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High Content Imaging Assays for IL-6-Induced STAT3 Pathway Activation in Head and Neck Cancer Cell Lines
Paul A Johnston1, Malabika Sen2, Yun Hua3
1Department of Pharmaceutical Sciences, School of Pharmacy, University of Pittsburgh, Room 586 Salk Hall, 3501 Terrace Street, Pittsburgh, PA, 15261, USA. paj18@pitt.edu.
Abstract:
In the canonical STAT3 signaling pathway, IL-6 receptor engagement leads to the recruitment of latent STAT3 to the activated IL-6 complex and the associated Janus kinase (JAK) phosphorylates STAT3 at Y705. pSTAT3-Y705 dimers traffic into the nucleus and bind to specific DNA response elements in the promoters of target genes to regulate their transcription. However, IL-6 receptor activation induces the phosphorylation of both the Y705 and S727 residues of STAT3, and S727 phosphorylation is required to achieve maximal STAT3 transcriptional activity. STAT3 continuously shuttles between the nucleus and cytoplasm and maintains a prominent nuclear presence that is independent of Y705 phosphorylation. The constitutive nuclear entry of un-phosphorylated STAT3 (U-STAT3) drives expression of a second round of genes by a mechanism distinct from that used by pSTAT3-Y705 dimers. The abnormally elevated levels of U-STAT3 produced by the constitutive activation of pSTAT3-Y705 observed in many tumors drive the expression of an additional set of pSTAT3-independent genes that contribute to tumorigenesis. In this chapter, we describe the HCS assay methods to measure IL-6-induced STAT3 signaling pathway activation in head and neck tumor cell lines as revealed by the expression and subcellular distribution of pSTAT3-Y705, pSTAT3-S727, and U-STAT3. Only the larger dynamic range provided by the pSTAT3-Y705 antibody would be robust and reproducible enough for screening.
Insights
Interleukin-6 (IL-6) signaling activates Signal transducer and activator of transcription 3 (STAT3) through Y705 phosphorylation, driving gene transcription. Unphosphorylated STAT3 (U-STAT3) also enters the nucleus, promoting distinct gene expression crucial for tumorigenesis.
Area of Science:
- Molecular Biology
- Cellular Signaling
- Cancer Biology
Background:
- The Signal transducer and activator of transcription 3 (STAT3) pathway is crucial in cellular responses to Interleukin-6 (IL-6).
- STAT3 phosphorylation occurs at Y705 and S727 residues, influencing transcriptional activity.
- Unphosphorylated STAT3 (U-STAT3) exhibits nuclear localization and regulates gene expression independently of Y705 phosphorylation.
Purpose of the Study:
- To investigate the distinct roles of phosphorylated STAT3 (pSTAT3) and U-STAT3 in IL-6-induced signaling.
- To establish high-content screening (HCS) assay methods for measuring STAT3 activation in head and neck tumors.
- To evaluate the utility of specific antibodies for detecting STAT3 modifications in cancer research.
Main Methods:
- Utilized high-content screening (HCS) assay methods.
- Measured expression and subcellular distribution of pSTAT3-Y705, pSTAT3-S727, and U-STAT3.
- Assessed IL-6-induced STAT3 signaling pathway activation in head and neck tumor cell lines.
Main Results:
- IL-6 receptor engagement activates STAT3, leading to Y705 phosphorylation and nuclear translocation of pSTAT3-Y705 dimers.
- STAT3 phosphorylation at S727 is necessary for maximal transcriptional activity.
- Constitutive activation of pSTAT3-Y705 leads to elevated U-STAT3, driving pro-tumorigenic gene expression independent of pSTAT3-Y705.
- The pSTAT3-Y705 antibody demonstrated a dynamic range suitable for screening.
Conclusions:
- STAT3 signaling involves distinct mechanisms mediated by phosphorylated and unphosphorylated forms.
- U-STAT3 plays a significant role in tumorigenesis through pSTAT3-independent gene regulation.
- HCS assays with appropriate antibodies, particularly for pSTAT3-Y705, are valuable for studying STAT3 activation in cancer.
