bims-pimaco Biomed News
on PI3K and MAPK signalling in colorectal cancer
Issue of 2021‒08‒29
seven papers selected by
Lucas B. Zeiger
Beatson Institute for Cancer Research

  1. Pharmacol Res. 2021 Aug 24. pii: S1043-6618(21)00390-X. [Epub ahead of print] 105806
      RAS proteins (HRAS, KRAS, NRAS) participate in many physiological signal transduction processes related to cell growth, division, and survival. The RAS proteins are small (188/189 amino acid residues) and they function as GTPases. These proteins toggle between inactive and functional forms; the conversion of inactive RAS-GDP to active RAS-GTP as mediated by guanine nucleotide exchange factors (GEFs) turns the switch on and the intrinsic RAS-GTPase activity stimulated by the GTPase activating proteins (GAPs) turns the switch off. RAS is upstream to the RAS-RAF-MEK-ERK and the PI3-kinase-AKT signaling modules. Importantly, the overall incidence of RAS mutations in all cancers is about 19% and RAS mutants have been a pharmacological target for more than three decades. About 84% of all RAS mutations involve KRAS. Except for the GTP/GDP binding site, the RAS proteins lack other deep surface pockets thereby hindering efforts to identify high-affinity antagonists; thus, they have been considered to be undruggable. KRAS mutations frequently occur in lung, colorectal, and pancreatic cancers, the three most deadly cancers in the United States. Studies within the last decade demonstrated that the covalent modification of KRAS C12, which accounts for about 10% of all RAS mutations, led to the discovery of an adjacent pocket (called the switch II pocket) that accommodated a portion of the drug. This led to the development of sotorasib as a second-line treatment of KRASG12C mutant non-small cell lung cancer. Considerable effort also has been expended to develop MAP kinase and PI3-kinase pathway inhibitors as indirect RAS antagonists.
    Keywords:  Adagrasib (PubChem CID: 138611145); Belvarafenib (PubChem CID: 89655386); Buparlisib (PubChem CID: 16654980); Cancer mutations; Lifirafenib (PubChem CID: 89670174); MAP kinase signaling pathway; Naporafenib (PubChem CID: 90456533); Non-small cell lung cancer; PI3-kinase signaling pathway; Pancreatic cancer; Pictilisib (PubChem CID: 17755052); RO5126766 (PubChem CID: 16719221); Sotorasib (PubChem CID: 137278711); Targeted covalent inhibitors; Tipifarnib (PubChem CID: 159324); Ulixertinib (PubChem CID: 11719003)
  2. J Biol Chem. 2021 Aug 18. pii: S0021-9258(21)00903-0. [Epub ahead of print] 101100
      mTOR complex 2 (mTORC2) signaling controls cell metabolism, promotes cell survival, and contributes to tumorigenesis, yet its upstream regulation remains poorly defined. While considerable evidence supports the prevailing view that amino acids activate mTOR complex 1 (mTORC1) but not mTORC2, several studies reported paradoxical activation of mTORC2 signaling by amino acids. We noted that after amino acid starvation of cells in culture, addition of an amino acid solution increased mTORC2 signaling. Interestingly, we found the pH of the amino acid solution to be alkaline, ∼pH 10. These observations led us to discover and demonstrate here that alkaline intracellular pH (pHi) represents a previously unknown activator of mTORC2. Using a fluorescent pH-sensitive dye (cSNARF1-AM) coupled with live-cell imaging, we demonstrate that culturing cells in media at alkaline pH induces a rapid rise in pHi, which increases mTORC2 catalytic activity and downstream signaling to the pro-growth and -survival kinase Akt. Alkaline pHi also activates AMPK, a canonical sensor of energetic stress. Functionally, alkaline pHi attenuates mTOR- and AMPK-mediated apoptosis caused by growth factor withdrawal. Collectively, these findings reveal that alkaline pHi increases mTORC2- and AMPK-mediated signaling to promote cell survival during conditions of growth factor limitation, analogous to the demonstrated ability of energetic stress to activate AMPK-mTORC2 and promote cell survival. As elevated pHi represents an under-appreciated hallmark of cancer cells, we propose that alkaline pHi stress sensing by AMPK-mTORC2 may contribute to tumorigenesis by enabling cancer cells at the core of a growing tumor to evade apoptosis and survive.
    Keywords:  AMPK; Akt; intracellular pH (pHi); mTORC2
  3. PLoS One. 2021 ;16(8): e0256774
      Cross talk between different signaling pathways is thought to be important for regulation of homeostasis of, as well as oncogenesis of, the intestinal epithelium. Expression of an active form of K-Ras specifically in intestinal epithelial cells (IECs) of mice (IEC-RasDA mice) resulted in the development of hyperplasia in the small intestine and colon of mice. IEC-RasDA mice also manifested the increased proliferation of IECs. In addition, the number of goblet cells markedly increased, while that of Paneth cells decreased in IEC-RasDA mice. Development of intestinal organoids was markedly enhanced for IEC-RasDA mice compared with control mice. Whereas, the expression of Wnt target genes was significantly reduced in the in intestinal crypts from IEC-RasDA mice compared with that apparent for the control. Our results thus suggest that K-Ras promotes the proliferation of IECs as well as generation of goblet cells. By contrast, Ras counter-regulates the Wnt signaling and thereby contribute to the proper regulation of intestinal epithelial cell homeostasis.
  4. Cancer Discov. 2019 Dec;9(12): OF8
      Distinct binding between RAS and RAF family members determines RAS signaling.
  5. Cancer Res. 2021 Aug 24. pii: canres.0206.2021. [Epub ahead of print]
      The SWI/SNF chromatin remodeling complexes control accessibility of chromatin to transcriptional and co-regulatory machineries. Chromatin remodeling plays important roles in normal physiology and diseases, particularly cancer. The ARID1A-containing SWI/SNF complex is commonly mutated and thought to be a key tumor suppressor in hepatocellular carcinoma (HCC), but its regulation in response to oncogenic signals remains poorly understood. mTOR is a conserved central controller of cell growth and an oncogenic driver of HCC. Remarkably, cancer mutations in mTOR and SWI/SNF complex are mutually exclusive in human HCC tumors, suggesting that they share a common oncogenic function. Here we report that mTOR complex 1 (mTORC1) interact with ARID1A and regulates ubiquitination and proteasomal degradation of ARID1A protein. The mTORC1-ARID1A axis promoted oncogenic chromatin remodeling and YAP-dependent transcription, thereby enhancing liver cancer cell growth in vitro and tumor development in vivo. Conversely, excessive ARID1A expression counteracted AKT-driven liver tumorigenesis in vivo. Moreover, dysregulation of this axis conferred resistance to mTOR-targeted therapies. These findings demonstrate that the ARID1A-SWI/SNF complex is a regulatory target for oncogenic mTOR signaling, which is important for mTORC1-driven hepatocarcinogenesis with implications for therapeutic interventions in HCC.
  6. Cancer Discov. 2019 Dec;9(12): OF5
      A drug trio consisting of BRAF, MEK, and EGFR inhibitors may become the new standard of care in BRAF-mutant metastatic colorectal cancer. In the phase III BEACON CRC trial, encorafenib, binimetinib, and cetuximab significantly extended overall and progression-free survival and increased the objective response rate compared with cetuximab plus chemotherapy.
  7. J Biol Chem. 2021 Aug 19. pii: S0021-9258(21)00906-6. [Epub ahead of print] 101103
      Heterodimeric Rag GTPases play a critical role in relaying fluctuating levels of cellular amino acids to the sensor mTOR complex 1 (mTORC1). Important mechanistic questions remain unresolved, however, regarding how guanine nucleotide binding enables Rag GTPases to transition dynamically between distinct Yoga-like structural poses that control activation state. Egri et al. identify a critical interdomain hydrogen bond within RagA and RagC that stabilizes their GDP-bound states. They demonstrate that this long-distance interaction controls Rag structure and function to confer appropriate amino acid sensing by mTORC1.