Characterization of DNA polymerase alpha from untransformed and pSV3.neo-transformed human fibroblasts

Replicative enzymes, DNA polymerase alpha (pol alpha), and in vitro ageing

Normal cells in culture are used to investigate the underlying mechanisms of DNA synthesis because they retain regulatory characteristics of the in vivo replication machinery. During the last few years new studies have identified a number of genetic changes that occur during in vitro ageing, providing insight into the progressive decline in biological function that occurs during ageing. Maintaining genomic integrity in eukaryotic organisms requires precisely coordinated replication of the genome during mitosis, which is the most fundamental aspect of living cells. To achieve this coordinated replication, eukaryotic cells employ an ordered series of steps to form several key protein assemblies at origins of replication. Major progress has recently been made in identifying the enzymes, and other proteins, of DNA replication that are recruited to origin sites and the order in which they are recruited during the process of replication. More than 20 proteins, including DNA polymerases, have been identified as essential components that must be preassembled at replication origins for the initiation of DNA synthesis. Of the polymerases, DNA polymerase alpha-primase (pol alpha) is of particular importance since its function is fundamental to understanding the initiation mechanism of eukaryotic DNA replication. DNA must be replicated with high fidelity to ensure the accurate transfer of genetic information to progeny cells, and decreases in DNA pol alpha activity and fidelity, which are coordinated with cell cycle progression, have been shown to be important facets of a probable intrinsic cause of genetic alterations during in vitro ageing. This has led to the proposal that pol alpha activity and function is one of the crucial determinants in ageing. In this review we summarize the current state of knowledge of DNA pol alpha function in the regulation of DNA replication and focus in particular on its interactive tasks with other proteins during in vitro ageing.

Generation and partial characterization of a transformed cetacean cell line

A primary epithelial cell line, DK1, established from renal tissue of a spontaneously aborted female Atlantic bottlenose dolphin was transfected with linearized pSV3.neo, an SV40 virus-derived plasmid encoding large tumor antigen (Tag). Transfected cells were grown in cetacean culture medium supplemented with 400 microg/ml geneticin (G418), and individual clones were selected using cloning rings. DKN1 was the first clone to be evaluated for future research use, and has been continuously cultured for 8 years. Intracellular cytokeratin and the expression of Tag were determined in DKN1, and cell growth was evaluated under different concentrations of l-glutamine, glutathione, and N-acetylcysteine. DKN1 cells did not require high levels of l-glutamine as previously reported for cetacean cells, and addition of antioxidants at the concentrations used in this study (2.0mM) decreased the rate of cell division. These data suggest strongly that these immortalized bottlenose dolphin epithelial cells have different levels of, and requirements for, glutathione than would be considered normal for terrestrial mammalian cells, do not require high levels of l-glutamine as previously suggested for dolphin cells, and exhibit decreased levels of cell growth and viability in high levels of the antioxidant GSH and its precursor, NAC.

Replicative enzymes and ageing: importance of DNA polymerase alpha function to the events of cellular ageing

A hallmark of cellular ageing is the failure of senescing cells to initiate DNA synthesis and transition from G1 into S phase of the cell cycle. This transition is normally dependent on or concomitant with expression of a set of genes specifying cellular proteins, some of which directly participate in DNA replication. Deregulation of this gene expression may play a pivotal role in the ageing process. The number of known enzymes and co-factors required to maintain integrity of the genome during eukaryotic DNA replication has increased significantly in the past few years, and includes proteins essential for DNA replication and repair, as well as for cell cycle regulation. In eukaryotic cells, ranging from yeast to man, a replicative enzyme essential for initiation of transcription is DNA polymerase alpha (pol alpha), the activity of which is coordinately regulated with the initiation of DNA synthesis. DNA pol alpha, by means of its primase subunit, has the unique ability to initiate de novo DNA synthesis, and as a consequence, is required for the initiation of continuous (leading-strand) DNA synthesis at an origin of replication, as well as for initiation of discontinuous (lagging-strand) DNA synthesis. The dual role of the pol alpha-primase complex makes it a potential interactant with the regulatory mechanisms controlling entry into S phase. The purpose of this review is to address the regulation and/or modulation of DNA pol alpha during ageing that may play a key role in the cascade of events which ultimately leads to the failure of old cells to enter or complete S phase of the cell cycle.

An accessory protein of DNA polymerase alpha declines in function with increasing age

Isoforms of DNA polymerase alpha (pol alpha/primase; pol alpha) were isolated from the livers of C57BL/6 mice either 3 months old (young) or 13 months old (mature). The 13-month-old mice were from two groups, one in which food was available ad libitum (AL), and one in which calories had been restricted to 60% of the AL intake (CR). The polymerases from young vs. mature and CR vs. AL mice differed in total and specific pol alpha activity, with the highest values exhibited by enzymes from 3-month-old mice. A more active isoform of pol alpha was typically isolated from CR animals than from AL animals. Differences in charge were used to chromatographically separate pol alpha into elution peaks exhibiting differing degrees of enzyme activity. DNA pol alpha isolated from tissues of mature mice exhibited a decline in activity which was not associated with decreased recoverable levels of pol alpha protein, but with a decline in the tendency of pol alpha to co-purify with an accessory protein, alpha AP, that binds double-stranded DNA (dsDNA). Low activity pol alpha isoforms which did not co-purify with alpha AP were stimulated by interaction with exogenous alpha AP. Pol alpha isoforms which co-purified with the dsDNA-binding accessory protein exhibited higher specific activity and less enhancement of activity upon interaction with exogenous alpha AP. Calorie restricted animals exhibited a pol alpha isoform that was more like pol alpha from younger animals in that it typically copurified with alpha AP, the DNA-binding accessory protein.

Differential expression of DNA polymerase alpha in normal and transformed human fibroblasts

The expression of DNA polymerase alpha (pol alpha) was studied in human fibroblast lines W138 (fetal lung) and GM3529 (skin, established from a 66 yr old donor), and their Simian virus 40 (SV40) large tumor antigen (TAg)-transformed corollaries, 2RA and 2-1 respectively. Both SV40-transformed and pSV3.neo (SV40-derived plasmid)-transformed cells express TAg, a virally encoded protein not expressed by the normal parent cell lines. Northern blot hybridization studies showed increased recovery of pol alpha mRNA from transformed cells compared with normal cells. This increase was correlated with increased pol alpha mRNA transcription as determined by nuclear run-on assays. Northern blot analyses also showed an increase in the instability of translationally active pol alpha mRNA in transformed cells. The results suggest that TAg, in addition to its dsDNA binding, pol alpha binding, retinoblastoma protein binding and helicase activities, may be involved either directly or indirectly in regulation of the steady state mRNA levels of pol alpha at the transcriptional level in both fetal and aged donor-derived transformed fibroblasts.

An accessory protein enhances both DNA binding and activity of DNA polymerase alpha isolated from normal, but not transformed, human fibroblasts

DNA polymerase alpha/primase (pol alpha) isolated from fibroblasts established from a 66-year-old human donor (GM3529) exhibited decreased specific activity compared with pol alpha from either fetal-derived fibroblasts (WI38), or pSV3.neo-transformed GM3529 fibroblasts. The pol alpha specific activity decrease was correlated with a decreased proliferative capacity frequently seen in cells from aged donors. Pol alpha isolated from pSV3.neo-transformed GM3529 cells (GM3529T) exhibited a single isoform with about 10-fold higher specific activity than pol alpha from GM3529 cells. GM3529T pol alpha was immunoreactive with both anti-pol alpha and anti-SV40 large tumor antigen. Polymerases from GM3529 and GM3529T cells were treated with a pol alpha accessory protein, alpha AP, isolated from L1210 cells. Pol alpha from GM3529T cells showed no increase in activity in the presence of alpha AP, while pol alpha isolated from GM3529 cells exhibited about an 8-fold increase in activity after treatment with alpha AP. Double stranded SV40 DNA containing multiple ori sequences exhibited a greater decrease in electrophoretic mobility in the presence of GM3529T pol alpha than when treated with GM3529 pol alpha. In the presence of pol alpha from either GM35229 or GM3529T cells SV40 dsDNA exhibited a decrease in electrophoretic mobility, and in each instance addition of alpha AP resulted in an even greater decrease in DNA mobility. These data indicate that alpha AP increased pol alpha binding to SV40 dsDNA, or that alpha AP bound the DNA in addition to previously bound pol alpha. GM3529 pol alpha also bound non-specific, non-SV40, dsDNA, whereas GM3529T pol alpha with associated TAg did not bind the non-viral dsDNA unless alpha AP was added to the preparation. While not all human diploid fibroblast cell lines derived from aged human donors necessarily exhibit decreased proliferative capacity compared with cells from young donors, decreased specific activity associated with a decline in cellular DNA synthesis is typical of pol alpha from cells derived from aged human donors. We suggest that a decrease in endogenous alpha AP interaction with pol alpha may account, in part, for the loss of DNA binding affinity and specific activity of pol alpha from GM3529 cells derived from an aged donor.

Age-related changes in expression and activity of DNA polymerase alpha: some effects of dietary restriction

DNA polymerase alpha (pol alpha) purified from human diploid fibroblasts (HDF) and from livers of C57BL/6N mice showed age-related decreases in: (1) mRNA levels; (2) the amount of enzyme isolated per cell; and (3) enzyme activity (HDF); as well as: a) the amount of enzyme isolated; b) the specific activity; and c) the enzyme fidelity (liver). Hepatic pol alpha from dietary restricted (DR) mice exhibited less of a decline in specific activity and copied synthetic DNA templates with relatively higher fidelity than did enzymes from animals fed ad libitum (AL). Pol alpha from fetal-derived HDF exhibited increased expression compared with aged donor-derived HDF, with both fetal and old cell pol alpha in normal cells being expressed at lower levels than in their transformed cell corollaries. Treatment of human pol alpha from aged donor-derived HDF with a pol alpha accessory protein isolated from log phase murine cells resulted in increased pol alpha binding of DNA and increased pol alpha activity. However, highly active pol alpha isolated from fetal-derived or transformed HDF, or from transformed murine cells, showed little or no activity enhancement in the presence of accessory protein. These data indicate that, as a function of increased age, there is a decrease in pol alpha expression and specific activity in HDF, as well as decreases in specific activity and fidelity of pol alpha in essentially amitotic murine hepatic tissues. Dietary restriction impedes the age-related declines in both activity and fidelity of hepatic pol alpha in mice. The data further indicate that transformation of slowly dividing HDF is associated with increased expression of pol alpha, but suggest that increased expression alone is not sufficient to explain the difference in polymerase activity levels between parental and transformed HDF. Lastly, the data suggest that interaction of pol alpha with an essential accessory protein may be altered as a function of age, an alteration that appears to be correlated with the decline in pol alpha DNA binding and specific activity.