Abstract 1372: A cell based high throughput screening assay for inhibitors of melanoma with high S100B

The calcium regulated S100B protein is over-expressed in many cancers including malignant melanoma where high levels of S100B correlate inversely with patient survival. It has been shown that increased S100B is not only a marker for cancer but may promote tumor survival by directly affecting the activity of several cell signaling proteins. For example, S100B has been shown to bind wild-type p53 in a calcium-dependent manner, sequestering it, and resulting in the loss of p53-dependent tumor suppression activities, and transient knockdown of S100B with siRNA has been shown to reverse this affect in melanoma cell lines. Therefore, the discovery of compounds that specifically inhibit the growth of melanoma cells with high levels of S100B may provide a useful therapeutic strategy. We have developed a cell based high throughput screening assay where the effect of small compounds on the growth of MALME-3M and WM115 melanoma cells with a stable shRNA to knockdown S100B is compared to the isogenically matched cell control containing a stably expressing a scrambled shRNA. Out of the greater than 15,000 compounds screened many inhibited growth of the melanoma cells, some had selectivity inhibiting the growth of the S100B high cells, and inversely high S100B was found to overcome the growth inhibition caused by several compounds. Previously, many S100B inhibitors found using an in vitro screening assay were found to function by chemically modifying Cys84 in S100B, so S100B might act as a sink to sequester some of the cysteine reactive compounds protecting cells from these compounds in some cases.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1372. doi:10.1158/1538-7445.AM2011-1372

In vitro screening and structural characterization of inhibitors of the S100B-p53 interaction

S100B is highly over-expressed in many cancers, including malignant melanoma. In such cancers, S100B binds wild-type p53 in a calcium-dependent manner, sequestering it, and promoting its degradation, resulting in the loss of p53-dependent tumor suppression activities. Therefore, S100B inhibitors may be able to restore wild-type p53 levels in certain cancers and provide a useful therapeutic strategy. In this regard, an automated and sensitive fluorescence polarization competition assay (FPCA) was developed and optimized to screen rapidly for lead compounds that bind Ca(2+)-loaded S100B and inhibit S100B target complex formation. A screen of 2000 compounds led to the identification of 26 putative S100B low molecular weight inhibitors. The binding of these small molecules to S100B was confirmed by nuclear magnetic resonance spectroscopy, and additional structural information was provided by x-ray crystal structures of several compounds in complexes with S100B. Notably, many of the identified inhibitors function by chemically modifying Cys84 in protein. These results validate the use of high-throughput FPCA to facilitate the identification of compounds that inhibit S100B. These lead compounds will be the subject of future optimization studies with the ultimate goal of developing a drug with therapeutic activity for the treatment of malignant melanoma and/or other cancers with elevated S100B.

Abstract 4125: The effects of S100B on cell signaling in malignant melanoma

Establishing the mechanism by which S100B effects ERK and its downstream proteins will provide valuable insight into the role of S100B in the progression of melanoma and potentially aid in developing new pharmacological drugs. The protein S100B is an effective prognostic marker for human malignant melanoma. Increased levels of serum S100B are predictive of disease stage, increased recurrence, and low overall survival. It has previously been found that S100B directly interacts with the tumor suppressor protein p53, down-regulating its protein levels, thereby allowing damaged cells to proliferate and mutations to propagate. S100B has also been implicated in several other signaling pathways, including the MAPK pathway. The initial hypothesis tested was that S100B inhibits the phosphorylation of ERK targets in a calcium-dependent manner. The S100B binding motif has been found to overlap with the ERK binding motif, suggesting that S100B binds the targets and physically prevents ERK from interacting with and subsequently phosphorylating them. ERK kinase assays revealed that the addition of S100B in the presence of calcium decreased ERK phosphorylation of RSK, but more dramatically Elk-1. Additionally, RSK was detected in S100B pull-downs in the presence of calcium; however, it was not detected in the presence of the calcium chelator EDTA. Over-expression of normal human S100B, but not a mutant S100B incapable of binding calcium, results in decreased phosphorylated RSK at Thr573, yet increased phosphorylated ERK, interestingly. Since a rise in serum S100B levels is indicative of tumor progression, it was also of interest to see if the addition of extracellular S100B alone is sufficient to elicit the same signaling response. Preliminary data shows an increase in phosphorylated ERK after the addition of low concentrations of extracellular S100B, as well as effects on ERK targets. We demonstrate that the calcium-binding protein S100B affects the MAPK pathway by simultaneously increasing phosphorylated ERK and decreasing phosphorylated RSK and Elk-1, together which could have a significant impact on cancer progression.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4125.