Adam D. Pierce

Glucose-activated RUNX2 phosphorylation promotes endothelial cell proliferation and an angiogenic phenotype

The runt‐related protein‐2 (RUNX2) is a DNA‐binding transcription factor that regulates bone formation, tumor cell metastasis, endothelial cell (EC) proliferation, and angiogenesis. RUNX2 DNA binding is glucose and cell cycle regulated. We propose that glucose may activate RUNX2 through changes in post‐translational phosphorylation that are cell cycle‐specific and will regulate EC function. Glucose increased cell cycle progression in EC through both G2/M and G1 phases with entry into S‐phase occurring only in subconfluent cells. In the absence of nutrients and growth factors (starvation), subconfluent EC were delayed in G1 when RUNX2 expression was reduced. RUNX2 phosphorylation, activation of DNA binding, and pRb phosphorylation were stimulated by glucose and were necessary to promote cell cycle progression. Glucose increased RUNX2 localization at focal subnuclear sites, which co‐incided with RUNX2 occupancy of the cyclin‐dependent kinase (cdk) inhibitor p21Cip1 promoter, a gene normally repressed by RUNX2. Mutation of the RUNX2 cdk phosphorylation site in the C‐terminal domain (S451A.RUNX2) reduced RUNX2 phosphorylation and DNA binding. Expression of this cdk site mutant in EC inhibited glucose‐stimulated differentiation (in vitro tube formation), monolayer wound healing, and proliferation. These results define a novel relationship between glucose‐activated RUNX2 phosphorylation, cell cycle progression, and EC differentiation. These data suggest that inhibition of RUNX2 expression or DNA binding may be a useful strategy to inhibit EC proliferation in tumor angiogenesis. J. Cell. Biochem. 113: 282–292, 2012.

Covalent Small Molecule Inhibitors of Ca(2+)-Bound S100B.

Elevated levels of the tumor marker S100B are observed in malignant melanoma, and this EF-hand-containing protein was shown to directly bind wild-type (wt) p53 in a Ca2+-dependent manner, dissociate the p53 tetramer, and inhibit its tumor suppression functions. Likewise, inhibiting S100B with small interfering RNA (siRNAS100B) is sufficient to restore wild-type p53 levels and its downstream gene products and induce the arrest of cell growth and UV-dependent apoptosis in malignant melanoma. Therefore, it is a goal to develop S100B inhibitors (SBiXs) that inhibit the S100B–p53 complex and restore active p53 in this deadly cancer. Using a structure–activity relationship by nuclear magnetic resonance approach (SAR by NMR), three persistent binding pockets are found on S100B, termed sites 1–3. While inhibitors that simultaneously bind sites 2 and 3 are in place, no molecules that simultaneously bind all three persistent sites are available. For this purpose, Cys84 was used in this study as a potential means to bridge sites 1 and 2 because it is located in a small crevice between these two deeper pockets on the protein. Using a fluorescence polarization competition assay, several Cys84-modified S100B complexes were identified and examined further. For five such SBiX–S100B complexes, crystallographic structures confirmed their covalent binding to Cys84 near site 2 and thus present straightforward chemical biology strategies for bridging sites 1 and 3. Importantly, one such compound, SC1982, showed an S100B-dependent death response in assays with WM115 malignant melanoma cells, so it will be particularly useful for the design of SBiX molecules with improved affinity and specificity.

Complex Formation between S100B Protein and the p90 Ribosomal S6 Kinase (RSK) in Malignant Melanoma Is Calcium-dependent and Inhibits Extracellular Signal-regulated Kinase (ERK)-mediated Phosphorylation of RSK

Background: S100B is overexpressed in malignant melanoma and contributes to cancer progression. Results: The S100B-RSK complex was found to be Ca2+-dependent, block phosphorylation of RSK at Thr-573, and sequester RSK to the cytosol. Conclusion: The Ca2+-dependent S100B-RSK complex provides a new link between the MAPK and Ca2+ signaling pathways. Significance: S100B inhibitors may restore normal MAPK and Ca2+ signaling in malignant melanoma. S100B is a prognostic marker for malignant melanoma. Increasing S100B levels are predictive of advancing disease stage, increased recurrence, and low overall survival in malignant melanoma patients. Using S100B overexpression and shRNAS100B knockdown studies in melanoma cell lines, elevated S100B was found to enhance cell viability and modulate MAPK signaling by binding directly to the p90 ribosomal S6 kinase (RSK). S100B-RSK complex formation was shown to be Ca2+-dependent and to block ERK-dependent phosphorylation of RSK, at Thr-573, in its C-terminal kinase domain. Additionally, the overexpression of S100B sequesters RSK into the cytosol and prevents it from acting on nuclear targets. Thus, elevated S100B contributes to abnormal ERK/RSK signaling and increased cell survival in malignant melanoma.

Regulation of RUNX2 transcription factor–DNA interactions and cell proliferation by vitamin D3 (cholecalciferol) prohormone activity

The fat‐soluble prohormone cholecalciferol (Vitamin D3) is a precursor of the circulating 25‐OH Vitamin D3, which is converted by 1α‐hydroxylase to the biologically active 1,25‐OH Vitamin D3. Active Vitamin D3 interacts with the Vitamin D receptor (VDR), a transcription factor that plays an important role in calcium mobilization and bone formation. RUNX2 is a DNA‐binding transcription factor that regulates target genes important in bone formation, angiogenesis, and cancer metastasis. Using computer‐assisted drug design (CADD) and a microtiter plate‐based DNA‐binding enzyme‐linked immunosorbent assay (D‐ELISA) to measure nuclear RUNX2 DNA binding, we have found that Vitamin D3 prohormones can modulate RUNX2 DNA binding, which was dose‐dependent and sensitive to trypsin, salt, and phosphatase treatment. Unlabeled oligonucleotide or truncated, dominant negative RUNX2 proteins were competitive inhibitors of RUNX2 DNA binding. The RUNX2 heterodimeric partner, Cbfβ, was detected in the binding complexes with specific antibodies. Evaluation of several RUNX2:DNA targeted small molecules predicted by CADD screening revealed a previously unknown biological activity of the inactive Vitamin D3 precursor, cholecalciferol. Cholecalciferol modulated RUNX2:DNA binding at nanomolar concentrations even in cells with low VDR. Cholecalciferol and 25‐OH Vitamin D3 prohormones were selective inhibitors of RUNX2‐positive endothelial, bone, and breast cancer cell proliferation, but not of cells lacking RUNX2 expression. These compounds may have application in modulating RUNX2 activity in an angiogenic setting, in metastatic cells, and to promote bone formation in disease‐mediated osteoporosis. The combination CADD discovery and D‐ELISA screening approaches allows the testing of other novel derivatives of Vitamin D and/or transcriptional inhibitors with the potential to regulate DNA binding and biological function.

Hyperglycemia and redox status regulate RUNX2 DNA-binding and an angiogenic phenotype in endothelial cells

Angiogenesis is regulated by hyperglycemic conditions, which can induce cellular stress responses, reactive oxygen species (ROS), and anti-oxidant defenses that modulate intracellular signaling to prevent oxidative damage. The RUNX2 DNA-binding transcription factor is activated by a glucose-mediated intracellular pathway, plays an important role in endothelial cell (EC) function and angiogenesis, and is a target of oxidative stress. RUNX2 DNA-binding and EC differentiation in response to glucose were conserved in ECs from different tissues and inhibited by hyperglycemia, which stimulated ROS production through the aldose reductase glucose-utilization pathway. Furthermore, the redox status of cysteine and methionine residues regulated RUNX2 DNA-binding and reversal of oxidative inhibition was consistent with an endogenous Methionine sulfoxide reductase-A (MsrA) activity. Low molecular weight MsrA substrates and sulfoxide scavengers were potent inhibitors of RUNX2 DNA binding in the absence of oxidative stress, but acted as antioxidants to increase DNA binding in the presence of oxidants. MsrA was associated with RUNX2:DNA complexes, as measured by a sensitive, quantitative DNA-binding ELISA. The related RUNX2 protein family member, RUNX1, which contains an identical DNA-binding domain, was a catalytic substrate of recombinant MsrA. These findings define novel redox pathways involving aldose reductase and MsrA that regulate RUNX2 transcription factor activity and biological function in ECs. Targeting of these pathways could result in more effective strategies to alleviate the vascular dysfunction associated with diabetes or cancer.

The Activation of Protein Kinase A by the Calcium-Binding Protein S100A1 Is Independent of Cyclic AMP

Biochemical and structural studies demonstrate that S100A1 is involved in a Ca2+-dependent interaction with the type 2α and type 2β regulatory subunits of protein kinase A (PKA) (RIIα and RIIβ) to activate holo-PKA. The interaction was specific for S100A1 because other calcium-binding proteins (i.e., S100B and calmodulin) had no effect. Likewise, a role for S100A1 in PKA-dependent signaling was established because the PKA-dependent subcellular redistribution of HDAC4 was abolished in cells derived from S100A1 knockout mice. Thus, the Ca2+-dependent interaction between S100A1 and the type 2 regulatory subunits represents a novel mechanism that provides a link between Ca2+ and PKA signaling, which is important for the regulation of gene expression in skeletal muscle via HDAC4 cytosolic–nuclear trafficking.

Novel protein–inhibitor interactions in site 3 of Ca2+-bound S100B as discovered by X-ray crystallography

Structure-based drug discovery is under way to identify and develop small-molecule S100B inhibitors (SBiXs). Such inhibitors have therapeutic potential for treating malignant melanoma, since high levels of S100B downregulate wild-type p53 tumor suppressor function in this cancer. Computational and X-ray crystallographic studies of two S100B-SBiX complexes are described, and both compounds (apomorphine hydrochloride and ethidium bromide) occupy an area of the S100B hydrophobic cleft which is termed site 3. These data also reveal novel protein-inhibitor interactions which can be used in future drug-design studies to improve SBiX affinity and specificity. Of particular interest, apomorphine hydrochloride showed S100B-dependent killing in melanoma cell assays, although the efficacy exceeds its affinity for S100B and implicates possible off-target contributions. Because there are no structural data available for compounds occupying site 3 alone, these studies contribute towards the structure-based approach to targeting S100B by including interactions with residues in site 3 of S100B.

Abstract 4201: Activation of p90 ribosomal S6 kinase (RSK) and downstream targets are directly regulated by S100B protein in malignant melanoma

S100B is an effective and extensively used prognostic marker for melanoma, with increasing serum levels of S100B being predictive of disease stage, increased recurrence, and low overall patient survival. We have recently identified a mechanism by which S100B alters ERK-RSK signaling to facilitate the progression of melanoma. Elucidation of this novel mechanism will aid the development of new pharmacological drugs to inhibit tumor progression. We have identified and characterized several ERK-RSK downstream targets regulated by S100B expression. These data are consistent with a mechanism in which elevated S100B binds directly to RSK in a calcium-dependent manner to prevent ERK-mediated phosphorylation of RSK. Our structural studies have further elucidated the S100B-RSK interaction as a novel drug target in melanoma cells and we are currently screening compounds for inhibitors of the S100B-RSK interaction. Citation Format: Adam D. Pierce. Activation of p90 ribosomal S6 kinase (RSK) and downstream targets are directly regulated by S100B protein in malignant melanoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4201. doi:10.1158/1538-7445.AM2014-4201

Abstract 5243: The novel calcium-dependent direct binding of S100B to p90 ribosomal S6 kinase (RSK) decreases MAP kinase-mediated RSK activation in malignant melanoma.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC S100B is an effective and extensively used prognostic marker for melanoma, with increasing serum S100B being predictive of disease stage, increased recurrence, and low overall patient survival. Establishing the mechanism by which S100B alters cell signaling provides insight into how it may facilitate the progression of melanoma and aid in developing new pharmacological drugs to inhibit cancer advancement. To evaluate specific roles for S100B in malignant melanoma, our S100B knock-down and over-expression studies established a correlation between S100B expression and cell viability. ERK phosphorylation was also found to correlate with S100B levels. We have discovered RSK, a downstream ERK target, as a novel binding partner of S100B. RSK is known to regulate MAPK signaling in melanoma but the mechanism is unclear. We observed an inverse relationship between activated and phosphorylated RSK and S100B levels. We used a Ca2+-binding mutant of S100B (E31A + E72A) to demonstrate that the S100B-RSK interaction was calcium-dependent. Pull-down experiments revealed the C-terminal domain of RSK to be necessary for S100B binding. In vitro, S100B was found to reduce ERK-dependent phosphorylation of RSK at Thr573. Our cellular fractionation and immuno-fluorescence studies showed that S100B protein prevented nuclear localization of phosphorylated RSK. These data are consistent with a mechanism in which elevated S100B binds directly to RSK in a calcium-dependent manner, preventing ERK-mediated phosphorylation of RSK and inhibiting RSK localization to the nucleus. We have identified the S100B-RSK interaction as a novel drug target in melanoma cells. We are currently screening compounds for inhibitors of the S100B-RSK interaction. Citation Format: Adam D. Pierce, Kira G. Hartman, David Weber. The novel calcium-dependent direct binding of S100B to p90 ribosomal S6 kinase (RSK) decreases MAP kinase-mediated RSK activation in malignant melanoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5243. doi:10.1158/1538-7445.AM2013-5243