Replicative senescence and differentiated gene expression in cultured adrenocortical cells

Human adrenocortical carcinoma cell lines

The human adrenal cortex secretes mineralocorticoids, glucocorticoids and adrenal androgens. These steroids are produced from unique cell types located within the three distinct zones of the adrenal cortex. Disruption of adrenal steroid production results in a variety of diseases that can lead to hypertension, metabolic syndrome, infertility and androgen excess. The adrenal cortex is also a common site for the development of adenomas, and rarely the site for the development of carcinomas. The adenomas can lead to diseases associated with adrenal steroid excess, while the carcinomas are particularly aggressive and have a poor prognosis. In vitro cell culture models provide important tools to examine molecular and cellular mechanisms controlling both the normal and pathologic function of the adrenal cortex. Herein, we discuss currently available human adrenocortical carcinoma cell lines and their use as model systems for adrenal studies.

Comparison of aldosterone production among human adrenocortical cell lines

Several human adrenocortical cell lines have been used as model systems for aldosterone production. However, these cell lines have not been directly compared with each other. Human adrenal cell lines SW13, CAR47, the NCI-H295 and its sub-strains and sub-clones were compared with regard to aldosterone production and aldosterone synthase (CYP11B2) expression. Culture media was collected 48 h after incubation, aldosterone secretion was measured and the data were normalized to the amount of cell protein. RNA was isolated for microarray analysis and quantitative RT-PCR (qPCR). The cell lines with the highest aldosterone production were further tested with regard to angiotensin II (Ang II) stimulation. Neither aldosterone nor CYP11B2 transcript were detected in SW13 or CAR47 cells. The aldosterone production by the NCI-H295, H295A, H295R-S1, H295R-S2, H295R-S3, HAC13, HAC15 and HAC50 were 119, 1, 6, 826, 18, 139, 412, and 1 334 (pmol/mg protein/48 h), respectively. H295A and H295R-S1 expressed less CYP11B2 than the commonly used H295R-S3 cells; while NCI-H295, H295R-S2, HAC13, HAC15 and HAC50 expressed 24-, 14-, 3-, 10-, and 35-fold higher CYP11B2 compared with the H295R-S3 cells. When treated with Ang II, NCI-H295, H295R-S2, HAC13, HAC15 and HAC50 showed significantly higher aldosterone production than the basal level (p<0.05). A comparison of the available human adrenal cell lines indicates that the H295R-S2 and the clonal cell lines, HAC13, HAC15 and HAC50 produced the highest levels of aldosterone and responded well to Ang II.

Adipogenesis and aging: does aging make fat go MAD?

In advanced old age, fat depot size declines while lipid is redistributed to muscle, bone marrow, and other tissues. Decreased fat depot size is related to reduced fat cell size and function and impaired differentiation of preadipocytes into fat cells. Reduced differentiation-dependent gene expression results from decreased abundance of the adipogenic transcription factors, CCAAT/enhancer binding alpha (C/EBPalpha) and peroxisome proliferator activated receptor gamma (PPARgamma). Increased expression of anti-adipogenic C/EBP family members contributes, perhaps due to cellular stress response pathway activation with aging. Hence, dysfunctional adipocyte-like cells appear in adipose tissue that are smaller and less insulin responsive than fully differentiated fat cells. Adipogenesis can be restored by overexpressing adipogenic transcription factors in preadipocytes from old animals. Redistribution of lipid to extra-adipose sites with aging could result from loss of lipid storage capacity in fat depots, altered fatty acid handling resulting in lipid accumulation, dysdifferentiation of mesenchymal precursors, such as muscle satellite cells and osteoblast precursors, into a partial adipocyte phenotype, or a combination of these mechanisms. Thus, accumulation of mesenchymal adipocyte-like default (MAD) cells in fat depots, muscle, bone marrow, and elsewhere is a potentially reversible process that could contribute to maldistribution of fat in old age.

Altered expression of C/EBP family members results in decreased adipogenesis with aging

Fat mass, adipocyte size and metabolic responsiveness, and preadipocyte differentiation decrease between middle and old age. We show that expression of CCAAT/enhancer binding protein (C/EBP)-α, a key regulator of adipogenesis and fat cell function, declined substantially with aging in differentiating preadipocytes cultured under identical conditions from rats of various ages. Overexpression of C/EBPα in preadipocytes cultured from old rats restored capacity to differentiate into fat cells, indicating that downstream differentiation-dependent genes maintain responsiveness to regulators of adipogenesis. C/EBPα-expression also decreased with age in fat tissue from three different depots and in isolated fat cells. The overall level of C/EBPβ, which modulates C/EBPα-expression, did not change with age, but the truncated, dominant-negative C/EBPβ-liver inhibitory protein (LIP) isoform increased in cultured preadipocytes and isolated fat cells. Overexpression of C/EBPβ-LIP in preadipocytes from young rats impaired adipogenesis. C/EBPδ, which acts with full-length C/EBPβ to enhance adipogenesis, decreased with age. Thus processes intrinsic to adipose cells involving changes in C/EBP family members contribute to impaired adipogenesis and altered fat tissue function with aging. These effects are potentially reversible.

Different kinetics of senescence in human fibroblasts and peritoneal mesothelial cells

Senescence has been reported for a wide variety of human cell types. In cultures of human fibroblasts the process is due to a percentage of the cells becoming senescent at each passage rather than all the cells entering senescence simultaneously at the end of the life span. By measuring the percentage of fibroblasts which are still cycling at each passage, a rate of decline in the growth fraction, which mirrors the rate of senescence, can be obtained. However, such an analysis has never been undertaken in multiple cell types using the same method to identify cycling cells. It is thus unknown if the rate of senescence is the same or different in cultures of different human cell types. To answer this question the rates of decline in the cycling fractions were simultaneously measured in two cultures of human cells (AGO7086A, peritoneal mesothelial cells; and 2DD, human dermal fibroblasts) which have practically identical in vitro life spans. 2DD fibroblasts showed a rate of decline of 0.89% cycling cells per population doubling when the data obtained were fitted to a simple linear equation. However, AGO7086A gave a decline of approximately 2.2% per population doubling. Thus mesothelial cells enter senescence significantly faster than fibroblasts (P < 0.001). This decline in the growth fraction was accompanied by an increasing fraction of mesothelial cells which retained detectable endogenous beta-galactosidase activity at pH 6. Such activity has previously been shown to be associated with senescent human fibroblasts. These findings suggest that the process of senescence has common features in different cell lineages but that the rate of the process can differ markedly between them.

Contribution of apoptosis to the phenotypic changes of adrenocortical cells in primary culture

The aim of this study was to evaluate the occurrence and physiological consequences of apoptosis in primary cultures of bovine adrenocortical cells (of fasciculata-reticularis origin). Under ACTH-free culture conditions, we observed apoptotic cells in the cell layer and the accumulation of apoptotic bodies in the culture medium. These were hardly detectable in ACTH-supplemented cultures. Under ACTH-free conditions, the DNA content of apoptotic bodies collected over 48 h represented up to 10-15% of that of the cell layer at the onset of the culture (as compared to 3% in ACTH-supplemented cultures). Past the fourth day of culture in the absence of ACTh, most cells lacked several markers of their originating fasciculata-reticularis phenotype and progressively evolved to an undifferentiated phenotype. The vast majority of the apoptotic bodies released during the first 4 days of culture were immunoreactive for P450 17 alpha. Inversely, during the same period of time, the proliferating cells (PCNA-positive) did not appear to express P450 17 alpha. Therefore, apoptosis could contribute, together with dedifferentiation, to the phenotype shift observed in ACTH-depleted cultures of adrenal fasciculata-reticularis cells. These observations also characterize this endocrine cell system as an in vitro model for the study of hormone-repressed apoptosis.

Effects of aging on ribosomal protein L7 messenger RNA levels in cultured rat preadipocytes

Ribosomal protein L7 mRNA is a cell cycle-independent message whose levels are lower in late passage "senescent" fibroblasts than early passage cultures. To determine whether decreases in L7 mRNA levels also occur during aging in tissues in vivo and whether reduced L7 mRNA is caused by terminal differentiation, we measured L7 and adipsin (a differentiation-dependent serine protease) mRNA levels in undifferentiated and differentiated preadipocytes and glyceraldehyde-3-phosphate dehydrogenase mRNA in differentiated preadipocytes cultured from perirenal fat depots of 3-, 17-, and 24-month-old male rats. L7 mRNA levels decreased with increasing age and were not affected by differentiation. In the same cultures, adipsin mRNA levels did not increase with age but did increase with differentiation, confirming that the preadipocytes exposed to enriched medium had, in fact, differentiated. Glyceraldehyde-3-phosphate dehydrogenase mRNA levels did not change with age indicating that the decrease in L7 mRNA was not a result of a general decrease in mRNA with age. These observations are consistent with the hypotheses that decreasing L7 mRNA levels are associated with aging and that late passage fibroblasts have features in common with senescence. The observations are not consistent with the hypothesis that senescent changes in cellular function are caused by terminal differentiation.

Changes in gene expression during senescence of adrenocortical cells in culture

Bovine adrenocortical cells undergo a process in which expression of steroid hydroxylases is lost progressively as a function of population doubling level (PDL) in culture. Each cytochrome P450 shows a characteristic rate of loss of expression as a function of PDL (in order of rates of loss: CYP11B >CYP21 >CYP17 >CYP11A). CYP11B and CYP21 require insulin-like growth factor I as well as cyclic AMP; these are the only factors required for induction in the primary culture. Middle- and later passage cells do not express CYP11B and CYP21 under the same conditions, but will do so when cells are grown in extracellular matrix Matrigel. In late-passage cells neither CYP17, CYP21, nor CYP11B are expressed, even in the presence of Matrigel; only CYP11A is expressed in late-passage cultures. When the different environmental factors required for induction of CYP11B and CYP21 are taken into account, induction of these genes disappears with the same kinetics as previously shown for CYP17 as a function of PDL. The primary cause of the loss of expression of these genes is likely to be a phenotypic switching event similar to that previously demonstrated for CYP17 by in situ hybridization. The mechanism of phenotypic switching is unknown. However, one HpaII site at -2.3 kb of CYP17 was methylated in the bovine adrenal cortex in vivo but showed rapid and complete demethylation when adrenocortical cells were placed in culture. This indicates a unique, reproducible, environmentally determined change in methylation, with as yet undetermined consequences. However, data from reporter constructs suggest that phenotypic switching does not result from a simple loss of regulatory factors that act within 2.5 kb of the promoter. Previous data suggested that SV40 T antigen may affect phenotypic switching, and thus that SV40 may be useful for the derivation of functional adrenocortical cell lines. Adaptation of methods previously used for bovine cells to human adrenocortical cells to produce SV40 T antigen-transfected clones yielded data indicating preservation of essential aspects of the human adrenocortical cell differentiated phenotype.

Activity of the CYP17 promoter in bovine adrenocortical cells before and after phenotypic switching

Cultured bovine adrenocortical cells reach replicative senescence after 100-120 population doublings in culture. Before reaching senescence, cells undergo high frequency phenotypic switching from CYP17+ to CYP17-, where '+' and '-' refer to the ability of intracellular cyclic AMP to induce expression of CYP17 (steroid 17 alpha-hydroxylase). We used luciferase reporter constructs to assess the activity of the CYP17 promoter in bovine adrenocortical cells before and after phenotypic switching. We constructed two plasmids containing -2544 to +29 and -488 to +29 of the 5' region of CYP17 linked to a promoterless luciferase gene. Because of technical difficulties with transient transfection of late-passage bovine adrenocortical cells, these experiments were performed using stable transfection. Cells at early passage (PDL 10) and late passage (PDL 55) were cotransfected with either of these two plasmids ligated to pSV3neo, and G418-resistant pools of clones were derived. The activity of the CYP17 promoter in these transfectants was tested by growing cells in complete medium until semiconfluent and then transferring them into defined medium with cholera toxin and insulin-like growth factor I for 6 h. Luciferase activity was consistently induced by cholera toxin/IGF-I over five passages in pooled clones derived by transfection of early passage cells with the -488 construct. Despite the lack of expression of the endogenous CYP17 gene in transfectants from late-passage cells, induced luciferase activity was higher in late-passage transfectants than early-passage transfectants for both the -2544 and -488 constructs.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional and morphological changes associated with the ageing of primary cultures of embryonic adrenal gland cells derived from the Pekin duck

The morphological and functional changes associated with ageing were studied in adrenal steroidogenic cells derived from duck embryos. Cells grown for not more than three days had structural characteristics similar to their counterparts in vivo; they contained numerous lipid droplets and mitochondria, an abundant smooth endoplasmic reticulum, an even network of microtubules, and microfilaments that formed extensive and elaborate systems of parallel stress fibers. After the 3rd day of growth in culture, many of the cells started to decrease in size and become elongated; the older cells showed less well-defined actin filaments and contained elongated mitochondria, fewer lipid droplets, less smooth endoplasmic reticulum, and swollen cisternae of rough endoplasmic reticulum. The proliferative capacity of the cells was the same when they were cultured in either the presence or the absence of 1-24 ACTH. After the first day of growth in culture, the steroidogenic capacity of the cells declined and the addition of 1-24 ACTH to the growth medium did not prevent changes in their structure and function. The decline in steroidogenic capacity occurred both in terms of the amount of hormone released into the culture medium and in the ability of the cells to respond when incubated in buffer containing 1-24 ACTH. Since the basal unstimulated rates of corticosteroid production also declined as the cells aged, it is probable that the steroidogenic deficiency occurs at a site distal to the corticotropin receptor; this is also consistent with the ultrastructural observations that suggest a relationship between the morphological changes and the decline in steroidogenic capacity as the cells age.

Demethylation of specific sites in the 5'-flanking region of the CYP17 genes when bovine adrenocortical cells are placed in culture

DNA methylation of CYP17 (steroid 17 alpha-hydroxylase) was studied in bovine adrenocortical cells, which lose the capacity to express this tissue-specific gene in culture by phenotypic switching. Restriction enzyme digestions, and sequencing of a lambda clone of a second CYP17 gene (CYP17A2), showed that there are at least three CYP17 genes in the bovine genome. Southern blotting of DNA digested with Msp I or Hpa II together with Eco RI was used to investigate the methylation status of Hpa II sites at -1.0 kb (H1), -1.8 kb (H2), and -2.3 kb (H4) in CYP17A1 and CYP17A2 and at -0.7 kb (H0) in CYP17A3. In cells and tissues other than white blood cells, H0 was nonmethylated whereas H1 was always methylated; H2 and H4 showed variation in methylation status among different cells and tissues. In particular, whereas H4 was methylated in the bovine adrenal cortex in vivo, there was a rapid and complete demethylation at H4 when adrenocortical cells were placed in culture. Sites downstream from H4 did not change methylation over the first six passages in culture; additionally, the coding region of CYP17 remained fully methylated under all conditions. In contrast to adrenocortical cells, DNA from fibroblasts was nonmethylated at H2, whereas all downstream sites were fully methylated. Digestion with another methylation-sensitive enzyme, Bsa HI, which has a site between H2 and H4, showed that this region is methylated in intact adrenal cortex but nonmethylated both in cultured adrenocortical cells and in fibroblasts. The specific changes in methylation at this site and at H4 in adrenocortical cells indicate a reproducible, environmentally determined change in methylation in adrenocortical cells when they are placed in culture.

The biochemistry of mammalian senescence

Senescence is a process which, until quite recently, has been the subject of little scientific investigation. Even the word "senescence" is difficult to define, and complex methodological pitfalls have impeded progress. In the past few years, there have been exciting advances in understanding the physiological, cell biological, biochemical, and molecular biological nature of senescence. Changes in membrane function, protein synthesis, DNA structure (including glycosylation, altered tertiary structure, free-radical effects, and loss of telomeric DNA), and changes in gene regulation with age are reviewed. Recent work on changes in responses to transcriptional regulatory proteins and cellular senescence factors, some of which have been identified, is particularly promising and leads to the conclusion that senescence, at least in part, is a programmed process. Despite these advances, the fundamental cause of senescence remains elusive but might, as in the case of other biological processes which are phylogenetically widespread, turn out to be quite simple, and perhaps, even modifiable.

Demethylation of satellite I DNA during senescence of bovine adrenocortical cells in culture

Over the finite proliferative life span of cultured bovine adrenocortical cells, satellite I DNA shows a progressive and extensive loss of methylation at CCGG sites. This was shown by Southern blotting after digestion with the methylation-sensitive enzyme HpaII alone, which provides a sensitive indicator of methylation loss, or digestion with the combination of EcoRI and HpaII, which provides a quantitative indication of loss of methylation. Bovine tissues, including adrenal cortex, all showed a much higher level of satellite methylation than cultured adrenocortical cells. After adrenocortical cells are placed in culture, some demethylation of satellite I is seen as early as 10 population doublings. By 80 population doublings, loss of satellite DNA methylation is extensive. The loss does not appear to prevent continued cell division, since an extended life span clone of bovine adrenocortical cells transfected with SV40 T antigen showed a similar pattern of extensive demethylation. Satellite demethylation has been reported in aging in vivo and the present cell culture system may provide an in vitro model for this form of genetic instability.

A model for phenotypic switching in adrenocortical cells senescing in culture

We have developed a cell model for loss of differentiated gene expression in cellular aging. Over long periods in culture, bovine adrenocortical cells lose expression of a specialized gene, steroid 17 alpha-hydroxylase. The decline in expression of 17 alpha-hydroxylase in mass cultures and clones of bovine adrenocortical cells is the result of a phenotypic switching process which yields a mixture of cells that can express 17 alpha-hydroxylase after induction with cyclic AMP and cells that are incapable of expression of 17 alpha-hydroxylase. In the experimental portion of the work, bovine adrenocortical cells were grown in culture in colonies, stimulated with cyclic AMP to induce 17 alpha-hydroxylase, then fixed and hybridized in situ with labeled 17 alpha-hydroxylase cDNA. Separate images of Giemsa-stained cell colonies and of their hybridization patterns were digitized and combined as stylized representations of the colonies for comparison with those produced by a computer simulation. A program for the simulation of growth of colonies of bovine adrenocortical cells in senescence with phenotypic switching of 17 alpha-hydroxylase is presented. Comparison between simulated colonies and real colonies shows that the model accurately simulates colony growth and phenotypic switching. It suggests that the probability of phenotypic switching per cell generation is in the range of 0.03 to 0.06. The major variable among colonies is division probability, consistent with observations in this and other cell culture systems that clones differ widely in replicative potential. Thus, phenotypic switching of 17 alpha-hydroxylase in adrenocortical cells may be modelled using simple assumptions.

Changes in gene expression and DNA methylation in adrenocortical cells senescing in culture

Recent experiments in cultured bovine adrenocortical cells show that the previously observed phenotypic switching of CYP17 (steroid 17 alpha-hydroxylase) expression is preceded at a much earlier time by changes in methylation in the CYP17 5' flanking region. Two CpG sites that are methylated in the adrenal cortex in vivo were observed to undergo rapid demethylation when adrenocortical cells were placed in culture. Two adjacent CpG sites that are also methylated in vivo did not demethylate; these two sites are completely nonmethylated in fibroblasts. All CpG sites downstream, in the promoter or coding region, are always methylated in all tissues and in bovine adrenocortical cells even after many population doublings in culture. In contrast to the specific and rapid demethylation of sites in CYP17, satellite I shows a slower and apparently random loss of methylation that extends over the entire replicative life span. These changes in methylation provide examples of genetic instability in cells that undergo senescence in culture. Future experiments will focus on the relationship of these events to the phenotypic switching process.

Regulation of steroid hydroxylases in normal and SV40 T antigen-transfected bovine adrenocortical cells in long-term culture

Over long periods of growth in culture, bovine adrenocortical cells lose the ability to express the steroid hydroxylase genes. For 17 alpha-hydroxylase, cells show a stochastic pattern of phenotypic switching from a state in which they express this gene in response to cyclic AMP to a state in which the gene is no longer inducible. Introducing SV40 T antigen into bovine adrenocortical cells greatly increases their replicative potential; steroid hydroxylase expression in these clones resembles that of the precursor cells before transfection. The other steroid hydroxylases (21-hydroxylase and 11 beta-hydroxylase) appear to undergo phenotypic switching like 17 alpha-hydroxylase. The loss of expression of these genes appears to be more rapid, but there are differences in the requirements of 21-hydroxylase and 11 beta-hydroxylase versus 17 alpha-hydroxylase for induction by cyclic AMP; additionally, growth of cells in extracellular matrix Matrigel was required for expression of 21-hydroxylase and 11 beta-hydroxylase in long-term cultures of either normal or SV40 T antigen-transfected cells. Understanding the molecular basis for the phenotypic switching of steroid hydroxylases that occurs in bovine adrenocortical cells may elucidate mechanisms for cellular senescence and for maintenance of tissue-specific functions during long-term growth in culture.

Cytotoxic effects of expression of human superoxide dismutase in bovine adrenocortical cells

Oxygen radicals and the cellular antioxidant enzymes may play a role in cellular senescence. We studied the feasibility of altering oxygen radical metabolism in a normal differentiated cell that undergoes senescence in culture by transfection of an expression vector containing human CuZn-SOD cDNA. Plasmid pRSV2-cSOD was constructed to contain the cDNA for human CuZn-SOD under the regulation of the Rous sarcoma virus long terminal repeat. Early passage cultures of bovine adrenocortical cells were cotransfected with pRSV2-cSOD and a plasmid (pSV3neo) allowing initial selection and continued growth of transfectants. Three passages after isolation of the polyclonal population, as cells grew to confluence, cultures showed focal cell death that spread outward to affect neighboring cells, so that by 72 h most cells had detached from the culture dish. Long term growth of the polyclonal population of transfectants without extensive cell death was achieved by continuous maintenance of low cell density during growth. Southern blot analysis of DNA from the pooled polyclonal population of transfected cells showed the presence of the expected 625 bp band from human CuZn-SOD. However, the intensity of this band indicated that only a minority of cells in the population had integrated the SOD plasmid, and DNA isolated from cells after 25 passages at low cell density showed plasmid sequences only of an altered form, suggesting that cells containing intact human SOD cDNA had been selectively lost from the population. When early passage low density transfectants were allowed to grow back to high cell density, cell death foci were again observed. Additionally, cell fusion with the formation of giant cells with massive multinucleation was observed by fluorescence microscopy after staining cultures with a DNA binding dye. In later stages of this process, the large nuclear mass in such a giant cell became fragmented as the cell detached from the dish and formed the center of a focus of cell death in the surrounding cells. Because cell death prevented the growth of large numbers of transfected cells, it was not possible to demonstrate the involvement of CuZn-SOD in the cytotoxic effect by direct means, but the control plasmid without the CuZn-SOD cDNA insert had no cytotoxic effect. Thus, the introduction of a vector for human CuZn-SOD in a normal differentiated cell caused a cytotoxic effect involving cell death, cell fusion, and nuclear fragmentation.