Abstract 252: Evaluating the role of STK33 kinase in mutant KRAS cells

Tumors harboring KRAS mutations remain an elusive target for oncology therapeutics. A recent publication (Scholl et al 2009) described a high throughput cellular RNAi screen which suggested a synthetic lethal relationship between STK33 and mutant KRAS. STK33 is a member of the calcium/calmodulin kinase family with a poorly characterized function. Although no genetic alterations in the STK33 gene have been reported in cancer, inhibiting STK33 kinase activity could offer a novel therapeutic approach to target mutant KRAS tumors. Thus, we sought to validate whether the survival of cancer cell lines with mutant KRAS is dependent on STK33, in parallel with a program to identify small molecule inhibitors of STK33 (Zhang Y et al AACR 2011). We focused on mutant KRAS leukemia cells because of their reported dependence on STK33. Using a panel of siRNA, 50-80% knockdown of STK33 at the RNA and protein level was achieved in NOMO-1KRASG13D and SKM-1KRASK117N cells. However we did not observe any alteration in cell viability. In addition, putative STK33 downstream signaling, (phosphorylation of p70 S6K thr389 and RPS6 ser235/236) was unaltered by modulation of STK33 expression. In contrast, knockdown of KRAS caused a significant reduction in both viability and signaling in these cell lines, confirming their dependence on KRAS. A panel of 27 cancer cell lines was screened with an siRNA library representing 1500 druggable genes. STK33 siRNA had no significant effect on viability regardless of KRAS mutational status. As expected, knockdown of KRAS significantly reduced cell viability, especially in mutant KRAS cells. A kinase dead mutant STK33 was used to test the dependence on STK33 kinase activity in mutant KRAS cell lines. Transient over-expression of kinase dead STK33 in PANC-1KRASG12D and DLD-1KRASG13D cells had no effect on viability, nor on the phosphorylation of p70 S6K thr389 and RPS6 ser235/236. In summary we have been unable to confirm a synthetic lethal relationship between STK33 and KRAS. Our data do not support inhibition of STK33 as a promising therapeutic approach for targeting mutant KRAS tumors.

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 252. doi:10.1158/1538-7445.AM2011-252

Abstract 253: Identification of STK33 kinase inhibitors for the validation of a synthetic lethal relationship between STK33 and mutant KRAS

The kinase activity of the serine/threonine kinase STK33 was recently shown to be required for the survival of cancer cell lines dependent on the KRAS oncogene. This suggests that an STK33 kinase inhibitor could be useful to treat cancer patients whose tumors harbor KRAS mutations. Our goal was to identify STK33 kinase inhibitors to explore the role of this kinase in mutant KRAS cancer cell lines. First, full-length wild-type or kinase dead (KD) STK33 proteins were prepared using a baculovirus/insect expression system and purified by chromatography. Preliminary experiments identified two peptides, CREBtide and p70S6K peptides, as potential substrates for STK33. Both peptides were therefore used to track STK33 kinase activity during purification steps. We found that phosphorylation of the p70S6K peptide correlated well with STK33 presence during purification, but the peptide was not phosphorylated by the kinase dead STK33. In contrast, CREBtide phosphorylation showed a poor correlation with STK33 positive fractions and also occurred in the presence of a kinase dead STK33. The p70S6K peptide was thus chosen as the substrate for final optimization of an STK33 kinase assay. Over 400 000 compounds were screened using a high throughput STK33 kinase assay. Multiple hits were identified and IC50s were determined for the top 1043 compounds. Multiple potent STK33 kinase inhibitors were identified, including 69 compounds with IC50s <100 nM and 6 with IC50s <10 nM. Since p70S6K and RPS6 phosphorylation were previously shown to be dependent on STK33 in mutant KRAS AML cell lines, we tested our STK33 inhibitors for their ability to inhibit these signaling nodes in cells. In our hands, the levels of p70S6K phosphorylation were too low to be measured accurately, thus we tested the effect of the 510 most potent STK33 kinase inhibitors on RPS6 phosphorylation in NB4 and SKM-1 mutant KRAS cell lines. While many compounds inhibited RPS6 phosphorylation, there was no correlation between the potency of the compounds in the STK33 enzyme assay and on RPS6 phosphorylation in cells. To assess whether the kinase activity of STK33 is required for the survival of mutant KRAS cell lines, we tested the effect of the 145 most potent STK33 kinase inhibitors on the survival of 4 AML cell lines, 2 mutant for KRAS (NB4 and SKM-1) and 2 wild-type for KRAS (U-937 and OCI-AML3). There was no correlation between the effect of the compounds on STK33 kinase activity and their effects on cell survival. Many compounds inhibited cell growth, but not in a KRAS-dependent manner. In summary, we have identified STK33 kinase inhibitors using a high throughput screen. These compounds had no selective growth inhibitory effects on KRAS dependent cells. We conclude that the STK33 kinase activity does not contribute to the survival of KRAS-dependent cell lines.

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 253. doi:10.1158/1538-7445.AM2011-253

Selective and potent Raf inhibitors paradoxically stimulate normal cell proliferation and tumor growth

Raf inhibitors are under clinical investigation, specifically in patients with tumor types harboring frequent activating mutations in B-Raf. Here, we show that cell lines and tumors harboring mutant B-Raf were sensitive to a novel series of Raf inhibitors (e.g., (V600E)B-Raf A375, IC(50) on cells = 2 nmol/L; ED(50) on tumor xenografts = 1.3 mg/kg). However, in cells and tumors with wild-type B-Raf, exposure to Raf inhibitors resulted in a dose-dependent and sustained activation of mitogen-activated protein kinase signaling. In some of these cell lines, Raf inhibition led to entry into the cell cycle, enhanced proliferation, and significantly stimulated tumor growth in vivo. Inhibition with structurally distinct Raf inhibitors or isoform-specific small interfering RNA knockdown of Raf showed that these effects were mediated directly through Raf. Either A-Raf or C-Raf mediated the Raf inhibitor-induced mitogen-activated protein kinase pathway activation in an inhibitor-specific manner. These paradoxical effects of Raf inhibition were seen in malignant and normal cells in vitro and in vivo. Hyperplasia of normal epithelial cells in the esophagus and the stomach was evident in mice with all efficacious Raf inhibitors (n = 8) tested. An implication of these results is that Raf inhibitors may induce unexpected normal cell and tumor tissue proliferation in patients.

Abstract 2681A: Structure-guided design of potent and selective inhibitors of B-Raf kinase displaying on-mechanism in vivo activity

The Ras-Raf-MAPK signaling pathway plays a central role in diverse cellular processes including differentiation, proliferation and survival. This pathway is activated in a large percentage of cancers as a result of mutations in Ras or in B-Raf Ser/Thr kinase. The activating mutation V600E in B-Raf is present in approximately 60% of melanomas and occurs with lower, yet still significant, frequency in human colon and thyroid cancers. Hence, V600EB-Raf has received considerable interest as a small-molecule drug discovery target within the pharmaceutical industry. A high-throughput screen of Amgen's internal kinase preferred library against a recombinant V600EB-Raf kinase domain enzyme identified a class of biarylamide compounds as potent inhibitors of this kinase, and potential starting points for medicinal chemistry efforts. However, these compounds suffered from poor kinase selectivity, particularly against tyrosine kinases from the Src-, VEGFR and PDGFR-families. Crystal structures of representative examples of this scaffold in B-Raf and other tyrosine kinases identified a unique pocket in B-Raf that could be accessed to achieve selective inhibitors of this enzyme. Consequently a novel series of isoquinoline-based compounds was derived which demonstrated potent inhibition against V600EB-Raf enzyme and inhibition of MAPK pathway signaling in cell lines harboring V600EB-Raf. Further optimization within this inhibitor class yielded compounds with favorable in vivo properties as demonstrated by robust inhibition of ERK phosphorylation in V600EB-Raf driven pharmacodynamic models. Crystallographic work supporting the optimization of this series of selective B-Raf inhibitors will be presented.

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 2681A.

Role of the Jun kinase pathway in the regulation of c-Jun expression and apoptosis in sympathetic neurons

When deprived of nerve growth factor (NGF), developing sympathetic neurons die by apoptosis. This death is associated with an increase in the level of c-Jun protein and is blocked by expression of a c-Jun dominant negative mutant. Here we have investigated whether NGF withdrawal activates Jun kinases, a family of stress-activated protein kinases that can stimulate the transcriptional activity of c-Jun by phosphorylating serines 63 and 73 in the transactivation domain and which can activate c-jun gene expression. We found that sympathetic neurons contained high basal levels of Jun kinase activity that increased further after NGF deprivation. In contrast, p38 kinase, another stress-activated protein kinase that can also stimulate c-jun gene expression, was not activated after NGF withdrawal. Consistent with Jun kinase activation, we found using a phospho-c-Jun-specific antibody that c-Jun was phosphorylated on serine 63 after NGF withdrawal. Furthermore, expression of a constitutively active form of MEK kinase 1 (MEKK1), which strongly activates the Jun kinase pathway, increased c-Jun protein levels and c-Jun phosphorylation and induced apoptosis in the presence of NGF. This death could be prevented by co-expression of SEKAL, a dominant negative mutant of SAPK/ERK kinase 1 (SEK1), an activator of Jun kinase that is a target of MEKK1. In contrast, expression of SEKAL alone did not prevent c-Jun expression, increases in c-Jun phosphorylation, or cell death after NGF withdrawal. Thus, activation of Jun kinase and increases in c-Jun phosphorylation and c-Jun protein levels occur at the same time after NGF withdrawal, but c-Jun levels and phosphorylation are regulated by an SEK1-independent pathway.

A c-Jun dominant negative mutant protects sympathetic neurons against programmed cell death

Sympathetic neurons depend on nerve growth factor (NGF) for survival and die by apoptosis in its absence. We have investigated the pattern of expression of the Jun and Fos family of transcription factors in dying sympathetic neurons using antibodies specific for each family member. When sympathetic neurons are deprived of NGF, the level of c-Jun protein significantly increases, whereas the levels of the other members of the Jun and Fos family remain relatively constant. c-Jun also becomes more phosphorylated, probably on its amino terminal transactivation domain. When microinjected into sympathetic neurons, an expression vector for a c-Jun dominant negative mutant protects them against NGF withdrawal-induced death, indicating that AP-1 activity is essential for neuronal cell death. Furthermore, overexpression of the full-length c-Jun protein is, in itself, sufficient to induce apoptosis in sympathetic neurons.