Mechanism of calmodulin inhibition of cAMP-dependent protein kinase activation of phosphorylation kinase

Synergistic radioprotection by gamma-tocotrienol and pentoxifylline: role of cAMP signaling

Purpose. This study was designed to determine the efficacy and mechanisms of radioprotection by the combination of gamma-tocotrienol (GT3) and pentoxifylline (PTX) against acute radiation injury. Materials and Methods. Post-irradiation survival was monitored to determine the most efficacious dose and time of administration of PTX. Dose reduction factor (DRF) was calculated to compare the radioprotective efficacy of the combination. To determine the mechanism of synergistic radioprotection by the combination, mevalonate or calmodulin were coadministered with the GT3-PTX combination. Mevalonate was used to reverse the inhibitory effect of GT3 on 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGCR), and calmodulin was used to reverse the inhibition of phosphodiesterase (PDE) by PTX. Results. The combination was most effective when 200 mg/kg of PTX was administered 15 min before irradiation along with 200 mg/kg of GT3 (-24 h) and resulted in a DRF of 1.5. White blood cells and neutrophil counts showed accelerated recovery in GT3-PTX-treated groups compared to GT3. Mevalonate had no effect on the radioprotection of GT3-PTX; calmodulin abrogated the synergistic radioprotection by GT3-PTX. Conclusion. The mechanism of radioprotection by GT3-PTX may involve PDE inhibition.

Cloning of a calmodulin kinase I homologue from Schizosaccharomyces pombe

By using (35)S-labeled calmodulin (CaM), we have isolated a full-length cDNA clone expressing the Schizosaccharomyces pombe homologue of calmodulin kinase I (CaMK-I), a gene we have named cmk1. It has been previously been shown in mammals that CaMK-I is a member of a CaM-dependent protein kinase cascade that ultimately regulates transcription factors such as ATF and cAMP-response element-binding protein. The cmk1 cDNA encodes a 335-amino acid protein with significant homology to mammalian CaMK-I, including a conserved sequence for phosphorylation by CaM kinase kinase. We have expressed the cmk1 cDNA in bacteria and yeast, and we have shown that it is a CaM-dependent protein kinase. A truncation mutant of cmk1 (d320) failed to bind CaM, indicating that the CaM-binding domain is at the extreme C terminus of the protein. The mRNA for cmk1 is expressed in a cell cycle-dependent manner, peaking at or near the G(1)/S boundary. Overexpression of wild-type cmk1 in S. pombe caused no apparent effects on growth and division. However, mutation of a predicted regulatory site (Thr-192) to aspartic acid resulted in hyperactivation of CMK1 activity in the presence of CaM and causes cell cycle arrest in vivo. Arrest is also accompanied by morphological defects. These results suggest the presence of a CaM-dependent protein kinase cascade in yeast and indicate that cmk1 may be important in cell cycle progression, a process known to be dependent on CaM in eukaryotic cells.