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L J Hansen

Publications and source records attributed to L J Hansen.

29 records · Page 2Linked to original sources
PubMed1990

Integration specificity of retrotransposons and retroviruses.

Analysis of in vivo integration patterns has provided no data to support the notion that retroelement integration is random. Rather, the diversity of insertion patterns of retroelements suggests numerous ways in which genomic DNA is identified for preferential targeting. These range from specific to general and include sequence content, removal of chromatin proteins, nuclear localization, distinctive topology, and association with particular trans-acting factors. Many are similar to mechanisms already demonstrated to affect activities of previously described recombinases. Moreover, such proposed targeting mechanisms could act directly or indirectly to influence integration site selection. A variety of observations are consistent with the preferential use of open chromatin for retroelement insertion. The site-specific retroelements insert into transcribed regions. In vitro studies with retroviruses and Ty1 have shown that naked DNA can function, at least under some conditions, as a target. The association of integration sites of retroviruses and regions in which DNase I hypersensitive sites exist and the preferential integration of Ty1 at the 5' ends of some genes might suggest that regions which do not have phased nucleosomes are targets for integration. Is targeting to transcriptionally active regions essentially passive, because they are not densely associated with chromatin proteins, or is targeting active in the sense of being a more specific process? Specific targets could be generated from DNA or protein motifs. Nucleosome-free regions are associated with a variety of nonnucleosome proteins, including topoisomerases, nuclear matrix proteins, transcription factors, or replication proteins. These then are candidates for proteins which target integration directly, by associating with the transposition complex or, indirectly, by inducing changes in the DNA. Polymerase III-transcribed genes, which are relatively defined targets of integration for some retrotransposon systems, probably exemplify several of these mechanisms. Promoter sequences may be directly involved in targeting some elements and positioning of the transcription complex may fix the integration sites of others. The most common sequence feature of characterized in vivo insertion sites is that they are AT-rich. This may reflect specificity of the IN protein, particularly the gypsylike elements, increased nicking of DNA, which is relatively weakly base-paired, as appears to be the case for the FLP recombinase (130), or simply the AT content of accessible regions in chromatin. Some of these questions will be resolved by the characterization of in vitro integration sites that have been recovered by physical means, rather than by biological assay. The insertion patterns of a number of retroposons suggest that retroelements can insert with a high degree of sequence specificity.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Progression through the cell cycle: an overview.

Tissues in adults can be maintained at constant mass or they can increase or decrease in size because of imbalances of synthetic and degradative processes acting at the cellular and molecular levels. Some size changes are caused by physiologic conditions to which the tissue must adjust. Alternatively, the balance may be distorted in favor of net tissue increase in pathologic situations such as cancer. Strict regulatory mechanisms are required to keep proliferation responsive to the organism's needs; these mechanisms may be defective in disease. Net tissue proliferation requires repeated rounds of cell duplication in excess of that necessary to counterbalance cell death. Duplication of a cell requires a net doubling of its every molecule and structure. The myriad of molecular events required for cell proliferation such as DNA duplication and its partitioning at mitosis are tightly regulated in normal cells. One may conceive of two classes of molecules: those required for "housekeeping," which constitute the cell's structural and functional machinery, and those such as growth factors, their receptors, and second messengers involved in signal transduction responsible for regulating the activities of the housekeeping molecules. These molecular events and the cascade of processes that control them can be organized within the sequence of the cell cycle. In this brief overview, we illustrate these issues with a few examples taken from very recent discoveries of novel proteins that appear to have major regulatory roles. Most of these results have been obtained with mammalian fibroblasts, but some have originated with discoveries made using two very different yeasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Ty3, a yeast retrotransposon associated with tRNA genes, has homology to animal retroviruses.

Ty3, a retrotransposon of Saccharomyces cerevisiae, is found within 20 base pairs (bp) of the 5' ends of different tRNA genes. Determination of the complete nucleotide sequence of one Ty3 retrotransposon (Ty3-2) shows that the element is composed of an internal domain 4,748 bp long flanked by long terminal repeats of the 340-bp sigma element. Three open reading frames (ORFs) longer than 100 codons are present in the sense strand. The first ORF, TYA3, encodes a protein with a motif found in the nucleic acid-binding protein of retroviruses. The second ORF, TYB3, has homology to retroviral pol genes. The deduced amino acid sequence of the reverse transcriptase domain shows the greatest similarity to Drosophila retrotransposon 17.6, with 43% identical residues. The inferred order of functional domains within TYB3--protease, reverse transcriptase, and endonuclease--resembles the order in Drosophila element 17.6 and in animal retroviruses but is different from that found in yeast elements Ty1 and Ty2. A second Ty3 element (Ty3-1) from a standard laboratory strain was overexpressed and shown to transpose.

Amino Acid Sequence↗
PubMed1987

Inhibitor of translational initiation in sea urchin eggs prevents mRNA utilization.

Actively reinitiating cell-free translation systems from sea urchin eggs and embryos have been developed. The extracts retain the overall differences in protein synthetic activity observed in intact eggs and embryos. The effect of combining extracts from eggs and embryos suggests the presence of a dominant inhibitor of translation in the egg. This inhibitor also prevents the initiation of translation in a cell-free system from rabbit reticulocytes. In the reticulocyte system, the egg inhibitor causes the accumulation of 48 S preinitiation complexes, as measured by the accumulation of initiator tRNA and globin mRNA on the small ribosomal subunit. Accumulation of this uncommon intermediate suggests either that the inhibitor prevents the binding of the 60 S ribosomal subunit, or that it prevents migration of the 40 S subunit from the 5' end of mRNA to the first AUG.

Animals↗
PubMed1987

Inhibitor of eukaryotic initiation factor 4F activity in unfertilized sea urchin eggs.

Extracts from unfertilized sea urchin eggs contain an inhibitor of translation that inhibits protein synthesis in cell-free translation systems from sea urchin embryos or rabbit reticulocytes. The inhibitory effects of egg extracts can be reversed by the addition of mammalian eukaryotic initiation factor 4F (eIF-4F) in both sea urchin embryo and reticulocyte systems, suggesting that the inhibitor inactivates this initiation factor. The accumulated data suggest that the ability of eIF-4F to recycle may be compromised. The addition of eIF-4F to cell-free translation systems from unfertilized sea urchin eggs also stimulates protein synthesis. However, the stimulation does not increase protein synthetic activity in the egg cell-free translation system to the levels observed in those produced from 2-hr embryos. This suggests that, although the unfertilized egg contains an inhibitor of eIF-4F and reduced levels of eIF-4F activity, inactivation of this component is only one of the factors involved in the low rate of maternal mRNA utilization found prior to fertilization.

Animals↗
PubMed1985

Demonstration of kallikrein in a rat pancreatic acinar cell carcinoma.

Kallikrein was identified immunohistochemically and biochemically in a transplantable pancreatic acinar cell carcinoma of the rat. The concentration of immunoreactive kallikrein in tumor homogenates was the same as in the pancreas. Kallikrein in tumor cells exists as a proenzyme and is released into blood in high concentrations. The impact of the presence of a kallikrein-producing tumor on other kallikrein-containing organs and other possibly interrelated systems was investigated. The concentration of kallikrein in the submandibular gland and pancreas of host rats was not significantly different from that of control rats. Urinary kallikrein secretion was significantly increased, although this may be a result of the high plasma glandular kallikrein concentration combined with kidney damage. The plasma concentration of kininogen, kininase, and renin was not significantly different from control rats. Rats with tumor had significantly lower blood pressure than did control animals, and blood pressure was inversely related to the concentration of glandular kallikrein in plasma. However, it was not proven that the low blood pressure was due to the high concentration of kallikrein. Nephrectomized tumor rats gave a smaller hypotensive response to kininase inhibition than was expected from their high concentration of circulating kallikrein. This may be explained by the absence of the "free kallikrein" fraction in plasma of host rats.

Amylases↗
PubMed1984

Differentiation of pancreatic acinar carcinoma cells cultured on rat testicular seminiferous tubular basement membranes.

The use of rat testicular seminiferous tubular basement membrane (STBM) segments as a model substratum for the in vitro maintenance of tumor cells dissociated from a transplantable pancreatic acinar rat carcinoma (J. K. Reddy and M. S. Rao, Science (Wash. DC), 198: 78-80, 1977) is described. Ultrastructurally pure, hollow tubular segments of STBM were prepared by mechanical disaggregation, DNase digestion, and deoxycholate treatment. Dissociated pancreatic acinar carcinoma cells adhered readily to STBM segments within 1 to 6 hr, and these STBM-tumor cell aggregates were maintained for up to 7 days in serum-free chemically defined medium supplemented with hydrocortisone, insulin, vitamin C, and soybean trypsin inhibitor. The tumor cells formed acinar-like clusters and displayed intercellular junctions and polarization of secretory granules toward the center of these clusters. By 4 days, virtually all cells of this acinar carcinoma maintained on STBM in supplemented chemically defined medium contained numerous secretory granules. Cell replication, as determined by [3H]thymidine autoradiography, ceased within 18 hr of attachment of neoplastic cells to STBM; however, all cells incorporated [3H]leucine as evidenced by light and electron microscopic autoradiography. In addition, two-dimensional analysis and fluorography of newly synthesized secretory proteins discharged by these cells in response to carbamylcholine revealed the presence of Mr 24,000 protein and 19 other secretory proteins characteristic of this tumor (L. J. Hansen, M. K. Reddy, and J. K. Reddy, Proc. Natl. Acad. Sci. USA, 80: 4379-4383, 1983). The culture system utilizing STBM and supplemented chemically defined medium should allow investigation of the effects of a variety of factors on morphogenesis, cytodifferentiation, and gene expression in pancreatic acinar tumors.

Animals↗
PubMed1983

Comparison of secretory protein and membrane composition of secretory granules isolated from normal and neoplastic pancreatic acinar cells of rats.

The diversity of cytodifferentiation in a transplantable rat pancreatic acinar carcinoma provides a biological model system for the study of regulatory molecular events that differ from those in normal acinar cells. Secretory (zymogen) granule proteins and granule membranes of neoplastic and normal pancreatic acinar cells were compared to determine the differences in gene expression between apparently well-differentiated secretory granule-containing neoplastic cells and normal cells. Nineteen proteins observed in two-dimensional polyacrylamide gels of normal secretory granule extracts were seen also in tumor granule extract profiles, with reduced but detectable amounts of lipase and four basic proteins. In addition, tumor granule extracts contained a new protein of Mr 24,000, designated p24, which was not detectable in normal extracts. Neoplastic granule membranes, while having phospholipid composition similar to that of normal membranes, lacked or contained a greatly reduced amount of a major glycoprotein of Mr 80,000 and three other proteins of Mrs 50,000, 37,000, and 36,000. The nature of p24 protein in secretory granule extracts and the significance of the reduction or absence of Mr 80,000 membrane glycoprotein in this tumor remain to be elucidated.

Animals↗
PubMed1981

Immunohistochemical localization of pancreatic exocrine enzymes in normal and neoplastic pancreatic acinar epithelium of rat.

The transplantable pancreatic acinar carcinomas of rat established recently provide useful model systems to examine the composition of secretory proteins as well as the secretory process in transformed pancreatic exocrine epithelium. The neoplastic acinar cells exhibit considerable variation in the extent of cytodifferentiation. In the present study the enzymatic profile of this heterogeneous tumor cell population has been investigated by the indirect immunofluorescent technique using antibodies against six pancreatic enzymes. By immunofluorescence, all neoplastic cells stained positively for the six enzymes tested: amylase, lipase, carboxypeptidase A, chymotrypsinogen, trypsinogen, and ribonuclease. Some variability in the intensity of immunofluorescence was noted, suggesting possible quantitative differences in the content of a given enzyme among tumor cells. These observations suggest that neoplastic acinar cells with or without secretory granules contain secretory proteins, but to a variable extent.

Amylases↗
PubMed1980

Secretion granules of transplantable pancreatic acinar carcinoma of rat.

Secretion granules of the rat transplantable pancreatic acinar carcinoma and of normal rat pancreas were isolated by differential centrifugation. Analysis of the granule content by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and isoelectric focusing procedures combined with specific enzyme assays indicated essential qualitative similarities between normal and neoplastic secretory proteins, suggesting retention of enzymic differentiation in this pancreatic acinar carcinoma. Accordingly, this epithelial tumor should serve as an important model for examination of regulatory mechanisms in cell differentiation and neoplasia.

Animals↗