Analysis of events associated with serum deprivation-induced apoptosis in C3H/Sol8 muscle satellite cells

Inhibitors of nuclear factor kappa B cause apoptosis in cultured macrophages

The precise role of the transcription factor nuclear factor kappa B (NF- κB) in the regulation of cell survival and cell death is still unresolved and may depend on cell type and position in the cell cycle. The aim of this study was to determine if three pharmacologic inhibitors of NF-κB, pyrrolidine dithiocarbamate, N-tosyl-L-lysl chloromethyl ketone and calpain I inhibitor, induce apoptosis in a murine macrophage cell line (RAW 264.7) at doses similar to those required for NF-κB inhibition. We found that each of the three inhibitors resulted in a dose- and time-dependent increase in morphologic indices of apoptosis in unstimulated, LPS-stimulated and TNF-stimulated cells. Lethal doses were consistent with those required for NF- κB inhibition. We conclude that nuclear NF-κB activation may represent an important survival mechanism in macrophages.

alpha-Skeletal muscle actin nemaline myopathy mutants cause cell death in cultured muscle cells

Nemaline myopathy is a neuromuscular disorder, characterized by muscle weakness and hypotonia and is, in 20% of the cases, caused by mutations in the gene encoding alpha-skeletal muscle actin, ACTA1. It is a heterogeneous disease with various clinical phenotypes and severities. In patients the ultrastructure of muscle cells is often disturbed by nemaline rods and it is thought this is the cause for muscle weakness. To search for possible defects during muscle cell differentiation we expressed alpha-actin mutants in myoblasts and allowed these cells to differentiate into myotubes. Surprisingly, we observed two striking new phenotypes in differentiating myoblasts: rounding up of cells and bleb formation, two features reminiscent of apoptosis. Indeed expression of these mutants induced cell death with apoptotic features in muscle cell culture, using AIF and endonuclease G, in a caspase-independent but calpain-dependent pathway. This is the first report on a common cellular defect induced by NM causing actin mutants, independent of their biochemical phenotypes or rod and aggregate formation capacity. These data suggest that lack of type II fibers or atrophy observed in nemaline myopathy patients may be also due to an increased number of dying muscle cells.

Increased expression of the pro-apoptotic Bcl2 family member PUMA is required for mitochondrial release of cytochrome C and the apoptosis associated with skeletal myoblast differentiation

We have previously shown that when skeletal myoblasts are cultured in differentiation medium (DM), roughly 30% undergo caspase 3-dependent apoptosis rather than differentiation. Herein, we investigate the molecular mechanism responsible for the activation of caspase 3 and the ensuing apoptosis. When 23A2 myoblasts are cultured in DM, caspase 9 activity is increased and pharmacological abrogation of caspase 9 activation impairs caspase 3 activation and apoptosis. Further, we detect a time dependent release of mitochondrial cytochrome C into the cytosol in roughly 30% of myoblasts. Inclusion of cycloheximide inhibits the release of cytochrome C, the activation of caspase 9 and apoptosis. These data indicate that the mitochondrial pathway plays a role in this apoptotic process and that engagement of this pathway relies on de novo protein synthesis. Through RT-PCR and immunoblot analysis, we have determined that the expression level of the pro-apoptotic Bcl2 family member PUMA is elevated when 23A2 myoblasts are cultured in DM. Further, silencing of PUMA inhibits the release of cytochrome C and apoptosis. Signaling by the transcription factor p53 is not responsible for the increased level of PUMA. Finally, myoblasts rescued from apoptosis by either inhibition of elevated caspase 9 activity or silencing of PUMA are competent for differentiation. These results indicate a critical role for PUMA in the apoptosis associated with skeletal myoblast differentiation and that a p53-independent mechanism is responsible for the increased expression of PUMA in these cells.

Signaling through the TRAIL receptor DR5/FADD pathway plays a role in the apoptosis associated with skeletal myoblast differentiation

Apoptosis rather than differentiation is a physiological process during myogenesis and muscle regeneration. When cultured myoblasts were induced to differentiate, we detected an increase in caspase 8 activity. Pharmacological inhibition of caspase 8 activity decreased apoptosis. Expression of a dominant-negative mutant of the adapter protein FADD also abrogated apoptosis, implicating a death ligand pathway. Treatment with TRAIL, but not Fas, induced apoptosis in these myoblasts. Accordingly, treatment with a soluble TRAIL decoy receptor or expression of a dominant-negative mutant of the TRAIL receptor DR5 abrogated apoptosis. While TRAIL expression levels remained unaltered in apoptotic myoblasts, DR5 expression levels increased. Finally, we also detected a reduction in FLIP, a death-receptor effector protein and caspase 8 competitive inhibitor, to undetectable levels in apoptotic myoblasts. Thus, our data demonstrate an important role for the TRAIL/DR5/FADD/caspase 8 pathway in the apoptosis associated with skeletal myoblast differentiation. Identifying the functional apoptotic pathways in skeletal myoblasts may prove useful in minimizing the myoblast apoptosis that contributes pathologically to a variety of diseases and in minimizing the apoptosis of transplanted myoblasts to treat these and other disease states.

Stem cell regulatory function mediated by expression of a novel mouse Oct4 pseudogene

Multiple pseudogenes have been proposed for embryonic stem (ES) cell-specific genes, and their abundance suggests that some of these potential pseudogenes may be functional. ES cell-specific expression of Oct4 regulates stem cell pluripotency and self-renewing state. Although Oct4 expression has been reported in adult tissues during gene reprogramming, the detected Oct4 signal might be contributed by Oct4 pseudogenes. Among the multiple Oct4 transcripts characterized here is a approximately 1 kb clone derived from P19 embryonal carcinoma stem cells, which shares a approximately 87% sequence homology with the parent Oct4 gene, and has the potential of encoding an 80-amino acid product (designated as Oct4P1). Adenoviral expression of Oct4P1 in mesenchymal stem cells promotes their proliferation and inhibits their osteochondral differentiation. These dual effects of Oct4P1 are reminiscent of the stem cell regulatory function of the parent Oct4, and suggest that Oct4P1 may be a functional pseudogene or a novel Oct4-related gene with a unique function in stem cells.

Differential signaling through NFkappaB does not ameliorate skeletal myoblast apoptosis during differentiation

During 23A2 skeletal myoblast differentiation, roughly 30% of the population undergoes apoptosis. Further, constitutive signaling by G12V:H-Ras or Raf:CAAX abrogates this apoptosis. In this study, we demonstrate an increase in NFkappaB activity in myoblasts that have survived and are expressing muscle-specific genes. NFkappaB activity is also elevated in myoblasts expressing constitutively active G12V:H-Ras but not Raf:CAAX. Expression of a dominant negative IkappaB (IkappaB-SR) sufficient to eliminate this elevated level of NFkappaB activity, in either the 23A2 myoblasts or their G12V:H-Ras-expressing counterparts, however, does not affect survival. Furthermore, expression of a constitutively active IkappaB kinase in 23A2 myoblasts does not protect these cells from the apoptosis associated with differentiation. Since signaling by IkappaB kinase can abrogate differentiation, this result demonstrates that abrogated differentiation and abrogated apoptosis are separable phenotypes.

No change in apoptosis in skeletal muscle exposed acutely or chronically to alcohol

The pathogenic mechanisms responsible for the deleterious changes in ethanol-exposed skeletal muscle are unknown, although apoptosis may be a causal process. We therefore investigated the responses of skeletal muscle to acute or chronic ethanol exposure in male Wistar rats. In acute studies, rats were dosed with ethanol (75 mmol (3.46 g)/kg BW) and killed after either 2.5 or 6 hours. In chronic studies, rats were fed ethanol as 35% of total dietary energy for 6 weeks. Apoptosis was determined by either DNA fragmentation or TUNEL (terminal deoxynucleotidyl transferase mediated dUTP nick end labelling) assays. The results showed that apoptosis was not increased in the ethanol-exposed muscle in both acute and chronic studies compared to appropriate controls.

Proliferation of myogenic progenitor cells following feeding in the sub-antarctic notothenioid fish Harpagifer bispinis

Feeding metabolism and the activation of myogenic progenitor cells were investigated in the fast myotomal muscle of the sub-Antarctic fish Hapagifer bispinis acclimatized to either simulated summer (10 degrees C; 18 h:6 h light:dark) or simulated winter (5 degrees C; 6 h:18 h light:dark) conditions. Ingestion of a single meal equivalent to 10% and 15% of body mass in simulated winter and summer groups, respectively, resulted in an average 2.6-fold and 3.6-fold increase in oxygen consumption, declining to 75% of peak values after 63 h and 46 h. In fasted individuals, the number of myogenic progenitor cells, identified by the expression of c-met, was not significantly different between simulated summer and winter fish, representing 6.6% and 5.8% of total myonuclei, respectively. However, the number of cells expressing myogenin was higher whereas the expression of MyoD was lower in winter than in summer groups. The ingestion of a single meal under winter and summer treatment regimes resulted in a significant increase in the number of cells expressing MyoD (51% and 111%) and PCNA (88% and 140%, respectively). This was followed by an increase in the abundance of c-met (74 and 85%) and myogenin (42 and 97%, respectively) positive cells, indicating the production of new myogenic progenitor cells and the commitment to differentiation of a number of them. These results show that the proliferation of myogenic progenitor cells can be induced by feeding in teleost fishes and that temperature and photoperiod influence the expression of myogenic regulatory factors.

Raf-induced effects on the differentiation and apoptosis of skeletal myoblasts are determined by the level of Raf signaling: abrogation of apoptosis by Raf is downstream of caspase 3 activation

We examined the effect of a constitutively active Raf protein (Raf-CAAX) on the differentiation and the coincident apoptosis of skeletal myoblasts. We found that a low level of Raf signaling leads to accelerated differentiation when compared to parental myoblasts, while a higher level of Raf signaling induces a transformed morphology and abrogates both differentiation and the coincident apoptosis. Raf signaling abrogates apoptosis without blocking the activation of caspase 3 and the subsequent cleavage of caspase 3 substrates. Eliminating the signal from Raf through MEK does not restore the ability to differentiate or to undergo apoptosis in the myoblasts with a high level of Raf signal, nor does it abrogate the accelerated differentiation observed in myoblasts with lower levels of Raf signal. Constitutive signaling through MEK is required, however, to maintain a transformed morphology. These results indicate that the effect of Raf on the differentiation and apoptosis of skeletal myoblasts is dictated by the level of Raf signaling, and that Raf signaling sufficient to abrogate the apoptosis coincident with differentiation does so downstream of caspase 3 signaling.

Potential role for Id myogenic repressors in apoptosis and attenuation of hypertrophy in muscles of aged rats

Aging attenuates the overload-induced increase in myogenic regulatory transcription factor (MRF) expression and the extent of muscle enlargement. To identify whether mRNA levels of repressors of the MRFs are greater in overloaded muscles from aged animals, overload was achieved in plantaris muscle of aged (33 mo; n = 14) and adult (9 mo; n = 17) rats. After 14 days, plantaris muscles in the overloaded limb were ~25% and 6% larger in adult and aged rats, respectively, compared with the contralateral limb. Hypertrophied muscles of adult rats had significantly greater levels of mRNA and protein levels for myogenin and MyoD compared with control muscles, but neither MRF increased with overload in muscles of aged rats. Muscles of aged rats had greater Id mRNA (150-700%) and protein repressor (200-6,000%) levels compared with adult rats. BAX and caspase 9 protein levels were 9,500% and 300% greater, respectively, in both control and hypertrophied muscles of aged rats compared with young adult rats. These data are consistent with the hypothesis that aging increases Id transcripts that activate apoptotic pathways involving BAX. This may contribute to sarcopenia by attenuating MRF protein levels in muscles of old animals.

Satellite cell proliferation and differentiation during postnatal growth of porcine skeletal muscle

Age-related changes in satellite cell proliferation and differentiation during rapid growth of porcine skeletal muscle were examined. Satellite cells were isolated from hindlimb muscles of pigs at 1, 7, 14, and 21 wk of age (4 animals/age group). Satellite cells were separated from cellular debris by using Percoll gradient centrifugation and were adsorbed to glass coverslips for fluorescent immunostaining. Positive staining for neural cell adhesion molecule (NCAM) distinguished satellite cells from nonmyogenic cells. The proportion of NCAM-positive cells (satellite cells) in isolates decreased from 1 to 7 wk of age. Greater than 77% of NCAM-positive cells were proliferating cell nuclear antigen positive at all ages studied. Myogenin-positive satellite cells decreased from 30% at 1 wk to 14% at 7 wk of age and remained at constant levels thereafter. These data indicate that a high percentage of satellite cells remain proliferative during rapid postnatal muscle growth. The reduced proportion of myogenin-positive cells during growth may reflect a decrease in the proportion of differentiating satellite cells or accelerated incorporation of myogenin-positive cells into myofibers.

Role of apoptosis in Duchenne's muscular dystrophy

We investigated the presence of apoptosis in muscle tissues from 24 patients (average age 5.44 +/- 1.81 years) with Duchenne's muscular dystrophy by in situ tailing of nuclear fragmentation. Muscle tissue from 4 children without histologic evidence of myopathy served as normal controls. Muscle fibers positive for nuclear DNA fragmentation were determined quantitatively by counting an area of at least 400 muscle fibers. Eleven of 24 specimens showed no nuclei with DNA fragmentation. On the other hand, 0.37 +/- 0.48% of fibers in patients with Duchenne's muscular dystrophy and none in controls had DNA fragmentation (P > .05). In this study, the percentage of apoptotic nuclei was higher in Duchenne's muscular dystrophy muscle than in normal controls. However, the difference did not reach a statistically significant level, and further studies with larger control groups are warranted.

Apoptosis and syncytial fusion in human placental trophoblast and skeletal muscle

Skeletal muscle fibers and placental villous trophoblast are the main representatives of syncytia in the human. Both syncytia are derived from fusion of mononucleated stem cells, show a high degree of differentiation, and have lost their generative potency. Consequently, for their growth both depend on fusion of additional stem cells. There is evidence that syncytial fusion is directly or indirectly related to apoptotic events: As early as in the differentiated stages of the mononucleated stem cells, initiation stages of the apoptosis cascade have been observed. After syncytial fusion progression of the cascade is retarded or blocked by a variety of mechanisms. In this review we emphasize the links between apoptosis cascade, differentiation pathways and syncytial fusion. It needs to be elucidated whether these processes simply take place in parallel, both temporally and spatially, or whether there are causal connections between apoptosis cascade and syncytial fusion. Based on recent data obtained for placental villous trophoblast, it is tempting to speculate that early molecular mechanisms of the apoptosis cascade are involved in differentiation and syncytial fusion. Data obtained in skeletal muscles support this assumption and reveal a considerable degree of homology in genesis, maintenance and turnover of both tissues.

Muscle fiber and satellite cell apoptosis in the aging human thyroarytenoid muscle: a stereological study with confocal laser scanning microscopy

OBJECTIVE

This study determines the role of changes in numerical densities of apoptotic myonuclei and satellite cells in age-related remodeling of the thyroarytenoid muscle (TA).

DESIGN

Changes in numerical densities of apoptotic myonuclei and satellite cells were estimated for the entire TA by using stereological techniques.

RESULTS

There was an increase in N(V apoptotic myonucleus, fiber type) (P < 0.05) and in the percentage of apoptotic myonuclei (P < 0.05) in type 1 but not in type 2 muscle fibers. There was also an increase in N(V apoptotic satellite cell, fiber type) (P < 0.05) and in the percentage of apoptotic satellite cells (P < 0.05) in type 1 fibers but not in type 2 fibers.

CONCLUSION

Apoptosis may contribute to age-related fiber loss and atrophy in the TA.

SIGNIFICANCE

Therapeutic techniques based on decreasing the frequency of apoptosis may block age-related fiber loss and atrophy in the TA.

Caspase 3 expression correlates with skeletal muscle apoptosis in Duchenne and facioscapulo human muscular dystrophy. A potential target for pharmacological treatment?

Apoptosis was detected in different muscular diseases, including severe dystrophin deficiency, but apoptotic mechanisms are not completely described in adult skeletal muscle. Studying patients affected by Duchenne muscular dystrophy (DMD) and by facio-scapulo-humeral dystrophy (FSHD) we showed an increase of apoptotic myonuclei, bax, and bcl-2-positive myofibers. Positive correlation was detected between apoptotic nuclei and bax expression (p < 0.01). Expression of caspases was analyzed by RNase protection. Caspase transcript was not detected in normal skeletal muscles. DMD muscles expressed caspase 8, 3, 5, 2, 7 and Granzyme B mRNAs. Low levels of caspase 6, 3, and Granzyme B transcripts were detected in FSHD patients. Tissue levels of caspase 3 protein significantly correlated with apoptotic myonuclei (p < 0.05) and with bax expression (p < 0.01). In all DMD cases the activity of caspase 3 was increased, while the FSHD samples were heterogeneous. These data indicate that human skeletal muscle fibers. during the dystrophic process, modulate the expression of caspases and that caspase 3 is involved in myofiber cell death. opening new perspective in the pharmacological treatments of muscular dystrophies, such as the use of caspase inhibitors.

Increase in ultraviolet sensitivity by overexpression of calpastatin in ultraviolet-resistant UVr-1 cells derived from ultraviolet-sensitive human RSa cells

Human RSa cells are highly sensitive to apoptotic-like cell death by ultraviolet irradiation (UV) while UVr-1 cells are their variant with an increased resistance to UV. Three days after UV at 10 J/m2, the viability of RSa cells was approximately 17% while that of UVr-1 cells was 65%. This different survival might reflect apoptotic cell death since apoptosis-specific DNA ladder was more clearly observed in RSa cells than in UVr-1 cells after UV. Addition of ALLN/calpain inhibitor I to the culture medium after UV resulted in similar survival (14 - 18%) between RSa and UVr-1 cells. Immunoblot analysis showed down-regulation of protein kinase CTheta, Src, Bax and mu-calpain after UV was more prominent in UVr-1 than in RSa cells. Activated mu-calpain appeared within 1 h post-UV only in UVr-1 cells. The expression of calpastatin, a specific endogenous inhibitor of calpain, was higher in RSa than in UVr-1 cells. To further examine the role of calpain in UV-induced cell death, cDNA of human calpastatin was transfected into UVr-1 cells. The results showed that overexpression of calpastatin suppressed down-regulation of Src, mu-calpain and Bax. Concomitantly, colony survival after UV was reduced in calpastatin-transfected cells as compared to vector control cells. Our results suggest that activation of calpain might account for, at least in part, the lower susceptibility to UV-induced cell death in UVr-1 cells.

The anti-death league: associative control of apoptosis in developing retinal tissue

Apoptosis, the major form of programmed cell death (PCD), is executed through a proteolytic cascade that can be differentially engaged by various extracellular signals. Modulation of both the sensitivity to PCD and of the actual sequence of apoptotic events is, therefore, strongly dependent on cell interactions. This paper reviews the use of a retinal explant preparation as a model of the organized nervous tissue, to study the effects of neural messengers in the control of sensitivity to apoptosis. Studies of retinal explants showed that dopamine, glutamate and nitric oxide may have anti-apoptotic effects upon developing retinal cells. At least the effects of nitric oxide are clearly paracrine. In addition, preliminary evidence has been gathered of a role for gap junctional communication in the control of sensitivity of retinal cells to the induction of apoptosis. These findings underscore the importance of selective cell interactions in the control of PCD in the developing nervous system.

Constitutive apoptosis in human neutrophils requires synergy between calpains and the proteasome downstream of caspases

Programmed cell death invariably requires the activation of proteolytic cascades that are not yet well defined but are initiated after apical caspase activation. We provide evidence that calpains and the proteasome function synergistically downstream of caspases to assist the constitutive apoptotic program of aging neutrophils, which plays an important role in resolution of inflammatory responses. Inhibitor studies indicated that "tethering" of preapoptotic senescent neutrophils to human macrophages required caspase activity. However, the development of morphological features characteristic of apoptosis, including nuclear morphology, PS exposure, surface protein shedding, and the capacity to be ingested by macrophages, required the downstream action of either calpains or the proteasome. Calpain activities were constitutively active in freshly isolated neutrophils and responsible for rearrangements in the protein composition and structure of the plasmalemmal cytoskeleton as they aged in culture and underwent apoptosis. This included a dissociation of protein(s) from F-actin, a candidate mechanism for increased susceptibility to cleavage, and a loss in immunodetectable alpha-actinin and ezrin, two actin-binding, membrane-anchoring proteins. These results clarify roles for different classes of proteases in a physiologically important form of constitutive apoptosis.

Induction of Akt-2 correlates with differentiation in Sol8 muscle cells

Phosphatidylinositol 3-kinase is involved in the regulation of muscle cell differentiation. The serine/threonine kinase Akt has been implicated in the signaling pathway downstream of PI3-kinase. Here we demonstrate that differentiation of Sol8 skeletal muscle cells is associated with a marked increase in endogenous Akt-2 protein. Myogenesis was induced by three different conditions: cell confluence, low serum or treatment with insulin or insulin-like growth factor-I. Differentiation by cell confluence resulted in an increase in the endogenous protein content and activation of Akt-2. Low serum conditions induced a dramatic raise in Akt-2 protein levels which correlates with the induction of the muscle cell differentiation marker myogenin. Treatment of Sol8 cells with the PI3-kinase inhibitor LY294002 prevented the expression of myogenin as effectively as the increase in Akt-2 content induced by low-serum conditions. Similarly, differentiation of Sol8 cells stimulated by 50 nM insulin or 10 nM IGF-I markedly increased Akt-2 protein levels. These results and the recent observation that active Akt translocates to the cell nucleus (J. Biol. Chem. 272, 30491-30497; 31515-31524, 1997) suggests that Akt-2 might play a crucial role in the initiation of the genetic program responsible for muscle cell differentiation.

Comparison of the effects of thyroxine and triiodothyronine on protein turnover and apoptosis in primary chick muscle cell cultures

Primary chick muscle cells were treated with physiological level of thyroxine (T4) or triiodothyronine (T3) to examine the effects of the hormones on growth, protein turnover, and apoptosis of the cells. Creatine kinase activity, as an index of differentiation, was increased by both T4 and T3. Even when the conversion from T4 to T3 was blocked by iopanoic acid, T4 increased creatine kinase activity. The rate of protein degradation estimated from [3H] tyrosine release was increased by T3 but not by T4. DNA cleavage and fragmentation, as indices of apoptosis, were induced by T3 but not by T4. These results show that T4 stimulates cell differentiation but not protein degradation and apoptosis in primary chick muscle cells, while all events are stimulated by T3.

Dystrophin deficient myotubes undergo apoptosis in mouse primary muscle cell culture after DNA damage

Apoptosis has been demonstrated to occur in differentiated myocardial muscle, neonatal skeletal muscle and skeletal myoblasts in response to injury. In this report, we studied differentiated normal and dystrophin deficient murine skeletal muscle cell cultures that have been injured by a pulse of cis-platinum (2 h). Forty-eight hours after DNA damage, dystrophin positive myotubes appeared almost normal though some myoblasts showed DNA fragmentation. On the other hand, dystrophin deficient myotubes presented progressive degeneration via apoptosis detected either by TUNEL or by nuclear morphology. Degeneration of mdx muscle fibers was confirmed by counting both the number of myotubes observed by contrast phase microscopy and myonuclei viewed by immunoreaction for MyoD. A 6-fold decrease in the number of muscle cells was observed in the dystrophin-deficient cell culture compared to the parental culture (P < 0.001). Direct evidence of degenerating myotubes displaying MyoD- and TUNEL-positive nuclei was obtained. Like myoblasts, differentiated dystrophin deficient myotubes were able to degenerate via apoptosis, showing that mature dystrophin deficient cells are fragile and undergo apoptosis when subjected to a mild injury which would normally be repaired in parental cells.

Bcl-2 expression identifies an early stage of myogenesis and promotes clonal expansion of muscle cells

We show that Bcl-2 expression in skeletal muscle cells identifies an early stage of the myogenic pathway, inhibits apoptosis, and promotes clonal expansion. Bcl-2 expression was limited to a small proportion of the mononucleate cells in muscle cell cultures, ranging from approximately 1-4% of neonatal and adult mouse muscle cells to approximately 5-15% of the cells from the C2C12 muscle cell line. In rapidly growing cultures, some of the Bcl-2-positive cells coexpressed markers of early stages of myogenesis, including desmin, MyoD, and Myf-5. In contrast, Bcl-2 was not expressed in multinucleate myotubes or in those mononucleate myoblasts that expressed markers of middle or late stages of myogenesis, such as myogenin, muscle regulatory factor 4 (MRF4), and myosin. The small subset of Bcl-2-positive C2C12 cells appeared to resist staurosporine-induced apoptosis. Furthermore, though myogenic cells from genetically Bcl-2-null mice formed myotubes normally, the muscle colonies produced by cloned Bcl-2-null cells contained only about half as many cells as the colonies produced by cells from wild-type mice. This result suggests that, during clonal expansion from a muscle progenitor cell, the number of progeny obtained is greater when Bcl-2 is expressed.

The many roles of c-Myc in apoptosis

The proto-oncogene c-myc encodes a transcription factor c-Myc, which is of great importance in controlling cell growth and vitality. The quantity of c-Myc is carefully controlled by many mechanisms, and its actions to induce and repress genes are modulated by interactions with other regulatory proteins. Understanding the kinetic and quantitative relationships that determine how and what genes c-Myc regulates is essential to understanding how Myc is involved in apoptosis. Reduction of c-myc expression and its inappropriate expression can be associated with cellular apoptosis. This review outlines the nature and regulation of the c-myc gene and of c-Myc and presents the systems and conditions in which Myc-related apoptotic events occur. Hypotheses of the mechanisms by which expression and repression of c-myc lead to apoptosis are discussed.

Proteolytic regulation of the zinc finger transcription factor YY1, a repressor of muscle-restricted gene expression

Regulated proteolysis has been postulated to be critical for proper control of cell functions. Muscle development, in particular, involves a great deal of structural adaptation and remodeling mediated by proteases. The transcription factor YY1 represses muscle-restricted expression of the sarcomeric alpha-actin genes. Consistent with this repressor function of YY1, the nuclear regulator is down-regulated at the protein level during skeletal as well as cardiac muscle cell differentiation. However, the YY1 message remains relatively unaltered throughout the myoblast-myotube transition, implicating a post-translational regulatory mechanism. We show that YY1 can be a substrate for cleavage by the calcium-activated neutral protease calpain II (m-calpain) and the 26 S proteasome. The calcium ionophore A23187 destabilized YY1 in cultured myoblasts, and the decrease in YY1 protein levels could be prevented by calpain inhibitor II and calpeptin. Treatment with the proteasome inhibitors MG132 and lactacystin resulted in the stabilization of YY1 protein, which is consistent with the finding that YY1 is readily polyubiquitinated in reticulocyte lysates. We further show that proteolytic targeting by calpain II and the proteasome involves different structural elements of YY1. This study thus illustrates two proteolytic pathways through which the transcriptional regulator can be differentially targeted under different cell growth conditions.

Chemical hypoxia triggers apoptosis of cultured neonatal rat cardiac myocytes: modulation by calcium-regulated proteases and protein kinases

Myocardial infarctions and stroke arise primarily as a result of hypoxia/ischemia-induced cell injury. However, the molecular mechanism of cardiac cell death due to hypoxia has not been elucidated. We showed here that chemical hypoxia induced by 1 mM azide triggered apoptosis of isolated neonatal rat ventricular cardiac myocytes but had no effect on cardiac fibroblasts. The azide-induced cardiomyocyte apoptosis could be characterized by a reversible initiation phase (0-46 h after azide exposure) during which cytosolic ATP levels remained little affected. This was followed by an irreversible execution phase (12-18 h) exhibiting prominent internucleosomal DNA fragmentation, cell membrane leakage, mitochondrial dysfunction, and increased calpain messenger RNA. Blocking extracellular calcium influx or intracellular calcium release was each effective in suppressing myocyte apoptosis. Cell death was also found to be mediated by calcium sensitive signal transduction events based on the use of specific antagonists. Consistent with the induction of calpain expression during apoptosis, blocking de novo protein synthesis and calpain activity inhibited cell death. These regulatory features coupled with the ease of the cell system suggest that the myocyte apoptosis model described here should be useful in the study of events leading to the demise of the myocardium.

Yeast two-hybrid cloning of a novel zinc finger protein that interacts with the multifunctional transcription factor YY1

Muscle-restricted transcription of sarcomeric actin genes is negatively controlled by the zinc finger protein YY1, which is down-regulated at the protein level during myogenic differentiation. To identify cellular proteins that might mediate the function/stability of YY1 in muscle cells, we screened an adult human muscle cDNA library using the yeast two-hybrid cloning system. We report the isolation and characterization of a novel protein termed YAF2 (YY1- associated factor 2) that interacts with YY1. The YAF2 cDNA encodes a 180 amino acid basic protein (pI 10.5) containing a single N-terminal C2-X10-C2 zinc finger. Lysine clusters are present that may function as a nuclear localization signal. Domain mapping analysis shows that the first and second zinc fingers of YY1 are targeted for YAF2 protein interaction. In contrast to the down-regulation of YY1, YAF2 message levels increase during in vitro differentiation of both rat skeletal and cardiac muscle cells. YAF2 appears to have a promyogenic regulatory role, since overexpression of YAF2 in C2 myoblasts stimulates myogenic promoter activity normally restricted by YY1. Co-transfection of YY1 reverses the stimulatory effect of YAF2. YAF2 also greatly potentiates proteolytic cleavage of YY1 by the calcium- activated protease m-calpain. The isolation of YAF2 may help in understanding the mechanisms through which inhibitors of myogenic transcription may be antagonized or eliminated by proteolysis during muscle development.