FHL2 mediates p53-induced transcriptional activation through a direct association with HIPK2

RNA sequencing of long-term label-retaining colon cancer stem cells identifies novel regulators of quiescence

Recent data suggest that therapy-resistant quiescent cancer stem cells (qCSCs) are the source of relapse in colon cancer. Here, using colon cancer patient-derived organoids and xenografts, we identify rare long-term label-retaining qCSCs that can re-enter the cell cycle to generate new tumors. RNA sequencing analyses demonstrated that these cells display the molecular hallmarks of quiescent tissue stem cells, including expression of p53 signaling genes, and are enriched for transcripts common to damage-induced quiescent revival stem cells of the regenerating intestine. In addition, we identify negative regulators of cell cycle, downstream of p53, that we show are indicators of poor prognosis and may be targeted for qCSC abolition in both p53 wild-type and mutant tumors. These data support the temporal inhibition of downstream targets of p53 signaling, in combination with standard-of-care treatments, for the elimination of qCSCs and prevention of relapse in colon cancer.

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Colon tumors contain therapy-resistant quiescent cancer stem cells (qCSCs)

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qCSC gene expression mirrors that of quiescent stem cells of the regenerating gut

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qCSCs are enriched for p53 signaling genes

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qCSC elimination may be achieved by inhibiting downstream targets of p53 signaling

Matrix mechanics controls FHL2 movement to the nucleus to activate p21 expression

Substrate rigidity affects many physiological processes through mechanochemical signals from focal adhesion (FA) complexes that subsequently modulate gene expression. We find that shuttling of the LIM domain (domain discovered in the proteins, Lin11, Isl-1, and Mec-3) protein four-and-a-half LIM domains 2 (FHL2) between FAs and the nucleus depends on matrix mechanics. In particular, on soft surfaces or after the loss of force, FHL2 moves from FAs into the nucleus and concentrates at RNA polymerase (Pol) II sites, where it acts as a transcriptional cofactor, causing an increase in p21 gene expression that will inhibit growth on soft surfaces. At the molecular level, shuttling requires a specific tyrosine in FHL2, as well as phosphorylation by active FA kinase (FAK). Thus, we suggest that FHL2 phosphorylation by FAK is a critical, mechanically dependent step in signaling from soft matrices to the nucleus to inhibit cell proliferation by increasing p21 expression.

Protein-protein interactions of the LIM-only protein FHL2 and functional implication of the interactions relevant in cardiovascular disease

FHL2 belongs to the LIM-domain only proteins and contains four and a half LIM domains, each of which are composed of two zinc finger structures. FHL2 exhibits specific interaction with proteins exhibiting diverse functions, including transmembrane receptors, transcription factors and transcription co-regulators, enzymes, and structural proteins. The function of these proteins is regulated by FHL2, which modulates intracellular signal transduction pathways involved in a plethora of cellular tasks. The present review summarizes the current knowledge on the protein interactome of FHL2 and provides an overview of the functional implication of these interactions in apoptosis, migration, and regulation of nuclear receptor function. FHL2 was originally identified in the heart and there is extensive literature available on the role of FHL2 in the cardiovascular system, which is also summarized in this review.

Transcriptional regulation of the tumor suppressor FHL2 by p53 in human kidney and liver cells

Four and a Half LIM protein 2 (FHL2) is a LIM domain only protein that is able to form various protein complexes and regulate gene transcription. Recent findings showed that FHL2 is a potential tumor suppressor gene that was down-regulated in hepatocellular carcinoma (HCC). Moreover, FHL2 can bind to and activate the TP53 promoter in hepatic cells. In this study, the activity of the two promoters of FHL2, 1a and 1b, were determined in the human embryonic kidney cell line HEK293 and the activation of these two promoters by p53 was investigated. Our results showed that the 1b promoter has a higher activity than the 1a promoter in HEK 293 cells but the 1a promoter is more responsive to the activation by p53 when compared with the 1b promoter. The regulation of FHL2 by p53 was further confirmed in liver cells by the overexpression of p53 in Hep3B cells and the knockdown of p53 in HepG2 cells. Combining promoter activity results of truncated mutants and predictions by bioinformatics tools, a putative p53 binding site was found in the exon 1a of FHL2 from +213 to +232. The binding between the p53 protein and the putative p53 binding site was then validated by the ChIP assay. Furthermore, the expression of FHL2 and TP53 were down-regulated in majority of HCC tumour samples (n = 41) and significantly correlated (P = 0.026). Finally, we found that the somatic mutation 747 (G→T), a hot spot mutation of the TP53 gene, is potentially associated with a higher expression of FHL2 in HCC tumour samples. Taken together, this is the first in-depth study about the transcriptional regulation of FHL2 by p53.

HIPK2: A tumour suppressor that controls DNA damage-induced cell fate and cytokinesis

In response to DNA-damage, cells have to decide between different cell fate programmes. Activation of the tumour suppressor HIPK2 specifies the DNA damage response (DDR) and tips the cell fate balance towards an apoptotic response. HIPK2 is activated by the checkpoint kinase ATM, and triggers apoptosis through regulatory phosphorylation of a set of cellular key molecules including the tumour suppressor p53 and the anti-apoptotic corepressor CtBP. Recent work has identified HIPK2 as a regulator of the ultimate step in cytokinesis: the abscission of the mother and daughter cells. Since proper cytokinesis is essential for genome stability and maintenance of correct ploidy, this finding sheds new light on the tumour suppressor function of HIPK2. Here we highlight the molecular mechanisms coordinating HIPK2 function and discuss its emerging role as a tumour suppressor.

Gene expression profiles classify human osteosarcoma xenografts according to sensitivity to doxorubicin, cisplatin, and ifosfamide

PURPOSE

In osteosarcoma, aggressive preoperative and postoperative multidrug chemotherapy given to all patients has improved patient survival rate to the present level of approximately 60%. However, no tumor marker is available that reliably can identify those patients who will or will not respond to chemotherapy.

EXPERIMENTAL DESIGN

In an attempt to find leads to such markers, we have obtained microarray gene expression profiles from a panel of 10 different human osteosarcoma xenografts and related the results to their sensitivity to ifosfamide, doxorubicin, and cisplatin.

RESULTS

The expression data identified genes with highly significant differential expression between poor and good responder xenografts to the three different drugs: 85 genes for doxorubicin, 74 genes for cisplatin, and 118 genes for ifosfamide. Technical validation with quantitative reverse transcription-PCR showed good correlation with the microarray expression data. Gene Ontology-guided analysis suggested that properties of the poorly responsive xenografts were resistance to undergo programmed cell death and, particularly for ifosfamide, a drive toward dedifferentiation and increased tumor aggressiveness. Leads toward metabolic alterations and involvement of mitochondrial pathways for apoptosis and stress response were more prominent for doxorubicin and cisplatin. Finally, small interfering RNA-mediated gene silencing of IER3 and S100A2 sensitized the human osteosarcoma cell line OHS to treatment with 4-hydroperoxyifosfamide.

CONCLUSIONS

The expression profiles contained several novel biomarker candidates that may help predict the responsiveness of osteosarcoma to doxorubicin, cisplatin, and ifosfamide. The potential of selected candidates will be further validated on clinical specimens from osteosarcoma patients.

The four-and-a-half-LIM protein 2 (FHL2) is overexpressed in gliomas and associated with oncogenic activities

Four-and-a-half-LIM protein 2 (FHL2) is a member of FHL protein family, which plays a crucial role in regulating gene expression, cell survival, and migration. Although its function in oncogenesis appears to be tumor type-specific, its roles in glioma formation and development are yet to be elucidated. In the present study, we demonstrated that the mRNA level of FHL2 was elevated in both low- and high-grade glioma samples. Overexpression of FHL2 stimulated the proliferation, anchorage-independent growth, and migration of human glioblastoma cells. Conversely, FHL2 knockdown by short hairpin RNA (shRNA-FHL2) inhibited glioblastoma cell proliferation and migration. Overexpression of FHL2 increased the tumorigenicity of glioblastoma cells in nude mice and decreased the mRNA levels of p53 and its downstream proapoptotic genes, including p21, Bcl2-associated protein X (Bax), and p53-upregulated modulator of apoptosis. It also enhanced the promoter activities of activator protein-1 (AP-1), human telomerase reverse transcriptase, and survivin genes. Together, these results provide the first evidence that FHL2 contributes to glioma carcinogenesis.

HIPKs: Jack of all trades in basic nuclear activities

Over the past decade several investigators have reported on the physical interaction of serine/threonine kinases of the homeodomain interacting-protein family (HIPKs) with increasing number of nuclear factors and on their localization in different nuclear sub-compartments. Although we are still far from a global understanding of the molecular consequences of HIPK subnuclear compartmentalization, the spatial description of particular interactions and posttranslational modifications promoted by these kinases on key cellular regulators might provide relevant insights. Here we will discuss the possible implications of the HIPK subnuclear localization in the regulation of gene transcription and in the cell response to stress.

Gene Ontology-driven transcriptional analysis of CD34+ cell-initiated megakaryocytic cultures identifies new transcriptional regulators of megakaryopoiesis

Differentiation of hematopoietic stem and progenitor cells is an intricate process controlled in large part at the level of transcription. While some key megakaryocytic transcription factors have been identified, the complete network of megakaryocytic transcriptional control is poorly understood. Using global gene expression microarray analysis, Gene Ontology-based functional annotations, and a novel interlineage comparison with parallel, isogenic granulocytic cultures as a negative control, we closely examined the mRNA level of transcriptional regulators in megakaryocytes derived from human mobilized peripheral blood CD34(+) hematopoietic cells. This approach identified 199 differentially expressed transcription factors or transcriptional regulators. We identified and detailed the transcriptional kinetics of most known megakaryocytic transcription factors including GATA1, FLI1, and MAFG. Furthermore, many genes with transcription factor activity or transcription factor binding activity were identified in megakaryocytes that had not previously been associated with that lineage, including BTEB1, NR4A2, FOXO1A, MEF2C, HDAC5, VDR, and several genes associated with the tumor suppressor p53 (HIPK2, FHL2, and TADA3L). Protein expression and nuclear localization were confirmed in megakaryocytic cells for four of the novel candidate megakaryocytic transcription factors: FHL2, MXD1, E2F3, and RFX5. In light of the hypothesis that transcription factors expressed in a particular differentiation program are important contributors to such a program, these data substantially expand our understanding of transcriptional regulation in megakaryocytic differentiation of stem and progenitor cells.

The sarcomere and the nucleus: functional links to hypertrophy, atrophy and sarcopenia

Skeletal muscle has a remarkable ability to rapidly adjust to changes in physiological requirements. This includes hypertrophic muscle growth and the atrophic loss of muscle mass, both of which occur in response to hormonal, endocrine and mechanical stimuli. In ageing muscle, sarcopenia (the loss of muscle fibres) can aggravate hormonally and mechanically induced atrophy. Hypertrophy and atrophy are associated with changes in sarcomeric protein composition and metabolic enzymes. The coordinated changes of transcriptional and splice mechanisms, protein turnover and cell fate integrates signalling pathways from hormone and cytokine receptors, as well as the sarcomere itself. This involves a number of proteins that shuttle between sarcomeric and nonsarcomeric localisations and thus convey signals from the contractile machinery to the nucleus. The M-band is emerging as a hub mainly for protein-kinase regulated ubiquitin signalling and protein turnover, whereas the I-band and Z-disk contain stretch-sensitive pathways involving transcriptional modifiers. Disruptions of these pathways can cause hereditary myopathies.

Transcriptional regulation of the p73 gene, a member of the p53 family, by early growth response-1 (Egr-1)

To elucidate the regulatory mechanism of p73 gene expression, we analyzed the human p73 promoter and found three putative Egr-1-binding sites located upstream of exon 1 (-1728, -321, and -38). The Egr-1 responsiveness of these sites was analyzed by transient transfection assays using 5'- and 3'-serial truncations of the p73 promoter, subcloned in a CAT reporter vector. The functional significance of the region was further confirmed by an electrophoretic mobility shift assay using the Egr-1 protein synthesized in vitro and a [32P]-labeled middle site sequence, followed by competition with unlabeled wild-type or mutant oligonucleotides and supershift assays using an anti-Egr-1 antibody. When induced by either the nitric oxide donor NOC-18 or the PPARgamma agonist troglitazone, Egr-1 bound to the p73 promoter, as assessed by chromatin immunoprecipitation assays, accompanied by increased expression of p73. MTT assays revealed that cell growth was significantly inhibited on treating the cells with troglitazone. Overall, our results provide direct evidence that Egr-1 positively regulated p73 expression by binding to its promoter in vivo, consistent with Egr-1 and p73 being involved in p53-independent tumor suppression.