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N A Berger

Publications and source records attributed to N A Berger.

At least 19 recordsLinked to original sources
PubMed1999

Increased sensitivity of human colon cancer cells to DNA cross-linking agents after GRP78 up-regulation.

We have shown earlier that pre-treatment of V79 Chinese hamster cells with 6-aminonicotinamide (6-AN) or 2-deoxyglucose (2-dG) results in over-expression of the Mr 78,000 glucose-regulated stress protein (GRP78) and the subsequent development of resistance to inhibitors of topoisomerase II. These phenomena also occur in V79-derived cell lines that are deficient in poly(ADP-ribose) (p(ADPR)) metabolism. In contrast, over-expression of GRP78 under the conditions outlined above is found to be associated with hypersensitivity to several clinically-relevant DNA cross-linking agents, namely, 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU), cisplatin, and melphalan. We have also previously shown that pre-treatment with 6-AN, an inhibitor of p(ADPR) metabolism, causes an increase in the life span in BCNU-treated mice bearing L1210 tumors. These observations prompted us to examine whether 6-AN pre-treatment can result in the over-expression of GRP78 in human colon cancer cell lines and, if so, whether this increase is associated with sensitization to DNA cross-linking agents outlined above. Following treatment of three colon cancer cell lines, HCT116, SW480, and VACO-8, for 48 h with 0.1 mM 6-AN, cytosolic GRP78 levels were elevated approximately 4.2 times, 8 times, and 2.5 times for each cell line respectively, as measured by Western immunoblotting. To determine sensitivity after GRP78 up-regulation, the cells were washed and grown for 412 h in growth medium devoid of 6-AN, before being treated with DNA cross-linking agents. The 412 h time period allowed p(ADPR) metabolism to return to normal while GRP78 levels remained elevated, thus allowing us to associate GRP78 over-expression with sensitivity to those agents. After treating cells for 1 h with BCNU, cisplatin, or melphalan, cell sensitivity was determined by clonogenic survival assay or a fluorescence-based cytotoxicity assay. Based on changes in IC50 values, 6-AN caused an increase in sensitivity for HCT116, SW480, and VACO-8 cells of 1.5, 2.3, and 1.0 times, respectively, for BCNU, 4.8, 3.8, and 2.6 for cisplatin, and 6.4, 3.7, and 2.2 times for melphalan. Thus, our results show that over-expression of GRP78 in human tumor cell lines is associated with increased sensitivity to clinically useful chemotherapy agents. This sensitization occurred in three different tumor cell lines, each bearing a separate genetic defect associated with altered sensitivity.

6-Aminonicotinamide↗
PubMed1999

Detection of poly(ADP-ribose) polymerase cleavage in response to treatment with topoisomerase I inhibitors: a potential surrogate end point to assess treatment effectiveness.

Cleavage of poly(ADP-ribose) polymerase (PARP) by caspases is a prominent characteristic of apoptosis or programmed cell death shown to be induced by topoisomerase (Topo) inhibitors. Because Topo I inhibitors have been shown to be effective in the treatment of some patients with colon cancer, we considered the possibility of using PARP cleavage as an early predictor of responsiveness to this class of agents. We show cleavage of PARP in response to treatment with Topo I inhibitors in colon cancer both in vitro and in vivo: (a) in vitro in SW480, HCT116, VACO5, VACO6, VACO8, VACO411, VACO425, and VACO451 human colon cancer cell lines treated with topotecan (TPT) or CPT-11; (b) in vivo in SW480, VACO451, and VRC5 colon cancer xenografts grown in athymic mice treated with TPT or CPT-11; and (c) in vivo in colon cancer samples from patients undergoing a Phase II clinical trial with CPT-11. Our results show a strong correlation between percentage of PARP cleavage and percentage of acridine orange-positive cells in colon cancer cell lines treated with 0.1 microM TPT for 24 and 48 h, confirming that PARP cleavage is a useful marker for programmed cell death in colon cancer cell lines. Results from experiments performed on colon cancer xenografts also show an association between PARP cleavage and response to treatment with TPT or CPT-11. The increase of PARP cleavage in xenografts and in clinical samples corresponding to treatment with Topo I inhibitors suggests that this procedure may have early predictive value to assess effectiveness of treatment. These results provide the basis for determining the validity of using PARP cleavage as an early marker of chemotherapeutic effectiveness in human samples.

Animals↗
PubMed1999

Poly(ADP-ribose) polymerase: a guardian of the genome that facilitates DNA repair by protecting against DNA recombination.

We have studied the clonogenic survival response to X-rays and MNNG of V79 Chinese hamster cells and two derivative cell lines, ADPRT54 and ADPRT351, deficient in poly(ADP-ribose) polymerase (PARP) activity. Under conditions of exponential growth, both PARP-deficient cell lines are hypersensitive to X-rays and MNNG compared to their parental V79 cells. In contrast, under growth-arrested, confluent conditions, V79 and PARP-deficient cells become similarly sensitive to X-rays and MNNG suggesting that PARP may be involved in the repair of X-ray or MNNG-induced DNA damage in logarithmically growing cells but not in growth-arrested confluent cells. This suggestion, however, creates a dilemma as to how PARP can be involved in DNA repair in only selected growth phases while it is functionally active in all growth phases. To explain these paradoxical results and resolve this dilemma we propose a hypothesis based on the consistent observation that inhibition of PARP results in a significant increase in sister chromatid exchange (SCEs). Thus, we propose that PARP is a guardian of the genome that protects against DNA recombination. We have extended this theme to provide an explanation for our results and the studies done by many others.

Animals↗
PubMed1998

Apoptosis induction by the glucocorticoid hormone dexamethasone and the calcium-ATPase inhibitor thapsigargin involves Bc1-2 regulated caspase activation.

The requirement for caspases (ICE-like proteases) were investigated in mediating apoptosis of WEHI7.2 mouse lymphoma cells in response to two death inducers with different mechanisms of action, the glucocorticoid hormone dexamethasone (DX) and the calcium-ATPase inhibitor thapsigargin (TG). Apoptosis induction by these agents followed different kinetics, and was closely correlated with in vivo activation of caspase-3 (CPP32/Yama/Apopain) and cleavage of the caspase target protein poly(ADP-ribose) polymerase (PARP). Caspase activation and PARP cleavage were inhibited by Bcl-2 overexpression. Cell extracts from DX- and TG-treated cells cleaved the in vitro synthesized baculovirus p35 ICE-like protease target, producing 25 and 10 kDa fragments. p35 cleavage was inhibited by mutating the active site aspartic acid to alanine, and by a panel of protease inhibitors that inhibit caspase-3-like proteases, including iodoacetamide, N-ethylmaleimide, and Ac-DEVD-cho. Treatment of cells in vivo with two cell permeant peptide fluoromethylketone inhibitors of caspase activity, Z-VAD-fmk and Z-DEVD-fmk, inhibited DX- and TG-induced apoptotic nuclear changes and maintained plasma membrane integrity, whereas the cathepsin inhibitor, Z-FA-fmk, and two calpain inhibitors failed to inhibit apoptosis. An unexpected observation was that due to the delayed time course of DX-induced apoptosis, optimal preservation of plasma membrane integrity was achieved by adding caspase inhibitors beginning 8 h after DX addition. In summary, the findings indicate that two diverse apoptosis-inducing signals converge into a common Bcl-2-regulated pathway that leads to caspase activation and apoptosis.

Animals↗
PubMed1998

Protease activation and cleavage of poly(ADP-ribose) polymerase: an integral part of apoptosis in response to photodynamic treatment.

Apoptosis induced by numerous cancer chemotherapeutic and other toxic agents has been shown to proceed through a cascade of proteases, now termed caspases, culminating in cleavage of a set of proteins. The ability of photodynamic treatment (PDT) with the phthalocyanine Pc 4 to activate cellular caspases has been assessed during the rapid apoptosis in murine lymphoma L5178Y-R cells. Cells were exposed to combinations of Pc 4 and activating red light that result in > or =90% cell death, as judged by a clonogenic assay. The rate of entry of cells into apoptosis was dose dependent. For 0.5 microM Pc 4 and either 2.1 or 3 kJ/m2, which kill 90 or 99.9% of the cells, oligonucleosomal fragmentation was visible on agarose gels as early as 60 or 30 min after PDT, respectively. To assess caspase activation, cells were harvested at various times after PDT, and cell proteins were subjected to electrophoresis and Western blot analysis, using an antibody to poly(ADP-ribose) polymerase (PARP). The cleavage of the normally Mr 116,000 PARP into fragments of Mr approximately 90,000 was observed at approximately the same time as the earliest DNA fragmentation. An antibody to the polymer, poly(ADP-ribose), did not recognize the Mr approximately 90,000 PARP cleavage products, in contrast to the parent enzyme. This analysis also revealed that levels of a poly(ADP-ribosylated) Mr 100,000 protein, tentatively identified as topoisomerase I, were maintained in cells after PARP was fully cleaved. Caspase-3 (and/or caspase-7) activity, as measured in cell lysates with the fluorogenic substrate DEVD-AMC, was elevated almost immediately after PDT. The cell-permeable, irreversible caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp(O-methyl)-fluoro-methylketone, inhibited PDT-induced apoptosis and PARP cleavage, whereas the inactive peptide analogue, benzyloxycarbonyl-Phe-Ala-fluoromethyl ketone, was without effect. The results indicate that PDT-induced apoptosis is mediated by activation of caspase-3 and/or other similar caspases.

Animals↗
PubMed1998

New isolation technique to study apoptosis in human intestinal epithelial cells.

Intestinal epithelial cells derive from stem cells at the base of the crypt and migrate along the crypt-lumen axis. Their life is terminated as they reach the luminal surface where they detach and are shed. Intestinal epithelial cells show evidence of apoptosis in the region of shedding, and cell death is thought to resemble a form of apoptosis called detachment-induced cell death, or anoikis. Human intestinal epithelial cells die rapidly in vitro due to loss of anchorage during isolation, making primary culture of these cells a goal that has not yet been reached. However, the molecular mechanisms underlying this process of anoikis are largely unknown. In this study, a novel protocol for the rapid, temperature-controlled isolation of highly purified human colonic epithelial cells from surgical specimens is described. Using this method, early molecular events of anoikis in nontransformed epithelial cells were studied. Intestinal epithelial cells were isolated at the beginning of the apoptotic cascade, before the activation of caspase 3 family members and cleavage of poly(ADP-ribose) polymerase and DNA fragmentation. Elucidating the molecular mechanisms of detachment-induced cell death may facilitate the establishment of long-term primary cultures of human intestinal epithelial cells and enhance our understanding of homeostasis in the intestinal epithelium.

Apoptosis↗
PubMed1997

Hypersensitivity to DNA cross-linking agents associated with up-regulation of glucose-regulated stress protein GRP78.

We have shown previously that NAD/poly(ADP-ribose) polymerase-deficient cells that overexpress Mr 78,000 glucose-regulated stress protein (GRP78) are resistant to topoisomerase II inhibitors, such as etoposide, m-amsacrine, and doxorubicin. However, these cells have been found to be hypersensitive to DNA cross-linking agents, including melphalan, cisplatin, and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). These observations prompted us to examine whether overexpression of GRP78 is associated with modulation of cytotoxicity of clinically useful DNA-cross-linking agents such as melphalan, BCNU, and cisplatin. We up-regulated GRP78 in V79 Chinese hamster cells by 2-5-fold using two independent approaches that include exposure to 6-aminonicotinamide, or 2-deoxyglucose. Subsequently, these GRP78-overexpressing cells were trypsinized, plated in regular medium without GRP78-inducing agents, and allowed a 5-h attachment time before being treated with melphalan, BCNU, or cisplatin for 1 h to determine clonogenic survivals. In addition, repair of DNA cross-links induced by those agents were determined by alkaline elution assay. Our results show that the GRP78-overexpressing V79 cells are hypersensitive to DNA cross-linking agents compared to the control V79 cells. Furthermore, repair of drug-induced DNA cross-links appears to be considerably slower in these cells relative to that found in control V79 cells. Thus, our results suggest that (a) up-regulation of GRP78 is associated with an impairment of DNA cross-link repair, (b) up-regulation of GRP78 is associated with potentiation of cytotoxicity induced by alkylating and platinating agents, and (c) up-regulation of GRP78 can be considered as a potentially useful tool to modulate the cytotoxicity of clinically useful alkylating and platinating agents.

6-Aminonicotinamide↗
PubMed1997

Factors affecting topotecan-induced programmed cell death: adhesion protects cells from apoptosis and impairs cleavage of poly(ADP-ribose)polymerase.

We have evaluated the influence of anchorage status together with endogenous levels of bcl-2 family members on the ability of the topoisomerase I inhibitor, topotecan (TPT), to induce programmed cell death (PCD) in human colon, breast, lymphoid, and cervical cancer cell lines. As part of this study, we assessed the use of measuring poly(ADP-ribose) polymerase (PARP) cleavage by Western blot, as an index of apoptosis, relative to measuring chromatin condensation by acridine orange analysis. Our results show a strong correlation between both assays, indicating that PARP cleavage is an accurate method to examine PCD. We have encountered a strong association between cell attachment and sensitivity to TPT-induced PCD. Cells growing attached to flasks appear to be relatively more resistant than suspension-growing cells in spite of endogenous bcl-2, bax, or bcl-x levels. Furthermore, we demonstrate that interference with attachment status alters the sensitivity of cells to TPT-induced PCD. Although cell attachment to ProNectin F confers protection against TPT-induced chromatin condensation and cleavage of PARP, cell detachment by poly(2-hydroxyethyl methacrylate) stimulates TPT-induced PCD and PARP cleavage.

Antineoplastic Agents↗
PubMed1997

Topotecan increases topoisomerase IIalpha levels and sensitivity to treatment with etoposide in schedule-dependent process.

To elucidate the effect of topoisomerase (Topo) I inhibitors in the modulation of Topo II levels and sensitivity to Topo II-directed drugs, athymic mice bearing SW480 human colon cancer xenografts were treated with simultaneous, subsequent, or distant doses of topotecan and etoposide. This in vivo study demonstrates that simultaneous administration of topotecan and etoposide results in an antagonistic response. In contrast, inhibition of Topo I by topotecan results in a compensatory increase in Topo II alpha levels associated with increasing sensitivity of tumors to subsequent treatment with the Topo II inhibitor etoposide. Furthermore, we show that Topo II alpha levels decline 5 days after the last dose of topotecan, resulting in restoration of the original response of the xenografts to etoposide. Thus, this study emphasizes the critical role of schedule dependency to optimize the effectiveness of combination chemotherapy with Topo I and Topo II inhibitors.

Animals↗
PubMed1996

Catalytic activity of poly(ADP-ribose) polymerase is necessary for repair of N-methylpurines in nontranscribed, but not in transcribed, nuclear DNA sequences.

The role of poly(ADP-ribose) polymerase (PADPRP) in nuclear DNA repair and other nuclear processes has been intensely studied and debated for decades. Recent studies have begun to shed new light on these arguments with firm experimental data for its role, primarily, as a remodeler of chromatin structure. Those studies imply that PADPRP plays an indirect role in DNA repair, serving to expose DNA to repair enzymes through chromatin remodeling. Only DNA that is tightly packaged would require PADPRP activity for its repair; while DNA in an open conformation would be accessible to DNA repair enzymes and not require PADPRP activity. The purpose of the current studies was to address the above hypothesis directly. Using quantitative Southern blot analysis, we studied repair in transcribed and nontranscribed nuclear DNA sequences in ADPRT 351 cells 95% deficient in PADPRP activity. Cells were exposed to methylnitrosourea (MNU) for 1 h and allowed to repair for 8 or 24 h. Densitometric scans of autoradiographs revealed that, when compared to their parental V79 cell line, ADPRT 351 cells 95% deficient in PADPRP activity were equally as efficient in repair of N-methylpurines in the transcribed sequence containing the dihydrofolate reductase gene. However, the ADPRT 351 cells were deficient in the ability to repair these lesions in the nontranscribed sequence containing the IgE gene compared to repair of the same sequence in the parental V79 cells. Nucleoid sedimentation assays demonstrated that the ADPRT 351 cells are deficient in repair across the entire genome when compared to the parental V79 cells. These studies indicate that PADPRP activity is not required for repair of N-methylpurines in transcribed nuclear DNA sequences but is necessary for the repair of these lesions in nontranscribed nuclear DNA sequences as well as across the entire genome since the DNA in a given cell is predominantly nontranscribed.

Alkylation↗
PubMed1996

Identification of a 116 kDa protein able to bind 1,3-bis(2-chloroethyl)-1-nitrosourea-damaged DNA as poly(ADP-ribose) polymerase.

SKI-1 is a 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)-resistant glioma cell line and SK-MG-1 is a BCNU-sensitive glioma cell line. Both cell lines do not express O6-methylguanine-DNA methyl transferase (MGMT) and exhibit comparable levels of 3-methyladenine DNA glycosylase. In order to detect DNA binding proteins involved in alternative DNA repair mechanisms of BCNU damage, we performed Southwestern analysis using a DNA probe damaged with BCNU and nuclear protein extracts from SKI-1 and SK-MG-1 cell lines. Both cell lines express a protein of M(r) 116,000 that is able to bind to BCNU-damaged DNA with higher specificity than to undamaged DNA. This protein was identified as poly(ADP-ribose) polymerase (PARP). Using glioma extracts depleted of PARP or using antibody to block the DNA binding domain of PARP no other protein binding to BCNU-treated probe was observed. Addition of methoxyamine, an inhibitor of DNA strand breaks, led to a significant reduction of PARP binding to BCNU-treated DNA. BCNU treatment of both glioma cell lines led to reduced nicotinamide adenine dinucleotide levels, indicating activation of PARP. Thus, the recognition and binding of PARP to BCNU-induced DNA nicks with concomitant PARP activation may be important processes that are involved in the initial stage of DNA repair of BCNU lesions in glial cells.

Antineoplastic Agents, Alkylating↗
PubMed1996

Wild-type p53 protein potentiates cytotoxicity of therapeutic agents in human colon cancer cells.

Wild-type p53 is induced by DNA damage. In different cell types, this induction is suggested either to facilitate DNA repair by inducing a cell cycle pause or to potentiate cell death via apoptosis. Wild-type p53 in different cell types has similarly been associated with either enhancement of or increased resistance to the cytotoxicity of many cancer therapeutic agents. We have constructed a colorectal cancer cell line bearing, in addition to endogenous mutant p53 alleles, an exogenous wild-type p53 allele that is under the regulatable control of the lac repressor. Induction of wild-type p53 by isopropyl-beta-thiogalactopyranoside in these cells induces a reversible growth arrest but does not induce cell death. However, we find that the induction of wild-type p53 powerfully potentiates the cytotoxicity of both irradiation and 5-fluorouracil, two agents that are used clinically in the treatment of colorectal cancer. We also find that induction of wild-type p53 potentiates the cytotoxicity of topotecan, a member of the camptothecin family of drugs that also has clinical activity against colon cancer. These findings suggest that the common loss of wild-type p53 in many colorectal cancers may play a role in the clinical resistance of these tumors to anticancer agents. Although some cancer cells may not be directly killed by p53 gene therapy, our findings suggest that genetic alteration of some cancers to induce wild-type p53 may increase their sensitivity to cytotoxic gene therapy.

Antineoplastic Agents↗
PubMed1995

Inhibition of etoposide (VP-16)-induced DNA recombination and mutant frequency by Bcl-2 protein overexpression.

Bcl-2 has been shown to inhibit apoptosis induced by several anticancer agents and to cause a dissociation between etoposide (VP-16)-induced protein-cross-linked DNA strand breaks and VP-16-induced cell death. We suggested previously that VP-16-induced cytotoxicity is mediated by a series of events leading from cleavable complex formation to aberrant DNA recombination, as measured by sister chromatid exchange (SCE) and Southern blot analysis of the hypoxanthine phosphoribosyl transferase (hprt) gene mutations. To further evaluate this hypothesis and to determine whether Bcl-2 could affect any steps leading to the aberrant DNA recombination process, we stably transfected an expression vector containing human Bcl-2 cDNA into V79 Chinese hamster cells. This transfection resulted in overexpression of the Bcl-2 gene product. We subsequently quantitated the relationship between VP-16-induced cytotoxicity, DNA strand breaks, SCE, and mutant frequency at the hprt locus in these Bcl-2-overexpressing cells. Two independent Bcl-2-overexpressing cell lines, BCL2/2 and BCL2/4, showed 3-5 times higher survival at 15 microM VP-16 compared with parental V79 cells or control NeoR cells that were obtained by transfecting V79 cells with the expression vector containing the G-418 resistance gene only. DNA single-strand breaks induced by VP-16 were similar in parental V79, control NeoR, BCL2/2, and BCL2/4 cells. In contrast, VP-16 induced significantly less SCE in Bcl-2-overexpressing cell lines compared with parental V79 and control NeoR cells. The SCE/chromosome induced by 15 microM VP-16 were 0.65, 0.42, 0.09, and 0.10, respectively, in V79, NeoR, BCL2/2, and BCL2/4. In addition, there was an excellent correlation between VP-16-induced SCE and cytotoxicity in all cell lines. Furthermore, VP-16-induced mutant frequencies at the hprt locus were 5-10 times less in BCL-2/2 and BCL-2/4 cells than those observed in the V79 or NeoR control cells. These results indicate that overexpression of Bcl-2 is associated with reduction in VP-16-induced genetic recombination, mutation, and cytotoxicity. Moreover, they suggest that Bcl-2 modulates cytotoxicity of VP-16 between cleavable complex formation and subsequent induction of DNA recombination events. Thus, our results provide important support for the hypothesis that VP-16-induced cytotoxicity is associated with aberrant recombination events, including gene deletions and rearrangements.

Animals↗
PubMed1995

Involvement of NAD-poly(ADP-ribose) metabolism in p53 regulation and its consequences.

We have used two different approaches to study the consequences of NAD/poly(ADP-ribose) deficiency on p53 expression and its activity in V79-derived cell lines. In the first approach, we have used two cell lines that are deficient in poly(ADP-ribose) (pADPR) synthesis because of deficiency in the enzyme poly(ADP-ribose) polymerase (PARP). In a second approach, we have used a cell line that is deficient in NAD/pADPR metabolism due to unavailability of NAD, the substrate for PARP. These NAD/PARP-deficient cell lines exhibit a significant reduction in both baseline p53 expression and its activity compared to their parental V79 cells. Furthermore, etoposide, a topoisomerase II inhibitor that was shown to cause an increase in p53 expression and subsequent apoptosis in V79 cells, failed to produce any significant increase in p53 expression or apoptotic DNA fragmentation in NAD/PARP-deficient cell lines. Thus, our studies suggest that NAD/pADPR synthesis may be involved in the regulation of p53 and its dependent pathways.

Animals↗
PubMed1995

Effect of inhibitors of poly(ADP-ribose) polymerase on the induction of GRP78 and subsequent development of resistance to etoposide.

We have recently demonstrated that cell lines deficient in poly(ADP-ribose) synthesis due to deficiency in the enzyme poly(ADP-ribose) polymerase (PADPRP) or depletion of its substrate NAD+ overexpress GRP78. Furthermore, this overexpression of GRP78 is associated with the acquisition of resistance to topoisomerase II-directed drugs such as etoposide (VP-16); (S. Chatterjee et al., Cancer Res., 54: 4405-4411, 1994). Thus, our studies suggest that interference with NAD+-PADPRP metabolism could provide an important approach to (a) define pathways of GRP78 induction, (b) study the effect of GRP78 on other cellular processes, (c) elucidate the mechanism of GRP78-dependent resistance to topoisomerase II targeted drugs, and (d) modulate responses to chemotherapy in normal and tumor tissues. However, in the in vivo situation, it is impractical to interfere with NAD+-PADPRP metabolism by mutational inactivation of PADPRP or by depletion of its substrate NAD+. Therefore, we have examined several inhibitors of NAD+-PADPRP metabolism including 3-aminobenzamide, PD128763, and 6-aminonicotinamide for their ability to reproduce the results obtained with cell lines deficient in NAD+-PADPRP metabolism relative to the induction of GRP78 and subsequent development of resistance to VP-16. Our studies show that 6-aminoicotinamide treatment is highly effective in the induction of GRP78 and subsequent development of resistance to VP-16, whereas treatment with 3-aminobenzamide or PD128763 does not induce GRP78 and thus does not result in VP-16 resistance.

6-Aminonicotinamide↗
PubMed1995

Topoisomerase I activity in squamous cell carcinoma of the head and neck.

In recent preclinical and clinical trials, topoisomerase I inhibitors have shown great promise as antitumor agents. These agents are most effective against tumors with high topoisomerase I activity. Therefore, determining topoisomerase I activity in advance may predict response to topoisomerase I inhibitors. Squamous cell carcinoma of the head and neck and normal tissue samples were obtained from 12 patients. Cellular extracts were prepared, and topoisomerase I activity assays were performed. The results suggest that topoisomerase I activity in squamous cell carcinoma of the head and neck is increased approximately sixtyfold compared to normal tissue. Increased activity often correlates with clinical responsiveness; these results predict that topoisomerase I inhibitors should be effective and selective against squamous cell carcinoma of the head and neck.

Aged↗