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Phosphorylation of RAB7 by TBK1/IKKε Regulates Innate Immune Signaling in Triple-Negative Breast Cancer

Jessica L Ritter1, Zehua Zhu2, Tran C Thai2

  • 1Breast Oncology Program, Dana-Farber/Brigham and Women's Cancer Center, Boston, MA.

Cancer Research
|October 31, 2019
PubMed

Insights

Triple-negative breast cancer (TNBC) involves PTEN mutations and immune signaling issues. New findings show Rab7 phosphorylation by TBK1/IKKε limits STING degradation, enhancing immune responses in PTEN-null TNBC.

Area of Science:

  • Oncology
  • Immunology
  • Cell Biology

Background:

  • Triple-negative breast cancer (TNBC) is characterized by PTEN mutations and innate immune signaling dysregulation.
  • Rab7 is a known substrate of PTEN phosphatase activity.
  • Innate immune signaling kinases TBK1/IKKε are implicated in TNBC pathogenesis.

Purpose of the Study:

  • To investigate the role of Rab7 as a substrate for TBK1/IKKε in TNBC.
  • To elucidate the mechanism by which PTEN loss and TBK1/IKKε activation converge on Rab7.
  • To explore the implications of Rab7 phosphorylation for STING regulation and innate immune signaling in TNBC.

Main Methods:

  • Phosphoproteomic analysis using stable isotope labeling with amino acids in cell culture to identify novel TBK1/IKKε substrates.
  • Expression of phosphomimetic and kinase-resistant Rab7 mutants in PTEN-null TNBC cells.
  • Assessment of Rab7 localization, STING degradation, and innate immune signaling activation (e.g., IRF3 targets).
  • Analysis of STING expression in PTEN-null TNBC tumors and cell lines.

Main Results:

  • Rab7 was identified as a novel substrate for TBK1/IKKε at serine-72 (S72).
  • Phosphorylation of Rab7-S72 by TBK1/IKKε in PTEN-null TNBC cells led to diffuse cytosolic localization and enhanced innate immune signaling.
  • This phosphorylation event limited lysosomal-mediated STING degradation, increasing downstream IRF3 target gene production (CXCL10, CCL5, IFNβ).
  • PTEN-null TNBC tumors and cell lines exhibited higher STING levels and hyperresponsiveness to STING agonists.

Conclusions:

  • Rab7 acts as a key convergence point for PTEN loss and TBK1/IKKε activation in TNBC.
  • Rab7 phosphorylation by TBK1/IKKε regulates STING turnover, impacting innate immune signaling.
  • These findings reveal a novel cross-talk mechanism with STING recycling and suggest potential therapeutic strategies targeting STING agonism in TNBC.

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