p16(Ink4a) overexpression in cancer: a tumor suppressor gene associated with senescence and high-grade tumors

p16(Ink4a) is a protein involved in regulation of the cell cycle. Currently, p16(Ink4a) is considered a tumor suppressor protein because of its physiological role and downregulated expression in a large number of tumors. Intriguingly, overexpression of p16(Ink4a) has also been described in several tumors. This review attempts to elucidate when and why p16(Ink4a) overexpression occurs, and to suggest possible implications of p16(Ink4a) in the diagnosis, prognosis and treatment of cancer.

The C-terminal SET domains of ALL-1 and TRITHORAX interact with the INI1 and SNR1 proteins, components of the SWI/SNF complex

The ALL-1 gene was discovered by virtue of its involvement in human acute leukemia. Its Drosophila homolog trithorax (trx) is a member of the trx-Polycomb gene family, which maintains correct spatial expression of the Antennapedia and bithorax complexes during embryogenesis. The C-terminal SET domain of ALL-1 and TRITHORAX (TRX) is a 150-aa motif, highly conserved during evolution. We performed yeast two hybrid screening of Drosophila cDNA library and detected interaction between a TRX polypeptide spanning SET and the SNR1 protein. SNR1 is a product of snr1, which is classified as a trx group gene. We found parallel interaction in yeast between the SET domain of ALL-1 and the human homolog of SNR1, INI1 (hSNF5). These results were confirmed by in vitro binding studies and by demonstrating coimmunoprecipitation of the proteins from cultured cells and/or transgenic flies. Epitope-tagged SNR1 was detected at discrete sites on larval salivary gland polytene chromosomes, and these sites colocalized with around one-half of TRX binding sites. Because SNR1 and INI1 are constituents of the SWI/SNF complex, which acts to remodel chromatin and consequently to activate transcription, the interactions we observed suggest a mechanism by which the SWI/SNF complex is recruited to ALL-1/trx targets through physical interactions between the C-terminal domains of ALL-1 and TRX and INI1/SNR1.

Trithorax and dCBP acting in a complex to maintain expression of a homeotic gene

Trithorax (Trx) is a member of the trithorax group (trxG) of epigenetic regulators, which is required to maintain active states of Hox gene expression during development. We have purified from Drosophila embryos a trithorax acetylation complex (TAC1) that contains Trx, dCBP, and Sbf1. Like CBP, TAC1 acetylates core histones in nucleosomes, suggesting that this activity may be important for epigenetic maintenance of gene activity. dCBP and Sbf1 associate with specific sites on salivary gland polytene chromosomes, colocalizing with many Trx binding sites. One of these is the site of the Hox gene Ultrabithorax (Ubx). Mutations in either trx or the gene encoding dCBP reduce expression of the endogenous Ubx gene as well as of transgenes driven by the bxd regulatory region of Ubx. Thus Trx, dCBP, and Sbf1 are closely linked, physically and functionally, in the maintenance of Hox gene expression.

The Drosophila trithorax gene encodes a chromosomal protein and directly regulates the region-specific homeotic gene fork head

The activity of the Drosophila gene trithorax is required to maintain the proper spatial pattern of expression of multiple homeotic genes of the Bithorax and Antennapedia complexes, trithorax encodes two large protein isoforms of > 400 kD. We have detected its products at 16 discrete sites on larval salivary gland polytene chromosomes, 12 of which colocalize with binding sites of several Polycomb group proteins. The intensity of trithorax protein binding is strongly decreased in larvae carrying mutations in another trithorax group gene ash-1, and in the Polycomb group gene pco/E(z). A strong trithorax binding site was found at the cytological location of the fork head gene, a region-specific homeotic gene not located within a homeotic complex. Further analysis showed that trithorax protein binds at ectopic sites carrying fork head sequences in transformed lines. Trithorax binding occurs within an 8.4-kb regulatory region that directs fork head expression in several embryonic tissues including salivary glands. Consistently, expression of endogenous fork head RNA is greatly reduced in trithorax mutant embryos and in larval tissues. These results show that trithorax maintains expression of target genes by interaction with their regulatory regions and that this interaction depends on the presence of at least some of the other trithorax and Polycomb group proteins.

Trithorax- and Polycomb-group response elements within an Ultrabithorax transcription maintenance unit consist of closely situated but separable sequences

In Drosophila, two classes of genes, the trithorax group and the Polycomb group, are required in concert to maintain gene expression by regulating chromatin structure. We have identified Trithorax protein (TRX) binding elements within the bithorax complex and have found that within the bxd/pbx regulatory region these elements are functionally relevant for normal expression patterns in embryos and confer TRX binding in vivo. TRX was localized to three closely situated sites within a 3-kb chromatin maintenance unit with a modular structure. Results of an in vivo analysis showed that these DNA fragments (each approximately 400 bp) contain both TRX- and Polycomb-group response elements (TREs and PREs) and that in the context of the endogenous Ultrabithorax gene, all of these elements are essential for proper maintenance of expression in embryos. Dissection of one of these maintenance modules showed that TRX- and Polycomb-group responsiveness is conferred by neighboring but separable DNA sequences, suggesting that independent protein complexes are formed at their respective response elements. Furthermore, we have found that the activity of this TRE requires a sequence (approximately 90 bp) which maps to within several tens of base pairs from the closest neighboring PRE and that the PRE activity in one of the elements may require a binding site for PHO, the protein product of the Polycomb-group gene pleiohomeotic. Our results show that long-range maintenance of Ultrabithorax expression requires a complex element composed of cooperating modules, each capable of interacting with both positive and negative chromatin regulators.

huASH1 protein, a putative transcription factor encoded by a human homologue of the Drosophila ash1 gene, localizes to both nuclei and cell-cell tight junctions

During animal development, regions of the embryo become committed to position-specific identities, which are determined by spatially restricted expression of Hox/homeotic genes. This expression pattern is initially established by the activity of the segmentation genes and is subsequently maintained during the proliferative stage through the action of transcription factors encoded by the trithorax (trx) and Polycomb (Pc) groups of genes. trithorax (trx)and ash1 (absent, small, or homeotic 1) are members of the Drosophila trx group. Their products are associated with chromosomes and are believed to activate transcription of target genes through chromatin remodeling. Recently, we reported molecular studies indicating that TRX and ASH1 proteins act in concert to bind simultaneously to response elements located at close proximity within the same set of target genes. Extension of these and other studies to mammalian systems required identification and cloning of the mammalian homologue of ash1 (the mammalian homologue of trx, ALL-1, was previously cloned). We have identified a human expressed sequence tag (EST) clone with similarity to the SET domain of Drosophila ASH1, and used it to clone the human gene. huASH1 resides at chromosomal band 1q21. The gene is expressed in multiple tissues as an approximately 10.5-kb transcript and encodes a protein of 2962 residues. The protein contains a SET domain, a PHD finger, four AT hooks, and a region with homology to the bromodomain. The last region is not present in Drosophila ASH1, and as such might confer to the human protein a unique additional function. Using several anti-huASH1 Ab for immunostaining of cultured cells, we found that the protein is distributed in intranuclear speckles, and unexpectedly also in intercellular junctions. Double-immunofluorescence labeling of huASH1 and several junctional proteins localized the huASH1 protein into tight junctions. The significance of huASH1 dual location is discussed. In particular, we consider the possibility that translocation of the protein between the junctional membrane and the nucleus may be involved in adhesion-mediated signaling.

Identification and characterization of the ARP1 gene, a target for the human acute leukemia ALL1 gene

ALL1, the human homologue of Drosophila trithorax, is directly involved in human acute leukemias associated with abnormalities at 11q23. Using the differential display method, we isolated a gene that is down-regulated in All1 double-knockout mouse embryonic stem (ES) cells. The gene, designated ARP1 (also termed RIEG, Ptx2, or Otlx2), is a member of a family of homeotic genes containing a short motif shared with several homeobox genes. Using a bacterially synthesized All1 polypeptide encompassing the AT-hook motifs, we identified a 0.5-kb ARP1 DNA fragment that preferentially bound to the polypeptide. Within this DNA, a region of approximately 100 bp was protected by the polypeptide from digestion with ExoIII and DNase I. Whole-mount in situ hybridization to early mouse embryos of 9.5-10.5 days indicated a complex pattern of Arp1 expression spatially overlapping with the expression of All1. Although the ARP1 gene is expressed strongly in bone marrow cells, no transcripts were detected in six leukemia cell lines with 11q23 translocations. These results suggest that ARP1 is up-regulated by the All1 protein, possibly through direct interaction with an upstream DNA sequence of the former. The results are also consistent with the suggestion that ALL1 chimeric proteins resulting from 11q23 abnormalities act in a dominant negative fashion.

Structure and expression pattern of human ALR, a novel gene with strong homology to ALL-1 involved in acute leukemia and to Drosophila trithorax

The ALL-1 gene is involved in human acute leukemia through chromosome translocations or internal rearrangements. ALL-1 is the human homologue of Drosophila trithorax. The latter is a member of the trithorax group (trx-G) genes which together with the Polycomb group (Pc-G) genes act as positive and negative regulators, respectively, to determine the body structure of Drosophila. We have cloned a novel human gene, ALR, which encodes a gigantic 5262 amino acid long protein containing a SET domain, five PHD fingers, potential zinc fingers, and a very long run of glutamines interrupted by hydrophobic residues, mostly leucine. The SET motif, PDH fingers, zinc fingers and two other regions are most similar to domains of ALL-1 and TRX. The first two motifs are also found in other trx-G and Pc-G proteins. The ALR gene was mapped to chromosome band 12q12-13, adjacent to the VDR gene. This region is involved in duplications and translocations associated with cancer. The analysis of ALR expression showed that its approximately 18 kb long mRNA is expressed, like ALL-1, in most adult tissues, including a variety of hematopoietic cells, with the exception of the liver. Whole mount in situ hybridization to early mouse embryos indicates expression in multiple tissues. Based on similarities in structure and expression pattern, ALR is likely to play a similar role to ALL-1 and trx, although its target genes have yet to be identified.

G1 cyclin/cyclin-dependent kinase-coordinated phosphorylation of endogenous pocket proteins differentially regulates their interactions with E2F4 and E2F1 and gene expression

Mitogenic stimulation leads to activation of G(1) cyclin-dependent kinases (CDKs), which phosphorylate pocket proteins and trigger progression through the G(0)/G(1) and G(1)/S transitions of the cell cycle. However, the individual role of G(1) cyclin-CDK complexes in the coordinated regulation of pocket proteins and their interaction with E2F family members is not fully understood. Here we report that individually or in concert cyclin D1-CDK and cyclin E-CDK complexes induce distinct and coordinated phosphorylation of endogenous pocket proteins, which also has distinct consequences in the regulation of pocket protein interactions with E2F4 and the expression of p107 and E2F1, both E2F-regulated genes. The up-regulation of these two proteins and the release of p130 and pRB from E2F4 complexes allows formation of E2F1 complexes not only with pRB but also with p130 and p107 as well as the formation of p107-E2F4 complexes. The formation of these complexes occurs in the presence of active cyclin D1-CDK and cyclin E-CDK complexes, indicating that whereas phosphorylation plays a role in the abrogation of certain pocket protein/E2F interactions, these same activities induce the formation of other complexes in the context of a cell expressing endogenous levels of pocket and E2F proteins. Of note, phosphorylated p130 "form 3," which does not interact with E2F4, readily interacts with E2F1. Our data also demonstrate that ectopic overexpression of either cyclin is sufficient to induce mitogen-independent growth in human T98G and Rat-1 cells, although the effects of cyclin D1 require downstream activation of cyclin E-CDK2 activity. Interestingly, in T98G cells, cyclin D1 induces cell cycle progression more potently than cyclin E. This suggests that cyclin D1 activates pathways independently of cyclin E that ensure timely progression through the cell cycle.

Nitrilase and Fhit homologs are encoded as fusion proteins in Drosophila melanogaster and Caenorhabditis elegans

The tumor suppressor gene FHIT encompasses the common human chromosomal fragile site at 3p14.2 and numerous cancer cell biallelic deletions. To study Fhit function we cloned and characterized FHIT genes from Drosophila melanogaster and Caenorhabditis elegans. Both genes code for fusion proteins in which the Fhit domain is fused with a novel domain showing homology to bacterial and plant nitrilases; the D. melanogaster fusion protein exhibited diadenosine triphosphate (ApppA) hydrolase activity expected of an authentic Fhit homolog. In human and mouse, the nitrilase homologs and Fhit are encoded by two different genes: FHIT and NIT1, localized on chromosomes 3 and 1 in human, and 14 and 1 in mouse, respectively. We cloned and characterized human and murine NIT1 genes and determined their exon-intron structure, patterns of expression, and alternative processing of their mRNAs. The tissue specificity of expression of murine Fhit and Nit1 genes was nearly identical. Because fusion proteins with dual or triple enzymatic activities have been found to carry out specific steps in a given biochemical or biosynthetic pathway, we postulate that Fhit and Nit1 likewise collaborate in a biochemical or cellular pathway in mammalian cells.

Trithorax and ASH1 interact directly and associate with the trithorax group-responsive bxd region of the Ultrabithorax promoter

Trithorax (TRX) and ASH1 belong to the trithorax group (trxG) of transcriptional activator proteins, which maintains homeotic gene expression during Drosophila development. TRX and ASH1 are localized on chromosomes and share several homologous domains with other chromatin-associated proteins, including a highly conserved SET domain and PHD fingers. Based on genetic interactions between trx and ash1 and our previous observation that association of the TRX protein with polytene chromosomes is ash1 dependent, we investigated the possibility of a physical linkage between the two proteins. We found that the endogenous TRX and ASH1 proteins coimmunoprecipitate from embryonic extracts and colocalize on salivary gland polytene chromosomes. Furthermore, we demonstrated that TRX and ASH1 bind in vivo to a relatively small (4 kb) bxd subregion of the homeotic gene Ultrabithorax (Ubx), which contains several trx response elements. Analysis of the effects of ash1 mutations on the activity of this regulatory region indicates that it also contains ash1 response element(s). This suggests that ASH1 and TRX act on Ubx in relatively close proximity to each other. Finally, TRX and ASH1 appear to interact directly through their conserved SET domains, based on binding assays in vitro and in yeast and on coimmunoprecipitation assays with embryo extracts. Collectively, these results suggest that TRX and ASH1 are components that interact either within trxG protein complexes or between complexes that act in close proximity on regulatory DNA to maintain Ubx transcription.

Expression profiles of acute lymphoblastic and myeloblastic leukemias with ALL-1 rearrangements

The ALL-1 gene is directly involved in 5-10% of acute lymphoblastic leukemias (ALLs) and acute myeloid leukemias (AMLs) by fusion to other genes or through internal rearrangements. DNA microarrays were used to determine expression profiles of ALLs and AMLs with ALL-1 rearrangements. These profiles distinguish those tumors from other ALLs and AMLs. The expression patterns of ALL-1-associated tumors, in particular ALLs, involve oncogenes, tumor suppressors, antiapoptotic genes, drug-resistance genes, etc., and correlate with the aggressive nature of the tumors. The genes whose expression differentiates between ALLs with and without ALL-1 rearrangement were further divided into several groups, enabling separation of ALL-1-associated ALLs into two subclasses. One of the groups included 43 genes that exhibited expression profiles closely linked to ALLs with ALL-1 rearrangements. Further, there were evident differences between the expression profiles of AMLs in which ALL-1 had undergone fusion to other genes and AMLs with partial duplication of ALL-1. The extensive analysis described here pinpointed genes that might have a direct role in pathogenesis.

The bithorax complex is regulated by trithorax earlier during Drosophila embryogenesis than is the Antennapedia complex, correlating with a bithorax-like expression pattern of distinct early trithorax transcripts

The trithorax gene is required throughout development to maintain expression of homeotic genes of the bithorax and Antennapedia complexes. We determined complete structures of maternal and zygotic alternatively spliced trithorax transcripts, and found that two RNA isoforms are expressed in a surprising manner in the early embryo. At syncytial blastoderm their expression is confined to the ventral region fated to become mesoderm. An additional broad domain of trithorax expression arises during cellularization and is quickly resolved into four pair-rule-like stripes in the posterior half of the embryo. This early expression pattern suggested the possibility that trithorax is involved in the very early regulation of homeotic genes expressed only in the posterior region of the embryo. Indeed, transcription of bithorax complex genes in the mesoderm and ectoderm is altered in strong trithorax mutants during germ band elongation, while the anteriorly expressed Antennapedia complex genes are affected only at late stages of embryonic development. In addition, in another mutant allele (trxE3), expression of bithorax complex genes is normal, while expression of Antennapedia complex genes is reduced. These results suggest that proper expression of genes in the two homeotic complexes is maintained by products of different trithorax RNAs at different times of embryogenesis.

Functional reconstruction of trans regulation of the Ultrabithorax promoter by the products of two antagonistic genes, trithorax and Polycomb

Maintenance of the "on-off" state of Drosophila homeotic genes in Antennapedia and bithorax complexes requires activities of the trithorax and Polycomb groups of genes. To identify cis-acting sequences for functional reconstruction of regulation by both trithorax and Polycomb, we examined the expression patterns of several Ubx-lacZ transgenes that carry upstream fragments corresponding to a region of approximately 50 kb. A 14.5-kb fragment from the postbithorax/bithoraxoid region of Ultrabithorax exhibited proper regulation by both trithorax and Polycomb in the embryonic central nervous system. Using a Drosophila haploid cell line for transient expression, we found that trithorax or Polycomb can function independently through this upstream fragment to activate or repress the Ultrabithorax promoter, respectively. Studies of deletion mutants of trithorax and Polycomb demonstrated that trithorax-dependent activation requires the central zinc-binding domain, while Polycomb-dependent repression requires the intact chromodomain. In addition, trithorax-dependent activity can be abrogated by increasing the amount of Polycomb, suggesting a competitive interaction between the products of trithorax and Polycomb. Deletion analysis of this fragment demonstrated that a 440-bp fragment contains response elements for both trithorax and Polycomb. Furthermore, we showed that the integrity of the proximal promoter region is essential for trithorax-dependent activation, implicating a long-range interaction for promoter activation.

Self-association of the SET domains of human ALL-1 and of Drosophila TRITHORAX and ASH1 proteins

The human ALL-1 gene is involved in acute leukemia through gene fusions, partial tandem duplications or a specific deletion. Several sequence motifs within the ALL-1 protein, such as the SET domain, PHD fingers and the region with homology to DNA methyl transferase are shared with other proteins involved in transcription regulation through chromatin alterations. However, the function of these motifs is still not clear. Studying ALL-1 presents an additional challenge because the gene is the human homologue of Drosophila trithorax. The latter is a member of the trithorax-Polycomb gene family which acts to determine the body pattern of Drosophila by maintaining expression or repression of the Antennapedia-bithorax homeotic gene complex. Here we apply yeast two hybrid methodology, in vivo immunoprecipitation and in vitro 'pull down' techniques to show self association of the SET motifs of ALL-1, TRITHORAX and ASH1 proteins (Drosophila ASH1 is encoded by a trithorax-group gene). Point mutations in evolutionary conserved residues of TRITHORAX SET, abolish the interaction. SET-SET interactions might act in integrating the activity of ALL-1 (TRX and ASH1) protein molecules, simultaneously positioned at different maintenance elements and directing expression of the same or different target genes.

Molecular genetic analysis of the Drosophila trithorax-related gene which encodes a novel SET domain protein

The products of the trithorax and Polycomb groups genes maintain the activity and silence, respectively, of many developmental genes including genes of the homeotic complexes. This transcriptional regulation is likely to involve modification of chromatin structure. Here, we report the cloning and characterization of a new gene, trithorax-related (trr), which shares sequence similarities with members of both the trithorax and Polycomb groups. The trr transcript is 9.6 kb in length and is present throughout development. The TRR protein, as predicted from the nucleotide sequence of the open reading frame, is 2431 amino acids in length and contains a PHD finger-like domain and a SET domain, two highly conserved protein motifs found in several trithorax and Polycomb group proteins, and in modifiers of position effect variegation. TRR is most similar in sequence to the human ALR protein, suggesting that trr is a Drosophila homologue of the ALR. TRR is also highly homologous to Drosophila TRITHORAX protein and to its human homologue, ALL-1/HRX. However, preliminary genetic analysis of a trr null allele suggests that TRR protein may not be involved in regulation of homeotic genes (i.e. not a member of the trithorax or Polycomb groups) or in position effect variegation.

ALL-1/MLL1, a homologue of Drosophila TRITHORAX, modifies chromatin and is directly involved in infant acute leukaemia

Rearrangements of the ALL-1/MLL1 gene underlie the majority of infant acute leukaemias, as well as of therapy-related leukaemias developing in cancer patients treated with inhibitors of topoisomerase II, such as VP16 and doxorubicin. The rearrangements fuse ALL-1 to any of >50 partner genes or to itself. Here, we describe the unique features of ALL-1-associated leukaemias, and recent progress in understanding molecular mechanisms involved in the activity of the ALL-1 protein and of its Drosophila homologue TRITHORAX.

Kinetic studies of the regulation of mitochondrial malate dehydrogenase by citrate

Mitochondrial malate dehydrogenase shows a complex regulation pattern in the presence of citrate. Previously published results indicate that this enzyme is activated by citrate in the NAD(+)----NADH direction and inhibited in the opposite direction. Moreover, high concentrations of L-malate or oxaloacetate produce deviations from the Michaelis-Menten behaviour. Results reported in this paper clearly show that citrate both activates and inhibits mitochondrial malate dehydrogenase in the same direction (NAD(+)----NADH), and in the same reaction medium, depending on substrate concentration. This surprising effect has made it necessary to propose a new kinetic mechanism that extends those previously suggested and allows us to explain both the citrate effect (activating or inhibitory) and the effect of high concentrations of L-malate and oxaloacetate.

Enhanced pancreatic tumor regression by a combination of adenovirus and retrovirus-mediated delivery of the herpes simplex virus thymidine kinase gene

We have evaluated the effectiveness of combining the different characteristics of retrovirus and adenovirus to apply the herpes simplex virus thymidine kinase gene (HSVtk) and ganciclovir (GCV) treatment for gene therapy of pancreatic cancer. Transduction of NP-18 human pancreatic cells in culture by either the adenoviral vector (ADV/tk) or the retroviral vector (Rv/tk) followed by GCV treatment resulted in a GCV dose-dependent cytotoxic effect. A bystander effect was determined, both in NP-18 cultures and in xenogeneic cell mixtures of NP-18 and PA317 cells. Studies in vivo indicated that the effectiveness of tumor regression after HSVtk gene transfer and GCV treatment was dependent first on the tumor size at the time of viral injection and secondly, in large tumors, on the type of virus administered. The administration of the viral combination (ADV/tk + vector producer cells VPC-Rv/tk) was the best approach tested and resulted in a dramatic reduction in tumor mass after 4 days of GCV treatment which was maintained for the treatment period. Remarkably, two animals presented a complete eradication of the tumor. Thus, the HSVtk/GCV system when administered using a viral combination (ADV/tk + VPC-Rv/tk), may be a promising suicide gene therapy for pancreatic carcinomas.

Adenoviral-mediated overexpression of human equilibrative nucleoside transporter 1 (hENT1) enhances gemcitabine response in human pancreatic cancer

Nucleoside-derived anticancer agents must be transported across the plasma membrane as a preliminary step to their conversion into active drugs. Hence, modulation of a specific nucleoside transporter may affect bioavailability and contribute significantly to sensitizing tumor cells to these anticancer agents. We have generated and functionally characterized a new recombinant adenovirus (Ad-hENT1) that has allowed us to overexpress the equilibrative nucleoside transporter hENT1 and to analyze its effects in human pancreatic tumor cells. Overexpression of hENT1 is associated with changes in cell cycle profile, in a variable manner depending on the particular cell type, thus suggesting a metabolic link between hENT1-mediated transport processes and the enzymatic machinery responsible for intracellular nucleoside metabolism. When assayed in vivo in a human pancreatic adenocarcinoma xenograft, intratumoral Ad-hENT1 injection improved the therapeutic response to gemcitabine. In summary, hENT1 overexpression is associated with alterations in nucleoside enzymatic machinery and cell cycle progression in cultured cells and enhances gemcitabine action in vivo.

Modulation of drug cytotoxicity by reintroduction of wild-type p53 gene (Ad5CMV-p53) in human pancreatic cancer

Chemotherapy does not significantly improve prognosis in pancreatic cancer. New therapeutical approaches involving p53 gene replacement appear to be very encouraging due to the key role of p53 in the cell response to DNA damage. Here, we have evaluated the effectiveness of combining wild-type p53 (wt-p53) gene reintroduction (Ad5CMV-p53) and exposure to two genotoxic drugs, gemcitabine and cisplatin, in several human pancreatic cell lines. The efficiency of the combinations was clearly dependent upon timing, as assessed by cell survival determinations. Although wt-p53 transduction before drug treatment induced chemoresistance, p53 transduction in cells treated previously with gemcitabine increased cytotoxicity. Cell cycle profiles showed significant decreases in the percentage of cells in the S phase as a consequence of arrests provoked by the expression of exogenous p53, reducing the number of cells susceptible to the drug. The sensitivity of cells to cisplatin, which has a lower degree of S-phase specificity, was not modified as much by p53 gene replacement. In contrast, the recognition of the previous drug-induced DNA damage by the newly expressed wt-p53 elicited increases in sub-G1 populations, consistent with the annexin determinations and bax/bcl-2 ratios observed. Experiments on subcutaneous pancreatic xenografts corroborated the effectiveness of this approach in vivo. Thus, the combination of p53 transduction and chemotherapy, under a correct schedule of administration, appears to be a very promising therapy for human pancreatic cancer.

Connexin-26 is a key factor mediating gemcitabine bystander effect

Gemcitabine is a nucleoside analogue with anticancer activity. Inside the cell, it is sequentially phosphorylated to generate the active drug. Phosphorylated nucleoside analogues have been shown to traffic through gap junctions. We investigated the participation of gap junctional intercellular communication (GJIC) as a possible mechanism spreading gemcitabine cytotoxicity in pancreatic tumors. Immunohistochemical analysis of pancreatic cancer biopsies revealed increased connexin 26 (Cx26) content but loss of connexins 32 (Cx32) and 43 (Cx43) expression. Cx26 abundance in neoplastic areas was confirmed by Cx26 mRNA in situ hybridization. Heterogeneity on the expression levels and the localization of Cx26, Cx32, and Cx43 were identified in pancreatic cancer cells and found to be associated with the extent of GJIC, and correlated with gemcitabine bystander cytotoxic effect. The abundance of Cx26 at the contact points in tumoral regions prompted us to study the involvement of Cx26 in the GJIC of gemcitabine toxic metabolites and their influence on the antitumoral effects of gemcitabine. Knockdown of Cx26 led to decreased GJIC and reduced gemcitabine bystander killing whereas overexpression of Cx26 triggered increased GJIC and enhanced the gemcitabine cytotoxic bystander effect. Gemcitabine treatment of mice bearing tumors, with a high GJIC capacity, resulted in a significant delay in tumor progression. Interestingly, gemcitabine administration in mice bearing tumors that overexpress Cx26 triggered a dramatic tumor regression of 50% from the initial volume. This study shows that Cx26 participates in the gap junction-mediated bystander cytoxic effect of gemcitabine and provides evidence that upregulation of Cx26 improves gemcitabine anticancer efficacy.

Retrovirus-mediated transfer of the herpes simplex virus thymidine kinase and connexin26 genes in pancreatic cells results in variable efficiency on the bystander killing: implications for gene therapy

Currently, there is no effective treatment for pancreatic cancer and prodrug-activating gene therapy with the herpes simplex virus thymidine kinase gene (HSV-tk) in combination with ganciclovir (GCV) has been suggested as a candidate approach against this disease. In the present study, we have evaluated the efficacy of the HSV-tk/GCV treatment in a panel of pancreatic tumor cells (NP-9, NP-18, NP-31) and the potentiation of the cytotoxic effect in combination with the overexpression of the connexin 26 gene (Cx26). Pancreatic cells transduced with a retrovirus containing the HSV-tk gene showed different sensitivities to GCV that seemed to be independent of HSV-tk expression levels. The extent of the bystander effect also varied among the pancreatic tumor cells and correlated with the level of gap junction intercellular communication (GJIC). Transduction of the pancreatic tumor cells with a retrovirus carrying the connexin 26 gene resulted in high levels of connexin 26 expression and in an increase in the GJIC that correlated to an extent in the bystander effect in both NP-9Cx26 and NP-18Cx26 cells. Neither an increment in GJIC nor an increase in the bystander killing was detected in NP-31Cx26. The bystander effect in NP-18 Cx26 cells was also prevented by the long term inhibitor of GJIC, 18-alpha-glycyrrhetinic acid (AGA). Together, these results demonstrate that pancreatic tumor cells are highly different as regards the susceptibility to HSV-tk/GCV treatment. Moreover, they indicate that overexpression of the Cx26 gene does not always correspond to an increase in GJIC although they clearly suggest the role of GJIC in mediating the bystander effect.

Adenovirus-mediated wt-p16 reintroduction induces cell cycle arrest or apoptosis in pancreatic cancer

Pancreatic cancer has long carried poor prognosis. The development of new therapeutic approaches is particularly urgent. Inactivation of the tumor-suppressor gene p16(INK4a/CDKN2), a specific inhibitor of the cyclin-dependent kinases CDK4 and CDK6, is the most common genetic alteration in human pancreatic cancer, making it an ideal target for gene replacement. Here we transfected tumor cells using a recombinant adenovirus containing the wt-p16 cDNA (Ad5RSV-p16). The overexpression of p16 decreased cell proliferation in all four human pancreatic tumor cell lines (NP-9, NP-18, NP-29, and NP-31). However, G1 arrest and senescence were observed in only three. In contrast, the fourth (NP-18) showed a significant increase in apoptosis. This differential behavior may be related to the differences found in the expression level of E2F-1. Experiments on subcutaneous pancreatic xenografts demonstrated the effectiveness of p16 in the inhibition of pancreatic tumor growth in vivo. Taken together, our results indicate that approaches involving p16 replacement are promising in pancreatic cancer treatment.

Anti-migratory and anti-angiogenic effect of p16: a novel localization at membrane ruffles and lamellipodia in endothelial cells

Recent evidence has established different functions for the tumor suppressor protein, p16(INK4A) aside from controlling the cell cycle. Here we report that cdk4/6 inhibition blocked both human umbilical vein endothelial cells (HUVEC) spreading on a vitronectin matrix and HUVEC migration on vitronectin. p16 can also act as an anti-angiogenic molecule in vitro since HUVEC and HMEC cells transfected with Ad-p16 or treated with Antennapedia p16 peptides are unable to differentiate on a Matrigel matrix. Both, p16, cyclin D1, cdk4 and cdk6 were immuno-colocalized with Ezrin, Rac, Vinculin, alphav-integrin, and FAK proteins in the ruffles and lamellipodia of migratory cells. Our results indicate that p16 is a key component of a new cytoplasmic pathway controlling angiogenesis of endothelial cells via the alphavbeta3-integrin-mediated migration.