidzein, an inactive inhibitor. Inhibition of NF-kappaB activation was not limited to myeloid cells, as it was observed with T cells and epithelial cells. Both the PTK inhibitors blocked the degradation of IkappaBalpha, the inhibitory subunit of NF-kappaB, and the consequent translocation of the p65 subunit without any significant effect on p50 or on c-Rel. The PTK inhibitors did not interfere with NF-kappaB binding to DNA. The NF-kappaB-dependent CAT reporter gene expression in transient transfection assays was also suppressed by the PTK inhibitors. Both PTK inhibitors abolished TNF-induced activation of N-terminal c-Jun kinase and mitogen-activated protein kinase kinase. Overall, our results suggest that a genistein- and erbstatin-sensitive PTK is involved in the pathway leading to NF-kappaB activation and gene expression by TNF and thus could be used as a target for development of antiinflammatory drugs.

Evidence for a synergistic role of two types of human tumor necrosis factor receptors for the ligand-dependent activation of the nuclear transcription factor NF-kappaB.

Tumor necrosis factor (TNF) is a multipotential cytokine that interacts with a wide variety of cells through two distinct receptors, referred to as the p60 and p80 receptors. Why there are two distinct receptors for the same ligand and whether these receptors mediate their signal independently or synergistically is not known. We examined the role of these two receptors in the ligand-dependent activation of a transcriptional factor, NF-kappaB, an early response (5-15 min) to TNF in human myeloid ML-1a cells. By using receptor type-specific antibodies, these cells were found to express almost equal amounts of both receptors. TNF-dependent activation of NF-kappaB could be blocked partially by both anti-p60 and anti-p80, suggesting that TNF mediates its effect independently through the p60 and p80 receptors. In comparison, the activation of NF-kappaB by lymphotoxin (LT), which shares receptors with TNF, was completely blocked by anti-p60, whereas anti-p80 had no effect. Anti-p60 but not anti-p80 by itself was found to activate NF-kappaB in a dose-dependent manner, but on a molar basis anti-p60 was found to be 100 times less potent than TNF. Interestingly, even though anti-p80 by itself was inactive, it potentiated the effect of anti-p60 synergistically, suggesting an interaction between the two types of TNF receptor. Thus, overall these results demonstrate that the two forms of TNF receptors could mediate their signal in both an independent and synergistic manner and that TNF mediates its signal through both forms of receptors, whereas LT mediates its signal through the p60 receptor.

Sanguinarine (pseudochelerythrine) is a potent inhibitor of NF-kappaB activation, IkappaBalpha phosphorylation, and degradation.

The nuclear factor NF-kappaB is a pleiotropic transcription factor whose activation results in inflammation, viral replication, and growth modulation. Due to its role in pathogenesis, NF-kappaB is considered a key target for drug development. In the present report we show that sanguinarine (a benzophenanthridine alkaloid), a known anti-inflammatory agent, is a potent inhibitor of NF-kappaB activation. Treatment of human myeloid ML-1a cells with tumor necrosis factor rapidly activated NF-kappaB, this activation was completely suppressed by sanguinarine in a dose- and time-dependent manner. Sanguinarine did not inhibit the binding of NF-kappaB protein to the DNA but rather inhibited the pathway leading to NF-kappaB activation. The reversal of inhibitory effects of sanguinarine by reducing agents suggests a critical sulfhydryl group is involved in NF-kappaB activation. Sanguinarine blocked the tumor necrosis factor-induced phosphorylation and degradation of IkappaBalpha, an inhibitory subunit of NF-kappaB, and inhibited translocation of p65 subunit to the nucleus. As sanguinarine also inhibited NF-kappaB activation induced by interleukin-1, phorbol ester, and okadaic acid but not that activated by hydrogen peroxide or ceramide, the pathway leading to NF-kappaB activation is likely different for different inducers. Overall, our results demonstrate that sanguinarine is a potent suppressor of NF-kappaB activation and it acts at a step prior to IkappaBalpha phosphorylation.

The p60 tumor necrosis factor (TNF) receptor-associated kinase (TRAK) binds residues 344-397 within the cytoplasmic domain involved in TNF signaling.

The p60 form of the tumor necrosis factor (TNF) receptor lacks motifs characteristic of tyrosine or serine/threonine protein kinases. Our recent observations have indicated that a p60 TNF receptor-associated kinase (p60-TRAK) from U-937 cells physically interacts with and causes the phosphorylation of the cytoplasmic domain of the TNF receptor. To define which region of the cytoplasmic domain is necessary for physical interaction with p60-TRAK, we constructed a series of deletions (grouped into three sets delta 1-delta 5, delta 6-delta 12, and delta 13-delta 16) of the p60 cytoplasmic domain, expressed them as glutathione S-transferase (GST) fusion proteins, and used them in affinity precipitations, followed by in vitro kinase assays. Our detailed analysis indicated that a serine-, threonine-, and proline-rich region (residues 243-274, delta 2) and the N-terminal half of the cytoplasmic domain (residues 243-323, delta 3) neither associated with p60-TRAK nor underwent phosphorylation. We found that out of 222 residues (205-426) in the cytoplasmic domain, only 54 (344-397, delta 12) were sufficient for binding p60-TRAK and for phosphorylation of the cytoplasmic domain. A region of approximately 30 residues (397-426) at the C-terminal end was found to interfere with optimal binding of the p60-TRAK activity. Thus, our results indicate that the minimal region of the cytoplasmic domain necessary for interacting with p60-TRAK and for phosphorylation resides within the domain previously reported to be needed for signaling the cytotoxic effect of TNF.

Retinoids downregulate both p60 and p80 forms of tumor necrosis factor receptors in human histiocytic lymphoma U-937 cells.

Because retinoids are known to modulate the growth and differentiation effects of tumor necrosis factor (TNF), we investigated the effect of all-trans-retinoic acid (RA) on the cell surface expression of TNF receptors in human histiocytic lymphoma U-937 cells. RA decreased the specific binding of 125I-labeled TNF to these cells in a dose- and time-dependent manner. The maximal decrease occurred when cells were treated with 1 mumol/L RA for 24 hours at 37 degrees C. Scatchard analysis of the binding indicated that the decrease by RA was caused by a decrease in receptor number and not by a decrease in affinity. The downmodulation of TNF receptors was also confirmed by covalent receptor-ligand cross-linking studies. Receptor-mediated internalization of the ligand was also found to be decreased on treatment of cells with RA. Northern blot analysis also indicated a decrease in the transcript of the receptor. By using antibodies specific to either the p60 or p80 form of the TNF receptor, we found that both receptors were downregulated by RA. RA treatment also decreased TNF receptors on acute monocytic leukemia cell line THP-1. Other analogues of RA, specifically 9-cis-RA, (E)-4-[2-(5,6,7,8- tetrahydro-2-naphthalenyl)-1-propenyl]-benzoic acid (TTNPB), and 3-methyl-TTNPB, which differ in their specificity towards different RA receptors, were also active in downregulating TNF receptors. 3-Methyl-TTNPB, which is more specific for the RXR form of the RA receptor, was found to be most potent. The downregulation of TNF receptors by RA correlated with the downmodulation of the antiproliferative effects of TNF against U-937 cells. Overall, our results indicate that RA downmodulates both the p60 and p80 form of the TNF receptor on cells of myeloid origin, which correlates with the cellular response.

Diminished responsiveness of senescent normal human fibroblasts to TNF-dependent proliferation and interleukin production is not due to its effect on the receptors or on the activation of a nuclear factor NF-kappa B.

The limited life span in culture of normal human diploid fibroblasts (HDF) has provided a model of cellular senescence. The short-term growth of these cells in culture is regulated by a number of different cytokines, including tumor necrosis factor (TNF), interleukin-1 (IL-1), and fibroblast growth factor (FGF). However, the effect of senescence on the responsiveness of HDF to these cytokines is not known. In the present report, we examined the effects of TNF on foreskin-derived HDF at different passage levels. We compared the response of HDF cells at population doubling (PD) 23 (young) with that of cells at PD 70 (senescent). Young cells proliferated in response to TNF in a dose-dependent manner. Under these conditions TNF had no effect on senescent HDF. The decrease in TNF responsiveness was found to be dependent on PD. The lack of response of senescent HDF was not unique to TNF, since FGF and IL-1 were also ineffective. In contrast to senescent HDF, TNF-dependent proliferation of young HDF could be further potentiated by IL-1 and FGF, suggesting an independent signaling mechanism. On exposure to TNF, senescent HDF produced IL-6 and IL-8, but to a much lower degree than that produced by young HDF. The diminished responsiveness of senescent HDF to TNF does not appear to be due to the difference in either receptor number or affinity, since senescent cells had two- to threefold higher number of TNF receptors than young HDF but the same affinity. TNF induced the activation of a nuclear transcriptional factor, NF-kappa B, equally in both young and senescent cells, which indicates the lack of a defect in the early events of TNF signal transduction in senescent fibroblasts. Overall, our results indicate that there is an age-dependent decline in TNF-induced proliferation and in the production of interleukins by fibroblasts; this unresponsiveness appears not to be due to TNF receptors or NF-kappa B activation. These results may have importance in understanding the diminished immune response, inflammation, and wound healing associated with aging.

Effect of tumor necrosis factors, interferons, interleukins, and growth factors on the activation of NF-kappa B: evidence for lack of correlation with cell proliferation.

The nuclear transcription factor NF-kappa B has been identified as a critical component in signal transduction pathways. We used an electrophoretic gel mobility shift assay to examine the activation of NF-kappa B in human U-937 cells treated with tumor necrosis factor (TNF), lymphotoxin (LT), interferons (IFN)-alpha, IFN-beta, and IFN-gamma, interleukins (IL)-1 beta, IL-4, and IL-6, leukemia inhibitory factor (LIF), basic fibroblast growth factor (FGF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and transforming growth factor-beta (TGF-beta). Only TNF, LT, and IL-1 activated NF-kappa B. Since interferons have been shown to induce TNF receptors and potentiate TNF-mediated cellular responses, we also measured the effect of interferons on TNF-induced activation of NF-kappa B. Under our conditions, all three IFNs potentiated the cytotoxic effects of TNF but had no effect on the TNF-dependent NF-kappa B activation. These results suggest overall that the activation of NF-kappa B is not a generalized mediator of signal transduction of most cytokines and also that NF-kappa B activation is not sufficient for antiproliferative effects mediated through certain cytokines.

Reconstitution of nuclear factor kappa B activation induced by tumor necrosis factor requires membrane-associated components. Comparison with pathway activated by ceramide.

Tumor necrosis factor (TNF) is known to induce the activation of a nuclear transcription factor, nuclear factor kappa B (NF-kappa B), in a wide variety of cell types. The post-receptor binding events that culminate in TNF-dependent NF-kappa B activation are not understood. To dissect this pathway, we developed a reconstitution system consisting of membrane, cytosolic, and post-nuclear fractions. Our results indicate that when incubated with the post-nuclear fraction derived from TNF-untreated cells, the membrane fraction from TNF-treated cells causes the activation of NF-kappa B with kinetics similar to that observed in intact cells. Under these conditions, the cytosolic fraction has no effect. This activation is tyrosine kinase-dependent since erbstatin completely abolished the effect. Furthermore, as revealed by immunoblotting, no degradation of the inhibitory subunit of NF-kappa B was observed. In this reconstitution system, we can also demonstrate the activation of NF-kappa B by ceramide, but this activation is not tyrosine kinase-dependent. Overall, our results indicate that intermediates required for NF-kappa B activation by TNF or ceramide are membrane-bound, but the mechanism of activation by TNF is most likely different from that of ceramide.

Tumor necrosis factor and lymphotoxin. Qualitative and quantitative differences in the mediation of early and late cellular response.

Tumor necrosis factor (TNF) is a 17-kDa protein produced by monocytes and a wide variety of other cell types in response to endotoxin and other cytokines. In contrast, lymphotoxin (LT) is a 25-kDa glycoprotein produced only by lymphocytes activated by mitogens. These two cytokines are 28% identical in their amino acid sequences. As they have common cell surface receptors, it is generally assumed that all cellular responses mediated through TNF are also mediated by LT and vice versa. In this report we tested this assumption, comparing the effect of TNF and LT on mediation of early (activation of the transcription factor NF-kappa B) and late (reduction of nitro blue tetrazolium, NBT) cellular responses in the human myelomonoblastic leukemic cell line ML-1a. Both qualitative and quantitative differences were found. LT was found to display 5-10 times more potent antiproliferative effects against murine fibroblasts than TNF. However, in ML-1a cells at concentrations wherein TNF activated NF-kappa B, LT did not. Higher concentrations (1,000-10,000 fold) of LT could activate NF-kappa B, but the activated complex was short lived (less than 1 h versus greater than 6 h when activated by TNF) and required longer treatment (15 min versus less than 5 min). TNF induced NBT-reducing activity in a dose-dependent manner, whereas LT was essentially inactive. Since both TNF and LT have been shown to bind to a common receptor, we tested whether the TNF-induced effects could be blocked by LT. LT inhibited both the early and late TNF-mediated cellular responses. By using receptor-blocking antibodies we found that both p60 and p80 forms of TNF receptors were functional for NBT-reducing activity, but TNF-dependent NF-kappa B activation required only the p60 receptor. Furthermore, we found that both TNF and LT bound with higher affinity to the p80 than to the p60 receptor. Thus, our overall results indicate that there are qualitative and quantitative differences in the action of TNF and LT, and these could be noted quite early in their signaling.

Staurosporine induces the cell surface expression of both forms of human tumor necrosis factor receptors on myeloid and epithelial cells and modulates ligand-induced cellular response.

Staurosporine, an inhibitor of protein kinase C, is commonly used to inhibit the growth factor-induced signal transduction pathway at the post-receptor level. In this report, we examined the effect of staurosporine on the constitutive expression of tumor necrosis factor (TNF) receptors in K562, a human erythroblastoid leukemic cell line. Exposure of these cells to staurosporine enhanced cell surface expression of TNF receptors by almost 7-fold in a dose- and time-dependent fashion. Maximum induction occurred at a concentration of 20 nM of the agent for 16 h at 37 degrees C. Induction of the TNF receptor was found to be temperature-dependent. No induction was observed at 22 or at 4 degrees C, suggesting the role of cell metabolism. Scatchard analysis indicated an increase in receptor number without any change in receptor affinity. TNF receptors were induced by staurosporine on a wide variety of human cells of both epithelial (primarily p60 receptors) and myeloid (mainly p80 receptor) origin. Receptor-specific antibodies showed that both TNF receptors were induced. The induction was abolished by inhibitors of protein synthesis, thus suggesting the de novo synthesis of the receptor. Furthermore, we found that staurosporine had no effect on the internalization or shedding of the receptor, but it induced the mRNA for both forms of the TNF receptor. Inhibitors of tyrosine kinases had no effect on the induction of TNF receptors. Modulation of the receptor number by staurosporine correlated with the enhancement of antiproliferative effects of TNF against different tumor cells. Thus, overall these results indicate that protein kinase C may be involved in the signal transduction of TNF not only at the postreceptor level but also at the receptor level.

Analysis of conformation and function of the chromatin of the brain of young and old rats.

Digestion of nuclei of the cerebral hemisphere of young (18-20 week) and old (90-97 week) rats by DNase I shows that the rate and extent of digestion is lower in the old. Time course analysis of the DNA fragments produced by DNase I by polyacrylamide gel electrophoresis shows that more of the lower base pair fragments are produced in the young. Also, the rate of production of these fragments is higher in the young than in the old. Assay of template-engaged RNA polymerase II (alpha-amanitin sensitive) shows that in the old it is only about 50% of that of the young. Addition of exogenous eukaryotic (wheat germ) RNA polymerase II is not able to restore transcription of the chromatin in the old to the level of the young. These data show that chromatin undergoes increasing condensation as a function of age, resulting in decreased transcriptional activity in old age.

Analysis of chromatin of the brain of young and old rats by nick-translation.

Chromatin of the brain of young (22-23 week) and old (118-119 week) rats has been analysed by nick-translation reaction following its digestion by DNaseI, EcoRI, MspI and HpaII. The incorporation of (3H)-dTMP in the old is only about 50 percent of that of the young. The difference in the incorporation following digestion of nuclei by MspI and HpaII that quantitate the degree of methylation of internal cytosines in the 5' CCGG 3' sequences, is nearly two-fold higher in the old. These data indicate that the chromatin undergoes increasing condensation as a function of age. One of the contributory factors may be increasing methylation of DNA. This may decrease the active fraction of chromatin.