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TPA inhibition of gap junctional intercellular communication in Syrian hamster embryo cells through two different
1Laboratory for Environmental and Occupational Cancer, Institute for Cancer Research, The Norwegian Radium Hospital, N-0310 Oslo 3, Norway.
Abstract:
Exposure of early passage Syrian hamster embryo cells to high TPA concentrations (8 or 16 mum) decreased gap junctional intercellular communication (GJIC) to about 20% of the control during the first hour. Subsequently, the GJIC capacity recovered to 70-80% after 4-10 hr. Exposure to lower TPA concentrations also reduced GJIC, but did not result in the subsequent rapid enhancement of communication. Treatment of the cells to different TPA concentrations down-regulated protein kinase C (PKC), but this down-regulation did not seem to explain fully the induced recurrence of GJIC following high TPA concentrations. The maximal down-regulation of PKC took place at TPA concentrations above 0.16 mum. Addition of 8 mum TPA to cells pretreated with 0.016 and 8 mum for 10 hr reduced GJIC from 80 to 40%, whereas pretreatment with 0.16 or 1.6 mum TPA made the cells refractory to further TPA-induced inhibition. The PKC inhibitor staurosporine suppressed the effect of TPA on GJIC in cells exposed to 0.016 mum TPA, whereas no suppression was observed in cells depleted of PKC. The results indicate that TPA is capable of inhibiting GJIC through two different mechanisms, a staurosporine-sensitive mechanism at low TPA concentrations (EC(50) = 0.18 mum) and a staurosporine-insensitive mechanism at higher concentrations (EC(50) = 7 mum).
Insights
Exposure to high TPA concentrations initially inhibits gap junctional intercellular communication (GJIC) but it recovers. Lower concentrations cause sustained inhibition via distinct mechanisms, involving protein kinase C (PKC) differently.
Area of Science:
- Cell Biology
- Molecular Toxicology
- Signal Transduction
Background:
- Gap junctional intercellular communication (GJIC) is vital for cell-cell signaling.
- Tumor promoters like TPA can disrupt GJIC, impacting cellular functions.
- Understanding TPA's effects on GJIC is crucial for toxicology and cancer research.
Purpose of the Study:
- To investigate the concentration-dependent effects of TPA on GJIC in Syrian hamster embryo cells.
- To elucidate the mechanisms underlying TPA-induced inhibition and recovery of GJIC.
- To differentiate the roles of protein kinase C (PKC) in TPA's effects on GJIC.
Main Methods:
- Exposure of early passage Syrian hamster embryo cells to varying TPA concentrations.
- Measurement of GJIC capacity over time using established assays.
- Assessment of protein kinase C (PKC) down-regulation and its correlation with GJIC.
- Utilizing PKC inhibitor staurosporine to probe TPA's mechanisms.
Main Results:
- High TPA concentrations (8-16 µM) transiently decreased GJIC, followed by recovery.
- Lower TPA concentrations induced sustained GJIC inhibition without recovery.
- TPA down-regulated PKC, but this did not fully explain GJIC recovery at high concentrations.
- Two distinct TPA-mediated GJIC inhibition mechanisms were identified: staurosporine-sensitive (low TPA) and staurosporine-insensitive (high TPA).
Conclusions:
- TPA exhibits dual mechanisms for inhibiting GJIC, dependent on concentration.
- PKC down-regulation is involved but does not solely account for GJIC modulation by TPA.
- The findings provide insights into the complex signaling pathways affected by tumor promoters.
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