HDAC2 Regulates Site-Specific Acetylation of MDM2 and Its Ubiquitination Signaling in Tumor Suppression

Histone deacetylases (HDACs) are promising targets for cancer therapy, although their individual actions remain incompletely understood. Here, we identify a role for HDAC2 in the regulation of MDM2 acetylation at previously uncharacterized lysines. Upon inactivation of HDAC2, this acetylation creates a structural signal in the lysine-rich domain of MDM2 to prevent the recognition and degradation of its downstream substrate, MCL-1 ubiquitin ligase E3 (MULE). This mechanism further reveals a therapeutic connection between the MULE ubiquitin ligase function and tumor suppression. Specifically, we show that HDAC inhibitor treatment promotes the accumulation of MULE, which diminishes the t(X; 18) translocation-associated synovial sarcomagenesis by directly targeting the fusion product SS18-SSX for degradation. These results uncover a new HDAC2-dependent pathway that integrates reversible acetylation signaling to the anticancer ubiquitin response.

Regulation of Active DNA Demethylation through RAR-Mediated Recruitment of a TET/TDG Complex

Retinoic acid (RA) plays important roles in development, growth, and homeostasis through regulation of the nuclear receptors for RA (RARs). Herein, we identify Hypermethylated in Cancer 1 (Hic1) as an RA-inducible gene. HIC1 encodes a tumor suppressor, which is often silenced by promoter hypermethylation in cancer. Treatment of cells with an RAR agonist causes a rapid recruitment of an RAR/RXR complex consisting of TDG, the lysine acetyltransferase CBP, and TET 1/2 to the Hic1 promoter. Complex binding coincides with a transient accumulation of 5fC/5caC and concomitant upregulation of Hic1 expression, both of which are TDG dependent. Furthermore, conditional deletion of Tdg in vivo is associated with Hic1 silencing and DNA hypermethylation of the Hic1 promoter. These findings suggest that the catalytic and scaffolding activities of TDG are essential for RA-dependent gene expression and provide important insights into the mechanisms underlying targeting of TET-TDG complexes.

Deconstruction of the SS18-SSX fusion oncoprotein complex: insights into disease etiology and therapeutics

Synovial sarcoma is a translocation-associated sarcoma where the underlying chromosomal event generates SS18-SSX fusion transcripts. In vitro and in vivo studies have shown that the SS18-SSX fusion oncoprotein is both necessary and sufficient to support tumorigenesis; however, its mechanism of action remains poorly defined. We have purified a core SS18-SSX complex and discovered that SS18-SSX serves as a bridge between activating transcription factor 2 (ATF2) and transducin-like enhancer of split 1 (TLE1), resulting in repression of ATF2 target genes. Disruption of these components by siRNA knockdown or treatment with HDAC inhibitors rescues target gene expression, leading to growth suppression and apoptosis. Together, these studies define a fundamental role for aberrant ATF2 transcriptional dysregulation in the etiology of synovial sarcoma.

Genetic deletion of Cyp26b1 negatively impacts limb skeletogenesis by inhibiting chondrogenesis

Cyp26b1, a retinoic acid (RA)-metabolising enzyme, is expressed in the developing limb bud, and Cyp26b1(-/-) mice present with severe limb defects. These malformations might be attributable to an RA-induced patterning defect; however, recent reports suggest that RA is dispensable for limb patterning. In this study, we examined the role of endogenous retinoid signalling in skeletogenesis using Cyp26b1(-/-) mice and transgenic mice in which Cyp26b1 is conditionally deleted under control of the Prrx1 promoter beginning at ~E9.5 (Prrx1Cre(+)/Cyp26b1(fl/fl)). We found that the limb phenotype in Prrx1Cre(+)/Cyp26b1(fl/fl) mice was less severe than that observed in Cyp26b1(-/-) animals and that a change in retinoid signalling contributed to the difference in phenotypes. We systematically examined the role of endogenous RA signalling in chondrogenesis and found that Cyp26b1(-/-) cells and limb mesenchymal cells treated with a CYP inhibitor, are maintained in a pre-chondrogenic state, exhibit reduced chondroblast differentiation and have modestly accelerated chondrocyte hypertrophy. Furthermore, Cyp26b1(-/-) mesenchyme exhibited an increase in expression of genes in a closely related tendogenic lineage, indicating that retinoid signals in the limb interfere with differentiation and maintain progenitor status. Together, these findings support an important function for RA in regulating the behaviour of mesenchymal progenitors, and their subsequent differentiation and maturation.

Pannexin 3 is a novel target for Runx2, expressed by osteoblasts and mature growth plate chondrocytes

Pannexins are a class of chordate channel proteins identified by their homology to insect gap junction proteins. The pannexin family consists of three members, Panx1, Panx2, and Panx3, and the role each of these proteins plays in cellular processes is still under investigation. Previous reports of Panx3 expression indicate enrichment in skeletal tissues, so we have further investigated this distribution by surveying the developing mouse embryo with immunofluorescence. High levels of Panx3 were detected in intramembranous craniofacial flat bones, as well as long bones of the appendicular and axial skeleton. This distribution is the result of expression in both osteoblasts and hypertrophic chondrocytes. Furthermore, the Panx3 promoter contains putative binding sites for transcription factors involved in bone formation, and we show that the sequence between bases -275 and -283 is responsive to Runx2 activation. Taken together, our data suggests that Panx3 may serve an important role in bone development, and is a novel target for Runx2-dependent signaling.

Mechanical force modulates scleraxis expression in bioartificial tendons

Following tendon injury, cartilage, bone and fat metaplasia are often observed, making the optimization of tenocyte differentiation an important clinical goal. In this study we examined the effect of static and cyclic mechanical loading on the expression of genes which play a role in tenocyte differentiation and function, namely scleraxis (Scx) and Type I collagen (Col1a1), and determined the effect of varying mechanical parameters including (1) static vs dynamic load, (2) increasing strain magnitude, (3) inclusion of 10 s rest periods, and (4) increasing cycle number. Cyclic loading resulted in a greater increase of tenocyte gene expression than static loading over 3 weeks in culture. Increasing strain levels potentiated the induction of tenocyte genes. The insertion of a 10 s rest periods further enhanced tenocyte gene expression, as did increasing repetition numbers. These results suggest that mechanical signaling exerts an important influence on the expression of genes which play a role in determining the tendon phenotype. Further work is required to confirm and extend these findings in primary cells such as resident tendon progenitor/stem cells, in order to provide an improved understanding of biology from which optimized rehabilitation programs can be developed.

Noncanonical activation of Akt/protein kinase B in {beta}-cells by the incretin hormone glucose-dependent insulinotropic polypeptide

Therapeutics based on the actions of the incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), have recently been introduced for the treatment of type 2 diabetes mellitus. The serine/threonine kinase Akt is a major mediator of incretin action on the pancreatic islet, increasing beta-cell mass and function and promoting beta-cell survival. The mechanisms underlying incretin activation of Akt are thought to involve an essential phosphoinositide 3-kinase-mediated phosphorylation of threonine 308, similar to the prototypical Akt activator, insulin-like growth factor-I (IGF-I). In this study, using activity assays on immunoprecipitated Akt, we discovered that GIP and GLP-1 were capable of stimulating Akt in the INS-1 beta-cell line and isolated mouse islets via a mechanism that did not require phosphoinositide 3-kinase or phosphorylation of Thr(308) and Ser(473), and this pathway involved the production of cAMP. Furthermore, we found that GIP stimulated anti-apoptotic signaling via this alternate mode of Akt activation. We conclude that incretins can activate Akt via a novel noncanonical mechanism that may provide an alternative therapeutic target for the treatment of type 2 diabetes mellitus and have broader implications for Akt physiology in human health and disease.

Roles of IP3R and RyR Ca2+ channels in endoplasmic reticulum stress and beta-cell death

OBJECTIVE

Endoplasmic reticulum (ER) stress has been implicated in the pathogenesis of diabetes, but the roles of specific ER Ca(2+) release channels in the ER stress-associated apoptosis pathway remain unknown. Here, we examined the effects of stimulating or inhibiting the ER-resident inositol trisphosphate receptors (IP(3)Rs) and the ryanodine receptors (RyRs) on the induction of beta-cell ER stress and apoptosis.

RESEARCH DESIGN AND METHODS

Kinetics of beta-cell death were tracked by imaging propidium iodide incorporation and caspase-3 activity in real time. ER stress and apoptosis were assessed by Western blot. Mitochondrial membrane potential was monitored by flow cytometry. Cytosolic Ca(2+) was imaged using fura-2, and genetically encoded fluorescence resonance energy transfer (FRET)-based probes were used to measure Ca(2+) in ER and mitochondria.

RESULTS

Neither RyR nor IP(3)R inhibition, alone or in combination, caused robust death within 24 h. In contrast, blocking sarco/endoplasmic reticulum ATPase (SERCA) pumps depleted ER Ca(2+) and induced marked phosphorylation of PKR-like ER kinase (PERK) and eukaryotic initiation factor-2alpha (eIF2alpha), C/EBP homologous protein (CHOP)-associated ER stress, caspase-3 activation, and death. Notably, ER stress following SERCA inhibition was attenuated by blocking IP(3)Rs and RyRs. Conversely, stimulation of ER Ca(2+) release channels accelerated thapsigargin-induced ER depletion and apoptosis. SERCA block also activated caspase-9 and induced perturbations of the mitochondrial membrane potential, resulting eventually in the loss of mitochondrial polarization.

CONCLUSIONS

This study demonstrates that the activity of ER Ca(2+) channels regulates the susceptibility of beta-cells to ER stress resulting from impaired SERCA function. Our results also suggest the involvement of mitochondria in beta-cell apoptosis associated with dysfunctional beta-cell ER Ca(2+) homeostasis and ER stress.

Bone sialoprotein expression enhances osteoblast differentiation and matrix mineralization in vitro

Bone sialoprotein (BSP) is an acidic, noncollagenous glycoprotein abundantly expressed in mineralized tissues. Although BSP is frequently used as a marker of osteoblast differentiation, the role of the protein in osteoblast function is unclear. BSP belongs to the SIBLING (Small Integrin-binding LIgand N-linked Glycoprotein) family of RGD-containing matrix proteins, several members of which have been shown to affect cell differentiation. The normal levels of BSP expression in osteoblasts were specifically altered by CMV-mediated adenoviral overexpression in primary osteoblasts or inhibition by an RNA interference-based strategy in the MC3T3E1 cell line. Alternatively, osteoblast cultures were supplemented with recombinant BSP protein. Quantitative real-time PCR was used to monitor the mRNA levels of the osteoblast-related transcription factors Osterix and Runx2 as well as the osteoblast-specific gene osteocalcin. As markers of osteoblast differentiation, alkaline phosphatase enzyme activity, Runx2-luciferase reporter activity and calcein incorporation into mineralized cultures were also measured. The overexpression of BSP increased osteoblast-related gene expression as well as calcium incorporation and nodule formation by osteoblast cultures. Similarly, supplementation of osteoblast cultures with recombinant BSP increased several markers of osteoblast differentiation. Conversely, suppression of BSP expression by small-hairpin RNA-encoding plasmids inhibited expression of osteoblast markers and nodule formation. Overexpression of several functional-domain mutants of BSP demonstrated that increases in osteoblast-related gene expression and matrix mineralization observed in BSP overexpression models are mediated by the integrin-binding RGD motif found near the C-terminus of the protein. These results demonstrate that BSP may serve as a matrix-associated signal directly promoting osteoblast differentiation resulting in the increased production of a mineralized matrix.

p38 MAP kinase signalling is required for hypertrophic chondrocyte differentiation

Longitudinal growth of endochondral bones is accomplished through the co-ordinated proliferation and hypertrophic differentiation of growth plate chondrocytes. The molecular mechanisms and signalling cascades controlling these processes are not well understood. To analyse the expression and roles of p38 mitogen-activated protein kinases in this process, we have established a micromass system for the reproducible hypertrophic differentiation of mouse mesenchymal limb bud cells. Our results show that all four mammalian p38 kinase genes are expressed during the chondrogenic programme, as well as their upstream regulators MKK3 (mitogen-activated protein kinase kinase 3) and MKK6. Treatment of micromass cultures with pharmacological inhibitors of p38 results in a marked delay in hypertrophic differentiation in micromass cultures, indicating a requirement for p38 signalling in chondrocyte differentiation. Inhibition of p38 kinase activity leads to reduced and delayed induction of alkaline phosphatase activity and matrix mineralization. In addition, p38 inhibition causes reduced expression of hypertrophic marker genes such as collagen X, matrix metalloproteinase 13 and bone sialoprotein. The function of p38 in hypertrophic differentiation appears to be mediated, at least in part, by the transcription factor myocyte enhancer factor 2C. In summary, we have demonstrated a novel requirement for p38 signalling in hypertrophic differentiation of chondrocytes and identified myocyte enhancer factor 2C as an important regulator of chondrocyte gene expression.

Inhibition of p38 MAPK signaling promotes late stages of myogenesis

Signaling through the p38 mitogen-activated protein kinases (MAPKs) is essential for cartilage formation in primary cultures of limb mesenchyme. Here we show that, concurrent with a decrease in chondrogenesis, inhibition of p38 in limb bud cultures dramatically promotes muscle development. Specifically, treatment of primary limb bud cultures with p38 inhibitors increases the expression of myogenic markers and causes a striking increase in formation of myotubes, which were detected using antibodies specific for myosin heavy chain. These results are surprising in that they contrast with several previous reports describing a requirement for p38 during myogenesis. Nonetheless, the enhanced myogenesis leads to the formation of an extensive network of contractile myofibers, and this enhanced myogenesis can be conferred upon myogenic cells from clonal populations, such as G8 or C2C12 cells, if they are co-cultured with the limb mesenchymal cells. We provide evidence for the maintenance and rapid organization of existing, somitic-derived limb myoblasts in response to p38 inhibitors. These findings imply a novel and unexpected role for p38 MAPK inhibition in myogenesis and highlight the importance of the limb bud microenvironment in promoting the progression of limb myoblasts.

Retinoid signalling and skeletal development

Metabolites of vitamin A, including retinoic acid (RA), comprise a class of molecules known to be important in development and homeostasis. RA functions through a class of nuclear hormone receptors, the RA receptors (RARs), to regulate gene transcription. In the developing mammalian limb, RA affects the differentiation of many cell lineages, including those of the chondrogenic lineage. In excess, RA is a potent teratogen, causing characteristic skeletal defects in a stage- and dose-dependent manner. Genetic analysis has shown that the absence of RARs leads to severe deficiencies in cartilage formation at certain anatomical locations while promoting ectopic cartilage formation at other sites. Expression of either a dominant-negative or a weak constitutively active RAR in the developing limbs of transgenic mice adversely affects chondrogenesis leading to skeletal malformations. Together, these results show that RAR-mediated signalling plays a fundamental role in skeletogenesis. This chapter will focus on the function of RARs in regulating chondroblast differentiation and the contribution of RA signalling to appositional and longitudinal growth of the skeletal primordia.