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Calmodulin involvement in TPA and DDT induced inhibition of intercellular communication
L Wärngård1, R Fransson, T B Drakenberg
1Department of Toxicology, Karolinska Institute, Stockholm, Sweden.
Abstract:
The organochlorine pesticide DDT is a liver tumour promoter and a potent inhibitor of intercellular communication. Present knowledge of the mechanism by which DDT inhibits intercellular communication is limited but it has been suggested that increased intracellular free calcium induced by DDT could be of importance. As the effects of calcium are closely associated with the multifunctional protein calmodulin (CaM) in most cells the potential binding of DDT to CaM and subsequent effects on CaM-stimulated Ca2+/Mg2+-ATPase activity were studied. DDT inhibited CaM-stimulated Ca2+/Mg2+-ATPase activity and bound to CaM in a manner similar to established CaM-inhibitors. Subsequently an in vitro assay for measuring inhibition of metabolic cooperation between 6-thioguanine (TG)-sensitive and TG-resistant Chinese hamster (V79) cells was used to investigate the possible involvement of CaM in the regulation of intercellular communication. Calmidazolium (CzM), a potent CaM inhibitor, was tested alone or in combination with the tumour promoters 12-O-tetradecanoyl phorbol-13-acetate (TPA) or DDT known inhibitors of intercellular communication. The results showed that CzM alone was without effect with regard to inhibition of metabolic cooperation but potentiated the response induced by TPA, an effect not noticed with DDT. These results suggest different mechanisms of action of TPA and DDT on metabolic cooperation and support the hypothesis that with calcium CaM may be of importance for drug-induced inhibition of intercellular communication and tumour promotion.
Insights
The organochlorine pesticide DDT inhibits intercellular communication by binding to calmodulin (CaM), a key protein in calcium signaling. This binding affects cellular processes, suggesting CaM
Area of Science:
- Toxicology
- Cell Biology
- Biochemistry
Background:
- The organochlorine pesticide DDT is a known liver tumor promoter and inhibitor of intercellular communication.
- The precise mechanism by which DDT disrupts intercellular communication is not fully understood, but increased intracellular calcium is a suspected factor.
- Calcium's effects are mediated by calmodulin (CaM), a multifunctional protein crucial for cellular signaling.
Purpose of the Study:
- To investigate the potential binding of DDT to calmodulin (CaM).
- To determine the effects of DDT binding on CaM-stimulated Ca2+/Mg2+-ATPase activity.
- To explore the role of CaM in regulating intercellular communication and tumor promotion by DDT and other agents.
Main Methods:
- Studied the binding of DDT to purified calmodulin (CaM).
- Assessed the impact of DDT on CaM-stimulated Ca2+/Mg2+-ATPase activity in vitro.
- Utilized an in vitro assay with Chinese hamster (V79) cells to measure metabolic cooperation inhibition.
- Tested the effects of calmidazolium (CzM), a CaM inhibitor, alone and in combination with DDT and TPA.
Main Results:
- DDT was found to bind to CaM, similar to known CaM inhibitors.
- DDT inhibited CaM-stimulated Ca2+/Mg2+-ATPase activity.
- Calmidazolium (CzM) alone did not inhibit metabolic cooperation but potentiated the effect of TPA.
- CzM did not potentiate the inhibitory effect of DDT on metabolic cooperation.
Conclusions:
- DDT's mechanism of inhibiting intercellular communication differs from that of TPA.
- The results support the hypothesis that calcium and calmodulin (CaM) play a role in drug-induced inhibition of intercellular communication and tumor promotion.
- Further research is warranted to elucidate the specific pathways involved.