Induction of a TRAIL-mediated suicide program by interferon alpha in primary effusion lymphoma

Gammaherpes viruses are often detected in lymphomas arising in immunocompromised patients. We have found that Azidothymidine (AZT) alone induces apoptosis in Epstein Barr Virus (EBV) positive Burkitt's lymphoma (BL) cells but requires interferon alpha (IFN-alpha) to induce apoptosis in Human Herpes Virus Type 8 (HHV-8) positive Primary Effusion Lymphomas (PEL). Our analysis of a series of AIDS lymphomas revealed that IFN-alpha selectively induced very high levels of the Death Receptor (DR) tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in HHV-8 positive PEL lines and primary tumor cells whereas little or no induction was observed in primary EBV+ AIDS lymphomas and EBV-Burkitt's lines. AZT and IFN-alpha mediated apoptosis in PEL was blocked by stable overexpression of dominant negative Fas Associated Death Domain (FADD), decoy receptor 2 (DcR2), soluble TRAIL receptor fusion proteins (DR-4 and DR-5) and thymidine. Trimeric TRAIL (in place of IFN-alpha) similarly synergized with AZT to induce apoptosis in HHV-8 positive PEL cells. This is the first demonstration that IFN-alpha induces functional TRAIL in a malignancy that can be exploited to effect a suicide program. This novel antiviral approach to Primary Effusion lymphomas is targeted and may represent a highly effective and relatively non-toxic therapy.

Identification of DNA replication and cell cycle proteins that interact with PCNA

The identity of DNA replication proteins and cell cycle regulatory proteins which can be found in complexes involving PCNA were investigated by the use of PCNA immobilized on Sepharose 4B. A column containing bovine serum albumin (BSA) bound to Sepharose was used as a control. Fetal calf thymus extracts were chromatographed on PCNA-Sepharose and BSA-Sepharose. The columns were washed and then eluted with 0.5 M KCl. The salt eluates were examined for the presence of both DNA replication proteins (Pol alpha, delta, straightepsilon, PCNA, RFC, RFA, DNA ligase I, NDH II, Topo I and Topo II) and cell cycle proteins (Cyclins A, B1, D1, D2, D3, E, CDK2, CDK4, CDK5 and p21) by western blotting with specific antibodies. The DNA replication proteins which bound to PCNA-Sepharose included DNA polymerase delta and straightepsilon, PCNA, the 37 and 40 kDa subunits of RFC, the 70 kDa subunit of RPA, NDH II and topoisomerase I. No evidence for the binding of DNA polymerase alpha, DNA ligase I or topoisomerase II was obtained. Of the cell cycle proteins investigated, CDK2, CDK4 and CDK5 were bound. This study presents strong evidence that PCNA is a component of protein complexes containing DNA replication, repair and cell cycle regulatory proteins.

Expression and characterization of the small subunit of human DNA polymerase delta

DNA polymerase delta is a heterodimer consisting of a catalytic subunit of 125 kDa and a small subunit of 50 kDa (p50). We have overexpressed p50 in Escherichia coli and have characterized the recombinant protein. p50 was readily overexpressed using the pET vector as an insoluble protein. A procedure was developed for its purification and renaturation. Examination of the physicochemical properties of renatured p50 showed that it is a monomeric protein with an apparent molecular weight of 60,000, a Stokes radius of 34 A, and a sedimentation coefficient of 4.1 S. Its physical properties were indistinguishable from p50 expressed as a soluble protein using the pTACTAC vector. Examination of the effects of recombinant p50 on the activity of DNA polymerase delta showed that p50 is able to slightly stimulate (about 2-fold) the activity of the recombinant 125-kDa catalytic subunit using poly(dA).oligo(dT) as a template in the absence of proliferating cell nuclear antigen. In the presence of proliferating cell nulear antigen, activity is stimulated about 5-fold. Seven stable hybridoma cell lines were established that produced monoclonal antibodies against p50. One of these antibodies (13D5) inhibited the activity of calf thymus DNA polymerase delta. This antibody, when coupled to a solid support, also was found to provide a method for the immunoafffinity purification of recombinant p50 and of DNA polymerase delta from calf thymus or HeLa extracts. Immunoprecipitation and enzyme-linked immunosorbent assays also confirmed that p50 interacts with the catalytic subunit of DNA polymerase delta.

A conserved region in the amino terminus of DNA polymerase delta is involved in proliferating cell nuclear antigen binding

Synthetic peptides to selected sequences in human DNA polymerase delta (pol delta) were used to identify the region involved in the interaction of pol delta to proliferating cell nuclear antigen. Peptides corresponding to sequences in five regions in the amino terminus of human pol delta and three in the carboxyl terminus, which are conserved with the yeast homologs of pol delta, were tested. These studies showed that the peptide corresponding to the N2 region (residues 129-149) selectively and specifically inhibited the PCNA stimulation of pol delta. This inhibition was relieved by titration with excess PCNA. The identification of the N-2 region as being involved in PCNA binding was supported by studies that demonstrated that the N2 peptide could bind PCNA. Deletion mutants of pol delta expressed in Sf9 cells provided evidence that the binding region for PCNA was located in the first 182 residues of the amino terminus. These studies provide reasonable evidence that residues within the region 129-149 of pol delta are involved in the binding site for PCNA.

Molecular cloning of the cDNA for the catalytic subunit of human DNA polymerase delta

The cDNA of human DNA polymerase delta was cloned. The cDNA had a length of 3.5 kb and encoded a protein of 1107 amino acid residues with a calculated molecular mass of 124 kDa. Northern blot analysis showed that the cDNA hybridized to a mRNA of 3.4 kb. Monoclonal and polyclonal antibodies to the C-terminal 20 residues specifically immunoblotted the human pol delta catalytic polypeptide. A multiple sequence alignment was constructed. This showed that human pol delta is closely related to yeast pol delta and the herpes virus DNA polymerases. The levels of pol delta message were found to be induced concomitantly with DNA pol delta activity and DNA synthesis in serum restimulated proliferating IMR90 cultured cells. The human pol delta gene was localized to chromosome 19 by Southern blotting of EcoRI digested DNA from a panel of rodent/human cell hybrids.

Induction of DNA polymerase activities in the regenerating rat liver

The levels of DNA polymerase alpha, DNA polymerase delta, and its accessory protein, proliferating cell nuclear antigen (PCNA) were examined in the regenerating rat liver. The levels of DNA polymerase alpha and delta activities in regenerating liver extracts were determined by the use of the DNA polymerase alpha specific inhibitor, BuAdATP [2-(p-n-butylanilino)-9-(2-deoxy-beta-D-ribofuranosyl) adenine 5'-triphosphate], and monoclonal antibodies. These reagents showed that the total DNA polymerase activities increased ca. 4-fold during regeneration and that the fraction of DNA polymerase delta activity at the peak was 40% of the total DNA polymerase activity. Immunoblots and inhibition studies using specific antibodies showed that DNA polymerase delta and epsilon and PCNA were concomitantly induced after partial hepatectomy. The levels of both DNA polymerase delta and epsilon and PCNA reached their maxima at 24-36 h post hepatectomy, i.e., at the same time that in vivo DNA synthesis reached its peak. Partial purification and characterization of DNA polymerases delta and epsilon from the regenerating rat liver were also performed. These observations suggest that the variation of DNA polymerase delta and epsilon and PCNA during liver regeneration is closely related to DNA synthesis and is consistent with their involvement in DNA replication.

Characterization of human DNA polymerase delta and its immunochemical relationships with DNA polymerase alpha and epsilon

DNA polymerase delta was purified from human placenta and its polymerase catalytic subunit identified as a 125-kDa polypeptide by activity staining. This 125-kDa form of DNA polymerase delta resembles that reported from calf thymus (Lee, M. Y. W. T., Tan, C.-K., Downey, K. M., and So, A. G. (1984) Biochemistry 23, 1906-1913) and differs in molecular properties from a previously described form isolated from human placenta (Lee, M. Y. W. T., and Toomey, N. L. (1987) Biochemistry 26, 1076-1085) and now referred to as DNA polymerase epsilon. The properties of DNA polymerase delta were further investigated to determine its relationships with DNA polymerase epsilon. The two enzymes differed in their response to proliferating cell nuclear antigen. Monoclonal antibodies against DNA polymerase delta were raised and used to examine its immunochemical relationships with DNA polymerase alpha and epsilon. These studies provided evidence that all three proteins are structurally distinct but share a common epitope(s). Immunofluorescence microscopy indicates that DNA polymerase delta and possibly also DNA polymerase epsilon are localized to the nucleus.

Immunochemical studies of DNA polymerase delta: relationships with DNA polymerase alpha

A panel of murine hybridoma cell lines which produce antibodies against polypeptides present in human placental DNA polymerase delta preparations was developed. Eight of these antibodies were characterized by virtue of their ability to inhibit DNA polymerase delta activity and immunoblot the 170-kDa catalytic polypeptide. Six of these eight antibodies inhibit DNA polymerase delta but not DNA polymerase alpha, showing that the two proteins are distinct. However, the other two monoclonal antibodies inhibited both DNA polymerase delta and alpha activities, providing the first evidence that these two proteins have a structural relationship. In addition to antibodies against the catalytic polypeptide we also identified 11 antibodies which recognize 120-, 100-, 88-, 75-, 62-, 36-, and 22-kDa polypeptides in DNA polymerase delta preparations, suggesting that these proteins might be part of a replication complex. The antibody to the 36-kDa polypeptide was shown to be directed against proliferating cell nuclear antigen/cyclin. These antibodies should prove useful for studies aimed at distinguishing between DNA polymerases alpha and delta and for the investigation of the functional roles of DNA polymerase delta polypeptides.

Human placental DNA polymerase delta: identification of a 170-kilodalton polypeptide by activity staining and immunoblotting

DNA polymerase delta was isolated from human placenta and identified as such on the basis of its association with a 3'- to 5'-exonuclease activity. The association of the polymerase and exonuclease activities was maintained throughout purification and attempted separations by physical or electrophoretic methods. Moreover, ratios of the two activities remained constant during the purification steps, and both activities were inhibited by aphidicolin, oxidized glutathione, and N-ethylmaleimide. The purified enzyme had an estimated molecular weight of 172,000, on the basis of a Stokes radius of 53.6 A and a sedimentation coefficient of 7.8 S. On sodium dodecyl sulfate (SDS) gel electrophoresis, polymerase delta preparations contained a band of ca. 170 kilodaltons (kDa) as well as several smaller polypeptides. The 170-kDa polypeptide was identified as the largest polypeptide component in the preparation possessing DNA polymerase activity by an activity staining procedure following gel electrophoresis in the presence of SDS. Western blotting of DNA polymerase delta with polyclonal antisera also revealed a single 170-kDa immunoreactive polypeptide. Monoclonal antibodies to KB cell polymerase alpha inhibited placental polymerase alpha but did not inhibit DNA polymerase delta, while the murine polyclonal antisera to polymerase delta inhibited delta but not alpha. These findings establish the existence of DNA polymerase delta in a human tissue and support the view that both its polymerase and its exonuclease activities may be associated with a single protein.

Differential effects of dimethylsulfoxide on the activities of human DNA polymerases alpha and delta

The effects of dimethylsulfoxide on the activities of purified human placental DNA polymerase alpha and DNA polymerase delta were examined. DNA polymerase alpha was inhibited by dimethylsulfoxide, whereas DNA polymerase delta was significantly activated, by as much as 6-fold. Kinetic data show that the effect of dimethylsulfoxide on DNA polymerase delta activity was due to a reduction in the apparent Km for its substrate, dTTP. This novel finding of the differential effects of dimethylsulfoxide on the activities of polymerases alpha and delta may be useful in their identification and differential assay.

Differential inhibition of human placental DNA polymerases delta and alpha by BuPdGTP and BuAdATP

The p-n-butylphenyl- and p-n-butylanilino- substituted analogs of dGTP and dATP, respectively, were tested as inhibitors of purified human placental DNA polymerases alpha and delta. It was observed that DNA polymerase alpha activity was potently inhibited by these analogs with I0.5 values as low as the nanomolar range, whereas DNA polymerase delta activity was poorly inhibited, with I0.5 values of ca. 100 micromolar. These results argue for a distinct identity of these two enzymes, and demonstrate the usefulness of these analogs as probes of DNA polymerase structures. In addition, these analogs provide a rapid method for the discrimination of the two enzyme activities and a means for the selective assay of DNA polymerase delta. Aphidicolin inhibited both DNA polymerases.