The nuclear matrix continues DNA synthesis at in vivo replicational forks.

Alkaline cesium chloride gradient analysis of in vivo [3H]bromodeoxyuridine-labeled and in vitro [alpha-32P]dCTP-labeled DNA was used to determine whether in vitro DNA synthesis in regenerating rat liver nuclei and nuclear matrices continued from sites of replication initiated in vivo. At least 70 and 50% of the products of total nuclear and matrix-bound in vitro DNA synthesis, respectively, were continuations of in vivo initiated replicational forks. The relationship of the in vitro DNA synthetic sites in total nuclei versus the nuclear matrix was examined by using [3H]bromodeoxyuridine triphosphate to density label in vitro synthesized DNA in isolated nuclei and [alpha-32P]dCTP to label DNA synthesized in isolated nuclear matrix. A minimum of about 40% of matrix-bound DNA synthesis continued from sites being used in vitro by isolated nuclei. Furthermore, nuclear matrices prepared from in vitro labeled nuclei were 5-fold enriched in DNA synthesized by the nuclei and were several-fold enriched, compared to total nuclear DNA, in a particularly high density labeled population of DNA molecules.

Enhanced proliferation and differentiation of human articular chondrocytes when seeded at low cell densities in alginate in vitro.

Dedifferentiated human articular chondrocytes exhibited a wide variation in their capacity to proliferate and redifferentiate in an alginate suspension culture system. The greatest extent of proliferation and redifferentiation was seen to be dependent on the formation of clonal populations of chondrocytes and correlated inversely with the initial cell seeding density. Redifferentiating chondrocytes seeded at low density (1 x 10(4) cells/ml alginate) compared with chondrocytes that were seeded at high density (1 x 10(6) cells/ml alginate) showed a nearly 3-fold higher median increase in cell number. a 19-fold greater level of type-II collagen mRNA expression, a 4-fold greater level of aggrecan mRNA expression, and a 6-fold greater level of sulfated glycosaminoglycan deposition at 4 weeks of culture. Matrix molecules from low-density cultures were assembled into chondrocyte-encapsulated, spherical extracellular matrices that were readily visualized in sections from 12-week cultures stained with antibodies against types I and II collagen and aggrecan. Ultrastructural analysis of 12-week low-density cultures confirmed the presence of thin collagen fibrils throughout the matrix.

Identification of 100 and 150 S DNA polymerase alpha-primase megacomplexes solubilized from the nuclear matrix of regenerating rat liver.

The majority of DNA polymerase alpha and primase activities bound to the nuclear matrix of regenerating rat liver were released into an extract by a mild sonication procedure. During maximal in vivo replication (22-h posthepatectomy) most of the solubilized alpha-polymerase and primase cosedimented at approximately 100 and 150 S as discrete megacomplexes with smaller amounts at 10 and 17 S. In contrast, high salt extracts obtained during nuclear matrix isolation as well as matrix extracts prepared just before the onset of in vivo replication (14-h posthepatectomy) were completely devoid of megacomplexes. In vitro incubation of the matrix extracts resulted in rapid dissolution of the megacomplexes to the 10 and 17 S forms. These relationships lead us to propose a dynamic assembly of the eucaryotic replisome which is initiated pre-replicatively as 10 and 17 S complexes and functionally expressed during in vivo replication as 100 and 150 S megacomplexes or "clustersomes."

Nuclear matrix-bound DNA primase. Elucidation of an RNA priming system in nuclear matrix isolated from regenerating rat liver.

Recent findings in purified systems demonstrate the universality of DNA polymerase-primase complexes which may function in the priming and continuation of eucaryotic DNA replication. In this report we characterize an in vitro, nuclear matrix-associated, priming and continuation system that can utilize either endogenous matrix-bound DNA or exogenous single-stranded DNA as template. 30-40% of total nuclear DNA primase activity was recovered in association with the isolated nuclear matrix fraction from regenerating rat liver. Matrix-bound primase catalyzed the alpha-amanitin, actinomycin D-resistant synthesis of oligonucleotide chains of 8-50 nucleotides on the endogenous template. At least a portion of the RNA primers were continued by DNA polymerase alpha with deoxynucleoside triphosphate incorporation up to 300-600 nucleotides. Nearest neighbor analysis revealed ribodeoxynucleotide covalent linkages in these RNA-DNA chains. The matrix-bound primase preferred single-stranded fd DNA as exogenous template over synthetic homopolymers and was strictly dependent on the presence of ribonucleoside triphosphates. Appropriate subfractionation revealed that the matrix-bound primase activity is exclusively localized in the nuclear matrix interior. The ability of primase and DNA polymerase to synthesize covalently linked RNA-DNA products demonstrates the potentially useful role of the nuclear matrix in vitro system for elucidating the organizational and functional properties of the eucaryotic replication apparatus in the cell nucleus.

Pre-replicative association of multiple replicative enzyme activities with the nuclear matrix during rat liver regeneration.

As a step toward the molecular elucidation of the putative replicational apparatus associated with the nuclear matrix, we have investigated the possible matrix association of several replicational related enzymes. In addition to the previously identified DNA polymerase alpha, DNA primase, 3'-5' exonuclease, RNase H, and DNA methylase were all recovered at significant levels (20-30% of total nuclear activity) in nuclear matrix isolated from regenerating rat liver during maximal in vivo replication (22 h post-hepatectomy). In contrast, DNA ligase was not detected on the nuclear matrix even though significant activity was present in isolated nuclei. Examination of the replicative dependency of these enzyme activities following partial hepatectomy revealed pre-replicative elevations which were distinct for each matrix-bound enzyme. A second late-replicative peak in DNA methylase is consistent with a role of this matrix-bound enzyme in the maintenance of the inheritable methylation pattern. Mild sonication resulted in a significant release of all of these activities except RNase H. A major portion of the matrix-solubilized DNA polymerase alpha, DNA primase, 3'-5' exonuclease, and DNA methylase activities cosedimented on sucrose gradients between approximately 8-12 S. Our results are consistent with the organization of at least a portion of these replicative enzymes into nuclear matrix-bound replicational complexes. We also propose a novel pre-replicative assembly model of the matrix-bound replicational apparatus in which DNA primase plays an initial and critical role.

Normal human mesothelial cells and fibroblasts transfected with the EJras oncogene become EGF-independent, but are not malignantly transformed.

The nature of the lesion in growth control exerted by the cancer-derived c-H-ras mutation, EJ-ras, and its transforming potential in diploid cells are both poorly understood. We introduced EJ-ras into normal, diploid human mesothelial cells and fibroblasts and obtained transfectants expressing p21EJ-ras. All clones examined were independent of EGF for rapid growth, and all secreted an EGF-like mitogen into the medium at levels sufficient to satisfy the EGF requirement of normal cells. The EJ-ras transfectants were not altered with respect to any other growth requirement, and they were not transformed. Eleven clones tested all retained a finite replicative lifespan which, in most cases, was the same as that of the parent cell strain. Three transfectants tested were not tumorigenic in nude mice. Thus p21EJ-ras can circumvent an important mitogenic signal pathway in human cells. Nevertheless, neither the secretion of an autocrine growth factor nor any other effect of p21EJ-ras serves to malignantly transform normal human cells, in contrast to the susceptibility of some established rodent cell lines to transformation by these mechanisms.

Enhanced processivity of nuclear matrix bound DNA polymerase alpha from regenerating rat liver.

Translocation of DNA during in vitro DNA synthesis on nuclear matrix bound replicational assemblies from regenerating rat liver was determined by measuring the processivity (average number of nucleotides added following one productive binding event of the polymerase to the DNA template) of nuclear matrix bound DNA polymerase alpha with poly(dT).oligo(A)10 as template primer. The matrix-bound polymerase had an average processivity (28.4 nucleotides) that was severalfold higher than the bulk nuclear DNA polymerase alpha activity extracted during nuclear matrix preparation (8.9 nucleotides). ATP at 1 mM markedly enhanced the activity and processivity of the matrix-bound polymerase but not the corresponding salt-soluble enzyme. The majority of the ATP-dependent activity and processivity enhancement was completed by 100 microM ATP and included products ranging up to full template length (1000-1200 nucleotides). Average processivity of the net ATP-stimulated polymerase activity exceeded 80 nucleotides with virtually all the DNA products greater than 50 nucleotides. Release of nuclear matrix bound DNA polymerase alpha by sonication resulted in a loss of ATP stimulation of activity and a corresponding decrease in processivity to a level similar to that of the salt-soluble polymerase (6.8 nucleotides). All nucleoside di- and triphosphates were as effective as ATP. Stimulation of both activity and processivity by the nonhydrolyzable ATP analogues adenosine 5'-O-(3-thiotriphosphate), 5'-adenylyl imidodiphosphate, and adenosine 5'-O-(1-thiotriphosphate) further suggested that the hydrolysis of ATP is not required for enhancement to occur.(ABSTRACT TRUNCATED AT 250 WORDS)