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Requirements for PKC-augmented JNK activation by MKK4/7

Pablo Lopez-Bergami1, Ze'ev Ronai

  • 1Signal Transduction Program, The Burnham Institute for Medical Research, 10901 N. Torrey Pines Road, La Jolla, CA 92037, USA.

The International Journal of Biochemistry & Cell Biology
|January 10, 2008
PubMed

Insights

Protein kinase C (PKC) enhances c-Jun N-terminal kinase (JNK) activation by MKK4/7. PKC phosphorylation must precede MKK4/7 action for optimal JNK activity, impacting UV-induced apoptosis.

Area of Science:

  • Cellular signaling pathways
  • Stress response mechanisms
  • Kinase regulation

Background:

  • c-Jun N-terminal kinases (JNKs) are crucial mediators of cellular responses to stress, DNA damage, and cytokines.
  • JNK activation is primarily regulated by upstream kinases MKK4 and MKK7.
  • Previous work indicated protein kinase C (PKC) can augment JNK activation, requiring the adaptor protein RACK1.

Purpose of the Study:

  • To elucidate the specific conditions required for protein kinase C (PKC)-dependent activation of c-Jun N-terminal kinases (JNKs).
  • To understand the temporal relationship between PKC phosphorylation and upstream kinase (MKK4/7) phosphorylation in JNK activation.
  • To investigate the functional consequences of inhibiting PKC-mediated JNK phosphorylation on UV-induced signaling and apoptosis.

Main Methods:

  • In vitro kinase assays to assess JNK phosphorylation and activation by PKC and MKK4/7.
  • Experimental manipulation of the order of kinase phosphorylation events (PKC vs. MKK4/7).
  • UV-irradiation of cells followed by analysis of JNK activation phases and apoptotic responses, with and without PKC inhibition.

Main Results:

  • PKC phosphorylation of JNK alone is insufficient for activation; it augments activation by MKK4 or MKK7.
  • For enhanced JNK activity, PKC phosphorylation of JNK must occur before phosphorylation by MKK4/7.
  • Inhibition of PKC phosphorylation of JNK impaired both early and late phases of UV-induced JNK activation and reduced JNK-mediated apoptosis.

Conclusions:

  • PKC-dependent phosphorylation of JNK is a critical regulatory step that potentiates JNK activation by canonical upstream kinases.
  • The timing of PKC phosphorylation relative to MKK4/7 phosphorylation is essential for maximal JNK activity.
  • These findings reveal key requirements for PKC in modulating JNK signaling pathways, influencing cellular responses to stress like UV irradiation and subsequent apoptosis.

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