Death-Defying Pathways Linking Cell Cycle and Apoptosis

Skeletal muscle characteristics are preserved in hTERT/cdk4 human myogenic cell lines

Background

hTERT/cdk4 immortalized myogenic human cell lines represent an important tool for skeletal muscle research, being used as therapeutically pertinent models of various neuromuscular disorders and in numerous fundamental studies of muscle cell function. However, the cell cycle is linked to other cellular processes such as integrin regulation, the PI3K/Akt pathway, and microtubule stability, raising the question as to whether genetic modification related to the cell cycle results in secondary effects that could undermine the validity of these cell models.

Results

Here we subjected five healthy and disease muscle cell isolates to transcriptomic analysis, comparing immortalized lines with their parent primary populations in both differentiated and undifferentiated states, and testing their myogenic character by comparison with non-myogenic (CD56-negative) cells. Principal component analysis of global gene expression showed tight clustering of immortalized myoblasts to their parent primary populations, with clean separation from the non-myogenic reference. Comparison was made to publicly available transcriptomic data from studies of muscle human pathology, cell, and animal models, including to derive a consensus set of genes previously shown to have altered regulation during myoblast differentiation. Hierarchical clustering of samples based on gene expression of this consensus set showed that immortalized lines retained the myogenic expression patterns of their parent primary populations. Of 2784 canonical pathways and gene ontology terms tested by gene set enrichment analysis, none were significantly enriched in immortalized compared to primary cell populations. We observed, at the whole transcriptome level, a strong signature of cell cycle shutdown associated with senescence in one primary myoblast population, whereas its immortalized clone was protected.

Conclusions

Immortalization had no observed effect on the myogenic cascade or on any other cellular processes, and it was protective against the systems level effects of senescence that are observed at higher division counts of primary cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0115-5) contains supplementary material, which is available to authorized users.

The Arabidopsis BET bromodomain factor GTE4 is involved in maintenance of the mitotic cell cycle during plant development

Bromodomain and Extra Terminal domain (BET) proteins are characterized by the presence of two types of domains, the bromodomain and the extra terminal domain. They bind to acetylated lysines present on histone tails and control gene transcription. They are also well known to play an important role in cell cycle regulation. In Arabidopsis (Arabidopsis thaliana), there are 12 BET genes; however, only two of them, IMBIBITION INDUCIBLE1 and GENERAL TRANSCRIPTION FACTOR GROUP E6 (GTE6), were functionally analyzed. We characterized GTE4 and show that gte4 mutant plants have some characteristic features of cell cycle mutants. Their size is reduced, and they have jagged leaves and a reduced number of cells in most organs. Moreover, cell size is considerably increased in the root, and, interestingly, the root quiescent center identity seems to be partially lost. Cell cycle analyses revealed that there is a delay in activation of the cell cycle during germination and a premature arrest of cell proliferation, with a switch from mitosis to endocycling, leading to a statistically significant increase in ploidy levels in the differentiated organs of gte4 plants. Our results point to a role of GTE4 in cell cycle regulation and specifically in the maintenance of the mitotic cell cycle.

Trypanosoma cruzi infection activates extracellular signal-regulated kinase in cultured endothelial and smooth muscle cells

Trypanosoma cruzi infection causes cardiomyopathy and vasculopathy. We examined the consequence of this infection for the mitogen-activated protein kinase (MAPK) pathways, which regulate cell proliferation in cultured human umbilical vein endothelial and vascular smooth muscle cells. Infection of these cells resulted in activation of extracellular signal-regulated kinases 1and 2 (ERK1/2) but not c-Jun N-terminal kinase or p38 MAPK. Treatment of these cells with the MAPK kinase inhibitor PD98059 prior to infection blocked the increase in phosphorylated ERK1/2 seen with infection. Heat-killed parasites did not activate ERK1/2, indicating that activation of ERK1/2 was dependent on infection of these cells by live parasites. Furthermore, transfection with dominant-negative Raf(301) or Ras(N17) constructs reduced the infection-associated levels of phospho-ERK1/2, indicating that the activation of ERK1/2 involved the Ras-Raf-ERK pathway. Infection also resulted in an increase in activator protein 1 (AP-1) activity, which was inhibited by transfection with a dominant-negative Raf(301) construct. T. cruzi-infected endothelial cells secreted endothelin-1 and interleukin-1beta, which activated ERK1/2 and induced cyclin D1 expression in uninfected smooth muscle cells. These data suggest a possible molecular paradigm for the pathogenesis of the vasculopathy and the cardiovascular remodeling associated with T. cruzi infection.

VDUP1 upregulated by TGF-beta1 and 1,25-dihydorxyvitamin D3 inhibits tumor cell growth by blocking cell-cycle progression

Vitamin D(3) upregulated protein 1 (VDUP1) is a 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) upregulated protein, and it is induced by various stresses. In human tumor tissues, VDUP1 expression was downregulated. Upon stimulation by growth-inhibitory signals such as TGF-beta1 and 1,25(OH)(2)D(3), its expression was rapidly upregulated as the cell growth was retarded. The transfection of VDUP1 in tumor cells reduced cell growth. The VDUP1 expression was also increased when the cell-cycle progression was arrested. Transfection of VDUP1 induced cell-cycle arrest at the G0/G1 phase, indicating that VDUP1 possesses a tumor-suppressive activity. In addition, it was found that VDUP1 interacted with promyelocytic leukemia zinc-finger, Fanconi anemia zinc-finger, and histone deacetylase 1, which are known to be transcriptional corepressors. VDUP1 itself suppressed IL-3 receptor and cyclin A2 promoter activity. Taken together, these results suggest that VDUP1 is a novel antitumor gene which forms a transcriptional repressor complex.

Activation of transcription factors AP-1 and NF-kappa B in murine Chagasic myocarditis

The myocardium of CD1 mice was examined for the activation of signal transduction pathways leading to cardiac inflammation and subsequent remodeling during Trypanosoma cruzi infection (Brazil strain). The activity of three pathways of the mitogen-activated protein kinases (MAPKs) was determined. Immunoblotting revealed a persistent elevation of phosphorylated (activated) extracellular-signal-regulated kinase (ERK), which regulates cell proliferation. During infection there was a transient activation of p38 MAPK but no activation of Jun N-terminal kinase. Early targets of activated ERK, c-Jun and c-Fos, were elevated during infection, as demonstrated by semiquantitative reverse transcription-PCR. Immunostaining revealed that the endothelium and the interstitial cells were most intensely stained with antibodies to c-Jun and c-Fos. Soon after infection, AP-1 and NF-kappa B DNA binding activity was increased. Protein levels of cyclin D1, the downstream target of ERK and NF-kappa B, were induced during acute infection. Immunostaining demonstrated increased expression of cyclin D1 in the vascular and endocardial endothelium, inflammatory cells, and the interstitial areas. Increased expression of the cyclin D1-specific phosphorylated retinoblastoma protein (Ser780) was also evident. Immunoblotting and immunostaining also demonstrated increased expression of proliferating cellular nuclear antigen that was predominantly present in the inflammatory cells, interstitial areas (i.e., fibroblasts), and endothelium. These data demonstrate that T. cruzi infection results in activation of the ERK-AP-1 pathway and NF-kappa B. Cyclin D1 expression was also increased. These observations provide a molecular basis for the activation of pathways involved in cardiac remodeling in chagasic cardiomyopathy.