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PIK3CA is a frequently mutated gene in cancer, including about ~15 to 20% of colorectal cancers (CRC). PIK3CA mutations lead to activation of the PI3K/AKT/mTOR signaling pathway, which plays pivotal roles in tumorigenesis. Here, we investigated the mechanism of resistance of PIK3CA-mutant CRC cell lines to gedatolisib, a dual PI3K/mTOR inhibitor. Out of a panel of 29 CRC cell lines, we identified 7 harboring one or more PIK3CA mutations; of these, 5 and 2 were found to be sensitive and resistant to gedatolisib, respectively. Both of the gedatolisib-resistant cell lines expressed high levels of active glycogen synthase kinase 3-beta (GSK3β) and harbored the same frameshift mutation (c.465_466insC; H155fs*) in TCF7, which encodes a positive transcriptional regulator of the WNT/β-catenin signaling pathway. Inhibition of GSK3β activity in gedatolisib-resistant cells by siRNA-mediated knockdown or treatment with a GSK3β-specific inhibitor effectively reduced the activity of molecules downstream of mTOR and also decreased signaling through the WNT/β-catenin pathway. Notably, GSK3β inhibition rendered the resistant cell lines sensitive to gedatolisib cytotoxicity, both in vitro and in a mouse xenograft model. Taken together, these data demonstrate that aberrant regulation of WNT/β-catenin signaling and active GSK3β induced by the TCF7 frameshift mutation cause resistance to the dual PI3K/mTOR inhibitor gedatolisib. Cotreatment with GSK3β inhibitors may be a strategy to overcome the resistance of PIK3CA- and TCF7-mutant CRC to PI3K/mTOR-targeted therapies.

Although MEK blockade has been highlighted as a promising antitumor drug, it has poor clinical efficacy in KRAS mutant colorectal cancer (CRC). Several feedback systems have been described in which inhibition of one intracellular pathway leads to activation of a parallel signaling pathway, thereby decreasing the effectiveness of single-MEK targeted therapies. Here, we investigated a bypass mechanism of resistance to MEK inhibition in KRAS CRC. We found that KRAS mutant CRC cells with refametinib, MEK inhibitor, induced MIF secretion and resulted in activation of STAT3 and MAPK. MIF knockdown by siRNA restored sensitivity to refametinib in KRAS mutant cells. In addition, combination with refametinib and 4-IPP, a MIF inhibitor, effectively reduced the activity of STAT3 and MAPK, more than single-agent treatment. As a result, combined therapy was found to exhibit a synergistic growth inhibitory effect against refametinib-resistant cells by inhibition of MIF activation. These results reveal that MIF-induced STAT3 and MAPK activation evoked an intrinsic resistance to refametinib. Our results provide the basis for a rational combination strategy against KRAS mutant colorectal cancers, predicated on the understanding of cross talk between the MEK and MIF pathways.

PIK3CA is a frequently mutated gene in cancer, including about ~15 to 20% of colorectal cancers (CRC). PIK3CA mutations lead to activation of the PI3K/AKT/mTOR signaling pathway, which plays pivotal roles in tumorigenesis. Here, we investigated the mechanism of resistance of PIK3CA‐mutant CRC cell lines to gedatolisib, a dual PI3K/mTOR inhibitor. Out of a panel of 29 CRC cell lines, we identified 7 harboring one or more PIK3CA mutations; of these, 5 and 2 were found to be sensitive and resistant to gedatolisib, respectively. Both of the gedatolisib‐resistant cell lines expressed high levels of active glycogen synthase kinase 3‐beta (GSK3β) and harbored the same frameshift mutation (c.465_466insC; H155fs*) in TCF7, which encodes a positive transcriptional regulator of the WNT/β‐catenin signaling pathway. Inhibition of GSK3β activity in gedatolisib‐resistant cells by siRNA‐mediated knockdown or treatment with a GSK3β‐specific inhibitor effectively reduced the activity of molecules downstream of mTOR and also decreased signaling through the WNT/β‐catenin pathway. Notably, GSK3β inhibition rendered the resistant cell lines sensitive to gedatolisib cytotoxicity, both in vitro and in a mouse xenograft model. Taken together, these data demonstrate that aberrant regulation of WNT/β‐catenin signaling and active GSK3β induced by the TCF7 frameshift mutation cause resistance to the dual PI3K/mTOR inhibitor gedatolisib. Cotreatment with GSK3β inhibitors may be a strategy to overcome the resistance of PIK3CA‐ and TCF7‐mutant CRC to PI3K/mTOR‐targeted therapies., What's new? Mutations in the PI3K/mTOR and WNT/β‐catenin pathways are common in colorectal cancer. Here, in colorectal cancer cells, the authors show that the frameshift mutation H155fs* in transcription factor TCF7 serves a critical role in mediating resistance to the PI3K/mTOR dual inhibitor gedatolisib. TCF7 H155fs* maintained mTOR signaling by inducing the active form of glycogen synthase kinase 3‐beta (GSK3β) and aberrant WNT/β‐catenin signaling. The latter conferred resistance to gedatolisib. Co‐treatment with a GSK3β inhibitor increased sensitivity to gedatolisib, a combination that synergistically inhibited colorectal tumor growth in mice. The findings shed light on gedatolisib resistance mechanisms and potential resistance biomarkers.

The cell-free DNA (cfDNA) represents a minimally invasive and alternative source of tumor DNA for molecular profiling. Despite next-generation sequencing (NGS) technique is qualified for genotyping cancer using cfDNA as a noninvasive method, it has caused problems as sequencing error and reproducibility. cfDNA in plasma and gDNA of Peripheral Blood Mononuclear Cells (PBMC) were isolated from each 54 advanced colorectal cancer patients. 39 available tumor tissues were isolated from same patients. Deep target-sequencing was performed with paired-end library enriched exons of 10 genes which are recurrently mutated in colorectal cancer. To reduce sequencing error, we devised ‘Denoising’ and calculated concordance of somatic variants between cfDNA and tumor tissue sequencing data. In addition, correlation of concordance data was analyzed with the clinical information. As a result, we selectively could detect clinically important somatic alteration among low/high variant allele frequency (0.31%~79.42%). For somatic alteration of 10 genes, sensitivity, specificity and accuracy were increased from 84.5%, 74.6% and 76.9% to 87.6%, 92.0% and 91.1% respectively after ‘Denoising’. On the other hand, patients with high cfDNA concentration(>50ng/ml) had higher somatic mutant fragments and larger metastatic lesion in liver than patients who have low cfDNA concentration. Our study showed that denoised deep target-sequencing is a suitable method for cfDNA genotyping and provides insights into strategies for monitoring metastatic lesion of advanced colorectal cancer. Citation Format: Jun-Kyu Kang, Hwang-Phill Kim, Seul-Ki Cheon, Ye-Lim Park, Yoojoo Lim, Sae-Won Han, Tae-You Kim. Noninvasive approach to assess metastatic lesion of advanced colorectal cancer by denoised deep target sequencing [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1800. doi:10.1158/1538-7445.AM2017-1800

KRAS is a frequently mutated gene in colorectal cancer. In addition, PIK3CA mutations commonly co-exist with KRAS mutations and lead to additive activation of the PI3K/mTOR signaling pathway. Here, we investigated preclinical activity of gedatolisib, a PI3K/mTOR dual inhibitor, to identify mechanism of inhibition of PI3K/mTOR in 28 colorectal cancer (CRC) cells. Cells specifically with PIK3CA mutation were sensitive while those with KRAS mutation were resistant to gedatolisib. 9 out of 28 CRC cells harbor PIK3CA & KRAS co-mutation and 7 out them were shown to be sensitive to gedatolisib. However, HCT15 and LS174T cells were resistant. We identified that resistant cell lines have high activity of GSK3B and TCF7 frameshift mutation (465insertC466;H155fs*), which functions as positive regulator of WNT/b-catenin pathway. The effects of GSK3B-knockdown showed decreased activity of mTOR downstream molecules in gedatolisib-treated resistant cells. Interestingly, these effects also caused a decrease in activity of WNT/b-catenin pathway. In addition, combination treatment of gedatolisib and CHIR-99021, GSK3B inhibitor, resulted in significantly enhanced cytotoxicity against gedatolisib-resistant TCF7 frameshift mutant cells. Taken together, these show that aberrant regulation of WNT/b-catenin pathway and high activity of GSK3B by TCF7 frameshift mutation cause resistance to PI3K/mTOR dual inhibitor. Inhibition of GSK3B activity in colorectal cancer cells with KRAS and PIK3CA co-mutations increases sensitivity to PI3K/mTOR dual inhibitor. Citation Format: Ye-Lim Park, Hwang-Phill Kim, Seul-Ki Cheon, Jun-Kyu Kang, Yoojoo Lim, Sang-Hyun Song, Sae-Won Han, Tae-you Kim. Activation of WNT/b-catenin signaling results in resistance to PI3K/mTOR dual inhibitor in co-existing KRAS and PIK3CA mutant colorectal cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5214. doi:10.1158/1538-7445.AM2017-5214

Although MEK blockade has been highlighted as a promising anti-tumor drug, it has poor clinical efficacy in KRAS mutant colorectal cancer. Several feedback systems have been described in which inhibition of one intracellular pathway leads to activation of a parallel signaling pathway, thereby decreasing the effectiveness of single-MEK targeted therapies. In this study, we describe a feedback mechanism in which MEK inhibition leads to activation of macrophage migration inhibitory factor (MIF)-induced stat3 signaling pathway in KRAS mutant colorectal cancer (CRC) cells. We found that KRAS mutant CRC cells with refametinib, MEK inhibitor, induced MIF secretion and resulted in activation of Stat3. MIF knockdown by siRNA partially restored sensitivity to refametinib in KRAS mutant cells. In addition, combination with refametinib and 4IPP, a MIF inhibitor, effectively reduced the activity of stat3 and MAPK, more than single agent treatment. As a result, combined therapy was found to exhibit a synergistic growth inhibitory effect against refametinib-resistant cells by downregulating MIF expression. These results reveal that MIF-induced stat3 activation evoked an intrinsic resistance to refametinib. Our results provide the basis for a rational combination strategy against KRAS mutant colorectal cancers, predicated on the understanding of cross-talk between the MEK and MIF pathways. Citation Format: Seul-Ki Cheon, Hwang-Phill Kim, Ye-Lim Park, Si Hyun Lee, Jun-Kyu Kang, Yoojoo Lim, Sang-Hyun Song, Sae-Won Han, Tae-You Kim. MIF-induced stat3 activation promotes resistance to MEK blockade in KRAS mutant colorectal cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3167. doi:10.1158/1538-7445.AM2017-3167