Analyzing phorbol ester effects on gap junctional communication: a dramatic inhibition of assembly

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Gap junctions are crucial for intercellular communication, mediating direct cell-to-cell transport of ions and small molecules.
• Assembly of functional gap junctions involves the precise arrangement of connexin proteins (Cx43) within the plasma membrane.
• 12-O-tetradecanoylphorbol-13-acetate (TPA) is a known activator of protein kinase C, involved in various cellular signaling pathways.

Purpose of the Study:

• To investigate the effect of TPA on the assembly of new gap junctions in Novikoff hepatoma cells.
• To determine whether TPA influences gap junction channel gating or the disassembly of pre-existing junctions.
• To elucidate the role of connexin 43 (Cx43) phosphorylation and plasma membrane concentration in TPA-mediated effects on gap junction assembly.

Main Methods:

• Novikoff hepatoma cells were dissociated and reaggregated in the presence or absence of TPA.
• Dye transfer assays (using Lucifer yellow) were employed to assess gap junction communication.
• Freeze-fracture electron microscopy was used to visualize gap junction particle aggregates.
• Western blotting and cell surface biotinylation were performed to analyze Cx43 phosphorylation and membrane levels.

Main Results:

• TPA (25 nM) dramatically inhibited the formation of new gap junctions, reducing dye transfer to 0.6% compared to controls (72-74%).
• Freeze-fracture electron microscopy revealed a complete absence of gap junctions in TPA-treated cells, while controls showed numerous particle aggregates.
• TPA did not affect dye transfer through pre-existing gap junctions in non-dissociated cells, indicating no impact on channel gating or disassembly.
• Short-term TPA treatment increased Cx43 phosphorylation but did not alter the total cellular Cx43 level or its plasma membrane concentration.

Conclusions:

• TPA significantly inhibits gap junction assembly, likely by affecting the process at the plasma membrane level.
• The phosphorylation of Cx43, induced by TPA, is implicated in the inhibition of gap junction assembly.
• The presence of Cx43 in the plasma membrane is insufficient for gap junction assembly, suggesting TPA interferes with crucial assembly steps mediated by protein kinase C.