Thienoquinolines as novel disruptors of the PKCε/RACK2 protein-protein interaction

Ten protein kinase C (PKC) isozymes play divergent roles in signal transduction. Because of sequence similarities, it is particularly difficult to generate isozyme-selective small molecule inhibitors. In order to identify such a selective binder, we derived a pharmacophore model from the peptide EAVSLKPT, a fragment of PKCε that inhibits the interaction of PKCε and receptor for activated C-kinase 2 (RACK2). A database of 330 000 molecules was screened in silico, leading to the discovery of a series of thienoquinolines that disrupt the interaction of PKCε with RACK2 in vitro. The most active molecule, N-(3-acetylphenyl)-9-amino-2,3-dihydro-1,4-dioxino[2,3-g]thieno[2,3-b]quinoline-8-carboxamide (8), inhibited this interaction with a measured IC₅₀ of 5.9 μM and the phosphorylation of downstream target Elk-1 in HeLa cells with an IC₅₀ of 11.2 μM. Compound 8 interfered with MARCKS phosphorylation and TPA-induced translocation of PKCε (but not that of PKCδ) from the cytosol to the membrane. The compound reduced the migration of HeLa cells into a gap, reduced invasion through a reconstituted basement membrane matrix, and inhibited angiogenesis in a chicken egg assay.

Abstract 3132: C6orf69 is down-modulated by protein kinase C epsilon and exhibits increased expression in breast cancers

Protein kinase C (PKC) comprises a serine/threonine kinase family of at least ten related isozymes. Protein kinase C epsilon (PKCε) has been shown previously to act as an oncogene, contributing to malignancy by tumor cell invasion, metastasis and inhibiting cell death. However, the precise function is not known at present. We investigated whether PKCε expression leads to alterations in the expression of other genes.

We found by gene array after overexpression of constitutively active PKCε that EST AA167550, of which 348 bases were known, was down-modulated. We obtained the full-length sequence of AA167550. It was identified as chromosome 6 open reading frame 69 gene (C6orf69). At least two distinct mRNAs were found to be transcribed from this gene. We generated an antibody and as shown by Western blotting, two proteins (47 and 40 kDa) were detected. The full length sequence of C6orf69 contains a BTB-BACK-Kelch domain usually involved in protein-protein interactions and implicated in the stability of actin filaments. C6orf69 co-immunoprecipitated with PKCε and bound to actin. PKCε/C6orf69 interaction was additionally confirmed by a yeast two-hybrid screen using C6orf69 full length as bait.

C6orf69 is expressed in many tissues except in heart, ovary, small intestine and leukocytes. In nine out of nine breast cancers, the expression levels were higher than in matched normal mammary tissue. C6orf69 contains four putative PKC phosphorylation sites. The C6orf69 protein is phosphorylated but is not a direct substrate of PKC isozymes, as demonstrated by in vitro PKC assays. Recently, BTB-domain containing proteins have been reported to regulate protein stability by recruiting proteins to the Cullin3 (Cul3) ubiquitin ligase leading to their degradation via ubiquitin/proteasome. By a yeast two-hybrid screen and co-immunoprecipitation we could demonstrate that C6orf69 is interacting specifically with Cul3. In addition, an in vivo ubiquitylation assay showed that overexpressed C6orf69 protein is efficiently polyubiquitylated, indicating to be a substrate of E3 ligase itself.

In summary, C6orf6 is down-modulated by PKCε, shows elevated expression in breast cancers compared to corresponding normal breast tissue, contains a BTB domain, interacts with PKCε, actin and Cul3. C6orf69 is phosporylated but not substrate of PKC isozymes and it is ubiquitylated.

This work was supported by grants P15039 and P19878 from the Austrian Science Fund (FWF).

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 3132. doi:10.1158/1538-7445.AM2011-3132

Protein kinase Cgamma in colon cancer cells: expression, Thr514 phosphorylation and sensitivity to butyrate-mediated upregulation as related to the degree of differentiation

Protein kinase C (PKC) isoenzymes are expressed and activated in a cell type-specific manner, and play an essential role in tissue-specific signal transduction. The presence of butyrate at millimolar concentrations in the colon raises the question of whether it affects the expression of PKC isoenzymes in the different cell types of the colonic epithelium. We investigated the protein expression levels of PKCgamma, Thr(514)-phosphorylated PKCgamma (pPKCgamma-Thr(514)), and their subcellular distribution as affected by butyrate in a set of colon cancer cell lines. Thr(514)-phosphorylation of de novo synthesized PKCgamma is the first step in priming of the inactive PKCgamma before its release into the cytoplasm. For immunoblot analysis, we employed three antibodies, one against an unmodified sequence, mapping within 50 amino acids at its C-terminus, a second against pPKCgamma-Thr(514), and a third against pPKCgamma-pan-Thr(514). The antibody against an unmodified C-terminal peptide epitope did not recognize pPKCgamma-Thr(514), suggesting that phosphorylation at this site interferes with the binding of the antibody to the C-terminus. Marked butyrate-induced upregulation of PKCgamma occurred in HT29 cells (model for colonocyte stem cells) and HT29-derived cell lines. However, in Caco2 and IEC-18 cells (models for differentiated intestinal epithelial cells), PKCgamma was insensitive to upregulation, and present exclusively as pPKCgamma-Thr(514). Lovo and SW480 expressed higher levels of PKCgamma. In HT29 cells, butyrate-induced upregulation of the non-phosphorylated PKCgamma was observed in both the membrane and the cytosolic fraction. In Caco2 cells, the Thr(514)-phosphorylated form was present at high levels in both fractions. The presence of unphosphorylated PKCgamma in HT29 cells, and its complete absence in Caco2 cells demonstrates a cell type-dependent differential coupling of Thr(514)-phosphorylation with de novo synthesis of PKCgamma in colon cancer cells.

Investigation of the marine compound spongistatin 1 links the inhibition of PKCalpha translocation to nonmitotic effects of tubulin antagonism in angiogenesis

The aims of the study were to meet the demand of new tubulin antagonists with fewer side effects by characterizing the antiangiogenic properties of the experimental compound spongistatin 1, and to elucidate nonmitotic mechanisms by which tubulin antagonists inhibit angiogenesis. Although tubulin-inhibiting drugs and their antiangiogenic properties have been investigated for a long time, surprisingly little is known about their underlying mechanisms of action. Antiangiogenic effects of spongistatin 1 were investigated in endothelial cells in vitro, including functional cell-based assays, live-cell imaging, and a kinome array, and in the mouse cornea pocket assay in vivo. Spongistatin 1 inhibited angiogenesis at nanomolar concentrations (IC(50): cytotoxicity>50 nM, proliferation 100 pM, migration 1.0 nM, tube formation 1.0 nM, chemotaxis 1.0 nM, aortic ring sprouting 500 pM, neovascularization in vivo 10 microg/kg). Further, a kinome array and validating data showed that spongistatin 1 inhibits the phosphorylation activity of protein kinase Calpha (PKCalpha), an essential kinase in angiogenesis, and its translocation to the membrane. Thus, we conclude that PKCalpha might be an important target for the antiangiogenic effects of tubulin antagonism. In addition, the data from the kinase array suggest that different tubulin antagonists might have individual intracellular actions.