Clonal expansion versus functional clonal inactivation: a costimulatory signalling pathway determines the outcome of T cell antigen receptor occupancy.
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Antigen-Presenting Cells↗
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T cell receptor (TCR) BV gene repertoires and clonal expansions of CD4 cells in patients with HIV infections.
Despite extensive investigation, the pathogenesis of T cell depletions that characterize AIDS has not been elucidated. To study this process further, we evaluated T cell antigen receptor beta-chain variable gene (TCRBV) repertoires in peripheral blood lymphocytes (PBL) of 23 HIV-infected patients. Expression levels of 28 TCRBV were determined by multiprobe RNase protection assay after polymerase chain reaction (PCR) amplifications. Abnormal expansions (> 2 s.d. from mean normal values) were frequent in HIV CD4, accounting for 26% of total measured TCRBV in this population. The number and magnitude of abnormalities among individuals were inversely proportional to their CD4 counts (P < 0.012 and P < 0.01, respectively). While abnormalities were not randomly distributed among TCRBV subfamilies, no particular genes were expanded or contracted among all patients. Only 14% of CD8 TCRBV were proportionally expanded (P < 0.01 compared with CD4), and there were limited concordances between paired CD8 and CD4 repertoires among individuals. CDR3 length analyses and TCRBV sequencing showed that most CD4 expansions comprised clonal or oligoclonal populations. Thus, T cell responses in HIV patients are characterized by severe TCRBV biases and clonal expansions among CD4 subsets, and these processes are exaggerated with disease progression. The heterogeneity and oligoclonality of the TCRBV expansions are consistent with responses to HIV-encoded or other conventional antigens rather than superantigenic effects. The presence of CD4 clonal proliferations in these patients may be important in the pathogenesis of HIV, and the absence or reduction of many T cell specificities due to oligoclonal expansions may increase susceptibility to opportunistic infections.
Acquired Immunodeficiency Syndrome↗
Clonal expansion but lack of subsequent clonal deletion of bacterial superantigen-reactive T cells in murine retroviral infection.
Several studies have suggested that activation-induced apoptosis of Ag-specific CD4+ T cells leads to depletion of this subset during HIV infection. The bacterial superantigen, staphylococcal enterotoxin A (SEA), is known to induce activation-induced apoptosis in the TCR V beta-bearing CD4+ T cells in the periphery after clonal expansion of these cells. The murine retroviral model of AIDS (MAIDS), which is induced by LP-BM5 murine leukemia virus, shares many common features with HIV infection in humans, except that CD4+ T cells increase progressively in susceptible strains. In this study, we challenged SEA to MAIDS mice and examined whether this retrovirus affects the fate of the SEA-reactive CD4+ T cells in vivo. At 4 wk post-infection with LP-BM5 murine leukemia virus, clonal expression and subsequent deletion of SEA-reactive CD4+V beta 3+ T cells occurred normally after SEA administration, whereas in vitro proliferative responses were severely impaired. At 8 wk postinfection, the in vivo expansion of CD4+V beta 3+ T cells was evident, but not followed by clonal deletion, as late as 14 days after SEA administration. This expanding subset in the infected mice expressed the Fas Ag in the same amount as the same subset in uninfected controls. These findings suggest that activation-induced apoptosis of superantigen-reactive CD4+ T cells is interfered with in vivo during the course of MAIDS, which is not attributable to underexpression of the Fas Ag by the CD4+ T cells.
Animals↗
Genetic instability and clonal expansion.
Inactivation of tumor suppressor genes can lead to clonal expansion. We study the evolutionary dynamics of this process and calculate the probability that inactivation of a tumor suppressor gene is preceded by mutations in genes that confer genetic instability. Unstable cells might have a slower rate of clonal expansion than stable cells because of an increased probability of generating lethal mutations or inducing apoptosis. We show that the different growth rates of genetically stable and unstable cells during clonal expansion represent, in general, only a small disadvantage for genetic instability. The intuitive reason for this conclusion is that robust clonal expansion, where cellular birth rates are significantly greater than death rates, occurs on a much faster time scale than waiting for those mutations that allow clonal expansion. Moreover, in special cases where clonal expansion is very slow, genetically unstable cells have a higher probability to accumulate additional mutations during clonal expansion that confer a selective advantage. Clonal expansion represents a major disadvantage for genetic instability only when inactivation of the tumor suppressor gene leads to a very small increase of the cellular reproductive rate that is cancelled by the increased mortality of unstable cells.
Animals↗
Cells that present both specific ligand and costimulatory activity are the most efficient inducers of clonal expansion of normal CD4 T cells.
Clonal expansion of naive CD4 T cells is a necessary step in most adaptive immune responses. Two distinct signals are required for clonal expansion to occur, ligation of T-cell receptors by an antigenic peptide bound to self major histocompatibility complex-encoded class II molecules (signal 1) and a costimulatory signal derived from an antigen-presenting cell (signal 2). To study whether these two signals need to be delivered by a single cell in order to induce clonal expansion of normal CD4 T cells, we have used anti-CD3 bound to Fc receptors as a ligand for the T-cell receptor to deliver signal 1 to all CD4T cells, and we have inactivated signal 2 with a newly generated monoclonal antibody or by using Fc receptor-positive cells that lack the costimulator. Costimulation was delivered by cells whose Fc receptors were blocked with anti-Fc receptor monoclonal antibody. Our results indicate that delivery of ligand and costimulator on one cell is at least 30-fold more efficient than separate delivery. No significant clonal expansion was observed when signals 1 and 2 were delivered by different cells. We have also carried out experiments using fibroblast transfectants that can deliver either or both of these two signals. These studies show that separate delivery of these two signals is at least 80-fold less efficient than their combined delivery by one cell. These findings may explain why tissues can express autoantigens and contain active antigen-presenting cells without inducing autoimmunity.
Animals↗
Aging and DNA damage are associated with the development of endothelial cell clonal expansion.
Endothelial dysfunction is a hallmark of vascular aging and a key contributor to cardiovascular disease. Although senescence has been widely studied as a terminal endothelial cell fate, recent evidence suggests that clonal expansion, the proliferative expansion of genetically identical cells, may also occur in aged tissues. We sought to determine whether endothelial clonal expansion increases with age, specifically at the atheroprone regions of the aorta, and to evaluate whether DNA damage promotes endothelial cell clonal expansion. Tamoxifen-inducible, endothelial-specific Cdh5-CreERT2 male and female mice were used to quantify clonal expansion in endothelial cells (ECs) across the aortic region in both young (4 mo) and aged (24 mo) mice. We further examined the effect of DNA damage by administering systemic doxorubicin (DOXO) to assess clonal dynamics in different aortic regions. Aging significantly increased EC clone size and the percentage of clonal ECs in atheroprone regions, particularly the minor arch, whereas only clone size increased in nonatheroprone regions. Systemic DOXO administration increased clone size across the aortic region without altering clonal recruitment, indicating selective amplification of preexisting clones. These findings suggest that clonal expansion is promoted by both aging and DNA damage. Clonal expansion may represent an underrecognized mechanism contributing to endothelial homogeneity and vascular remodeling during aging and in response to sublethal genomic stress.NEW & NOTEWORTHY Aging reshapes the vascular endothelium in unexpected ways. Using lineage tracing in mice, we show that endothelial cells undergo age-dependent clonal expansion, particularly in atheroprone regions exposed to disturbed blood flow. This process is amplified by DNA damage and reflects the selective expansion of preexisting clones rather than increased recruitment. Endothelial clonal expansion may represent an underrecognized mechanism driving vascular remodeling during aging and genotoxic stress.
Animals↗
Non-malignant clonal expansions of CD8+ memory T cells in aged individuals.
CD8(+) T cells provide a major line of defense against intracellular pathogens. Upon encounter with antigen, CD8(+) T cells go through three distinct phases involving proliferation, contraction, and differentiation to become eventually long-lived CD8(+) memory T cells. CD8(+) memory T cells provide long-term protection against infection by intracellular pathogens. CD8(+) memory T-cell proliferation and survival are regulated by many factors, including cytokines, and CD8(+) memory T cells are stably maintained over a period of months to years. In aged humans and mice, however, there are significant alterations to the CD8(+) memory T-cell compartment with frequent development of monoclonal expansions of CD8(+) memory T cells in healthy individuals. Interestingly, CD8(+) clonal expansions are not malignant and do not progress to lymphomas, suggesting that these cells must still be under certain constraints. In this review, we discuss our current understanding of factors that contribute to and regulate these CD8(+) clonal expansions as well as the impact of CD8(+) clonal expansions on immune function of the aged. In addition, we discuss similarities and differences between CD8(+) clonal expansions observed in humans and mice, and we postulate that CD8(+) clonal expansions represent a spectrum of biological outcomes ranging from antigen-driven to antigen-independent phenomena.
Aging↗
B lymphocytic clonal expansion in rheumatoid arthritis.
OBJECTIVE: To learn whether rheumatoid factor (RF), HLA-DR4, or current therapy influences clonal expansion of B lymphocytes (B cells) in persons with rheumatoid arthritis (RA). METHODS: We measured clonal expansion by analysis of cell surface staining for immunoglobulin light chains. Double staining methods detected a B cell marker (CD19) plus either kappa or lambda on peripheral blood lymphocytes from subjects with RA (n = 26) and controls (n = 26). The difference between frequency histograms of surface kappa and lambda staining was determined by the Kolmogorov-Smirnov statistic D that represents the fraction of clonally expanded B cells. RESULTS: The mean D value in RA was over 50% higher than in the controls [0.225 +/- SD 0.155 versus 0.144 +/- 0.025 (p < 0.0001)]. Ten subjects with RA had values exceeding +2 SD for controls (p = 0.0007). Mean D correlated with RF titer (Spearman's rank correlation coefficient rSp + 0.53, p = 0.006). All 10 high D values were found in the RA subgroups with positive serum tests for RF and with the HLA-DR4 positive genotype. The channel of maximal difference between kappa and lambda staining was higher in the RA group than in controls, showing that clonal expansion was most marked among brightly staining cells. Patients with RA currently receiving low dose methotrexate (MTX) tended to have higher D values than those not receiving MTX (mean 0.29 versus 0.18, respectively, p < 0.025). The RA group currently receiving MTX had a higher frequency of abnormal D values (7 of 11 versus 3 of 15 not currently receiving MTX, p = 0.03). This probably reflects preferential use of MTX for severely affected individuals. Confirmatory studies to detect clonal immunoglobulin gene rearrangements were attempted in selected individuals with high D values, but none was demonstrated in total leukocytic or B cell enriched fractions. CONCLUSION: Findings consistent with B cell clonal expansion occur in about 40% of persons with RA, particularly in the subgroups with positive serum tests for RF and with the HLA-DR4 genotype. However, the clonal expansion level must be below the sensitivity of confirmatory methods.
Adult↗
UVB-induced apoptosis drives clonal expansion during skin tumor development.
The mechanism by which a single mutant cell clonally expands is usually assumed to involve an additional mutation in a cell cycle regulatory gene. An alternative mechanism for driving clonal expansion is apoptosis, which might create vacant stem cell compartments that can be repopulated by mutant cells. This model predicts that in a mouse with reduced apoptotic capacity (i) more mutated cells will appear initially but (ii) these cells will expand into clones more slowly than in wild-type animals. To test this hypothesis for ultraviolet B (UVB)-induced skin carcinogenesis, we examined UVB-induced p53 mutant clones and tumors in a transgenic (Tg) mouse (K14-Survivin) with skin-specific expression of the apoptosis inhibitor Survivin. To limit the effects of Survivin on apoptosis, without affecting epidermal proliferation or differentiation, we used Survivin expression levels and UVB doses that resulted in a 2-fold reduction in keratinocyte apoptosis. After 5 weeks of chronic UVB irradiation, newly created p53 mutant keratinocyte clones (indicative of initial mutation frequency) were 1.4-fold more frequent in K14-Survivin mice (P = 4 x 10(-6)). As predicted, this effect was reversed for clones growing by clonal expansion, which were rarer in Tg skin by 1.7-fold (P = 0.047). At 10 weeks large expanding Tg clones were rarer by a magnitude approaching the apoptosis differential (approximately 2-fold, P = 4 x 10(-5)). Survivin expression also retarded clonal expansion at later stages of tumor development. By 20 weeks 95% of animals carried tumors (primarily papillomas), which were 1.6-fold rarer in apoptosis-defective Tg mice (P = 0.03). In contrast, the rate of tumors attaining large size (> or =3 mm, P = 0.048) and converting to carcinoma was increased approximately 2-fold in Tg mice. Thus, Survivin-regulated apoptosis appears to suppress two stages that involve new mutations, initiation and malignant conversion, yet drives clonal expansion of existing p53 mutant cells.
Animals↗
DNA synthesis and mitotic clonal expansion is not a required step for 3T3-L1 preadipocyte differentiation into adipocytes.
Upon differentiation induction of 3T3-L1 preadipocytes by a hormone mixture containing 1-isobutyl-3-methylxanthine, dexamethasone, and insulin, the preadipocytes undergo approximately 2 rounds of mitotic clonal expansion, which just precedes the adipogenic gene expression program and has been thought to be an essential early step for differentiation initiation. By inducing 3T3-L1 preadipocytes with each individual hormone, it was determined that the mitotic clonal expansion was induced only by insulin and not by 1-isobutyl-3-methylxanthine or dexamethasone. Cell number counting and fluorescence-activated cell-sorting analysis indicated that a significant fraction of 3T3-L1 preadipocytes differentiated into adipocytes without mitotic clonal expansion when induced with the combination of 1-isobutyl-3-methylxanthine and dexamethasone. Furthermore, when normally induced 3T3-L1 preadipocytes were treated with PD98059 (an inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1) to block the activation of extracellular signal-regulated kinase (Erk) 1 and Erk2, the mitotic clonal expansion was blocked, but adipocyte differentiation was not affected. These observations were confirmed by bromodeoxyuridine labeling. The differentiated adipocytes induced with 1-isobutyl-3-methylxanthine and dexamethasone or standard hormone mixture plus PD98059 were not labeled by bromodeoxyuridine. Thus, it is evident that 3T3-L1 preadipocytes could differentiate into adipocytes without DNA synthesis and mitotic clonal expansion. Our results also suggested that activation of Erk1 and Erk2 is essential to but not sufficient for induction of mitotic clonal expansion.
1-Methyl-3-isobutylxanthine↗
CDR3 spectratyping identifies clonal expansion within T-cell subpopulations that demonstrate therapeutic antitumor activity.
BACKGROUND: Antigen-specific T cells undergoing clonal expansion share common rearrangements of the variable complementary determining region 3 (CDR3) of the T-cell receptor (TCR), which can be identified using polymerase chain reaction-based V beta (VB) spectratyping. The purpose of this study was to determine whether CDR3 spectratyping identifies clonal expansion within tumor-draining lymph node (TDLN) subpopulations with antitumor therapeutic activity. METHODS: Recently sensitized T cells from 4T1 murine mammary carcinoma TDLN were fractionated based on CD62L (L-selectin) surface expression before RNA isolation and culture. L-selectinlow and L-selectinhigh TDLN were analyzed for T-cell receptor usage by immunophenotyping and CDR3 spectratyping, and then culture activated with anti-CD3/IL-2 to assess therapeutic efficacy after adoptive transfer. RESULTS: Adoptive transfer experiments confirmed that mice treated with culture-activated L-selectinlow TDLN cells exhibited delayed subcutaneous tumor growth and prolonged survival as compared to control or L-selectinhigh-treated mice (P < .01). CDR3 spectratyping demonstrated oligoclonal skewing of the CDR3 regions within several VB families including VB3, VB5.2, and VB17 in L-selectinlow but not in L-selectinhigh TDLN. Although fluorescence-activated cell sorter analysis demonstrated the highest percentage of cells expressing VB13 usage in both populations, CDR3 spectratyping did not identify the presence of clonal expansion. CONCLUSIONS: These data suggest that CDR3 spectratyping may be useful in identifying T cells undergoing clonal expansion that demonstrate antitumor therapeutic activity.
Animals↗
Rapamycin-sensitive phase of 3T3-L1 preadipocyte differentiation after clonal expansion.
Inhibition of insulin-induced 3T3-L1 preadipocyte differentiation by rapamycin has been attributed to a blockade of the early critical clonal expansion phase of the adipogenic program. Rapamycin binds to, and inhibits, mTOR (mammalian target of rapamycin), leading to diminution of p70 S6 kinase activity and eukaryotic initiation factor 4E binding protein 1 (eIF4E-BP1) function. Our objective was to determine if rapamycin-sensitive pathways exist subsequent to the clonal expansion phase. We determined that the mitotic clonal expansion was complete by day 4 of the differentiation protocol, based on the response to Ara-C (cytosine beta-D-arabinofuranoside), which only inhibits differentiation when administered during this phase. Treatment of differentiating 3T3-L1 cells with rapamycin, starting on day 4, exerted potent negative effects on glycerol phosphate dehydrogenase activity, and triacylglycerol accumulation, as well as on the protein expression of adipogenic transcription factors, C/EBPalpha and PPARgamma. Insulin-stimulated p70 S6 kinase activity, and its inhibition by rapamycin, were comparable in preadipocytes at day 0 vs. day 4 post-differentiation. We conclude that a component of the adipogenic program, operating after the completion of clonal expansion, is inhibited by rapamycin, suggesting an ongoing need for mTOR function in this process.
3T3 Cells↗
T cell receptor Vbeta repertoire usage and clonal expansion of T cells in chronic myelogenous leukemia.
BACKGROUND: In general, it is very important to understand the state of T cell immune response against tumor cells in leukemia patients and it is especially critical to assess the T cell repertoire of untreated patients. As we know, few studies have dealt with the distribution of oligoclonal T cells in leukemia, so we investigated the distribution and clonality of TCR Vbeta repertoire of T cells in patients with chronic myelogenous leukemia (CML) in chronic phase. METHODS: The complementarity determining region 3 (CDR3) of TCR Vbeta24 subfamily genes were amplified in peripheral blood mononuclear cells from 27 cases with CML using reverse transcription-polymerase chain reaction (RT-PCR). In order to observe the distribution of TCR Vbeta repertoire, the PCR products were further analyzed by genescan technique to evaluate clonality of the detectable TCR Vbeta T cells. The PCR products of the oligoclonal T cells from three cases were analyzed by direct sequencing to define the sequence of CDR3. RESULTS: The expression pattern of TCR Vbeta repertoire in different individuals are different. Vbeta2-21 subfamilies could be detected in CML cases. The frequent usage Vbeta repertoire in CML was Vbeta1, Vbeta2 or Vbeta13. Most of the PCR products from 27 patients displayed polyclonality, while a part of the PCR products from 21 out of 27 samples displayed clonal expansion pattern. The clonal expanded T cells in CML could be found in Vbeta16 subfamilies. The frequent usage of Vbeta genes in clonal expansion was Vbeta3, Vbeta13 or Vbeta21. Multiple Vbeta clonal expansion was a general phenomenon in the same patient. The CDR3 sequence of Vbeta21 oligoclonal T cells from 3 cases showed some difference in splice regions and in the usage of J segments. CONCLUSIONS: These results indicated that clonal expanded T cells could be found in patients with CML and were tendentious in Vbeta3, Vbeta13 and Vbeta21 subfamilies that may be related to the specific immune response for leukemia cell associated antigen.
Clone Cells↗
Frequent intracellular clonal expansions of somatic mtDNA mutations: significance and mechanisms.
It has been proposed that age-dependent accumulation of somatic mutations in mtDNA is responsible for some aspects of the aging process. However, most cells contain hundreds to thousands of mtDNA molecules. Any nascent somatic mutant therefore appears as a single copy among a majority of wild-type species. A single mutant molecule is unlikely to influence the physiology of the cell and thus cannot play a role in the aging process. To affect cellular physiology, the nascent somatic mutants must somehow accumulate clonally in the cell to significant levels. The evidence supporting the view that, indeed, clonal expansion of mtDNA mutations is a widespread process in various human tissues, and the mechanisms by which clonal expansions may affect the aging process, are reviewed. Originally, clonal expansion was demonstrated for mtDNA with large deletions in muscle. Cell-by-cell analysis of human cardiomyocytes and buccal epithelial cells revealed that clonal expansion affects point mtDNA mutations as well as deletions. Expansions are not limited to muscle, but likely are present in most tissues, and almost every cell of an aged tissue is likely to be affected by an expansion. While the very existence of clonal expansion is beyond doubt, the mechanisms driving this process are largely controversial. The hypotheses explaining expansion includes random or various selective mechanisms, or both. We show that the spectra of expanded point mutations are drastically different in cardiomyocytes and epithelial cells. This suggests that the mechanisms of expansion in these tissues are different. In particular, we propose random segregation and positive selection models for epithelial and muscle cells, respectively.
Aging↗
Differential regulation of activation, clonal expansion, and antibody secretion in human B cells.
Limiting dilution analysis, hemolytic plaque assay, and ELISA procedures were used to study the recruitment, clonal expansion, and antibody secretion in human TNP-specific B cells activated in the presence of TNP-ovalbumin (TNP-OA), pokeweed mitogen (PWM), or regulatory T cells. TNP-OA-responsive, hapten-specific PFC precursor cells occupy approximately 0.5% of all sIgM+/sIgD+ B cells in cord blood, bone marrow, peripheral blood, and tonsil. The PWM-responsive, hapten-specific PFC precursor pool is 70 to 90% smaller and does not express sIgD. Antigen-reactive B cells go through a minimum of three divisions in culture (six to nine PFC per clone), and antibody secretory rates of about 10(4) molecules IgM/cell/hr are achieved. In contrast, PWM-induced clone sizes were at least 60 PFC per clone, with antibody secretory rates of approximately 6 to 7 X 10(4) molecules IgM/cell/hr. Addition of high-dose carrier-primed suppressor T cells to limit dilution cultures reduced PFC precursor cell recruitment by up to 99%. However, in the few clones escaping from suppression, both clonal expansion and antibody secretory rates were much higher than in suppressor cell-free cultures, generating 30 to 60% of the antibody secreted in controls but with consequently much more restricted clonal diversity. When limiting dilution cultures were compared with standard microcultures of 2 X 10(5) cells, both clonal expansion and antibody secretory rates were much lower than expected, with a culture efficiency calculated to be 10 to 20% of that in low-density cultures. Our data suggest that the B cell subsets activated by antigen and by mitogen differ in their abilities for clonal expansion and antibody secretion. The hapten-specific and -responsive B cell family is expressed early in ontogeny, and in adults it is distributed evenly throughout the body. These limiting dilution experiments revealed that the primary effect of regulatory T cells is a drastic reduction in clonal diversity, and much less a mere reduction in overall response magnitude.
Aging↗
PPARgamma1 synthesis and adipogenesis in C3H10T1/2 cells depends on S-phase progression, but does not require mitotic clonal expansion.
Adipogenesis is typically stimulated in mouse embryo fibroblast (MEF) lines by a standard hormonal combination of insulin (I), dexamethasone (D), and methylisobutylxanthine (M), administered with a fresh serum renewal. In C3H10T1/2 (10T1/2) cells, peroxisome proliferator-activated receptor gamma1 (PPARgamma1) expression, an early phase key adipogenic regulator, is optimal after 36 h of IDM stimulation. Although previous studies provide evidence that mitotic clonal expansion of 3T3-L1 cells is essential for adipogenesis, we show, here, that 10T1/2 cells do not require mitotic clonal expansion, but depend on cell cycle progression through S-phase to commit to adipocyte differentiation. Exclusion of two major mitogenic stimuli (DM without insulin and fresh serum renewal) from standard IDM protocol removed mitotic clonal expansion, but sustained equivalent PPARgamma1 synthesis and lipogenesis. Different S-phase inhibitors (aphidicolin, hydroxyurea, l-mimosine, and roscovitin) each arrested cells in S-phase, under hormonal stimulation, and completely blocked PPARgamma1 synthesis and lipogenesis. However, G2/M inhibitors effected G2/M accumulation of IDM stimulated cells and prevented mitosis, but fully sustained PPARgamma1 synthesis and lipogenesis. DM stimulation with or without fresh serum renewal elevated DNA synthesis in a proportion of cells (measured by BrdU labeling) and accumulation of cell cycle progression in G2/M-phase without complete mitosis. By contrast, standard IDM treatments with fresh serum renewal caused elevated DNA synthesis and mitotic clonal expansion while achieved equivalent level of adipogenesis. At most, one-half of the 10T1/2 mixed cell population differentiated to mature adipocytes, even when clonally isolated. PPARgamma was exclusively expressed in the cells that contained lipid droplets. IDM stimulated comparable PPARgamma1 synthesis and lipogenesis in isolated cells at low cell density (LD) culture, but in about half of the cells and with sensitivity to G1/S, but not G2/M inhibitors. Importantly, growth arrest occurred in all differentiating cells, while continuous mitotic clonal expansion occurred in non-differentiating cells. Irrespective of confluence level, 10T1/2 cells differentiate after progression through S-phase, where adipogenic commitment induced by IDM stimulation is a prerequisite for PPARgamma synthesis and subsequent adipocyte differentiation.
1-Methyl-3-isobutylxanthine↗
Are there differences in the cell cycle of normal and malignant cells? An approach to the analysis of the clonal expansion of cells in malignant epithelial proliferations in situ.
The Williams-Bjerknes model for clonal expansion of transformed cells in epithelia was used to examine a series of related questions about the growth of in situ proliferations in epidermis. Bowens disease, actinic keratosis and lentigo maligna lesions were reconstructed on a three-dimensional basis, and all were found to behave differently: the evidence supported the proposal that the 'carcinogenic advantage' enjoyed by transformed cells was proliferative in nature, and indeed varied between the three lesions. We conclude that the clonal expansion of a single cell, with different proliferative carcinogenic advantage, can explain the three-dimensional appearances of these in situ proliferations; furthermore, the well-known 'multifocality' of in situ epidermal proliferations can also be explained by the clonal expansion of a single transformed cell.
Animals↗
Mitotic clonal expansion during preadipocyte differentiation: calpain-mediated turnover of p27.
Evidence is presented that calpain, a calcium-activated protease, degrades the cyclin-dependent kinase inhibitor, p27, during the mitotic clonal expansion phase of 3T3-L1 preadipocyte differentiation. Calpain activity is required during an early stage of the adipocyte differentiation program. Thus, inhibition of calpain with N-acetyl-Leu-Leu-norleucinal (ALLN) blocks clonal expansion and acquisition of the adipocyte phenotype only when added between 12 and 24 h after the induction of differentiation. Likewise, inhibition of calpain by overexpression of calpastatin, the specific endogenous inhibitor of calpain, prevents 2-day post-confluent preadipocytes from reentering the cell cycle triggered by the differentiation inducers. Inhibition of calpain with ALLN causes preadipocytes to arrest just prior to S phase and prevents phosphorylation of the retinoblastoma gene product, DNA replication, clonal expansion, and subsequent adipocyte differentiation but does not affect the expression of immediate early genes (i.e. fos, jun, C/EBPbeta, and C/EBPdelta). Inhibition of calpain by either ALLN or by overexpression of calpastatin blocks the degradation of p27. p27 is degraded in vitro by cell-free extracts from clonally expanding preadipocytes that contain "active" calpain but not by extracts from pre-mitotic preadipocytes that do not. This action is inhibited by calpastatin or ALLN. Likewise, p27 in preadipocyte extracts is a substrate for purified calpain; this proteolytic action was inhibited by heat inactivation, EGTA, or ALLN. Thus, extracellular signals from the differentiation inducers appear to activate calpain, which degrades p27 allowing density-dependent inhibited preadipocytes to reenter the cell cycle and undergo mitotic clonal expansion.
3T3 Cells↗