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Heterologous desensitization of response mediated by selective PKC-dependent phosphorylation of G(i-1) and G(i-2)

K S Murthy1, J R Grider, G M Makhlouf

  • 1Departments of Medicine and Physiology, Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298-0711, USA.

American Journal of Physiology. Cell Physiology
|September 26, 2000
PubMed

Insights

Protein kinase C (PKC) selectively phosphorylates G alpha(i-1) and G alpha(i-2) proteins, leading to heterologous desensitization in smooth muscle. This phosphorylation limits G protein signaling and affects responses mediated by both alpha and beta-gamma subunits.

Area of Science:

  • Biochemistry
  • Cellular signaling
  • Pharmacology

Background:

  • Protein kinase C (PKC) plays a crucial role in cellular signal transduction.
  • G proteins are key mediators of cellular responses to external stimuli.
  • Heterologous desensitization is a process where activation of one receptor leads to reduced responsiveness of another.

Purpose of the Study:

  • To investigate the mechanism by which PKC induces heterologous desensitization in smooth muscle.
  • To identify the specific G protein subunits targeted by PKC.
  • To elucidate the impact of PKC activation on G protein-mediated signaling pathways.

Main Methods:

  • Intestinal smooth muscle cells were pretreated with PKC activators (phorbol ester, cholecystokinin octapeptide, calyculin A).
  • Specific G alpha-subunits (Galpha(i-1), Galpha(i-2), Galpha(i-3), Galpha(o)) were assessed for phosphorylation.
  • Inhibition of adenylyl cyclase and activation of phospholipase C (PLC)-beta were measured.
  • PKC inhibitors (calphostin C, bisindolylmaleimide) and selective PKC inhibitors were used.
  • Receptor binding and effector enzyme activity were evaluated.

Main Results:

  • PKC activation selectively phosphorylated Galpha(i-1) and Galpha(i-2), but not Galpha(i-3) or Galpha(o).
  • This phosphorylation blocked adenylyl cyclase inhibition mediated by G(i-1) and G(i-2) coupled receptors.
  • Responses mediated by G protein beta-gamma subunits, such as PLC-beta activation, were also inhibited.
  • Desensitization was reversed by PKC inhibitors and prevented by PKC downregulation.
  • Short-term PKC activation did not affect receptor binding or effector enzyme activity.

Conclusions:

  • Selective phosphorylation of Galpha(i-1) and Galpha(i-2) by PKC is a key mechanism for heterologous desensitization in smooth muscle.
  • PKC-induced desensitization involves both G alpha and beta-gamma subunit signaling pathways.
  • The observed desensitization is attributed to reduced reassociation and availability of heterotrimeric G proteins.

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