Nuclear and mitochondrial DNA synthesis and energy metabolism in primary rat glial cell cultures

Effect of growth factors and steroids on transglutaminase activity and expression in primary astroglial cell cultures

Type-2 transglutaminase (TG-2) is a multifunctional enzyme involved in the regulation of cell differentiation and survival that recently has been shown to play an emerging role in astrocytes, where it is involved in both proliferation and differentiation processes. Growth factors (GFs) such as EGF, basic fibroblast growth factor, insulin-like growth factor-I (IGF-I), and insulin (INS) are trophic and mitogenic peptides that participate in neuron-glia interactions and stimulate neuronal and astroglial proliferation and differentiation. Steroid hormones such as glucocorticoids and estrogens also play a pivotal role in neuronal and astroglial proliferation and differentiation and are key hormones in neurodegenerative and neuroprotective processes. We investigated the effects of the interaction of GFs with dexamethasone (DEX) or 17beta-estradiol (E(2)) on TG-2 activity and their expression in cultured astrocytes. We observed a significant increase in TG-2 activity and expression in astroglial cells treated for 24 hr with IGF-I, EGF, or INS. Priming of the cells with DEX or E(2), for 48 hr also led to an increase in TG-2 levels. When growth factors were present in the last 24 hr of the steroid treatment, a reduction in TG-2 expression and activity and a different subcellular TG-2 distribution were found. Our data indicate that steroid hormone-GF interaction may play an important role in astroglial function. The effect on TG-2 could be part of the regulation of intracellular pathways associated with the astrocyte response observed in physiological conditions and, possibly, also in neuropathological diseases.

Effect of growth factors on DNA labeling and cytoskeletal protein expression in 17-beta-estradiol and basic fibroblast growth factor pre-treated astrocyte cultures

Astrocytes react to all noxae which damage neurons. Their reactions include degeneration, hypertrophy, hyperplasia and fibre formation. Growth factors inducing proliferation and differentiation of both neurons and astrocytes in culture play a pivotal role in the dynamic flow of signaling molecules between neurons and astroglia. Estrogens as well influence astroglia and are neuroprotectants. This study has investigated the interactions between growth factors and estrogens on DNA labeling and cytoskeletal protein [glial fibrillary acidic protein (GFAP) and vimentin] expression in 22 DIV astrocyte cultures treated for 24 or 36 h under different experimental conditions. Contemporary addition of 17-beta-estradiol (E2) with two or three growth factors for 24 h, significantly stimulated methyl-[3H]thymidine incorporation into DNA from 22 days in vitro (DIV) astrocyte cultures. This effect reached a peak when E2 was co-added with epidermal growth factor (EGF), basic fibroblast growth factor (bFGF) and insulin. In astrocyte cultures treated for 36 h with E2 and EGF + insulin or bFGF + insulin added in the last 12 h, DNA labeling was remarkably increased. The parallel cyclin Dl expression positively correlated with ERK2 activation. Western blot analysis for cytoskeletal proteins showed also changes of both GFAP and vimentin expression. The above data suggest the occurrence of a scheduled interaction between "competence" or "progression" growth factors and estrogens on DNA labeling and cytoskeletal protein expression during astroglial cell proliferation and differentiation in culture. A better understanding of the mechanisms of these interactions may contribute to develop strategies for controlling astroglial reaction in cerebrovascular disease including stroke and hypertensive brain damage.

Effect of growth factors on nuclear and mitochondrial ADP-ribosylation processes during astroglial cell development and aging in culture

Epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), insulin-like growth factor-I (IGF-I) and insulin (INS) are powerful mitogens and may regulate gene expression in cultured astrocytes by ADP-ribosylation process. Nuclear poly-ADP ribose polymerase (PARP) and mitochondrial monoADP-ribosyltransferase (ADPRT) are the key enzymes involved in poly-ADP-ribosylation and mono ADP-ribosylation, respectively. In this investigation the effect of EGF, bFGF, IGF-I or INS on nuclear PARP and mitochondrial ADPRT activities were assessed in nuclei and mitochondria purified from developing (30 DIV) or aging (90 and 190 DIV) primary rat astrocyte cultures. A marked increase of PARP activity in bFGF or IGF-I treated astroglial cell cultures at 30 DIV was found. Nuclear PARP and mitochondrial ADPRT activities were greatly stimulated by treatment with EGF or INS alone or together in astrocyte cultures at 30 DIV. Nuclear PARP and mitochondrial ADPRT activities showed a more remarkable increase in control untreated astrocyte cultures at 190 DIV than at 90 DIV. These findings suggest that ADP-ribosylation process is involved in DNA damage and repair during cell differentiation and aging in culture. Twelve hours treatment with EGF, INS or bFGF significantly stimulated nuclear PARP and mitochondrial ADPRT activities in 190 DIV aging astrocyte cultures. The above results indicate that EGF, INS and bFGF may play a crucial role in the post-translational modification of chromosomal proteins including ADP-ribosylation process in in vitro models. This suggests that growth factors regulate genomic stability in glial cells during development and maturation, stimulating nuclear and mitochondrial ADP-ribosylation processes in developing or aging astrocyte cultures.

Effect of 17-beta estradiol and epidermal growth factor on DNA and RNA labeling in astroglial cells during development, maturation and differentiation in culture

Growth factors stimulate astroglial and neuronal proliferation and differentiation in culture. Estrogens markedly influence astroglia, and are key factors participating in neurodegeneration. The aim of the present study was to investigate interactions between estradiol (E2) and epidermal growth factor (EGF) during astroglia development, maturation and differentiation in culture. DNA or RNA labeling in 16 or 40 or 60 days in vitro (DIV) astrocyte cultures treated for 24 or 48 h with EGF and/or E2 was evaluated. A significant increase in DNA labeling in 16 DIV astrocyte cultures treated for 24 h with EGF (5 ng/ml) and E2 (1 nM) was found. EGF (5 or 10 ng/ml) addition in the last 24 h in 48 h E2 (1 or 5 nM)-treated astrocyte cultures at 16 DIV caused a slight, but significant increase in DNA labeling. No differences in RNA labeling were observed in 16 DIV astrocyte cultures treated for 24 or 48 h with EGF (5 or 10 ng/ml) in the presence of E(2) (1 or 5 nM). A significant stimulation in DNA labeling was shown in 40 DIV astrocyte cultures treated for 48 h with E2 (1 or 5 nM) in the presence of EGF (5 or 10 ng/ml) added in the last 24 h. In well differentiated astroglial cell cultures (60 DIV), DNA labeling was remarkably increased after 24 h treatment with 1 nM E2 or 5 ng/ml EGF. Co-addition of 1 nM E2 and 5 ng/ml EGF for 24 h reduced [methyl-(3)H]thymidine incorporation, when data are compared to E2- or EGF-treated cultures. Addition of EGF in the presence of E2 for 48 h or only in the last 24 h caused a significant decrease of [methyl-(3)H]thymidine incorporation in comparison with EGF-treated cultures at 60 DIV or with untreated cultures. Treatment of cultures for 24 h with EGF (5 or 10 ng/ml) alone or in combination with E2 (1 or 5 nM) induced a strong increase of RNA labeling in 60 DIV astrocyte cultures. Addition for 48 h of E2 (1 or 5 nM) or EGF (5 or 10 ng/ml) alone or in association stimulated significantly RNA labeling in astrocyte cultures at 60 DIV. When 60 DIV astrocyte cultures were treated for 48 h with E2 (1 or 5 nM) in the presence of EGF (5 or 10 ng/ml) added only in the last 24 h, a potentiating effect of RNA labeling was observed. The above results suggest that interaction between growth factors and estrogens may contribute to regulate astroglia development, maturation and differentiation.

Mitochondrial DNA synthesis studied autoradiographically in various cell types in vivo

It is generally accepted that mitochondria are able to proliferate even in postmitotic cells due to their natural turnover and also to satisfy increased cell energy requirements. However, no detailed studies are available, particularly with respect to specific cell types. Since [3H]-thymidine is incorporated not only into nuclear (n) DNA but also into the DNA of cytoplasmic mitochondria, an autoradiographic approach was developed at the light microscopy level in order to study basic questions of mitochondrial (mt) proliferation in organs of rodents in situ via the cytoplasmic incorporation of [3H]-thymidine injected into the animals 1 h before sacrifice. Experiments carried out on mice after X-irradiation showed that cytoplasmic labeling was not due to a process such as unscheduled nuclear DNA synthesis (nUDS). Furthermore, half-lives of mitochondria between 8-23 days were deduced specifically in relation to cell types. The phase of mtDNA synthesis was about 75 min. Finally, mt proliferation was measured in brain cells of mice as a function of age. While all neurons showed a decreasing extent of mtDNA synthesis during old age, nUDS decreased only in distinct cell types of the cortex and hippocampus. We conclude that the leading theories explaining the phenomenon of aging are closely related, i.e., aging is due to a decreasing capacity of nDNA repair, which leads to unrepaired nDNA damage, or to an accumulation of mitochondria with damaged mtDNA, which leads to a deficit of cellular energy production.

Effect of CDP-choline treatment on mitochondrial and synaptosomal protein composition in different brain regions during aging

Several age-dependent modifications of inner mitochondrial membrane and synaptosomal plasma membrane proteins from different brain regions of 4-, 12-, 18- and 24-month-old male Wistar rats, were observed. Some proteins, identified by immunoblotting assay as various subunits of mitochondrial respiratory chain complexes and calmodulin, were particularly impaired. Chronic treatment with CDP-choline at a dose of 20 mg/kg body weight per day for 28 days caused significant changes in the amounts of several of the above mentioned proteins. Most of the proteins, which decreased during aging, showed a significant increase after CDP-choline treatment compared with the corresponding control values at the same age. The effect of CDP-choline might be due to: the increased availability of cytidylic nucleotides, which in the brain are present in limited amounts compared to the other nucleotides; the increased content of total adenine nucleotides; the improvement of brain energy metabolism.

Effect of hypoxia on mitochondrial protein composition of cerebral cortex during aging

The effect of hypoxia on the protein composition of mitochondria from cerebral cortex of rats at 4, 12, and 24 months of age was investigated. The proteins were separated by electrophoresis on SDS polyacrylamide gels and the percent content was evaluated by measuring the optical density of the stained gels. The results demonstrate that hypoxic treatment causes a decrease in the amount of some proteins as follows: the 90 and the 16 kDa Mw proteins at 4 months; the 82 and the 79 kDa Mw proteins at 24 months; the 52-49, 35 and 20 kDa at all ages investigated; the 44 kDa protein at 4 and 12 months and the 28 kDa protein at 4 and 24 months of age. Our results show that hypoxic conditions affect mitochondrial protein composition to a greater extent than aging alone.

Effect of trophic factors, released after hippocampal injury, on astroglial cell proliferation

An increase in astrocyte mitogenic factors and in some specific astroglial enzymatic activities after neuronal injury has been observed. Our study is concerned with the effect of the intracerebral administration of ibotenic acid (IBO) into the rat hippocampus. IBO injection causes a selective degeneration of neurons while sparing afferent fibers. We observed a transient increase in glutamine synthetase activity, a well-known astroglial marker, reaching a peak at 9-15 days after injury in lesioned hippocampus. We investigated the presence of astrocyte mitogenic factors at various times after toxin injection. Crude extracts, prepared from lesioned hippocampi 4, 9, and 14 days after IBO injection, were tested for the ability to stimulate [methyl-3H]thymidine incorporation into rat astroglial cell cultures. Crude extracts prepared 9 and 14 days after IBO injection showed a higher mitogenic activity compared to extracts prepared 4 days after lesion. Mitogenic activity of injured brain extracts was suppressed by heat inactivation (100 degrees C for 10 min).

Antioxidant enzymatic activities and resistance to oxidative stress in primary and subcultured rat astroglial cells

The survival of neural tissues depends in part on the balance between the formation of free radicals due to oxidative metabolism and the transformation of the free radicals to non-toxic compounds. Serial subculture of rat glial cells as described here resulted in a decrease of the specific activities of several antioxidant enzymes and a glial specific marker for astrocytes. Thus, there was an increased susceptibility to oxidative stress in cultures by the third passage. These subcultured glial cell cultures may represent a useful model for the study of free radical induced neural damage that may be relevant to CNS trauma and aging.

Age-related changes of ribonuclease activities in various regions of the rat central nervous system

Acid (pH 5.5), free, and latent alkaline (pH 7.4) RNases were assayed in homogenates of temporal cortex, hypothalamus, hippocampus, and cervicothoracic segments of spinal cord of rats at three different ages (5, 14, and 25 months old). Free alkaline RNase activity was lower (two- to fivefold) than the acid activity. Both free and inhibitor-bound alkaline RNases remained unchanged with age in all CNS regions examined. This result also indirectly indicates no change of RNase-inhibitor complex throughout aging. In contrast, the acid RNase activity showed a significant increase during aging in all tissues, with exception of the hypothalamus. Because this enzyme is localized mainly in the lysosomes, this result might be due to an increased lysosomal activity and/or to the release of hydrolases into the cytoplasm from these organelles, undergoing shrinkage and degeneration in aged animals.

Effect of epidermal growth factor on the labeling of the various RNA species and of nuclear proteins in primary rat astroglial cell cultures

This study investigated the effects of epidermal growth factor (EGF) on the labeling of various RNA species and of nuclear proteins in primary rat astroglial cell cultures. After 12 hours of EGF treatment in serum-free medium or chemically defined medium, significant increase in RNA labeling, and also in acid-soluble radioactivity and RNA content, was observed. The ratio RNA/DNA was significantly higher in EGF-treated cultures compared with controls. Ribosomal RNAs (28S and 18S), polyadenylated, and nonpolyadenylated RNAs showed a higher specific radioactivity in EGF-treated cultures. Among the nuclear proteins, the labeling of basic proteins was enhanced by EGF treatment, whereas that of total nuclear acidic protein (NHPs) was less modified, except for some NHPs separated by gel electrophoresis with a molecular weight (MW) approximately 95-83 and 44 kd, which were significantly more labeled in EGF-treated cultures.

Effect of epidermal growth factor and insulin on DNA, RNA, and cytoskeletal protein labeling in primary rat astroglial cell cultures

The effect of epidermal growth factor (EGF) and insulin on DNA, RNA, and cytoskeletal protein labeling in primary rat astroglial cell cultures was investigated. Cultures were grown for 15-30 days in vitro in 10% fetal calf serum (FCS)-supplemented medium and then maintained in serum-free basal medium (DMEM) supplemented with fatty acid-free bovine serum albumin (BSA) for a starvation period of 24 hr before the addition of factors. The effect of factors was tested at different times (4, 10, 22, and 28 hr). At each time, [methyl-3H]thymidine or [5,6-3H]uridine was added to the control and treated cells; the incubation time after the addition of labeled precursors was 2 hr at 37 degrees C. The results obtained indicated that the addition of EGF or FCS significantly stimulated [methyl-3H]thymidine incorporation into DNA, reaching the maximum effect after 22 hr. EGF alone significantly stimulated [3H]uridine incorporation into RNA, and this effect was already maximum at 4 hr and remained constant up to 22 hr. The addition of insulin alone caused a slight increase in nucleic acid labeling for short times (4-10 hr). In contrast with EGF, no detectable stimulation of incorporation of labeled precursors after insulin treatment for 22 hr was observed. On the other hand, the addition of insulin in the presence of EGF induced an increase of the values observed with EGF alone on macromolecular synthesis at all the times studied. Furthermore, a decrease in cell number was observed in confluent cultures maintained for 1 week in medium containing DMEM + BSA in comparison to serum-supplemented (DMEM + BSA + FCS) cultures.