A Bispecific Molecule Targeting CD40 and Tumor Antigen Mesothelin Enhances Tumor-Specific Immunity

Agonistic CD40 monoclonal antibodies (mAb) have demonstrated some clinical activity, but with dose-limiting toxicity. To reduce systemic toxicity, we developed a bispecific molecule that was maximally active in the presence of a tumor antigen and had limited activity in the absence of the tumor antigen. LB-1 is a bispecific molecule containing single-chain Fv domains targeting mouse CD40 and the tumor antigen mesothelin. LB-1 exhibited enhanced activity upon binding to cell-surface mesothelin but was less potent in the absence of mesothelin binding. In a mouse model implanted with syngeneic 4T1 tumors expressing cell-surface mesothelin, LB-1 demonstrated comparable antitumor activity as an agonistic CD40 mAb but did not cause elevation of serum cytokines and liver enzymes, as was observed in anti-CD40-treated mice. The results from our study of LB-1 were used to develop a human cross-reactive bispecific molecule (ABBV-428) that targeted human CD40 and mesothelin. ABBV-428 demonstrated enhanced activation of antigen-presenting cells and T cells upon binding to cell-surface mesothelin, and inhibition of cultured or implanted PC3 tumor cell growth after immune activation. Although expression of cell-surface mesothelin is necessary, the bispecific molecules induced immune-mediated antitumor activity against both mesothelin⁺ and mesothelin^- tumor cells. ABBV-428 represents a class of bispecific molecules with conditional activity dependent on the binding of a tumor-specific antigen, and such activity could potentially maximize antitumor potency while limiting systemic toxicity in clinical studies.

CD20-directed small modular immunopharmaceutical, TRU-015, depletes normal and malignant B cells

PURPOSE

CD20-directed therapy with rituximab is effective in many patients with malignant lymphoma or follicular lymphoma. However, relapse frequently occurs within 1 year, and patients become increasingly refractory to retreatment. Our purpose was to produce a compact, single-chain CD20-targeting immunotherapeutic that could offer therapeutic advantages in the treatment of B-cell lymphoma.

EXPERIMENTAL DESIGN

Rituximab is a chimeric antibody containing two heavy chains and two light chains. Here, we describe the properties of TRU-015, a small modular immunopharmaceutical specific for CD20, encoded by a single-chain construct containing a single-chain Fv specific for CD20 linked to human IgG1 hinge, CH2, and CH3 domains but devoid of CH1 and CL domains.

RESULTS

TRU-015 mediates potent direct signaling and antibody-dependent cellular cytotoxicity but has reduced size and complement-mediated cytotoxicity activity compared with rituximab. TRU-015 is a compact dimer of 104 kDa that comigrates with albumin in size exclusion chromatography and retains a long half-life in vivo. TRU-015 induced growth arrest in multiple B lymphoma cell lines in vitro and showed effective antitumor activity against large, established subcutaneous Ramos or Daudi xenograft tumors in nude mice. TRU-015 also showed rapid, dose-dependent, and durable depletion of peripheral blood B cells following single-dose administration to nonhuman primates.

CONCLUSION

These results indicate that TRU-015 may improve CD20-directed therapy by effectively depleting embedded malignant B cells and nonmalignant pathogenic B cells and do so with reduced complement activation.

Centrifugal elutriation to obtain synchronous populations of cells

Counterflow centrifugal elutriation is a noninvasive method for separating large numbers of cells on the basis of their size and mass. For mammalian cells, this method is useful for separating mixed populations of cells, in particular cells at different stages of the cell division cycle without perturbing cell metabolism or using synchronizing agents. This unit describes a method for separating 2 x 10(8) cells using the standard JE-6B rotor or larger numbers of cells in the JE-5.0 rotor. To verify the purity and to characterize the cell cycle positions of cells in the elutriated populations, the unit includes protocols for measuring nascent DNA synthesis by [(3)H]thymidine incorporation and for detecting DNA synthesis and content by propidium iodide flow cytometry alone or in combination with bromodeoxyuridine incorporation.

Engineered anti-CD70 antibody with multiple effector functions exhibits in vitro and in vivo antitumor activities

Antigens expressed on malignant cells in the absence of significant expression on normal tissues are highly desirable targets for therapeutic antibodies. CD70 is a TNF superfamily member whose normal expression is highly restricted but is aberrantly expressed in hematologic malignancies including non-Hodgkin lymphoma (NHL), Hodgkin disease, and multiple myeloma. In addition, solid tumors such as renal cell carcinoma, nasopharyngeal carcinoma, thymic carcinoma, meduloblastoma, and glioblastoma express high levels of this antigen. To functionally target CD70-expressing cancers, a murine anti-CD70 monoclonal antibody was engineered to contain human IgG1 constant domains. The engineered antibody retained the binding specificity of the murine parent monoclonal antibody and was shown to induce Fc-mediated effector functions including antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity, and antibody-dependent cellular phagocytosis in vitro. Further, administration of this antibody significantly prolonged survival of severe combined immunodeficient (SCID) mice bearing CD70+ disseminated human NHL xenografts. Survival of these mice was dependent upon the activity of resident effector cells including neutrophils, macrophages, and natural killer (NK) cells. These data suggest that an anti-CD70 antibody, when engineered to contain human IgG1 constant domains, possesses effector cell–mediated antitumor activity and has potential utility for anticancer therapy.

Lymphocyte activation antigen CD70 expressed by renal cell carcinoma is a potential therapeutic target for anti-CD70 antibody-drug conjugates

Metastatic renal cell carcinoma (RCC) is an aggressive disease refractory to most existing therapeutic modalities. Identifying new markers for disease progression and drug targets for RCC will benefit this unmet medical need. We report a subset of clear cell and papillary cell RCC aberrantly expressing the lymphocyte activation marker CD70, a member of the tumor necrosis factor superfamily. Importantly, CD70 expression was found to be maintained at the metastatic sites of RCC. Anti-CD70 antibody-drug conjugates (ADC) consisting of auristatin phenylalanine phenylenediamine (AFP) or monomethyl auristatin phenylalanine (MMAF), two novel derivatives of the anti-tubulin agent auristatin, mediated potent antigen-dependent cytotoxicity in CD70-expressing RCC cells. Cytotoxic activity of these anti-CD70 ADCs was associated with their internalization and subcellular trafficking through the endosomal-lysosomal pathway, disruption of cellular microtubule network, and G2-M phase cell cycle arrest. The efficiency of drug delivery using anti-CD70 as vehicle was illustrated by the much enhanced cytotoxicity of antibody-conjugated MMAF compared with free MMAF. Hence, ADCs targeted to CD70 can selectively recognize RCC, internalize, and reach the appropriate subcellular compartment(s) for drug release and tumor cell killing. In vitro cytotoxicity of these ADCs was confirmed in xenograft models using RCC cell lines. Our findings provide evidence that CD70 is an attractive target for antibody-based therapeutics against metastatic RCC and suggest that anti-CD70 ADCs can provide a new treatment approach for advanced RCC patients who currently have no chemotherapeutic options.

Lysosomal trafficking and cysteine protease metabolism confer target-specific cytotoxicity by peptide-linked anti-CD30-auristatin conjugates

The chimeric anti-CD30 monoclonal antibody cAC10, linked to the antimitotic agents monomethyl auristatin E (MMAE) or F (MMAF), produces potent and highly CD30-selective anti-tumor activity in vitro and in vivo. These drugs are appended via a valine-citrulline (vc) dipeptide linkage designed for high stability in serum and conditional cleavage and putative release of fully active drugs by lysosomal cathepsins. To characterize the biochemical processes leading to effective drug delivery, we examined the intracellular trafficking, internalization, and metabolism of the parent antibody and two antibody-drug conjugates, cAC10vc-MMAE and cAC10vc-MMAF, following CD30 surface antigen interaction with target cells. Both cAC10 and its conjugates bound to target cells and internalized in a similar manner. Subcellular fractionation and immunofluorescence studies demonstrated that the antibody and antibody-drug conjugates entering target cells migrated to the lysosomes. Trafficking of both species was blocked by inhibitors of clathrin-mediated endocytosis, suggesting that drug conjugation does not alter the fate of antibody-antigen complexes. Incubation of cAC10vc-MMAE or cAC10vc-MMAF with purified cathepsin B or with enriched lysosomal fractions prepared by subcellular fractionation resulted in the release of active, free drug. Cysteine protease inhibitors, but not aspartic or serine protease inhibitors, blocked antibody-drug conjugate metabolism and the ensuing cytotoxicity of target cells and yielded enhanced intracellular levels of the intact conjugates. These findings suggest that in addition to trafficking to the lysosomes, cathepsin B and perhaps other lysosomal cysteine proteases are requisite for drug release and provide a mechanistic basis for developing antibody-drug conjugates cleavable by intracellular proteases for the targeted delivery of anti-cancer therapeutics.

Enhanced Activity of Monomethylauristatin F through Monoclonal Antibody Delivery: Effects of Linker Technology on Efficacy and Toxicity

We have previously shown that antibody-drug conjugates (ADCs) consisting of cAC10 (anti-CD30) linked to the antimitotic agent monomethylauristatin E (MMAE) lead to potent in vitro and in vivo activities against antigen positive tumor models. MMAF is a new antimitotic auristatin derivative with a charged C-terminal phenylalanine residue that attenuates its cytotoxic activity compared to its uncharged counterpart, MMAE, most likely due to impaired intracellular access. In vitro cytotoxicity studies indicated that mAb-maleimidocaproyl-valine-citrulline-p-aminobenzyloxycarbonyl-MMAF (mAb-L1-MMAF) conjugates were >2200-fold more potent than free MMAF on a large panel of CD30 positive hematologic cell lines. As with cAC10-L1-MMAE, the corresponding MMAF ADC induced cures and regressions of established xenograft tumors at well tolerated doses. To further optimize the ADC, several new linkers were generated in which various components within the L1 linker were either altered or deleted. One of the most promising linkers contained a noncleavable maleimidocaproyl (L4) spacer between the drug and the mAb. cAC10-L4-MMAF was approximately as potent in vitro as cAC10-L1-MMAF against a large panel of cell lines and was equally potent in vivo. Importantly, cAC10-L4-MMAF was tolerated at >3 times the MTD of cAC10-L1-MMAF. LCMS studies indicated that drug released from cAC10-L4-MMAF was the cysteine-L4-MMAF adduct, which likely arises from mAb degradation within the lysosomes of target cells. This new linker technology appears to be ideally suited for drugs that are both relatively cell-impermeable and tolerant of substitution with amino acids. Thus, alterations of the linker have pronounced impacts on toxicity and lead to new ADCs with greatly improved therapeutic indices.

Preclinical antilymphoma activity of a humanized anti-CD40 monoclonal antibody, SGN-40

SGN-40 is a humanized IgG1 antihuman CD40 that is currently in a phase I clinical trial for the treatment of multiple myeloma. As surface CD40 expression on B-lineage cells is maintained from pro-B cells to plasma cells, SGN-40 may be applicable to treatment of other B-cell neoplasias, including non-Hodgkin's lymphoma. In this study, we examined potential in vitro and in vivo anti-B-lineage lymphoma activity of SGN-40. Recombinant SGN-40 was expressed and purified from Chinese hamster ovary cells and characterized based on binding affinity, specificity, and normal B-cell stimulation. The ability of SGN-40 to target neoplastic B cells was examined in vitro by proliferation inhibition, cytotoxicity, and antibody-dependent cell cytotoxicity assays and in vivo by human lymphoma xenograft models. Recombinant SGN-40 showed high affinity, Kd of approximately 1 nmol/L, and specific binding to CD40. Whereas SGN-40 was a weak agonist in stimulating normal B-cell proliferation in the absence of IL-4 and CD40L, it delivered potent proliferation inhibitory and apoptotic signals to, and mediated antibody-dependent cytotoxicity against, a panel of high-grade B-lymphoma lines. These in vitro antilymphoma effects were extended to disseminated and s.c. xenograft CD40 tumor models. In these xenograft models, the antitumor activity of SGN-40 was comparable with that of rituximab. The preclinical in vitro and in vivo antilymphoma activity of SGN-40 observed in this study provides a rationale for the clinical testing of SGN-40 in the treatment of CD40+ B-lineage lymphomas.

Signaling via the anti-CD30 mAb SGN-30 sensitizes Hodgkin's disease cells to conventional chemotherapeutics

SGN-30, a monoclonal antibody with activity against CD30+ malignancies, is currently in phase II clinical evaluation for treatment of Hodgkin's disease (HD) and anaplastic large cell lymphoma. The mechanisms underlying SGN-30's antitumor activity were investigated using cDNA array of L540 cells. SGN-30 treatment activated NF-kappaB and modulation of several messages including the growth regulator p21WAF1/CIP1 (p21) and cellular adhesion marker ICAM-1. p21 protein levels increased coincident with growth arrest and Annexin V/PI staining in treated HD cells. To determine if SGN-30-induced growth arrest would sensitize tumor cells to chemotherapeutics used against HD, L540cy and L428 cells were exposed to SGN-30 in combination with a panel of cytotoxic agents and resultant interactions quantified by the Combination Effects Method. Interactions between SGN-30 and all cytotoxic agents examined were additive or better. These in vitro data translated to increased efficacy of SGN-30 and bleomycin against L540cy tumor xenografts. In addition to direct cell killing, SGN-30 affects growth arrest and drug sensitization through growth regulating and proapoptotic machinery. Importantly, these data suggest that SGN-30 can enhance the efficacy of standard chemotherapies used to treat patients with CD30+ malignancies.

In vivo drug-linker stability of an anti-CD30 dipeptide-linked auristatin immunoconjugate

Effective antibody-drug conjugates (ADC) combine high drug-linker stability in circulation and efficient intratumoral release of drug. Conjugation of monomethyl auristatin E (MMAE) to the anti-CD30 monoclonal antibody (mAb), cAC10, produced a selective and potent ADC against CD30(+) anaplastic large cell lymphoma and Hodgkin's disease models. This ADC, cAC10-valine-citrulline-MMAE, uses a protease-sensitive dipeptide linker designed to release MMAE by lysosomal cathepsin B in target cells but maintain a stable linkage and attenuate drug potency in circulation. To evaluate ADC stability in vivo, we developed methods for measuring drug/mAb ratios at progressive times in plasma from ADC-treated mice and nonhuman primates. Anti-idiotype mAb permitted the capture and quantitation of mAb cAC10, whereas antidrug mAb and MMAE-conjugated horseradish peroxidase reporter provided quantitative detection of conjugated drug following its in vitro release by cathepsin B. These data were validated by an alternative ELISA using anti-idiotype and anti-MMAE mAbs for capture and detection, respectively. Both methods differentiated ADC with variable levels of drug loading and were subsequently applied to stability studies in severe combined immunodeficient mice and cynomolgus monkeys. Evaluation of ADC from mouse circulation showed the linker half-life to be approximately 144 hours (6.0 days), significantly greater than that reported for disulfide- or hydrazone-linked ADCs in mice or human trials. In cynomolgus monkey, the apparent linker half-life was approximately 230 hours (9.6 days), suggesting that the drug-linker will be highly stable in humans. These data represent the longest reported drug-linker half-life to date and provide the basis for the pronounced specificity and antitumor activity of cAC10-valine-citrulline-MMAE.

In vivo Drug-Linker Stability of an Anti-CD30 Dipeptide-Linked Auristatin Immunoconjugate

Effective antibody-drug conjugates (ADC) combine high drug-linker stability in circulation and efficient intratumoral release of drug. Conjugation of monomethyl auristatin E (MMAE) to the anti-CD30 monoclonal antibody (mAb), cAC10, produced a selective and potent ADC against CD30+ anaplastic large cell lymphoma and Hodgkin's disease models. This ADC, cAC10-valine-citrulline-MMAE, uses a protease-sensitive dipeptide linker designed to release MMAE by lysosomal cathepsin B in target cells but maintain a stable linkage and attenuate drug potency in circulation. To evaluate ADC stability in vivo, we developed methods for measuring drug/mAb ratios at progressive times in plasma from ADC-treated mice and nonhuman primates. Anti-idiotype mAb permitted the capture and quantitation of mAb cAC10, whereas antidrug mAb and MMAE-conjugated horseradish peroxidase reporter provided quantitative detection of conjugated drug following its in vitro release by cathepsin B. These data were validated by an alternative ELISA using anti-idiotype and anti-MMAE mAbs for capture and detection, respectively. Both methods differentiated ADC with variable levels of drug loading and were subsequently applied to stability studies in severe combined immunodeficient mice and cynomolgus monkeys. Evaluation of ADC from mouse circulation showed the linker half-life to be ∼144 hours (6.0 days), significantly greater than that reported for disulfide- or hydrazone-linked ADCs in mice or human trials. In cynomolgus monkey, the apparent linker half-life was ∼230 hours (9.6 days), suggesting that the drug-linker will be highly stable in humans. These data represent the longest reported drug-linker half-life to date and provide the basis for the pronounced specificity and antitumor activity of cAC10-valine-citrulline-MMAE.

Efficient elimination of B-lineage lymphomas by anti-CD20-auristatin conjugates

The anti-CD20 antibody rituximab is useful in the treatment of certain B-cell malignancies, most notably non-Hodgkin's lymphoma. Its efficacy has been increased when used in combination with chemotherapy, yet anti-CD20 monoclonal antibodies (mAbs) directly conjugated with drugs such as doxorubicin (Dox) have failed to deliver drug or to demonstrate antitumor activity. We have produced anti-CD20 antibody-drug conjugates that possess potent antitumor activity by using the anti-mitotic agent, monomethyl auristatin E (MMAE), linked via the lysosomally cleavable dipeptide, valine-citrulline (vc). Two anti-CD20 conjugates, rituximab-vcMMAE and 1F5-vcMMAE, were selectively cytotoxic against CD20(+) B-lymphoma cell lines, with IC(50) values ranging from 50 ng/mL to 1 microg/mL. Unlike rituximab, which showed diffuse surface localization, rituximab-vcMMAE capped and was internalized within 4 hours after binding to CD20(+) B cells. Internalization of rituximab-vcMMAE was followed by rapid G(2)-M phase arrest and onset of apoptosis. Anti-CD20 antibody-drug conjugates prepared with Dox were internalized and localized as with rituximab-vcMMAE, yet these were not effective for drug delivery (IC(50) > 50 microg/mL). Consistent with in vitro activity, rituximab-vcMMAE showed antitumor efficacy in xenograft models of CD20-positive lymphoma at doses where rituximab or rituximab-Dox conjugates were ineffective. These data indicate that anti-CD20-based antibody-drug conjugates are effective antitumor agents when prepared with a stable, enzyme-cleavable peptide linkage to highly potent cytotoxic agents such as MMAE.

Secondary mAb−vcMMAE Conjugates Are Highly Sensitive Reporters of Antibody Internalization via the Lysosome Pathway

Monoclonal antibodies (mAb) selectively recognizing tumor surface antigens are an important and evolving approach to targeted cancer therapy. One application of therapeutic mAbs is drug targeting via mAb-drug conjugate (ADC) technology. Identification of mAbs capable of internalizing following antigen binding has been accomplished by tracking decline of surface-bound mAb or by internalization of a secondary mAb linked to a toxin. These methods may not be sufficiently sensitive for screening nor wholly predictive of the mAbs' capacity for a specific drug delivery. We have developed a highly selective and sensitive method to detect mAbs for cell internalization and drug delivery. This system uses secondary anti-human or anti-murine mAbs conjugated to the high-potency drug monomethyl auristatin E (MMAE) via a highly stable, enzymatically cleavable linker. Prior studies of this drug linker technology demonstrated internalization of a primary ADC leads to trafficking to lysosomes, drug release by lysosomal cathepsin B, and ensuing cell death. A secondary antibody--drug conjugate (2 degrees ADC) capable of binding primary mAbs bound to the surface of antigen-positive cells has comparable drug delivery capability. The system is sufficiently sensitive to detect internalizing mAbs in nonclonal hybridoma supernatants and is predictive of the activity of subsequently produced primary ADC. Because of their high extracellular stability, the noninternalized 2 degrees ADC are 100--1000-fold less toxic to cells over extended periods of time, permitting an assay in which components can be added without need for separate wash steps. This homogeneous screening system is amenable to medium-throughput screening applications and enables the early identification of mAbs capable of intracellular trafficking for drug delivery and release.

cAC10-vcMMAE, an anti-CD30-monomethyl auristatin E conjugate with potent and selective antitumor activity

The chimeric monoclonal antibody cAC10, directed against CD30, induces growth arrest of CD30+ cell lines in vitro and has pronounced antitumor activity in severe combined immunodeficiency (SCID) mouse xenograft models of Hodgkin disease. We have significantly enhanced these activities by conjugating to cAC10 the cytotoxic agent monomethyl auristatin E (MMAE) to create the antibody-drug conjugate cAC10-vcMMAE. MMAE, a derivative of the cytotoxic tubulin modifier auristatin E, was covalently coupled to cAC10 through a valine-citrulline peptide linker. The drug was stably attached to the antibody, showing only a 2% release of MMAE following 10-day incubation in human plasma, but it was readily cleaved by lysosomal proteases after receptor-mediated internalization. Release of MMAE into the cytosol induced G2/M-phase growth arrest and cell death through the induction of apoptosis. In vitro, cAC10-vcMMAE was highly potent and selective against CD30+ tumor lines (IC50 less than 10 ng/mL) but was more than 300-fold less active on antigen-negative cells. In SCID mouse xenograft models of anaplastic large cell lymphoma or Hodgkin disease, cAC10-vcMMAE was efficacious at doses as low as 1 mg/kg. Mice treated at 30 mg/kg cAC10-vcMMAE showed no signs of toxicity. These data indicate that cAC10-vcMMAE may be a highly effective and selective therapy for the treatment of CD30+ neoplasias.

Development of potent monoclonal antibody auristatin conjugates for cancer therapy

We describe the in vitro and in vivo properties of monoclonal antibody (mAb)-drug conjugates consisting of the potent synthetic dolastatin 10 analogs auristatin E (AE) and monomethylauristatin E (MMAE), linked to the chimeric mAbs cBR96 (specific to Lewis Y on carcinomas) and cAC10 (specific to CD30 on hematological malignancies). The linkers used for conjugate formation included an acid-labile hydrazone and protease-sensitive dipeptides, leading to uniformly substituted conjugates that efficiently released active drug in the lysosomes of antigen-positive (Ag+) tumor cells. The peptide-linked mAb-valine-citrulline-MMAE and mAb-phenylalanine-lysine-MMAE conjugates were much more stable in buffers and plasma than the conjugates of mAb and the hydrazone of 5-benzoylvaleric acid-AE ester (AEVB). As a result, the mAb-Val-Cit-MMAE conjugates exhibited greater in vitro specificity and lower in vivo toxicity than corresponding hydrazone conjugates. In vivo studies demonstrated that the peptide-linked conjugates induced regressions and cures of established tumor xenografts with therapeutic indices as high as 60-fold. These conjugates illustrate the importance of linker technology, drug potency and conjugation methodology in developing safe and efficacious mAb-drug conjugates for cancer therapy.

The anti-CD30 monoclonal antibody SGN-30 promotes growth arrest and DNA fragmentation in vitro and affects antitumor activity in models of Hodgkin's disease

The leukocyte activation marker CD30 is highly expressed on the Reed Sternberg cells of Hodgkin's disease (HD). On normal tissues, CD30 has a restricted expression profile limited to activated T cells, activated B cells, and activated natural killer cells. This expression profile makes CD30 an ideal target for monoclonal antibody (mAb)-based therapies of Hodgkin's disease. CD30 mAbs have been shown to be effective in in vitro and in vivo models of hematologic malignancies such as anaplastic large cell lymphoma, yet these mAb have not been efficacious in HD models. We have found that a mAb against CD30, AC10, was able to inhibit the growth of HD cell lines in vitro. To generate a more clinically relevant molecule, the variable regions from AC10 were cloned into an expression construct containing the human gamma1 heavy chain and kappa light chain constant regions. The resulting chimeric antibody, designated SGN-30, retained the binding and in vitro growth-inhibitory activities of the parental antibody. Treatment of HD cell lines with SGN-30 in vitro resulted in growth arrest in the G(1) phase of the cell cycle and DNA fragmentation consistent with apoptosis in the HD line L540cy. Severe combined immunodeficient mouse xenograft models of disseminated HD treated with SGN-30 produced significant increases in survival. Similarly, xenograft models of localized HD demonstrated dose-dependent reduction in tumor mass in response to SGN-30 therapy. SGN-30 is being developed for the treatment of patients who have HD that is refractory to initial treatment or who have relapsed and have limited therapeutic options.

Protease-mediated fragmentation of p-amidobenzyl ethers: a new strategy for the activation of anticancer prodrugs

A new anticancer prodrug activation strategy based on the 1,6-elimination reaction of p-aminobenzyl ethers is described. Model studies were undertaken with the N-protected peptide benzyloxycarbonyl-valine-citrulline (Z-val-cit), which was attached to the amino groups of p-aminobenzyl ether derivatives of 1-naphthol and N-acetylnorephedrine. The amide bond that formed was designed for hydrolysis by cathepsin B, a protease associated with rapidly growing and metastatic carcinomas. Upon treatment with the enzyme, the Z-val-cit-p-amidobenzyl ether of 1-naphthol (2) underwent peptide bond hydrolysis with the rapid release of 1-naphthol. The aliphatic Z-val-cit-p-amidobenzyl ether of N-acetylnorephedrine (5) also underwent amide bond hydrolysis, but without the ensuing elimination of N-acetylnorephedrine. On the basis of these results, the phenolic anticancer drugs etoposide (6) and combretastatin A-4 (7) were attached to the Z-val-cit-p-amidobenzyl alcohol through ether linkages, forming the peptide-drug derivatives 8 and 9, respectively. Both compounds were stable in aqueous buffers and serum and underwent ether fragmentation upon treatment with cathepsin B, resulting in the release of the parent drugs in chemically unmodified forms. The released drugs were 13-50 times more potent than were the prodrug precursors on a panel of cancer cell lines. In contrast, the corresponding carbonate derivative of combretastatin A-4 (13) was unstable in aqueous environments and was as cytotoxic as combretastatin A-4. This result extends the use of the self-immolative p-aminobenzyl group for the fragmentation of aromatic ethers and provides a new strategy for anticancer prodrug development.

Identification and activities of human carboxylesterases for the activation of CPT-11, a clinically approved anticancer drug

CPT-11 is a clinically approved anticancer drug used for the treatment of advanced colorectal cancer. Upon administration, the carbamate side chain of the drug is hydrolyzed, resulting in the release of SN-38, an agent that has approximately 1000-fold increased cytotoxic activity. Since only a very small percentage of the injected dose of CPT-11 is converted to SN-38, there is a significant opportunity to improve its therapeutic efficacy and to diminish its systemic toxicity by selectively activating the drug within tumor sites. We envisioned that a mAb-human enzyme conjugate for CPT-11 activation would be of interest, particularly since the conjugate would likely be minimally immunogenic, and the prodrug is clinically approved. Toward this end, it was necessary to identify the most active human enzyme that could convert CPT-11 to SN-38. We isolated enzymes from human liver microsomes based on their abilities to effect the conversion and identified human carboxylesterase 2 (hCE-2) as having the greatest specific activity. hCE-2 was 26-fold more active than human carboxylesterase 1 and was 65% as active as rabbit liver carboxylesterase, the most active CPT-11 hydrolyzing enzyme known. The anti-p97 mAb 96.5 was linked to hCE-2, forming a conjugate that could bind to antigen-positive cancer cells and convert CPT-11 to SN-38. Cytotoxicity assays established that the conjugate led to the generation of active drug, but the kinetics of prodrug activation (48 pmol x min(-1) x mg(-1) was insufficient for immunologically specific prodrug activation. These results confirm the importance of hCE-2 for CPT-11 activation and underscore the importance of enzyme kinetics for selective prodrug activation.

Oncostatin M in the anti-inflammatory response

Oncostatin M (OM) is a pleiotropic cytokine of the interleukin 6 family, whose in vivo properties and physiological function remain in dispute and poorly defined. These in vivo studies strongly suggest that OM is anabolic, promoting wound healing and bone formation, and anti-inflammatory. In models of inflammation OM is produced late in the cytokine response and protects from lipopolysaccharide (LPS)-induced toxicities, promoting the re-establishment of homoeostasis by cooperating with proinflammatory cytokines and acute phase molecules to alter and attenuate the inflammatory response. Administration of OM inhibited bacterial LPS-induced production of tumour necrosis factor alpha and septic lethality in a dose dependent manner. Consistent with these findings, in animal models of chronic inflammatory disease OM potently suppressed inflammation and tissue destruction in murine models of rheumatoid arthritis and multiple sclerosis. T cell function and antibody production were not impaired by OM treatment. Taken together, these data indicate that the activities of this cytokine in vivo are anti-inflammatory without concordant immunosuppression.

Selective tumor sensitization to taxanes with the mAb-drug conjugate cBR96-doxorubicin

The chimeric monoclonal antibody cBR96 conjugated to doxorubicin (cBR96-Dox) is selectively internalized by a wide variety of human carcinomas expressing an extended form of Lewis Y antigen (Le(y)). Endocytosis is followed by cleavage and release of free doxorubicin from the endocytic vesicles and subsequent cytotoxicity. Combination studies with standard anti-cancer agents, undertaken to further increase the potency of this targeted therapy, identified significant synergistic anti-tumor activity of cBR96-Dox and either of the taxanes paclitaxel or docetaxel. Treatment with cBR96-Dox 24 hr prior to paclitaxel resulted in a steady increase in the percentage of G(2) tumor cells and corresponding increase in sensitivity to taxanes. Cell cycle analysis indicated the cBR96-delivered doxorubicin was most effective against S-phase cells, yet cells exposed to even subtoxic levels progressed to and arrested in G(2), at a point of high sensitivity to the anti-tubulin agent paclitaxel. The synergy obtained by staged combination of cBR96-Dox and paclitaxel in vitro was reflected in significant anti-tumor efficacy in vivo against xenograft models of human lung and breast tumors that could not be achieved by either agent alone. The staged combination elicited significant or complete regressions of established human Le(y)-positive tumor xenografts using significantly reduced drug levels. Taken together, these data demonstrate a mechanistic approach to the selective elimination of Le(y)-positive tumors by using targeted doxorubicin followed by taxane treatment.

Agonistic properties and in vivo antitumor activity of the anti-CD40 antibody SGN-14

Ligation of CD40 is essential for primary B-cell activation and expansion and yet has suppressive or apoptotic effects on some CD40-expressing neoplasia. SGN-14 is a monoclonal antibody that binds to the human CD40 receptor. Here we report that SGN-14, in the presence of interleukin 4, provided a modest level of stimulation of peripheral blood B cells, as measured by proliferation. Stimulation was greatly enhanced in the presence of nonproliferating CD40 ligand-expressing cells. The enhanced agonistic activity could be attributed to a dose-dependent increase in CD40L binding to CD40 in the presence of SGN-14. In contrast to its proliferative effect on primary B cells, SGN-14 inhibited the growth of B-cell-derived tumor lines in vitro, and this growth inhibition was enhanced in the presence of CD40L-expressing cells. In vivo, SGN-14 showed significant antitumor activity in treating human B-cell lymphoma and multiple myeloma xenografted severe combined immunodeficient mice. Antitumor activity was not diminished by blunting murine natural killer activity, suggesting that CD40 ligation contributes to the antitumor efficacy of SGN-14. On the basis of these activities, SGN-14 is being pursued for therapeutic use in treating patients with CD40-expressing hematological malignancies.

Effect of flap modifications on human FEN1 cleavage

The flap endonuclease, FEN1, plays a critical role in DNA replication and repair. Human FEN1 exhibits both a 5' to 3' exonucleolytic and a structure-specific endonucleolytic activity. On primer-template substrates containing an unannealed 5'-tail, or flap structure, FEN1 employs a unique mechanism to cleave at the point of annealing, releasing the 5'-tail intact. FEN1 appears to track along the full length of the flap from the 5'-end to the point of cleavage. Substrates containing structural modifications to the flap have been used to explore the mechanism of tracking. To determine whether the nuclease must recognize a succession of nucleotides on the flap, chemical linkers were used to replace an interior nucleotide. The nuclease could readily traverse this site. The footprint of the nuclease at the time of cleavage does not extend beyond 25 nucleotides on the flap. Eleven-nucleotide branches attached to the flap beyond the footprinted region do not prevent cleavage. Single- or double-thymine dimers also allow cleavage. cis-Platinum adducts outside the protected region are moderately inhibitory. Platinum-modified branch structures are completely inert to cleavage. These results show that some flap modifications can prevent or inhibit tracking, but the tracking mechanism tolerates a variety of flap modifications. FEN1 has a flexible loop structure through which the flap has been proposed to thread. However, efficient cleavage of branched structures is inconsistent with threading the flap through a hole in the protein.

Re-entry into the cell cycle: a mechanism for neurodegeneration in Alzheimer disease

Several recent findings demonstrated increased expression of cell cycle-related proteins in the degenerating neurons found in Alzheimer disease. We hypothesize that this apparent attempt to re-enter the cell cycle is a neuronal response to external growth stimuli that leads to an abortive re-entry into the cell cycle. However, since neurons of adults apparently lack the capacity both to divide in vivo and in vitro, it is possible that they lack the components necessary to complete the cell division process. Nonetheless, the importance of these findings is that they provide an explanation for the increased phosphorylation of cytoskeletal proteins such as tau and neurofilaments that represent the most striking intracellular changes in the disease. Further, it is our contention that inappropriate reentry into the cell cycle and interrupted mitotic processes are significant factors not only in the cytoskeletal pathology but also in the neuronal degeneration that characterizes the pathology of Alzheimer disease.

Regulation of inflammatory responses by oncostatin M

Oncostatin M (OM) is a pleiotropic cytokine produced late in the activation cycle of T cells and macrophages. In vitro it shares properties with related proteins of the IL-6 family of cytokines; however, its in vivo properties and physiological function are as yet ill defined. We show that administration of OM inhibited bacterial LPS-induced production of TNF-alpha and lethality in a dose-dependent manner. Consistent with these findings, OM potently suppressed inflammation and tissue destruction in murine models of rheumatoid arthritis and multiple sclerosis. T cell function and Ab production were not impaired by OM treatment. Taken together these data indicate the activities of this cytokine in vivo are antiinflammatory without concordant immunosuppression.

Regulation of Inflammatory Responses by Oncostatin M

Oncostatin M (OM) is a pleiotropic cytokine produced late in the activation cycle of T cells and macrophages. In vitro it shares properties with related proteins of the IL-6 family of cytokines; however, its in vivo properties and physiological function are as yet ill defined. We show that administration of OM inhibited bacterial LPS-induced production of TNF-α and lethality in a dose-dependent manner. Consistent with these findings, OM potently suppressed inflammation and tissue destruction in murine models of rheumatoid arthritis and multiple sclerosis. T cell function and Ab production were not impaired by OM treatment. Taken together these data indicate the activities of this cytokine in vivo are antiinflammatory without concordant immunosuppression.

Abnormal expression of the cell cycle regulators P16 and CDK4 in Alzheimer's disease

In this study, we demonstrate that two important regulators of the cell cycle, cyclin-dependent kinase-4 and its inhibitor p16, are increased in the brains of cases of Alzheimer's disease patients compared with age-matched controls. Both proteins are increased in the pyramidal neurons of the hippocampus, including those neurons containing neurofibrillary tangles and granulovacuolar degeneration. As p16 is not normally found in terminally differentiated neurons, it seems paradoxical that it is increased in Alzheimer's disease unless it is responding to increases in cyclin-dependent kinase-4 or other cell cycle regulators. Induction of the latter, a protein that signals re-entry and progression through the cell cycle, may itself be the consequence of alpha response to a growth stimulus. Re-entry into the cell cycle is likely deleterious in terminally differentiated neurons and may contribute to the biochemical abnormalities, such as oxidative stress and hyperphosphorylated tau protein, as well as the neuronal degeneration characteristic of the pathology of Alzheimer's disease.

Cyclin G2 is up-regulated during growth inhibition and B cell antigen receptor-mediated cell cycle arrest

Human cyclin G2 together with its closest homolog cyclin G1 defines a novel family of cyclins (Horne, M. C., Goolsby, G. L., Donaldson, K. L., Tran, D., Neubauer, M., and Wahl, A. F. (1996) J. Biol. Chem. 271, 6050-6061). Cyclin G2 is highly expressed in the immune system where immunologic tolerance subjects self-reactive lymphocytes to negative selection and clonal deletion via apoptosis. Here we investigated the effect of growth inhibitory signals on cyclin G2 mRNA abundance in different maturation stage-specific murine B cell lines. Upon treatment of wild-type and p53 null B cell lines with the negative growth factor, transforming growth factor beta1, or the growth inhibitory corticosteroid dexamethasone, cyclin G2 mRNA levels were increased in a time-dependent manner 5-14-fold over control cell levels. Unstimulated immature B cell lines (WEHI-231 and CH31) and unstimulated or IgM B cell receptor (BCR) -stimulated mature B cell lines (BAL-17 and CH12) rapidly proliferate and express low levels of cyclin G2 mRNA. In contrast, BCR-stimulated immature B cell lines undergo growth arrest and coincidentally exhibit an approximately 10-fold increase in cyclin G2 transcripts and a decrease in cyclin D2 message. Costimulation of WEHI-231 and CH31 cells with calcium ionophores and protein kinase C agonists partially mimics anti-IgM stimulation and elicits a strong up-regulation of cyclin G2 mRNA and down-regulation of cyclin D2 mRNA. Signaling mutants of WEHI-231 that are deficient in the phosphoinositide signaling pathway and consequently resistant to the BCR stimulus-induced growth arrest did not display a significant increase in cyclin G2 or decrease in cyclin D2 mRNAs when challenged with anti-IgM antibodies. The two polyclonal activators lipopolysaccharide and soluble gp39, which inhibit the growth arrest response of immature B cells, suppressed cyclin G2 mRNA expression induced by BCR stimulation. These results suggest that in murine B cells responding to growth inhibitory stimuli cyclin G2 may be a key negative regulator of cell cycle progression.

Separation by counterflow centrifugal elutriation and analysis of T- and B-lymphocytic cell lines in progressive stages of cell division cycle

The method of counterflow centrifugal elutriation (CCE) facilitates the non-invasive separation of proliferating cells into the progressive stages of the cell division cycle. We present here detailed protocols for the separation of primary lymphocytes and lymphocytic cell lines including Jurkat, a mature human T-cell line, Ramos, a human B-cell line, WEHI-231, a murine B-cell lymphoma, and stimulated human peripheral T-cells into progressive stages of the cell division cycle by counterflow centrifugal elutriation. Protocols for using the elutriator to concentrate large volumes of cells prior to separation, the preparation of highly enriched lymphocyte populations at progressive stages through the cell division cycle and conversion parameters from low to high volume rotors are described. Simple dual-staining methods of BrdUrd incorporation and propidium iodide staining for DNA content and subsequent flow cytometry are detailed. Together with [3H]thymidine incorporation data these provide a very accurate determination of cell cycle position of the separated populations.

Mechanism of tracking and cleavage of adduct-damaged DNA substrates by the mammalian 5'- to 3'-exonuclease/endonuclease RAD2 homologue 1 or flap endonuclease 1

The mammalian 5'- to 3'-exonuclease/endonuclease, called RAD2 homologue 1 or flap endonuclease 1, has a unique cleavage activity, dependent on specific substrate structure. On a primer-template, in which the primer has an unannealed 5'-tail, endonucleolytic cleavage near the annealing point releases the tail intact. Entering at the 5'-end, the nuclease tracks along the entire tail to the point of cleavage. Genetic analyses suggest that this nuclease removes DNA adducts in vivo (Sommers, C. H., Miller, E. J., Dujon, B., Prakash, S., and Prakash, L. (1995) J. Biol. Chem. 270, 4193-4196). Micrococcal nuclease footprinting shows that after tracking the nuclease protects a region of the tail 25 nucleotides long, adjacent to the cleavage site. Substrates with adducts at specific locations were used to assess the mechanism of RAD2 homologue 1 nuclease tracking and its ability to cleave modified DNA. Either a conventional cis-diamminedichloroplatinum (II) (CDDP) or a bulky CDDP derivative was placed within or beyond the region protected by the nuclease. The nuclease cleaved the tail of both substrates. In contrast, a CDDP adduct just adjacent to the expected cleavage point was inhibitory. A CDDP adduct at the very 5'-end of the tail was also cleaved. The nuclease could remove tails containing adducts on the sugar-phosphate backbone. Apparently, the nuclease is designed to slide over various types of damage on single stranded DNA and then cut past the damaged site.

Activation of human monocytes through CD40 induces matrix metalloproteinases

The activation of monocytes/macrophages to secrete pro-inflammatory cytokines and matrix metalloproteinases (MMPs) is critically important in the development of chronic inflammatory diseases. However, the consequence of interactions between activated T cells and monocytes in these inflammatory processes is not well understood. In this study we have investigated the induction of MMPs in human monocytes by activated T cells. We show that fixed cells and the cell membranes from a T cell line, BMS-2, that expresses high levels of the CD40 ligand gp39 (also called TRAP, TBAM, or CD40L) stimulate both the expression of mRNA and the production of MMPs by human monocytic cells. Activation of monocytes by the human T cells could be significantly inhibited by a F(ab')2 fragment of a neutralizing Ab specific for human gp39, but not by an Ab that recognizes murine gp39. Furthermore, recombinant soluble gp39 (sgp39) alone induced marked increases in the levels of a 92-kDa metalloproteinase (gelatinase) in both the human monocytic cell line, THP-1, and peripheral human monocytes, and induction was blocked by the anti-human gp39 Ab. Pretreatment with IFN-gamma significantly enhanced gp39 induction of MMPs in THP-1 cells but not in peripheral monocytes. Up-regulation of mRNA for the 92-kDa MMP by gp39 could be detected within 6 h of stimulation and was maximal 24 h after treatment. MMP enzymatic activity was detectable in the culture medium 12 to 18 h following stimulation of the cells and remained high through 48 h. These results suggest the interaction of T cells with monocytes/macrophages via the gp39-CD40 counter receptors may be significant in development or maintenance of chronic inflammatory lesions.

Activation of human monocytes through CD40 induces matrix metalloproteinases

The activation of monocytes/macrophages to secrete pro-inflammatory cytokines and matrix metalloproteinases (MMPs) is critically important in the development of chronic inflammatory diseases. However, the consequence of interactions between activated T cells and monocytes in these inflammatory processes is not well understood. In this study we have investigated the induction of MMPs in human monocytes by activated T cells. We show that fixed cells and the cell membranes from a T cell line, BMS-2, that expresses high levels of the CD40 ligand gp39 (also called TRAP, TBAM, or CD40L) stimulate both the expression of mRNA and the production of MMPs by human monocytic cells. Activation of monocytes by the human T cells could be significantly inhibited by a F(ab')2 fragment of a neutralizing Ab specific for human gp39, but not by an Ab that recognizes murine gp39. Furthermore, recombinant soluble gp39 (sgp39) alone induced marked increases in the levels of a 92-kDa metalloproteinase (gelatinase) in both the human monocytic cell line, THP-1, and peripheral human monocytes, and induction was blocked by the anti-human gp39 Ab. Pretreatment with IFN-gamma significantly enhanced gp39 induction of MMPs in THP-1 cells but not in peripheral monocytes. Up-regulation of mRNA for the 92-kDa MMP by gp39 could be detected within 6 h of stimulation and was maximal 24 h after treatment. MMP enzymatic activity was detectable in the culture medium 12 to 18 h following stimulation of the cells and remained high through 48 h. These results suggest the interaction of T cells with monocytes/macrophages via the gp39-CD40 counter receptors may be significant in development or maintenance of chronic inflammatory lesions.

Cyclin G1 and cyclin G2 comprise a new family of cyclins with contrasting tissue-specific and cell cycle-regulated expression

We describe the isolation and characterization of cDNAs encoding full-length human and murine cyclin G1 and a novel human homologue of this cyclin designated cyclin G2. Cyclin G1 is expressed at high levels in skeletal muscle, ovary, and kidney. Following an initial up-regulation from early G1 to G1/S phase, cyclin G1 mRNA is constitutively expressed throughout the cell cycle in T and B cell lines. In contrast, in stimulated peripheral T cells, cyclin G1 mRNA is maximal in early G1 phase and declines in cell cycle progression. Cyclin G1 levels parallel p53 expression in murine B lymphocytes; however, in several human Burkitt's lymphomas, murine lymphocytes treated with transforming growth factor-beta, early murine embryos, and several tissues of p53 null mice, cyclin G1 levels are either inverse of p53 levels or expressed independent of p53. The cyclin G1 homologue, cyclin G2, exhibits 60% nucleotide sequence identity and 53% amino acid sequence identity with cyclin G1, and like cyclin G1, exhibits closest sequence identity to the cyclin A family. Distinct from cyclin G1, the amino acid sequence for cyclin G2 shows a PEST-rich sequence and a potential Shc PTB binding site. Cyclin G2 mRNA is differentially expressed compared to cyclin G1, the highest transcript levels seen in cerebellum, thymus, spleen, prostate, and kidney. In contrast to the constitutive expression of cyclin G1 in lymphocytes, cyclin G2 mRNA appears to oscillate through the cell cycle with peak expression in late S phase.

Loss of normal p53 function confers sensitization to Taxol by increasing G2/M arrest and apoptosis

The anticancer agent paclitaxel (Taxol) stabilizes tubulin polymerization resulting in arrest in mitosis and apoptotic cell death. Normal human fibroblasts depleted of functional p53 by SV40 T antigen or HPV-16 E6, and primary embryo fibroblasts from p53 null mice showed seven- to ninefold increased cytotoxicity by paclitaxel. Reduced levels of p53 correlated with increased G2/M phase arrest, micronucleation, and p53-independent paclitaxel-induced apoptosis. Surviving cells with intact p53 progressed through mitosis and transiently accumulated in the subsequent G1 phase, coincident with increased p53 and p21cip1,waf1 protein levels. These results are in contrast to studies linking p53 loss with resistance to DNA damaging anticancer agents.

Stimulation of CD40 with purified soluble gp39 induces proinflammatory responses in human monocytes

CD40 is a glycoprotein of about 50 kDa that plays a crucial role in B cell growth and differentiation. It is found on the surface of B cells, follicular dendritic cells, monocytes, and some endothelial, epithelial, and carcinoma cells. Engagement of CD40 with anti-CD40 mAbs, gp39 expressed on the cell surface or soluble forms of gp39, primes B cells to efficiently respond to subsequent stimulatory signals leading to B cell proliferation, differentiation, and isotype switching. Peripheral monocytes also express CD40 on the cell surface and expression in increased following treatment with IFN-gamma. Using a soluble murine CD8/human gp39 fusion protein (sgp39) we have found that CD40 plays a crucial role in the regulation of monocyte function. Stimulation of human peripheral monocytes with sgp39 induced homotypic aggregation and significantly increased the expression of several cell-surface proteins including CD54, MHC class II, CD86, and CD40. Soluble gp39 also dramatically enhanced monocyte survival, preventing the onset of apoptosis that normally occurs upon withdrawal of serum. Finally, in the absence of any costimulatory molecules, sgp39 stimulated monocytes to produce TNF-alpha, IL-1 beta, IL-6, and IL-8. These results suggest that ligation of CD40 on human monocytes induces phenotypic changes that would be expected to influence T cell activation by the monocyte and also to enhance or prolong inflammatory responses.

Stimulation of CD40 with purified soluble gp39 induces proinflammatory responses in human monocytes

CD40 is a glycoprotein of about 50 kDa that plays a crucial role in B cell growth and differentiation. It is found on the surface of B cells, follicular dendritic cells, monocytes, and some endothelial, epithelial, and carcinoma cells. Engagement of CD40 with anti-CD40 mAbs, gp39 expressed on the cell surface or soluble forms of gp39, primes B cells to efficiently respond to subsequent stimulatory signals leading to B cell proliferation, differentiation, and isotype switching. Peripheral monocytes also express CD40 on the cell surface and expression in increased following treatment with IFN-gamma. Using a soluble murine CD8/human gp39 fusion protein (sgp39) we have found that CD40 plays a crucial role in the regulation of monocyte function. Stimulation of human peripheral monocytes with sgp39 induced homotypic aggregation and significantly increased the expression of several cell-surface proteins including CD54, MHC class II, CD86, and CD40. Soluble gp39 also dramatically enhanced monocyte survival, preventing the onset of apoptosis that normally occurs upon withdrawal of serum. Finally, in the absence of any costimulatory molecules, sgp39 stimulated monocytes to produce TNF-alpha, IL-1 beta, IL-6, and IL-8. These results suggest that ligation of CD40 on human monocytes induces phenotypic changes that would be expected to influence T cell activation by the monocyte and also to enhance or prolong inflammatory responses.

Lineage-specific induction of B cell apoptosis and altered signal transduction by the phosphotyrosine phosphatase inhibitor bis(maltolato)oxovanadium(IV)

Protein tyrosine phosphorylation is known to play key roles in lymphocyte signal transduction, and phosphotyrosine phosphatases (PTP) can act as both positive and negative regulators of these lymphocyte signals. We sought to examine the role of PTP further in these processes by characterizing the effects of bis(maltolato)-oxovanadium(IV) (BMLOV), previously known to be a nontoxic insulin mimetic agent in vivo. BMLOV was found to be a potent phosphotyrosine phosphatase inhibitor. BMLOV induced cellular tyrosine phosphorylation in B cells in a pattern similar to that observed following antigen receptor stimulation, whereas little tyrosine phosphorylation was induced in T cells. In B cells, BMLOV treatment resulted in tyrosine phosphorylation of Syk and phospholipase C gamma 2, while sIgM-induced signals were inhibited. By contrast, T cell receptor signals were moderately increased by BMLOV, and the cells displayed greater induction of IL-2 receptor without toxicity. The compound selectively induced apoptosis in B cell lymphoma and myeloid leukemia cell lines, but not in T cell leukemia or colon carcinoma cells. Interleukin-4 plus anti-CD40 antibody treatment of normal human peripheral B cells rescued the cells from BMLOV-induced death. These results suggest that phosphotyrosine phosphatase inhibitors can activate B cell signal pathways in a lineage-specific manner, resulting in desensitization of receptor-mediated signaling and induction of apoptosis.

Activation of p34cdc2 coincident with taxol-induced apoptosis

Toxicity elicited by the antitumor compound taxol has been linked to irreversible tubulin polymerization, cell cycle block at mitosis, and cell death from apoptosis. We have used pulsed drug exposure of synchronized populations to identify two points, one in transition from G0 to G1 and the other at G2/M of cell cycle, that are most sensitive to taxol-induced cell killing. By analyzing these lesions separately, we have differentiated events related to mitotic block from those that may contribute to apoptosis. The taxol lesion forms rapidly and stably in transition or mitotic cells, because secondary washes to remove residual drug will decrease cytotoxicity except for cells in these populations. Both G2/M cells and G0/G1 transition cells synchronously initiated apoptotic DNA fragmentation within 20 h of pulsed taxol treatment, indicating that a sustained mitotic block is not requisite to initiate cell death. Apoptosis was inhibited by cyclohexamide and by 2-aminopurine and sodium orthovanadate; thus, cell cycle progression appeared requisite for cell death. Taxol treatment of G0/G1 or G2/M cells clearly leads to a block of mitosis followed by a perturbation of tyrosine phosphoprotein regulation; however, protein tyrosine phosphorylation correlated with mitotic block rather than time after drug exposure. Conversely, p34cdc2 kinase activation does not occur at mitotic block but rather 20 h after drug exposure and coincident with DNA fragmentation. Taken together, these results suggest that mitotic block may not be a sufficient signal for taxol-induced apoptosis and that the taxol lesion initiates apoptosis via a phosphoregulation pathway possibly involving the p34cdc2 kinase.

Cytotoxicity of the anticancer agents cisplatin and taxol during cell proliferation and the cell cycle

The overt effects of the anti-cancer drugs cisplatin (cis-DDP) and taxol appear to be DNA modification and microtubule stabilization respectively, yet the mechanisms by which these drugs elicit tumor cell death are not well understood. In this report cell sensitivities to cis-DDP and taxol were accurately determined as a function of cell proliferation and cell cycle stage. Quiescent fibroblasts restimulated to synchronously enter the cell cycle become maximally sensitive to cis-DDP immediately preceding DNA synthesis, and resistance increases with onset of DNA synthesis. Mid-log proliferating cells were separated into progressive stages of the cell cycle by centrifugal elutriation or by double thymidine (dThd) block. Cells staged by either method are maximally sensitive to cis-DDP in G1, just prior to the onset of DNA synthesis and minimally sensitive in peak DNA synthesis, with entry into S phase resulting in a 2-fold decrease in sensitivity. Cells that remained blocked at the G1/S phase boundary during cis-DDP treatment remain maximally sensitive after release. Sensitivity to taxol increases at 2 points: transiently during transition of normal cells from quiescence to proliferation and steadily as proliferating cells progress from early G1 to late G2. This 3-fold increase in taxol sensitivity through the cell cycle is rapidly reversed upon cell division. Synchronous cells treated with either drug at points of maximum sensitivity initiate apoptotic DNA fragmentation 12-14 hr post-exposure to drug.

Activity of calf thymus DNA helicase E on cis-diamminedichloroplatinum (II)-damaged DNA

Calf DNA helicase E (hel E) is a moderately processive, 3' to 5' helicase, active on nicked DNA, that we have proposed to have a role in DNA repair (Turchi, J. J., Murante, R. S., and Bambara, R. A. (1992) Nucleic Acids Res. 20, 6075-6080). Here we have examined its activity on a series of cis-diamminedichloroplatinum (II) (cis-DDP)-modified DNA substrates. Hel E was capable of efficiently displacing a primer strand containing, in an internal position, a cis-DDP-modified dGG. In a two-primer model system, calf DNA polymerase epsilon could successfully extend an upstream primer through a cis-DDP-modified down-stream primer, to the end of the complementary template strand, in a reaction dependent on hel E. However, the translocation of hel E was blocked by cis-DDP modification of the template strand. Primer displacement was completely prevented if the modified site was located just upstream of the primer. The DNA-dependent ATPase activity of helicase E was also reduced by cis-DDP modification of the template DNA. Substrate competition experiments indicated that cis-DDP-modified DNA templates did not sequester hel E. Substrate titration experiments suggested that there is a short delay without ATP hydrolysis before dissociation of helicase E from cis-DDP-modified template sites. Interestingly, hel E could displace a primer if the cis-DDP modification was on the template within the annealed region. Possible explanations for this are discussed. Taken together, these results are consistent with the proposal that hel E participates in DNA repair by displacing segments of damaged DNA.

Effects of the anticancer drug cis-diamminedichloroplatinum(II) on the activities of calf thymus DNA polymerase epsilon

DNA polymerase (pol) epsilon is essential for DNA replication and is thought to be a component of DNA repair systems in eukaryotic cells. The activities of pol epsilon have been examined using a series of synthetic oligonucleotides designed with cis-diamminedichloroplatinum(II) (cis-DDP)-modified specific guanine residues. Pol epsilon was incapable of synthesis over cis-DDP-modified single guanine or adjacent guanine residues present in the template strand. Both single and double guanines modified by cis-DDP present at the 3'-OH end of a primer strand completely inhibited the synthetic activity of pol epsilon and, in addition, sequestered pol epsilon at the platinated 3'-OH termini. The sequestering of pol epsilon on cis-DDP modified DNA may interfere with the function of this enzyme in DNA repair in vivo. The intrinsic 3' to 5' proofreading exonuclease activity of pol epsilon was also examined. Pol epsilon was capable of degrading a single-strand template with internal cis-DDP-modified guanines up to, but not through, the platinated nucleotides. A single platinated guanosine was sufficient to block the 3' to 5' exonuclease activity of pol epsilon. These results suggest that cis-DDP-DNA adducts inhibit DNA synthesis mediated by DNA polymerase epsilon and that platinated sites can arrest the nuclease of pol epsilon, a function exhibited during DNA repair.

Binding of a sequence-specific single-stranded DNA-binding factor to the simian virus 40 core origin inverted repeat domain is cell cycle regulated

The inverted repeat domain (IR domain) within the simian virus 40 origin of replication is the site of initial DNA melting prior to the onset of DNA synthesis. The domain had previously been shown to be bound by a cellular factor in response to DNA damage. We demonstrate that two distinct cellular components bind opposite strands of the IR domain. Replication protein A (RPA), previously identified as a single-stranded DNA binding protein required for origin-specific DNA replication in vitro, is shown to have a preference for the pyrimidine-rich strand. A newly described component, IR factor B (IRF-B), specifically recognizes the opposite strand. IRF-B binding activity in nuclear extract varies significantly with cell proliferation and the cell cycle, so that binding of IRF-B to the IR domain is negatively correlated with the onset of DNA synthesis. Loss of IRF-B binding from the nucleus also occurs in response to cellular DNA damage. UV cross-linking indicates that the core binding component of IRF-B is a protein of ca. 34 kDa. We propose that RPA and IRF-B bind opposite strands of the IR domain and together may function in the regulation of origin activation.

Cell cycle-dependent phosphorylation of human DNA polymerase alpha

The expression of DNA polymerase alpha, a principal chromosome replication enzyme, is constitutive during the cell cycle. We show in this report that DNA polymerase alpha catalytic polypeptide p180 is phosphorylated throughout the cell cycle and is hyperphosphorylated in G2/M phase. The p70 subunit is phosphorylated only in G2/M phase. This cell cycle-dependent phosphorylation is due to cell cycle-dependent kinase(s) and not to phosphatase(s). In vitro evidence indicates the involvement of p34cdc2 kinase in the mitotic phosphorylation of DNA polymerase alpha. Tryptic phosphopeptide maps demonstrate that peptides phosphorylated in vitro are identical to those phosphorylated in vivo. DNA polymerase alpha from mitotic cells is found to have lower affinity for single-stranded DNA than does polymerase alpha from G1/S phase cells. These results imply that the mitotic phosphorylation of polymerase alpha may affect its physical interaction with other replicative proteins and/or with DNA at the replication fork.

Gene expression of human DNA polymerase alpha during cell proliferation and the cell cycle

We studied the expression of the human DNA polymerase alpha gene during cell proliferation, during cell progression through the cell cycle, and in transformed cells compared with normal cells. During the activation of quiescent cells (G0 phase) to proliferate (G1/S phases), the steady-state mRNA levels, rate of synthesis of nascent polymerase protein, and enzymatic activity in vitro exhibited a substantial and concordant increase prior to the peak of in vivo DNA synthesis. In transformed cells, the respective values were amplified greater than 10-fold. In actively growing cells separated into discrete stages of the cell cycle by counterflow elutriation or by mitotic shakeoff, levels of steady-state transcripts, translation rates, and enzymatic activities of polymerase alpha were constitutively and concordantly expressed at all stages of the cell cycle, with only a moderate elevation prior to the S phase and a slight decline in the G2 phase. These findings support the conclusion that the regulation of human DNA polymerase alpha gene expression is at the transcriptional level and strongly suggest that the regulatory mechanisms that are operative during the entrance of a cell into the mitotic cycle are fundamentally different from those that modulate polymerase alpha expression in continuously cycling cells.

Human DNA polymerase alpha gene expression is cell proliferation dependent and its primary structure is similar to both prokaryotic and eukaryotic replicative DNA polymerases

We have isolated cDNA clones encoding the human DNA polymerase alpha catalytic polypeptide. Studies of the human DNA polymerase alpha steady-state mRNA levels in quiescent cells stimulated to proliferate, or normal cells compared to transformed cells, demonstrate that the polymerase alpha mRNA, like its enzymatic activity and de novo protein synthesis, positively correlates with cell proliferation and transformation. Analysis of the deduced 1462-amino-acid sequence reveals six regions of striking similarity to yeast DNA polymerase I and DNA polymerases of bacteriophages T4 and phi 29, herpes family viruses, vaccinia virus and adenovirus. Three of these conserved regions appear to comprise the functional active site required for deoxynucleotide interaction. Two putative DNA interacting domains are also identified.

Cell cycle dependent activities of DNA polymerases alpha and delta in Chinese hamster ovary cells

The activities of DNA polymerases alpha and delta, in extracts from Chinese hamster ovary (CHO) cells, were assayed in order to determine whether these polymerases are regulated during the cell cycle. An exponential population of CHO cells was separated into enriched populations of G-1, S, and G-2/M phases of cell cycle by centrifugal elutriation. Total cell homogenates from each population were assayed for DNA polymerase activity by measuring labeled nucleotide incorporation into the exogenous templates oligo(dT).poly(dA) and DNase I activated calf thymus DNA. In these experiments, specific DNA polymerase inhibitors were added to assays of the cellular extracts to allow for the independent measurement of activities of DNA polymerases alpha and delta. Comparisons of total DNA polymerase activity from cellular extracts, sampled from each portion of the cell cycle, demonstrated no significant change with respect to the concentration of total protein. However, results indicate that the activity of DNA polymerase delta increases with respect to that of DNA polymerase alpha in the G-2/M portion of the cell cycle. This difference in relative activities of DNA polymerases alpha and delta suggests a coordinate regulation of a specific species of DNA polymerase during the cell cycle.

Properties of two forms of DNA polymerase delta from calf thymus

Purified calf thymus DNA polymerases delta I and II each have an associated 3' to 5' exonuclease but otherwise resemble DNA polymerase alpha in size, biochemical kinetic parameters, and the presence of DNA primase [Crute, J. J., Wahl, A. F., & Bambara, R. A. (1986) Biochemistry 25, 26-36]. Here we demonstrate a functional association of polymerase and exonuclease with each delta form. Furthermore, we show that the exonuclease can be dissociated from DNA polymerase delta I but does not appear to be removable from DNA polymerase delta II. Polymerases delta I, delta II, and alpha are equally sensitive to the inhibitor aphidicolin, suggesting a similarity in active site structure. In comparison with DNA polymerase alpha and delta II, DNA polymerase delta I has intermediate sensitivity to 2-(p-n-butylanilino)-2'-deoxyadenosine 5'-triphosphate (BuAdATP) or N2-(p-n-butylphenyl)-2'-deoxyguanosine 5'-triphosphate (BuPdGTP). The activity of the DNA primase of the delta II enzyme is insensitive to BuAdATP whereas 1.0 microM of this inhibitor will decrease the activity of the DNA primase of the alpha and delta I enzymes approximately 50%. Two monoclonal antibodies that potently inhibit DNA polymerase alpha are only slightly inhibitory to DNA polymerase delta I and are ineffective at inhibiting DNA polymerase delta II. DNA polymerase delta II had been previously found to be nearly inactive on nuclease-treated calf thymus DNA, relative to its activity on homopolymeric DNA. We find that addition of purified calf histone proteins or spermidine can greatly enhance synthesis by this enzyme on activated calf DNA.

Purification of a calf thymus DNA-dependent adenosinetriphosphatase that prefers a primer-template junction effector

A purification procedure has been developed that resolves four chromatographically distinct DNA-dependent ATPase activities from calf thymus tissue. One of these activities has been purified to a nearly homogeneous protein, as judged by polyacrylamide gel electrophoresis. This protein has a specific activity of 18 mumol of ATP hydrolyzed per minute per milligram of protein and is active only in the presence of a DNA effector. The DNA-dependent ATPase activity is greatest in the presence of DNA containing a 3'-hydroxyl primer-template junction with a segment of adjacent single strand, i.e., a DNA polymerase substrate. Primer-template effectors that have had the 3'-hydroxyl group eliminated by the addition of a dideoxyribonucleotide are less active as cofactors for ATP hydrolysis than effectors which retain the 3'-hydroxyl group. Other DNAs can serve as cofactors, but with a reduced rate of ATP hydrolysis. DNA cofactors which are single stranded are much more effective at promoting ATPase activity than completely double-stranded cofactors, although the effectiveness of single-stranded DNA decreases as the length of the oligonucleotide decreases. An RNA/DNA hybrid does not promote ATPase activity. These data suggest that ATPase A may be involved in the recognition of primer-template junctions and the elongation phase of DNA synthesis.

Purification and characterization of two new high molecular weight forms of DNA polymerase delta

Two high molecular weight DNA polymerases, which we have designated delta I and delta II, have been purified from calf thymus tissue. Using Bio Rex-70, DEAE-Sephadex A-25, and DNA affinity resin chromatography followed by sucrose gradient sedimentation, we purified DNA polymerase delta I 1400-fold to a specific activity of 10 000 nmol of nucleotide incorporated h-1 mg-1, and DNA polymerase delta II was purified 4100-fold to a final specific activity of 30 000 nmol of nucleotide incorporated h-1 mg-1. The native molecular weights of DNA polymerase delta I and DNA polymerase delta II are 240 000 and 290 000, respectively. Both enzymes have similarities to other purified delta-polymerases previously reported in their ability to degrade single-stranded DNA in a 3' to 5' direction, affinity for an AMP-hexane-agarose matrix, high activity on poly(dA) X oligo(dT) template, and relative resistance to the polymerase alpha inhibitors N2-(p-n-butylphenyl)dATP and N2-(p-n-butylphenyl)dGTP. These two forms of DNA polymerase delta also share several common features with alpha-type DNA polymerases. Both calf DNA polymerase delta I and DNA polymerase delta II are similar to calf DNA polymerase alpha in molecular weight, are inhibited by the alpha-polymerase inhibitors N-ethylmaleimide and aphidicolin, contain an active DNA-dependent RNA polymerase or primase activity, display a similar extent of processive DNA synthesis, and are stimulated by millimolar concentrations of ATP. We propose that calf DNA polymerase delta I, which also has a template specificity essentially identical with that of calf DNA polymerase alpha, could be an exonuclease-containing form of a DNA replicative enzyme.

Inhibition of mammalian DNA polymerases by hematoporphyrin derivative and photoradiation

Hematoporphyrin derivative (HPD) plus photoradiation caused the inactivation of DNA polymerases from calf thymus and R3230AC rat mammary tumor. Photosensitization of purified DNA polymerase-alpha as well as two forms of DNA polymerase-delta (I and II) from calf thymus were evaluated. Although all polymerase enzyme forms were inactivated at 70 micrograms HPD/ml, DNA polymerase-delta II was the most sensitive, displaying a 90% inactivation under conditions that did not cause significant inactivation of the other polymerase forms. Unlike DNA polymerase-alpha, the delta-forms have an associated 3'- to 5'-exonuclease activity. The exonuclease associated with DNA polymerase-delta II was uniquely sensitive to a low level of HPD and light exposure. DNA polymerase-delta II can be distinguished from other polymerase forms in cell extracts by its relative insensitivity to the polymerase inhibitor N2-(p-n-butylphenyl)deoxyadenosine 5'-triphosphate. In cytosols prepared from calf thymus and R3230AC rat mammary tumors, DNA polymerase-delta II was preferentially inhibited by HPD plus light. Furthermore, in experiments in which tumor-bearing rats were administered HPD prior to preparation of tumor cytosols, DNA polymerase-delta II was specifically inactivated by exposure to light. These results are discussed in view of their possible role in cancer therapy, and the potential use of HPD as a specific inhibitory agent of DNA polymerase-delta II is suggested.