Effect of thyroid hormone on the synthesis of sialosyl galactosylceramide (GM4) in myelinogenic cultures of cells dissociated from embryonic mouse brain

Ceramide glycosylation catalyzed by glucosylceramide synthase and cancer drug resistance

Glucosylceramide synthase (GCS), converting ceramide to glucosylceramide, catalyzes the first reaction of ceramide glycosylation in sphingolipid metabolism. This glycosylation by GCS is a critical step regulating the modulation of cellular activities by controlling ceramide and glycosphingolipids (GSLs). An increase of ceramide in response to stresses, such as chemotherapy, drives cells to proliferation arrest and apoptosis or autophagy; however, ceramide glycosylation promptly eliminates ceramide and consequently, these induced processes, thus protecting cancer cells. Further, persistently enhanced ceramide glycosylation can increase GSLs, participating in selecting cancer cells to drug resistance. GCS is overexpressed in diverse drug-resistant cancer cells and in tumors of breast, colon, and leukemia that display poor response to chemotherapy. As ceramide glycosylation by GCS is a rate-limiting step in GSL synthesis, inhibition of GCS sensitizes cancer cells to anticancer drugs and eradicates cancer stem cells. Mechanistic studies indicate that uncoupling ceramide glycosylation can modulate gene expression, decreasing MDR1 through the cSrc/β-catenin pathway and restoring p53 expression via RNA splicing. These studies not only expand our knowledge in understanding how ceramide glycosylation affects cancer cells but also provide novel therapeutic approaches for targeting refractory tumors.

Triiodothyronine regulates somatostatin gene expression in cultured fetal rat cerebrocortical cells

The effect of triiodothyronine (T3) on somatostatin (SS) mRNA levels in cultured fetal rat cerebrocortical cells was studied. Two different experimental approaches were sought. They differed in the length of time in which cells were deprived of thyroid hormones prior to the addition of exogenous T3. When the cells were not deprived of thyroid hormones, T3 caused a dose-related decrease in SS mRNA content at all doses tested. However, when the cells were deprived of T3 for 24 h, a biphasic effect was observed. These findings suggest that T3 regulates SS gene expression in fetal cultured cerebrocortical cells.

Up-regulation of embryonic NCAM in an EC cell line by retinoic acid

The impact of retinoic acid (RA) on the expression of the neural cell adhesion molecules (NCAMs) and their developmentally regulated polysialic acid (PSA) moiety was studied in embryonal carcinoma (EC) cell lines. These cell lines are known to be capable of RA-induced differentiation into neurons (murine P19 cells) or parietal endoderm (murine F9 cells), respectively. Monoclonal antibodies were employed to monitor expression of NCAM and PSA. F9 and P19 cells were both found to express NCAM but only P19 cells carried the highly polysialylated "embryonic form" of NCAM (E-NCAM). The amount of NCAM in aggregated P19 cells but not in F9 cells was dramatically increased upon treatment with RA. Since NCAMs play an important role in cell interactions during embryogenesis it is tempting to speculate that the regulative impact of RA on NCAMs is related to its morphogenic property.

Insulin: its binding to specific receptors and its stimulation of DNA synthesis and 2',3'-cyclic nucleotide phosphohydrolase activity in cerebral cells cultured from embryonic mouse brain

The presence and specificity of insulin receptors was investigated in culture cells obtained from 15-16 days old embryonic mouse cerebra. Developmental studies suggested that the maximum insulin binding occurred at about 11 days in vitro (DIV). Scatchard analysis of binding data revealed two types of binding sites. One type of receptor was the high affinity type (Kd = 7.77 x 10(-9) M; number of receptor sites, Bmax = 350 fmol/mg protein) while the other type was of low affinity type (Kd = 5.75 X 10(-8)M; Bmax = 1150 fmol/mg protein). The specificity of receptors for insulin was also confirmed by showing that [125I]insulin was displaced by non-radioactive insulin but not by glucagon or growth hormone. Insulin displayed a clear dose-dependent stimulation of thymidine incorporation. It also stimulated the activity of the enzyme 2', 3'-cyclic nucleotide phosphohydrolase (CNPase), which is specifically associated with myelin produced from oligodendroglia. Thus insulin has a positive influence on the proliferation and differentiation of brain cells.

Cellular and molecular aspects of myelin protein gene expression

The cellular and molecular aspects of myelin protein metabolism have recently been among the most intensively studied in neurobiology. Myelination is a developmentally regulated process involving the coordination of expression of genes encoding both myelin proteins and the enzymes involved in myelin lipid metabolism. In the central nervous system, the oligodendrocyte plasma membrane elaborates prodigious amounts of myelin over a relatively short developmental period. During development, myelin undergoes characteristic biochemical changes, presumably correlated with the morphological changes during its maturation from loosely-whorled bilayers to the thick multilamellar structure typical of the adult membrane. Genes encoding four myelin proteins have been isolated, and each of these specifies families of polypeptide isoforms synthesized from mRNAs derived through alternative splicing of the primary gene transcripts. In most cases, the production of the alternatively spliced transcripts is developmentally regulated, leading to the observed protein compositional changes in myelin. The chromosomal localizations of several of the myelin protein genes have been mapped in mice and humans, and abnormalities in two separate genes appear to be the genetic defects in the murine dysmyelinating mutants, shiverer and jimpy. Insertion of a normal myelin basic protein gene into the shiverer genome appears to correct many of the clinical and cell biological abnormalities associated with the defect. Most of the dysmyelinating mutants, including those in which the genetic defect is established, appear to exhibit pleiotropy with respect to the expression of other myelin genes. Post-translational events also appear to be important in myelin assembly and metabolism. The major myelin proteins are synthesized at different subcellular locations and follow different routes of assembly into the membrane. Prevention of certain post-translational modifications of some myelin proteins can result in the disruption of myelin structure, reminiscent of naturally occurring myelin disorders. Studies on the expression of myelin genes in tissue culture have shown the importance of epigenetic factors (e.g., hormones, growth factors, and cell-cell interactions) in modulating myelin protein gene expression. Thus, myelinogenesis has proven to be very useful system in which to examine cellular and molecular mechanisms regulating the activity of a nervous system-specific process.

Investigations on myelinogenesis in vitro: II. The occurrence and regulation of protein kinases by thyroid hormone in primary cultures of cells dissociated from embryonic mouse brain

The occurrence and regulation by thyroid hormone of four protein kinases (cyclic AMP independent and dependent, calcium/calmodulin stimulated, and calcium/phosphatidyl serine stimulated protein kinases) was studied in primary cultures of cells dissociated from embryonic mouse brain. Serum from a thyroidectomized calf, which contained low levels of L-3,5,3'-triiodothyronine, T3 (less than 25 ng/100 ml), and thyroxine, T4 (less than 1 microgram/100 ml) was used in the culture medium in place of normal calf-serum (T3, 130 ng/100 ml; T4 5.9 micrograms/100 ml) to render the cultures responsive to exogenously added T3. Cultures grown in hypothyroid calf-serum containing medium had less cAMP dependent and independent protein kinase activity than control cultures grown in normal calf-serum containing medium. However, this activity was restorable to a considerable degree if the cultures grown in hypothyroid calf serum containing medium were supplemented with L-3,5,3'-triiodothyronine (T3). The presence of calcium/calmodulin stimulated protein kinase was also distinctly observed. In comparison, the activity of calcium/phosphatidyl serine stimulated protein kinase was less than the other protein kinases.

Investigations on myelinogenesis in vitro: I. The occurrence of endogenous protein kinase and its role in the phosphorylation of myelin basic proteins in "myelin-like membranes" isolated from cerebral cell cultures

The presence of a protein kinase capable of phosphorylating endogenous as well as exogenously added myelin basic proteins has been demonstrated in a myelin-like membrane fraction isolated from reaggregating and surface adhering, primary cultures of cells dissociated from embryonic mouse brain. Only the large and small components of myelin basic proteins were found to be phosphorylated when myelin-like membrane fraction was incubated with [gamma-32P]ATP. The protein kinase endogenous to the myelin-like membrane fraction was mainly of the cyclic AMP independent type. There was very little cyclic AMP dependent or cyclic GMP dependent protein kinase activities in this myelin-like fraction. Although the myelin basic proteins were the only endogenous proteins phosphorylated, protein kinase of the myelin-like membrane was capable of catalyzing the phosphorylation of exogenous substrates, such as histones.

Regulation of dipeptidyl aminopeptidase I and angiotensin converting enzyme activities in cultured murine brain cells by cortisol and thyroid hormone

Cultures of dissociated brain cells from 15-day-old fetal mice were grown in the presence and absence of 20 or 50 nM triiodothyronine (T3), 30 or 300 nM cortisol, and 30 nM cortisol plus 50 nM T3 added to chemically defined media or in media supplemented with 15% serum from control and hypothyroid calves. The specific activities of five lysosomal enzymes--N-acetyl galactosaminidase, beta-glucuronidase, beta-galactosidase, cathepsin B, and dipeptidyl aminopeptidase I (DAP-I)--were higher in cells grown in calf serum than in cells grown in defined media. Of these enzymes, only DAP-I was elevated in activity when the cells were grown in hypothyroid calf serum instead of control calf serum. Elevation of DAP-I activity was reversed by addition of 20 nM T3 to hypothyroid calf serum. The enzymatic properties of DAP-I were similar whether the cells were grown in control or hypothyroid calf serum and were similar to those reported for human fibroblasts and the purified enzyme. When the cells were grown in defined media, cortisol decreased the activities of all lysosomal enzymes, with 300 nM cortisol being more effective than 30 nM cortisol. Addition of 50 nM T3 to 30 nM cortisol decreased DAP-I activity more than 30 nM cortisol alone, but 50 nM T3 alone in defined media did not alter DAP-I levels. The reduction of DAP-I activity in these cells by T3 required cortisol, unidentified components in serum, or both.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigations on myelinogenesis in vitro: developmental expression of myelin basic protein mRNA and its regulation by thyroid hormone in primary cerebral cell cultures from embryonic mice

The concentration of myelin basic protein (MBP) mRNA in primary cultures of cells dissociated from embryonic mouse cerebra and grown in the presence of varying amounts of thyroid hormone was measured using a 32P-labeled cDNA probe and a dot-blot procedure. The cDNA probe contained 1.85 kilobases of the gene for MBP. The concentration of mRNA specific for MBP in control cells grown on a medium containing normal (euthyroid) calf serum increased with increasing age of culture. The greatest increase occurred between 15 and 35 days in culture (5.25-fold increase); whereas between 35 and 50 days in culture, the rate of accumulation slowed to yield a net increase of MBP mRNA of only 10%. The quantity of MBP mRNA was drastically diminished at all ages studied when the cells were grown from the sixth day onward on a medium containing hypothyroid calf serum. Although the amount of MBP mRNA in hypothyroid-treated cells did increase, the change in concentration was less (3.43-fold), and it peaked earlier (at 30 days). Unlike the euthyroid cells, after 30 days the MBP mRNA actually fell in the hypothyroid-treated cells. If hypothyroid media were supplemented with triiodothyronine (T3) on the eighth day in culture, the quantity of MBP mRNA in the cells was restored almost completely to the levels found in the control euthyroid cells at all ages. Therefore, the regulation of the synthesis of MBP by thyroid hormone is at least in part a pretranslational event; that is, thyroid hormone adjusts the concentration of the mRNA specific for MBP.

Growth and lipid composition of rat brain glial cells cultured in lipoprotein deficient serum

The purpose of this study was to examine the effects of substituting lipoprotein deficient serum (LPDS) for complete fetal calf serum (FCS) in culture media on the growth and lipid composition of cells dissociated from 1 to 2-day-old rat brain. The results show that in FCS cultures DNA, protein and all lipids increase with an increase in the number of days in culture. Substitution of LPDS for FCS in the culture media caused a slower increase in each of these constituents. Esterified cholesterol remained unaltered with time in LPDS cultures but increased continuously in FCS cultures. Substitution of LPDS for FCS reduced the DNA:protein ratio, and unesterified cholesterol:phospholipid ratio but the protein:phospholipid ratio and the proportion of individual phospholipids were not affected. The data indicate that removal of low density lipoprotein (LDL) from serum used in culture media reduces cell proliferation and causes alterations in cellular lipid composition specifically ratio of cholesterol:phospholipids.

A new myelin-deficient mutant hamster: biochemical and morphological studies

Biochemical and morphological studies were done on a new trembling mutant hamster CBB. The yield of myelin from the mutant was 30 and 40% of the control at 46 and 140 days of age, respectively, but myelin composition and 2',3'-cyclic nucleotide-3'-phosphohydrolase (CNPase) activity were normal. Morphologically, about 18% of the axons were myelinated in the mutant optic nerve at 46 days of age, in which the myelinated fibers were those with larger diameters (more than 0.6 micron), while the control had a peak at 0.4 micron in diameter. The ultrastructure and thickness of compact myelin lamellae in the mutant were normal. Myelination and the structure of peripheral nerve myelin appeared normal. The results indicate that the essential defect is the delay and arrest of myelination in the CNS, which is probably caused by either a decreased rate of synthesis of myelin components in oligodendrocytes or a defect in the oligodendrocyte-axon recognition in smaller axons.

Developmental study on the regulation of neurotransmitter-sensitive adenylate cyclase systems in primary cerebral cell cultures from embryonic mice

An ontogenetic study of the effect of various neurohormones and other activators on adenylate cyclase systems was carried out using cultures of cells from 15-d-old embryonic mouse brain. Dopamine stimulated the enzyme activity at earlier culture ages (i.e. 4 and 10 d) but had little stimulatory effect at later ages (i.e. 20 and 33 d). Further, this stimulation at the earlier ages was blocked by the dopaminergic blocker, fluphenazine, but not by alpha and beta-adrenergic antagonists. In contrast to dopamine, isoproterenol (a beta-adrenergic agonist) had little stimulatory effect at earlier ages, but its ability to stimulate cyclase activity increased with age. This increase in all age groups was blocked by propranolol (a beta-adrenergic antagonist). Epinephrine-sensitive enzyme activity showed a steady increase with age, which could be blocked with propranolol except in 4-d-old cultures, where it was blocked instead by fluphenazine. Because the cultures are relatively enriched in neurons at earlier ages and in glia in later ages, the results suggest a predominantly neuronal localization for the dopamine sensitive adenylate cyclases and a glial localization of the isoproterenol and epinephrine sensitive adenylate cyclases. Histamine, serotonin, calcium/calmodulin and chloroadenosine were either only slightly or not at all stimulatory.

Investigations on myelinogenesis in vitro: a study of the critical period at which thyroid hormone exerts its maximum regulatory effect on the developmental expression of two myelin associated markers in cultured brain cells from embryonic mice

Cultures of cells dissociated from embryonic mouse brain were used to assess the period in which thyroid hormone exerts its maximum influence on the regulation of the expression of two myelin associated metabolites, sulfolipids and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP-ase). Cultures were grown for a specified number of days on a medium containing normal calf serum and then a portion were switched to a medium containing hypothyroid calf serum for 2 days. One half of these cultures were then supplemented with 50 nM triiodothyronine and growth was continued in all cultures for 3 more days. The cells were then assayed for CNP-ase activity and for their ability to incorporate 35SO4 into sulfolipids. Studies with both myelin markers showed that in the earlier culture ages of 5, 8, and 11 days, thyroid hormone was able to fully restore the activities when added to cultures grown on hypothyroid calf-serum. In contrast, in the intermediate age range (15, 19, and 22 days) the restoration was partial, while in the higher ages, there was practically negligible restoration with T3. Since the culture system eliminates the possibility of a blood brain barrier and drastically decreases the complicity of other hormones, the lack of a myelinogenic response to thyroid hormone after a certain age must be attributed to the loss of sensitivity of the oligodendroglia to T3 possibly through genetic programming.

Thyroid hormones reversibly suppress somatostatin secretion and immunoreactivity in cultured neocortical cells

Thyroid hormone effects on brain somatostatin-like immunoreactivity (SRIF-LI) were studied in an in vitro model system. Serum was removed from the nutrient culture medium of fetal day-18 rat cerebral cortex cells maintained in primary, long-term, dispersed monolayer culture. Chronic administration of either T3 or T4 in serum-free medium was associated with suppressed release of SRIF-LI into the culture medium (36-43 h accumulation), cell content of peptide and acute release in response to potassium-induced depolarization. Suppression was dose-dependent with an IC50 of less than 1 nM for T3. The most dramatic effects were observed for K+-induced release. Thirty-five to 50% suppression was typically observed with T3 at a near maximum dose (3 nM). Reverse T3 and diiodotyrosine were less potent and effective than T3. TRIAC and diiodothyronine also possessed significant suppressive activity. T3 suppression of release depended on duration of pretreatment. Administered for less than 16 h, T3 failed to significantly suppress K+-induced release, but significant suppression was observed for pretreatment periods of 16 h or longer. Indirect fluorescent immunohistochemical examination revealed a reduction in the number of cells positively stained for SRIF-LI in T3-treated dishes relative to controls. Upon removal of T3 and subsequent recovery in serum supplemented medium for 24 h, T3-treated and control cells exhibited similar levels of SRIF-LI release and cell content. T3-treated and control cells incorporated [3H] leucine into trichloracetic acid precipitable counts to similar extents. Dexamethasone and several sex steroids failed to modify the effects of T3 and did not independently influence SRIF-LI levels. Acute cycloheximide administration did not reverse T3 effects. The data indicate that primary brain cell cultures may be useful models to examine direct peripheral hormone actions on nervous tissue. Thyroid hormones suppress SRIF-LI levels in a dose, time and structure-dependent manner, which appears to be reversible. The findings are consistent with a possible integration of peripheral hormone and brain peptide physiology.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Towards an improved serum-free, chemically defined medium for long-term culturing of cerebral cortex tissue

The present study describes a series of experiments which have led to a substantially improved serum-free, chemically defined medium (CDM) for long-term culturing of reaggregated fetal rat cerebral cortex tissue. A reduction of the original medium concentrations of the hormones insuline, T3 and corticosterone, on the one hand, and an enrichment of the medium with the vitamins A, C and E, the unsaturated fatty acids linoleic and linolenic acid, and biotin, L-carnitine, D(+)-galactose, glutathione (reduced) and ethanolamine, on the other hand, formed the most important chemical adjustments of the medium. With the aid of this CDM (encoded R12), the light- and electron microscopic architecture of the tissue could be kept in a good condition (superior to that seen earlier in serum-supplemented medium) up to 23 days in vitro. From that time on, the neuronal network lying between the reaggregates degenerated for the largest part, while a portion of the large neurons (probably pyramidal cells) plus some of the neuronal network within the reaggregates degenerated too. This degeneration process continued during the following weeks, but the reaggregates nevertheless retained most of their mass, so that both small and large neuronal cell bodies (visible in transparent regions at the edge of the reaggregates) remained in good condition up to at least 103 DIV. Stout, thick nerve bundles interconnecting the reaggregates, also survived up to this point. Electron microscopic evaluation of such 'aged' reaggregates revealed degenerating as well as healthy regions. The latter had indeed retained healthy-looking pyramidal and non-pyramidal neurons, embedded within a dense neuropil which was often traversed by myelinated axons. The numerical synapse density in such selected, healthy tissue regions reached its maximum during the sixth week in vitro, followed by a rapid decrease and a stabilization at about half the peak values. The present culture system has opened the possibility for performing controlled quantitative studies on the relationship between structure and function of cerebral cortex tissues during development and aging, on its dependence on nutrients, hormones and drugs, and on special factors synthesized by the tissue and released into the nutrient medium.

Regulation of myelin basic protein (arginine) methyltransferase by thyroid hormone in myelinogenic cultures of cells dissociated from embryonic mouse brain

The ontogenetic expression of myelin basic protein (arginine) methyltransferase in myelinogenic cultures of cells dissociated from embryonic mouse brain is highly dependent on the presence of thyroid hormone. Restoration of myelin basic protein methyltransferase to normal activities occurred 16 h after the addition of 100 nM L-3,5,3'-triiodothyronine to hypothyroid medium. These data demonstrate that thyroid hormone can regulate a posttranslational event. On the other hand, histone (arginine) methyltransferase has a different temporal activity pattern, which is not coordinated with myelination, and is not influenced by the lack of thyroid hormone. These data, which suggest the existence of two methyltransferases, were substantiated by demonstrating that the total amount of methylation of added myelin basic protein and histone is the same whether they are incubated together or separately. The requirement of thyroid hormone for the expression of the myelin basic protein methyltransferase and not for histone methyltransferase suggests that thyroid hormone preferentially regulates myelin-associated events in these cultures.

Regulation of arylsulphatase A and sulphogalactolipid turnover by cortisol in myelinogenic cultures of cells dissociated from embryonic mouse brain

Myelinogenic cultures of cells dissociated from embryonic mouse brain were used to study the regulation of myelination-associated molecules by cortisol. Cortisol in physiological concentrations (0.03 microM) caused an increased accumulation of myelination-associated sulphogalactolipids. It also stimulated the myelin- and oligodendroglia-specific cyclic nucleotide phosphohydrolase. The increase in sulphogalactolipid content was caused by a cortisol-concentration-dependent inhibition in arylsulphatase A activity and not by an increase in either cerebroside sulphotransferase activity or an increase in availability of adenosine 3'-phosphate 5'-phosphosulphate. Of several steroid hormones tested only the glucocorticoid types brought about these changes. The relationship between net sulphogalactolipid accumulation and arylsulphatase A inhibition induced by cortisol was confirmed by sulphogalactolipid turnover studies. Depending on whether a single-phase or a two-phase decay calculation is used, the turnover of sulphogalactolipid with cortisol present was decreased at 22 days in culture by either 62% or 65% respectively of that without cortisol. This decrease in turnover can be attributed completely to the decrease of arylsulphatase activity by cortisol to 63% of the value for normal cells grown under the same conditions.

Investigations on myelinogenesis in vitro: regulation of 5'-nucleotidase activity by thyroid hormone in cultures of dissociated cells from embryonic mouse brain

The developmental pattern of the myelin-associated 5'-nucleotidase and its regulation by L-3,3',5,-triiodothyronine (T3) have been demonstrated in a culture system of cells dissociated from embryonic mouse brain. Hypothyroid calf serum containing low levels of T3 (31 ng/100 ml), and thyroxine, T4 (less than 1 microgram/ml), was used in the culture medium in place of normal calf serum (T3, 103 ng/100ml; T4, 5.7 micrograms/ml) to render the cultures responsive to exogenously added T3. By means of T3 supplementation, the lower levels of enzyme activity observed in the cultures grown in the presence of hypothyroid calf-serum containing medium could be restored to a considerable extent although not completely to normal values. Half-maximal stimulatory effect was obtained at 3.9 X 10(-8)M T3 concentration. Among the various substrates tested, 5'-AMP, 5'-UMP and 5'-CMP were equally good, while 5'-GMP yielded approximately half the activity.

Cell proliferation in growing cultures of dissociated embryonic mouse brain: macromolecule and ornithine decarboxylase synthesis and regulation by hormones and drugs

Primary cultures of cells dissociated from embryonic mouse brain were demonstrated to be a useful system for studying cell proliferation and its regulation. Ornithine decarboxylase activity was closely correlated with the rate of DNA and RNA synthesis during cell growth, suggesting that the enzyme is as good an indicator of cell proliferation in these cultures as it is in vivo. Both DNA synthesis and ornithine decarboxylase activity were stimulated by insulin. The enzyme was stimulated five- to sixfold by insulin and approximately twofold by butyrate, cis-retinoic acid, and 12-O-tetradecanoylphorbol-13-acetate. No effect on the enzyme activity was observed with triiodothyronine, hydrocortisone, growth hormone, cyclic AMP, or cyclic GMP.