Annexin A6 is a scaffold for PKCα to promote EGFR inactivation

Area of Science:

• Cell Biology
• Molecular Oncology
• Biochemistry

Background:

• Protein kinase Cα (PKCα) phosphorylates epidermal growth factor receptor (EGFR) at T654, inhibiting EGFR tyrosine phosphorylation (pY-EGFR) and downstream signaling.
• Upregulation of PKCα in cancers often doesn't correlate with EGFR inactivation, suggesting unknown regulatory factors.
• The precise mechanisms governing PKCα's ability to downregulate EGFR remain unclear.

Purpose of the Study:

• To investigate the role of annexin A6 (AnxA6) in regulating PKCα-mediated EGFR phosphorylation.
• To elucidate the molecular mechanisms by which AnxA6 influences EGFR signaling in cancer cells.
• To determine if AnxA6 can serve as a scaffold for PKCα to inactivate EGFR.

Main Methods:

• Ectopic expression of AnxA6 in EGFR-overexpressing cancer cell lines (A431, head and neck, breast).
• RNA interference (RNAi) to knockdown PKCα.
• Analysis of EGFR tyrosine phosphorylation, T654 phosphorylation, internalization, and degradation.
• Investigation of AnxA6-EGFR and AnxA6-PKCα interactions using protein binding assays.
• Expression of AnxA6 mutants and T654A EGFR mutant.

Main Results:

• Ectopic AnxA6 expression reduced pY-EGFR and increased T654-EGFR phosphorylation, decreasing EGFR activation, internalization, and degradation.
• PKCα knockdown in AnxA6-expressing cells restored pY-EGFR, clonogenic growth, and EGFR degradation.
• AnxA6 interacts with EGFR, promotes PKCα membrane association and EGFR/PKCα complex formation.
• AnxA6's N-terminal mutant lacking PKCα recruitment ability did not affect T654-EGFR phosphorylation or EGFR/PKCα association.
• AnxA6 overexpression did not inhibit phosphorylation of the T654A EGFR mutant.
• AnxA6 plasma membrane anchoring was sufficient to recruit PKCα independently of EGF or Ca(2+).

Conclusions:

• AnxA6 acts as a novel scaffold protein for PKCα.
• AnxA6 enhances PKCα-mediated EGFR inactivation by promoting PKCα membrane targeting and EGFR/PKCα complex formation.
• AnxA6 plays a significant role in downregulating EGFR signaling in cancer cells, offering potential therapeutic insights.