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Toll-like receptors 2 and 4 activate STAT1 serine phosphorylation by distinct mechanisms in macrophages

Sang Hoon Rhee1, Bryan W Jones, Vladimir Toshchakov

  • 1The Pulmonary Center, Boston University School of Medicine, Boston, Massachusetts 02118, USA.

The Journal of Biological Chemistry
|April 11, 2003
PubMed

Insights

Toll-like receptor (TLR) engagement distinctively phosphorylates STAT1. TLR4 triggers tyrosine phosphorylation via PI3K, while both TLR2 and TLR4 induce serine phosphorylation through p38 MAPK and other pathways.

Area of Science:

  • Immunology
  • Molecular Biology
  • Cell Signaling

Background:

  • Toll-like receptors (TLRs) are crucial for innate immunity, initiating signaling cascades upon pathogen recognition.
  • STAT1 (Signal Transducer and Activator of Transcription 1) is a key transcription factor involved in immune responses.
  • TLR engagement activates diverse intracellular pathways, but the specific mechanisms for STAT1 activation remain incompletely understood.

Purpose of the Study:

  • To elucidate the distinct signaling pathways governing STAT1 phosphorylation at serine 727 (Ser-727 STAT1) and tyrosine 701 (Tyr-701 STAT1) following TLR2 and TLR4 engagement.
  • To identify the key kinases and adapter proteins involved in these differential phosphorylation events in murine macrophages.

Main Methods:

  • Murine macrophages were stimulated with TLR2 and TLR4 agonists.
  • Specific inhibitors were used to block key signaling molecules, including LY294002 (PI3K inhibitor), SB203580 (p38 MAPK inhibitor), and rottlerin (PKC-delta inhibitor).
  • Western blotting and analysis of protein phosphorylation were performed to assess STAT1 activation and pathway involvement.

Main Results:

  • TLR4 engagement induced both delayed Tyr-701 STAT1 and rapid Ser-727 STAT1 phosphorylation.
  • TLR2 engagement primarily induced rapid Ser-727 STAT1 phosphorylation.
  • PI3K inhibition blocked TLR4-induced Tyr-701 STAT1 phosphorylation, while p38 MAPK inhibition blocked Ser-727 STAT1 phosphorylation.
  • MyD88 was required for TLR2-induced Ser-727 STAT1 phosphorylation, but not solely for TLR4-induced Ser-727 STAT1 phosphorylation.
  • Protein kinase C-delta (PKC-delta) inhibition blocked TLR4-induced Ser-727 STAT1 phosphorylation, but not TLR2-induced phosphorylation.

Conclusions:

  • STAT1 tyrosine and serine phosphorylation are distinct outcomes of TLR engagement in macrophages.
  • p38 MAPK, PKC-delta, and a novel TLR2-specific pathway are critical for Ser-727 STAT1 phosphorylation.
  • These findings reveal differential signaling mechanisms downstream of TLRs, impacting immune cell activation and function.

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