c-Myc-mediated regulation of telomerase activity is disabled in immortalized cells.

Myc overexpression is a hallmark of human cancer and promotes transformation by facilitating immortalization. This function has been linked to the ability of c-Myc to induce the expression of the catalytic subunit of telomerase, telomerase reverse transcriptase (TERT), as ectopic expression of TERT immortalizes some primary human cell types. c-Myc up-regulates telomerase activity in primary mouse embryonic fibroblasts (MEFs) and myeloid cells. Paradoxically, Myc overexpression also triggers the ARF-p53 apoptotic program, which is activated when MEFs undergo replicative crises following culture ex vivo. The rare immortal variants that arise from these cultures generally suffer mutations in p53 or delete Ink4a/ARF, and Myc greatly increases the frequency of these events. Alternative reading frame (ARF)- and p53-null MEFs have increased telomerase activity, as do variant immortal clones that bypass replicative crisis. Similarly, immortal murine NIH-3T3 fibroblasts and myeloid 32D.3 and FDC-P1.2 cells do not express ARF and have robust telomerase activity. However, Myc overexpression in these immortal cells results in remarkably discordant regulation of TERT and telomerase activity. Furthermore, in MEFs and 32D.3 cells TERT expression and telomerase activity are regulated independently of endogenous c-Myc. Thus, the regulation of TERT and telomerase activity is complex and is also regulated by factors other than Myc, ARF, or p53.

Functional analysis of human replication protein A in nucleotide excision repair.

Human replication protein A (RPA) is a three-subunit protein complex (70-, 34-, and 11-kDa subunits) involved in DNA replication, repair, and recombination. Both the 70- (p70) and 34-kDa (p34) subunits interact with Xeroderma pigmentosum group A complementing protein (XPA), a key protein involved in nucleotide excision repair. Our deletion analysis indicated that no particular domain(s) of RPA p70 was essential for its interaction with XPA, whereas 33 amino acids from the C terminus of p34 (p34Delta33C) were necessary for the XPA interaction. Furthermore, mutant RPA lacking the p34 C terminus failed to interact with XPA, suggesting that p34, not p70, is primarily responsible for the interaction of RPA with XPA. RPA stimulated the interaction of XPA with UV-damaged DNA through an RPA-XPA complex on damaged DNA sites because (i) the RPA mutant lacking the C terminus of p34 failed to stimulate an XPA-DNA interaction, and (ii) the ssDNA binding domain of RPA (amino acids 296-458) was necessary for the stimulation of the XPA-DNA interaction. Two separate domains of p70, a single-stranded DNA binding domain and a zinc-finger domain, were necessary for RPA function in nucleotide excision repair. The mutant RPA (RPA:p34Delta33C), which lacks its stimulatory effect on the XPA-DNA interaction, also poorly supported nucleotide excision repair, suggesting that the XPA-RPA interaction on damaged DNA is necessary for DNA repair activity.

In vitro analysis of UV-damage-induced inhibition of replication.

We investigated DNA-damage-induced inhibition of replication by using an in vitro system, with which both replication and repair can be examined simultaneously. The system contains non-irradiated simian virus 40 (SV40) origin-containing DNA, UV-irradiated circular duplex DNA lacking an SV40 origin, and cell extracts that support both replication and repair activities. Using this system, we show that replication is significantly inhibited in the presence of UV-irradiated, but not non-irradiated, DNA and, to a lesser extent, repair activity is also inhibited by the presence of replication activity. In contrast, replication activity was not affected by UV-damaged DNA when the reactions were carried out with purified replication proteins, suggesting that protein factor(s) in the cell extracts are involved in the inhibition of replication that is triggered by DNA damage. Inhibition was efficiently reversed by the combined actions of proteins involved in both repair and replication, suggesting that the inhibition of replication observed in our system may be caused by the recruitment of replication proteins to damaged DNA sites.

Human xeroderma pigmentosum group A protein interacts with human replication protein A and inhibits DNA replication.

Human replication protein A (RPA; also known as human single-stranded DNA binding protein, or HSSB) is a multisubunit complex involved in both DNA replication and repair. While the role of RPA in replication has been well studied, its function in repair is less clear, although it is known to be involved in the early stages of the repair process. We found that RPA interacts with xeroderma pigmentosum group A complementing protein (XPAC), a protein that specifically recognizes UV-damaged DNA. We examined the effect of this XPAC-RPA interaction on in vitro simian virus 40 (SV40) DNA replication catalyzed by the monopolymerase system. XPAC inhibited SV40 DNA replication in vitro, and this inhibition was reversed by the addition of RPA but not by the addition of DNA polymerase alpha-primase complex, SV40 large tumor antigen, or topoisomerase I. This inhibition did not result from an interaction between XPAC and single-stranded DNA (ssDNA), or from competition between RPA and XPAC for DNA binding, because XPAC does not show any ssDNA binding activity and, in fact, stimulates RPA's ssDNA binding activity. Furthermore, XPAC inhibited DNA polymerase alpha activity in the presence of RPA but not in RPA's absence. These results suggest that the inhibitory effect of XPAC on DNA replication probably occurs through its interaction with RPA.

Genes of the linear mitochondrial DNA of Williopsis mrakii: coding sequences for a maturase-like protein, a ribosomal protein VAR1 homologue, cytochrome oxidase subunit 2 and methionyl tRNA.

The mitochondrial DNA (mtDNA) in some yeasts has a linear structure with inverted terminal repeats closed by a single-stranded loop. These mtDNAs have generally a constant gene order, beginning with a small ribosomal RNA gene at the right end and terminating with a cytochrome oxidase subunit 2 gene (COX2) at the left end, independently of the wide variation in genome size. In the mtDNAs from several species of the genus Williopsis, we found an additional open reading frame, ORF1, which was homologous to the Saccharomyces cerevisiae RF1 gene encoding a group I intron maturase-like protein. ORF1 genes from W. mrakii and W. suaveolens were mapped and sequenced. Next to ORF1, COX2 and methionyl tRNA genes were present on the opposite strand. The same relative positions of genes in the mtDNAs so far examined suggests that the constancy of gene order is generally conserved also at the level of individual tRNA genes. We identified another open reading frame, ORF2, in W. mrakii mtDNA. It was mapped next to the cytochrome oxidase subunit 3 gene. Rich in adenine-thymine bases, ORF2 appears to be a homologue of the VAR1 gene which codes for a small ribosomal subunit protein in S. cerevisiae mitochondria. Nucleotide sequences data have been deposited in the EmBL data library under the following Accession Numbers: X66594 (Apocytochrome b and ORF2 genes of W. mrakii), X66595 (ORF1, tRNA-Met and COX2 genes of W. mrakii), X73415 (tRNA-Met and COX2 genes of W. suaveolens), X73416 (ORF1 gene of W. suaveolens) and X73414 (tRNA-Met and COX2 genes of P. jadinii).

Linear mitochondrial DNAs of yeasts: frequency of occurrence and general features.

In most yeast species, the mitochondrial DNA (mtDNA) has been reported to be a circular molecule. However, two cases of linear mtDNA with specific termini have previously been described. We examined the frequency of occurrence of linear forms of mtDNA among yeasts by pulsed-field gel electrophoresis. Among the 58 species from the genera Pichia and Williopsis that we examined, linear mtDNA was found with unexpectedly high frequency. Thirteen species contained a linear mtDNA, as confirmed by restriction mapping, and labeling, and electron microscopy. The mtDNAs from Pichia pijperi, Williopsis mrakii, and P. jadinii were studied in detail. In each case, the left and right terminal fragments shared homologous sequences. Between the terminal repeats, the order of mitochondrial genes was the same in all of the linear mtDNAs examined, despite a large variation of the genome size. This constancy of gene order is in contrast with the great variation of gene arrangement in circular mitochondrial genomes of yeasts. The coding sequences determined on several genes were highly homologous to those of the circular mtDNAs, suggesting that these two forms of mtDNA are not of distant origins.

Linear mitochondrial DNAs of yeasts: closed-loop structure of the termini and possible linear-circular conversion mechanisms.

The terminal structure of the linear mitochondrial DNA (mtDNA) from three yeast species has been examined. By enzymatic digestion, alkali denaturation, and sequencing of cloned termini, it was shown that in Pichia pijperi and P. jadinii, both termini of the linear mtDNA were made of a single-stranded loop covalently joining the two strands, as in the case of vaccinia virus DNA. The left and right loop sequences were in either of two orientations, suggesting the existence of a flip-flop inversion mechanism. Contiguous to the terminal loops, inverted terminal repeats were present. The mtDNA from Williopsis mrakii seems to have an analogous structure, although terminal loops could not be directly demonstrated. Electron microscopy revealed the presence, among linear molecules, of a small number of circular DNAs, mostly of monomer length. Linear and circular models of replication are considered, and possible conversion mechanisms between linear and circular forms are discussed. A flip-flop inversion mechanism between the inverted repeat sequences within a circular intermediate may be involved in the generation of the linear form of mtDNA.

Characterization of enkephalins and related peptides in rat hypophysial portal blood.

Rat hypophysial portal blood, collected from the pituitary stalk, was extracted and enkephalins were assayed by different RIA. Met-Enk-IR and Leu-Enk-IR levels were 1635 +/- 470 pg/ml and 125 +/- 50 pg/ml, respectively. Using HPLC characterization, the presence in portal blood of Met-Enk, Leu-Enk, proenkephalins fragments and dynorphin1-17 has been demonstrated. An unidentified Met-Enk-IR peptide has also been found.

Adrenal medullary opiate receptors. Pharmacological characterization in bovine adrenal medulla and a human pheochromocytoma.

We have characterized the opiate binding sites on the membranes of bovine adrenal medulla and human pheochromocytoma, using 3H-labeled D-Ala2-D-Leu5-enkephalin ( [3H]DADLE), [3H]etorphine, and [3H]ethylketocyclazocine ( [3H]EKC). Binding was stereoselective in both membrane preparations. Association and dissociation kinetics showed that steady state was achieved after 20-25 min of incubation at 37 degrees. Saturation experiments were performed in the absence or in the presence of morphiceptin (1 microM), which masks the mu sites, D-Ser2-Leu-enkephalin-Thr6 (100 nM), which masks delta sites, or DADLE (5 microM), which was found to mask the delta, mu, and benzomorphan receptor. Taking into consideration the affinities of the three radioligands used (DADLE identifying the delta and mu sites when used in the nanomolar range; etorphine identifying the delta, mu, and benzomorphan sites; EKC identifying the delta, mu, kappa, and benzomorphan receptors) we have characterized pharmacologically the opiate sites present on bovine and human membranes. Human pheochromocytoma membranes contained (a) mu binding sites (15 fmoles/mg of protein, KD [3H]etorphine 1.0 nM, [3H]EKC 5.4 nM, [3H]DADLE 5.6 nM); (b) kappa sites (41 fmoles/mg of protein, KD [3H]EKC 1.0 nM); (c) benzomorphan sites (115 fmoles/mg of protein, KD [3H]etorphine and [3H]EKC 1.0 nM). On bovine membranes we have detected (a) delta binding sites (10 fmoles/mg of protein, KD [3H]DADLE 0.7 nM); (b) mu sites (24 fmoles/mg of protein, KD [3H]DADLE 2.9 nM, [3H]etorphine 0.2 nM, [3H]EKC 3.4 nM); (c) kappa sites (12 fmoles/mg of protein, KD [3H]EKC 0.4 nM); (d) benzomorphan sites (80 fmoles/mg of protein, KD [3H]etorphine 0.2 nM, [3H]EKC 1.3 nM); (e) a residual high-affinity (20 fmoles/mg of protein, KD 0.2 nM) site identified by [3H]etorphine in the presence of 5 microM DADLE. The relative proportions of benzomorphan sites were equal in both tissues (65% of the high-affinity sites) whereas kappa receptors were more abundant on human membranes (25%) than on bovine membranes (9% of the high-affinity sites).

Opiate binding sites spectrum on bovine adrenal medullas and six human pheochromocytomas.

Opiate binding sites have been characterized on membranes from bovine adrenal medullas and six human pheochromocytomas. In human tumors, large variations in site distribution were observed. Kappa and benzomorphan sites represented the majority of the sites detected. The heterogeneity of the opiate sites on these tissues could explain the observed differences in the pharmacological responses to opiates of cultured cells from these tissues. Furthermore, adrenal medullas could be a good model for the study of the kappa site action at the cellular level.