Differential regulation of CXC ligand 1 transcription in melanoma cell lines by poly(ADP-ribose) polymerase-1.

The continuous production of the CXC ligand 1 (CXCL1) chemokine by melanoma cells is a major effector of tumor growth. We have previously shown that the constitutive expression of this chemokine is dependent upon transcription factors nuclear factor-kappa B (NF-kappaB), stimulating protein-1 (SP1), high-mobility group-I/Y (HMGI/Y), CAAT displacement protein (CDP) and poly(ADP-ribose) polymerase-1 (PARP-1). In this study, we demonstrate for the first time the mechanism of transcriptional regulation of CXCL1 through PARP-1 in melanoma cells. In its inactive state, PARP-1 binds to the CXCL1 promoter in a sequence-specific manner and prevents binding of NF-kappaB (p65/p50) to its element. However, activation of the PARP-1 enzymatic activity enhances CXCL1 expression, owing to the loss of PARP-1 binding to the CXCL1 promoter, accompanied by enhanced binding of p65 to the promoter. The delineation of the role of NF-kappaB-interacting factors in the putative CXCL1 enhanceosome will provide key information in developing strategies to block constitutive expression of this and other chemokines in cancer and to develop targeted therapy.

Inhibition of poly(ADP-ribose) polymerase activity is insufficient to induce tetraploidy.

Poly(ADP-ribose) polymerase (PARP) knockout mice are resistant to murine models of human diseases such as cerebral and myocardial ischemia, traumatic brain injury, diabetes, Parkinsonism, endotoxic shock and arthritis, implicating PARP in the pathogenesis of these diseases. Potent selective PARP inhibitors are therefore being evaluated as novel therapeutic agents in the treatment of these diseases. Inhibition or depletion of PARP, however, increases genomic instability in cells exposed to genotoxic agents. We recently demonstrated the presence of a genomically unstable tetraploid population in PARP(-/-) fibroblasts and its loss after stable transfection with PARP cDNA. To elucidate whether the genomic instability is attributable to PARP deficiency or lack of PARP activity, we investigated the effects of PARP inhibition on development of tetraploidy. Immortalized wild-type and PARP(-/-) fibroblasts were exposed for 3 weeks to 20 microM GPI 6150 (1,11b-dihydro-[2H:]benzopyrano[4,3,2-de]isoquinolin-3-one), a novel small molecule specific competitive inhibitor of PARP (K(i) = 60 nM) and one of the most potent PARP inhibitors to date (IC(50) = 0.15 microM). Although GPI 6150 initially decreased cell growth in wild-type cells, there was no effect on cell growth or viability after 24 h. GPI 6150 inhibited endogenous PARP activity in wild-type cells by approximately 91%, to about the residual levels in PARP(-/-) cells. Flow cytometric analysis of unsynchronized wild-type cells exposed for 3 weeks to GPI 6150 did not induce the development of tetraploidy, suggesting that, aside from its catalytic function, PARP may play other essential roles in the maintenance of genomic stability.

Selective isolation of domains of chromatin proximal to both carcinogen-induced DNA damage and poly-adenosine diphosphate-ribosylation.

Poly-adenosine diphosphate (ADP)-ribosylation of nuclear proteins has been demonstrated previously to be activated in vivo by the presence of DNA single-strand breaks and has thus been implicated to play an important role in altering chromatin structure during cellular recovery from DNA damage. Based upon these considerations, a novel immunofractionation method, using antipoly(ADP-ribose) coupled to Sepharose, has been used to enrich for those limited domains of chromatin undergoing poly-ADP-ribosylation. We have used three independent methods to verify the presence of significant levels of single-strand DNA breaks adjacent to polynucleosomes engaged in ADP-ribosylation.

Effect of 1-methyl-1-nitrosourea on poly(adenosine diphosphate-ribose) polymerase activity at the nucleosomal level.

The stimulation of poly(adenosine diphosphate ribose) [poly(ADP-ribose)] polymerase activity at the nuclear level after damage of HeLa cells by 1-methyl-1-nitrosourea has been previously reported. We have observed a similar activation of the enzyme after treatment of cells with MNU at the nucleosomal level of chromatin (greater than 1N). This stimulation of enzyme activity did not occur through an inhibition of the glycohydrolase enzyme which cleaves poly(ADP-ribose), or elongation of poly(ADP-ribose) chains, or an increased biosynthesis of enzyme protein. The increased activity appears to be a consequence of the generation of more acceptor sites on nuclear proteins for initiation of poly(ADP-ribose) synthesis. The data indicate that MNU increased the accessibility of nucleosome core histones for modification by poly(ADP) ribosylation.

A putative role for nicotinamide adenine dinucleotide-promoted nuclear protein modification in the antitumor activity of N-methyl-N-nitrosourea.

Incubation of HeLa cells with the anticancer agent N-methyl-N-nitrosourea (MNU) results in: (a) depression of intracellular nicotinamide adenine dinucleotide levels; (b) stimulation of the chromatin-associated, chromosomal protein-modifying enzyme polyadenosine diphosphoribose [poly(ADP-ribose)] polymerase, which uses nicotinamide adenine dinucleotide as substrate; and (c) some fragmentation of cellular DNA. DNase treatment of HeLa nuclei in vitro also stimulates poly(ADP-ribose) polymerase activity, but not in nuclei derived from MNU-treated cells unless they have been subsequently incubated to allow for recovery from MNU damage. DNA polymerase activity is stimulated in vitro by poly(ADP) ribosylation of nuclear proteins. By using intact nuclei derived from MNU-treated HeLa cells, the repair via elongation of single-strand DNA breaks is demonstrated in vitro. This repair is dependent on DNA polymerase activity and is enhanced by adenosine diphosphate ribosylation of histones. Inhibition of poly(ADP-ribose) polymerase with nicotinamide results in extensive degradation of MNU-damaged DNA. Taken as a whole, these results suggest that poly(ADP-ribose) polymerase may play a role in the repair of alkylation damage to cellular DNA and that the inhibition of this enzyme in vivo might be exploited to potentiate the antitumor and carcinogenic activities of MNU.

Poly(ADP-ribosyl)ation of p53 during apoptosis in human osteosarcoma cells.

Spontaneous apoptosis in human osteosarcoma cells was observed to be associated with a marked increase in the intracellular abundance of p53. Immunoprecipitation and immunoblot analysis revealed that, together with a variety of other nuclear proteins, p53 undergoes extensive poly(ADP-ribosyl)ation early during the apoptotic program in these cells. Subsequent degradation of poly(ADP-ribose) (PAR), attached to p53 presumably by PAR glycohydrolase, the only reported enzyme to degrade PAR, was apparent concomitant with the onset of proteolytic processing and activation of caspase-3, caspase-3-mediated cleavage of poly(ADP-ribose) polymerase (PARP), and internucleosomal DNA fragmentation during the later stages of cell death. The decrease in PAR covalently bound to p53 also coincided with the marked induction of expression of the p53-responsive genes bax and Fas. These results suggest that poly(ADP-ribosyl)ation may play a role in the regulation of p53 function and implies a regulatory role for PARP and/or PAR early in apoptosis.

Enhanced poly(adenosine diphosphate ribose) polymerase activity and gene expression in Ewing's sarcoma cells.

Ewing's sarcoma (ES) is a highly malignant childhood bone tumor and is considered curable by moderate doses of radiotherapy. The addition of chemical inhibitors of the activity of the nuclear enzyme poly(adenosine diphosphate ribose) [poly(ADPR)] polymerase to ES cells in culture results in increased cell killing, a phenomenon called "inhibitor sensitization." Since poly(ADPR) polymerase is thought to be associated with DNA repair, it has been suggested that ES cells and other inhibitor-sensitized cells may have a reduced capacity for polymer synthesis resulting in deficient postirradiation recovery. We present here the unexpected observation that in comparison to other cell lines tested, ES cells exhibit a high enzyme activity, higher constitutive levels of the protein, and elevated levels of its mRNA transcript for poly(ADPR) polymerase. No gross amplifications or rearrangements of the gene were observed; however, regulation of poly(ADPR) polymerase in these tumor cells takes place at the level of the gene transcript.

Nitrosourea interaction with chromatin and effect on poly(adenosine diphosphate ribose) polymerase activity.

Poly(adenosine diphosphate ribose) polymerase, a chromatin-bound enzyme, was stimulated 150 to 200% after treatment of HeLa cells with methylnitrosourea (MNU). In contrast, a slight inhibitory effect on enzyme activity was observed after treatment of cells with various concentrations of chloroethylnitrosoureas. To define precisely the differential effects of nitrosoureas on the enzyme activity, their interactions with chromatin substructure were studied. A nonrandom, in vivo alkylation of chromatin DNA by equimolar concentrations of MNU and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) was revealed by digestion of nuclei from drug-treated cells with micrococcal nuclease and DNase I. [methyl-14C]MNU interacted preferentially with the more accessible regions of chromatin, the internucleosome linkers, whereas, the [chloroethyl-14C]CCNU alkylated the nucleosomal core DNA to a greater extent. These two drugs also differed in their extent of covalent modification of histone and nonhistone chromosomal protein. The binding of MNU to histones was greater than of CCNU. CCNU mainly affected nonhistone proteins. This difference in the reactivity of methyl and chloroethyl nitrosoureas with chromatin may relate to their differential effect on poly(adenosine diphosphate ribose) polymerase activity, as well as to their carcinogenic and antitumor properties.

Induction of apoptosis in proliferating human endothelial cells by the tumor-specific antiangiogenesis agent combretastatin A-4.

The antiangiogenic, tubulin-binding drug combretastatin A-4 exhibits a selective toxicity for proliferating endothelial cells in vitro and induces vascular shutdown in tumor models in vivo. The mechanism of combretastatin A-4 cytotoxicity has now been investigated with cultured proliferating human umbilical vein endothelial cells by examining various markers of apoptosis. Incubation of cells with 0.1 mM combretastatin A-4 induced the conversion (first detected after 6 h) of the CPP32 proenzyme to active caspase-3, a cysteine protease that plays an important role in apoptosis in many cell types; the drug also increased caspase-3 activity. Another early event observed was the binding of annexin V to 50% of the cells 8 h after drug treatment. Internucleosomal DNA fragmentation, another hallmark of apoptosis, was detected in cells incubated with 0.1 mM combretastatin A-4 for 24 h. Staining with Hoechst 33258 revealed that about 75% of cells exhibited a nuclear morphology characteristic of apoptosis after incubation with drug for 24 h. Incubation of cells for up to 8 h with combretastatin A-4 did not induce the release of lactate dehydrogenase or increase the uptake of propidium iodide, both indicators of membrane integrity. These results indicate that the selective cytotoxic effect of combretastatin A-4 is mediated by the induction of apoptosis rather than by necrosis and may provide an enhanced clinical strategy in cancer chemotherapy with this new agent.

Influence of hydrocortisone on the binding of nitrosoureas to nuclear chromatin subfractions.

The effects of steroid-induced modifications of chromatin structure on the extent and sites of chloroethylnitrosourea binding to chromatin were studied using log-phase HeLa cells. The cells were exposed to 0.1 to 2.0 microM hydrocortisone for 22 hr; this resulted in depressed DNA synthesis while transcriptional activity was stimulated. Hydrocortisone had no effect upon cellular or nuclear uptake of the two nitrosoureas under study, 0.6 mM chlorozotocin or 1-(2-chloroethyl-3-cyclohexyl-1-nitrosourea). Both drugs were found to alkylate transcriptional chromatin preferentially, as demonstrated by DNase II and DNase I digestion. This alkylation was stimulated 2-fold by the same micromolar concentrations of hydrocortisone, 0.1 to 2.0 microM, which stimulated transcription. The extent of nuclear RNA alkylation, determined using RNase T2 as a probe, was found to contribute less than 20% of total chromatin alkylation and was unaffected by steroid pretreatment. Instead, the increased alkylation within these chromatin subfractions was attributed to a steroid-induced alteration of chromatin structure. Electron microscopic examination of HeLa nuclear morphology revealed a hydrocortisone-induced disaggregation of nuclear membrane-associated heterochromatin resulting in a more heterogeneous, less condensed distribution of chromatin. Such data are consistent with a relaxation of the supercoiled chromatin structure, resulting in increased transcription and increased accessibility of potential target sites for nitrosourea alkylation.

Irreversible binding of poly(ADP)ribose polymerase cleavage product to DNA ends revealed by atomic force microscopy: possible role in apoptosis.

During apoptosis, DNA undergoes fragmentation and caspase-3 cleaves poly(ADP-ribose) polymerase (PARP) into both a 24-kDa fragment containing the DNA binding domain and an 89-kDa fragment containing the catalytic and automodification domains. Atomic force microscopy revealed that recombinant full-length PARP bound to plasmid DNA fragments and linked them into chainlike structures. Automodification of PARP in the presence of NAD+ resulted in its dissociation from the DNA fragments, which, nevertheless, remained physically aligned. A recombinant 28-kDa fragment of PARP containing the DNA binding domain but lacking the automodification domain irreversibly bound to and linked DNA fragments in the absence or presence of NAD+. Identical results were obtained on incubation of internucleosomal DNA fragments from apoptotic cells with the products of cleavage of recombinant PARP by purified caspase-3. The 24-kDa product of PARP cleavage by caspase-3 may contribute to the irreversibility of apoptosis by blocking the access of DNA repair enzymes to DNA strand breaks.

Poly(ADP-ribose) polymerase upregulates E2F-1 promoter activity and DNA pol alpha expression during early S phase.

E2F-1, a transcription factor implicated in the activation of genes required for S phase such as DNA pol alpha, is regulated by interactions with Rb and by cell-cycle dependent alterations in E2F-1 abundance. We have shown that depletion of poly(ADP-ribose) polymerase (PARP) by antisense RNA expression downregulates pol alpha and E2F-1 expression during early S phase. To examine the role of PARP in the regulation of pol alpha and E2F-1 gene expression, we utilized immortalized mouse fibroblasts derived from wild-type and PARP knockout (PARP-/-) mice as well as PARP-/- cells stably transfected with PARP cDNA [PARP-/-(+PARP)]. After release from serum deprivation, wild-type and PARP-/-(+PARP) cells, but not PARP-/- cells, exhibited a peak of cells in S phase by 16 h and had progressed through the cell cycle by 22 h. Whereas [3H]thymidine incorporation remained negligible in PARP-/- cells, in vivo DNA replication maximized after 18 h in wild-type and PARP-/-(+PARP) cells. To investigate the effect of PARP on E2F-1 promoter activity, a construct containing the E2F-1 gene promoter fused to a luciferase reporter gene was transiently transfected into these cells. E2F-1 promoter activity in control and PARP-/-(+PARP) cells increased eightfold after 9 h, but not in PARP-/- cells. PARP-/- cells did not show the marked induction of E2F-1 expression during early S phase apparent in control and PARP-/-(+PARP) cells. RT - PCR analysis and pol alpha activity assays revealed the presence of pol alpha transcripts and a sixfold increase in activity in both wild-type and PARP-/-(+PARP) cells after 20 h, but not in PARP-/- cells. These results suggest that PARP plays a role in the induction of E2F-1 promoter activity, which then positively regulates both E2F-1 and pol alpha expression, when quiescent cells reenter the cell cycle upon recovery from aphidicolin exposure or removal of serum.

The E7 protein of human papillomavirus type 16 sensitizes primary human keratinocytes to apoptosis.

The 'high risk' human papillomaviruses are associated with the development of anogenital carcinomas and their E6 and E7 genes possess immortalizing and transforming functions in several in vitro culture systems. Recently the E6 gene has also been shown to enhance the apoptosis of human mammary epithelial cells. To determine the apoptotic activity of these oncogenes in the natural host cell, we infected genital keratinocytes with retroviruses expressing either HPV-16 E6, E7, or both the E6 and E7 (E6/7) genes. Apoptosis was quantitated under normal growth conditions or when induced by tumor necrosis factor alpha/cycloheximide or sulfur mustard. In contrast to previous findings with mammary epithelial cells, the E6 gene did not significantly augment either spontaneous or induced apoptosis. E6 also did not suppress apoptosis in normal keratinocytes (despite dramatically reducing their p53 levels), suggesting that p53-independent events mediated this effect. In contrast, E7 increased both spontaneous and induced apoptosis as well as the cellular levels of p53 and p21 protein. Interestingly, co-expression of E6 abrogated E7-facilitated apoptosis by tumor necrosis factor alpha nearly completely, but had only a minor protective effect on sulfur mustard induced apoptosis in these cells, demonstrating at least in part the p53-dependence and -independence of these two apoptotic pathways. Finally, our results indicate that the apoptosis of normal and E7-expressing keratinocytes is differentially affected by E6 expression and that E7, when unaccompanied by E6, sensitizes keratinocytes to apoptosis.

Poly(ADP-ribose) polymerase facilitates the repair of N-methylpurines in mitochondrial DNA.

This study was designed to test the hypothesis that poly(ADP-ribose) polymerase (PARP) plays a role in the repair of damage to mitochondrial DNA (mtDNA). A rat insulinoma cell line was transfected with a PARP antisense vector that was under the control of a dexamethasone promoter. Transfected cells were selected for stable integration of the antisense vector. Several cell lines containing the antisense vector were isolated. For these studies, one of these lines (clone 5) was chosen for further evaluation. When cells were treated with dexamethasone for 72 h, PARP activity was diminished by 60%. Western blot analysis revealed a concomitant reduction in PARP protein. When clone 5 cells were exposed to the simple methylating agent methylnitrosourea (MNU) without previous treatment with dexamethasone, repair of lesions in mtDNA was found to be similar to that seen in wild-type cells or in wild-type cells treated with dexamethasone. However, when clone 5 cells were pretreated with dexamethasone for 72 h, repair of MNU-induced damage was significantly inhibited. To ascertain whether the PARP activity that was inhibited by the antisense treatment was the same as that found in the nucleus, repair studies were performed on fibroblasts derived from PARP knockout mice and their normal wild-type controls. Attenuated repair was also seen in the cells in which the gene for PARP was inactivated. These are the first studies to demonstrate that PARP can facilitate the repair of simple alkylation damage to mtDNA.

Poly(ADP-ribosyl)ation of p53 in vitro and in vivo modulates binding to its DNA consensus sequence.

The tumor-suppressor p53 undergoes extensive poly(ADP-ribosyl)ation early during apoptosis in human osteosarcoma cells, and degradation of poly(ADP-ribose) (PAR) attached to p53 coincides with poly(ADP-ribose)polymerase-1, (PARP-1) cleavage, and expression of p53 target genes. The mechanism by which poly(ADP-ribosyl)ation may regulate p53 function has now been investigated. Purified wild-type PARP-1 catalyzed the poly(ADP-ribosyl) of full-length p53 in vitro. In gel supershift assays, poly(ADP-ribosyl)ation suppressed p53 binding to its DNA consensus sequence; however, when p53 remained unmodified in the presence of inactive mutant PARP-1, it retained sequence-specific DNA binding activity. Poly(ADP-ribosyl)ation of p53 by PARP-1 during early apoptosis in osteosarcoma cells also inhibited p53 interaction with its DNA consensus sequence; thus, poly(ADP-ribosyl)ation may represent a novel means for regulating transcriptional activation by p53 in vivo.

Engineered human skin model using poly(ADP-ribose) polymerase antisense expression shows a reduced response to DNA damage.

Poly(ADP-ribose) polymerase (PADPRP) modifies nuclear proteins in response to DNA-damaging agents. The principal organ subject to exposure to many of these agents is the skin. To understand the role of PADPRP in the maintenance of the epidermis, a model system has been developed in which we have selectively lowered the levels of this enzyme by the use of induced expression of antisense RNA. Human keratinocyte lines were stably transfected with the cDNA for human PADPRP in the antisense orientation under an inducible promoter. Induction of this antisense RNA in cultured cells selectively lowers the levels of PADPRP mRNA, protein, and enzyme activity. Induction of antisense RNA also led to a reduction in the levels of PADPRP in individual cell nuclei, as well as the loss of the ability of cells to synthesize and modify proteins by poly(ADP-ribose) polymer in response to DNA damage. When keratinocyte clones containing the antisense construct or empty vector alone were grafted onto nude mice, they formed histologically normal human skin. The PADPRP antisense construct was also inducible in vivo by the topical application of dexamethasone to the reconstituted epidermis. In addition, poly(ADP-ribose) polymer could be induced and detected in vivo following the topical application of a DNA alkylating agent to the grafted transfected skin layers. Accordingly, a model system has been developed in which the levels of PADPRP can be selectively manipulated in human keratinocytes in cell culture, and potentially in reconstituted epidermis as well. This system will be a useful tool to study the role of PADPRP and DNA repair in general in essential biologic processes in the epidermis.

PARP determines the mode of cell death in skin fibroblasts, but not keratinocytes, exposed to sulfur mustard.

Sulfur mustard is cytotoxic to dermal fibroblasts as well as epidermal keratinocytes. We demonstrated that poly(ADP-ribose) polymerase (PARP) modulates Fas-mediated apoptosis, and other groups and we have shown that PARP plays a role in the modulation of other types of apoptotic and necrotic cell death. We have now utilized primary dermal fibroblasts, immortalized fibroblasts, and keratinocytes derived from PARP(-/-) mice and their wildtype littermates (PARP(+/+)) to determine the contribution of PARP to sulfur mustard toxicity. Following sulfur mustard exposure, primary skin fibroblasts from PARP-deficient mice demonstrated increased internucleosomal DNA cleavage, caspase-3 processing and activity, and annexin V positivity, compared to those derived from PARP(+/+) animals. Conversely, propidium iodide staining, PARP cleavage patterns, and random DNA fragmentation revealed a dose-dependent increase in necrosis in PARP(+/+) but not PARP(-/-) cells. Using immortalized PARP(-/-) fibroblasts stably transfected with the human PARP cDNA or with empty vector alone, we show that PARP inhibits markers of apoptosis in these cells as well. Finally, primary keratinocytes were derived from newborn PARP(+/+) and PARP(-/-) mice and immortalized with the E6 and E7 genes of human papilloma virus. In contrast to fibroblasts, keratinocytes from both PARP(-/-) and PARP(+/+) mice express markers of apoptosis in response to sulfur mustard exposure. The effects of PARP on the mode of cell death in different skin cell types may determine the severity of vesication in vivo, and thus have implications for the design of PARP inhibitors to reduce sulfur mustard pathology.

Sulfur mustard induces markers of terminal differentiation and apoptosis in keratinocytes via a Ca2+-calmodulin and caspase-dependent pathway.

Sulfur mustard (SM) induces vesication via poorly understood pathways. The blisters that are formed result primarily from the detachment of the epidermis from the dermis at the level of the basement membrane. In addition, there is toxicity to the basal cells, although no careful study has been performed to determine the precise mode of cell death biochemically. We describe here two potential mechanisms by which SM causes basal cell death and detachment: namely, induction of terminal differentiation and apoptosis. In the presence of 100 microM SM, terminal differentiation was rapidly induced in primary human keratinocytes that included the expression of the differentiation-specific markers K1 and K10 and the cross-linking of the cornified envelope precursor protein involucrin. The expression of the attachment protein, fibronectin, was also reduced in a time- and dose-dependent fashion. Features common to both differentiation and apoptosis were also induced in 100 microM SM, including the rapid induction of p53 and the reduction of Bcl-2. At higher concentrations of SM (i.e., 300 microM), formation of the characteristic nucleosome-sized DNA ladders, TUNEL-positive staining of cells, activation of the cysteine protease caspase-3/apopain, and cleavage of the death substrate poly(ADP-ribose) polymerase, were observed both in vivo and in vitro. Both the differentiation and the apoptotic processes appeared to be calmodulin dependent, because the calmodulin inhibitor W-7 blocked the expression of the differentiation-specific markers, as well as the apoptotic response, in a concentration-dependent fashion. In addition, the intracellular Ca2+ chelator, BAPTA-AM, blocked the differentiation response and attenuated the apoptotic response. These results suggest a strategy for designing inhibitors of SM vesication via the Ca2+-calmodulin or caspase-3/PARP pathway.

Model systems for the study of the role of PADPRP in essential biological processes.

The nuclear enzyme poly(ADP-ribose) polymerase (PADPRP) is implicated in a number of cellular processes, including DNA repair, replication, and differentiation. We have been using several model systems to examine these potential roles of PADPRP. A human keratinocyte model system has been developed in which stable lines of epidermal cells contain an inducible construct harboring the antisense cDNA to PADPRP. When PADPRP antisense RNA is induced in culture, endogenous PADPRP mRNA, protein, and enzymatic activity is lowered, and the pattern of poly(ADP) ribosylation in response to alkylating agents is altered. When keratinocyte clones containing the antisense construct or empty vector alone were grafted onto nude mice, they formed histologically normal human skin. The PADPRP antisense construct was also inducible in vivo by the topical application of dexamethasone to the reconstituted epidermis, as determined by in situ hybridization. In addition, poly(ADP-ribose) polymer could be induced and detected in vivo following the topical application of a DNA alkylating agent to the grafted transfected skin layers. Thus, a model system has been developed in which the levels of PADPRP can be selectively manipulated in human keratinocytes in cell culture, and potentially in reconstituted epidermis as these keratinocyte lines can be grafted to nude mice, whereupon they form a histologically and immunocytochemically normal human epidermis. Another system that we have been utilizing to examine the role of PADPRP in proliferation and differentiation is stable lines of mouse preadipocytes that contain an inducible antisense PADPRP RNA. Similar to the keratinocyte system, these cells can inducibly express antisense PADPRP RNA, and subsequently lower levels of endogenous PADPRP.(ABSTRACT TRUNCATED AT 250 WORDS)