Irradiation of Epithelial Carcinoma Cells Upregulates Calcium-Binding Proteins That Promote Survival under Hypoxic Conditions.

Hypoxia is thought to promote tumor radio-resistance via effects on gene expression in cancer cells that modulate their metabolism, proliferation, and DNA repair pathways to enhance survival. Here we demonstrate for the first time that under hypoxic condition A431 epithelial carcinoma cells exhibit increased viability when exposed to low-dose γ-irradiation, indicating that radiotherapy can promote tumor cell survival when oxygen supply is limited. When assessed using iTRAQ quantitative proteomics and Western blotting, irradiated tumor cells were observed to significantly up-regulate the expression of calcium-binding proteins CALM1, CALU, and RCN1, suggesting important roles for these mediators in promoting tumor cell survival during hypoxia. Accordingly, shRNA-knockdown of CALM1, CALU, and RCN1 expression reduced hypoxic tumor cell resistance to low-dose radiation and increased apoptosis. These data indicate that γ-irradiation of hypoxic tumor cells induces up-regulation of calcium-binding proteins that promote cancer cell survival and may limit the efficacy of radiotherapy in the clinic.

Electrostatic control of the overall shape of calmodulin: numerical calculations.

The paper reports the results of numerical calculations of the pKa's of the ionizable groups and the electrostatic interactions between calmodulin lobes in three different states of calmodulin: calcium-free, peptide-free; calcium-loaded, peptide-free; and calcium-loaded, peptide-bound. NMR and X-ray studies revealed that in these states the overall structure of calmodulin adopts various conformations referred as: disordered semi-compact, extended and compact conformations, respectively. In addition, a new X-ray structure was recently reported (Structure, 2003, 11, 1303) showing that calcium-loaded, peptide-free calmodulin can also adopt a compact conformation in addition to the well known extended conformation. The calculated energy changes of calcium-loaded, peptide-free calmodulin along the pathway connecting these two conformations provide a possible explanation for this structural plasticity. The effect of pH and organic compounds in the solution phase on the preference of calmodulin to adopt compact or extended conformations may be thus rationalized. Analysis of the contribution of the ionization changes to the energy of association of calmodulin lobes suggested that the formation of the compact forms requires protonation of several acidic residues. However, two different protonation scenarios are revealed: a protonation due to internal lobe organization and thus independent of the lobes association, and a protonation induced by the lobes association resulting to a proton uptake. In addition, the role of the individual residues on the energy of association of calmodulin lobes is calculated in two compact conformations (peptide-free and peptide-bound) and is shown that a set of residues always plays a dominant role in inter-domain interactions.

Resonance energy transfer in a calcium concentration-dependent cameleon protein.

We report investigations of resonance energy transfer in the green fluorescent protein and calmodulin-based fluorescent indicator constructs for Ca(2+) called cameleons using steady-state and time-resolved spectroscopy of the full construct and of the component green fluorescent protein mutants, namely ECFP (donor) and EYFP (acceptor). EYFP displays a complicated photophysical behavior including protonated and deprotonated species involved in an excited-state proton transfer. When EYFP is excited in the absorption band of the protonated species, a fast nonradiative deactivation occurs involving almost 97% of the excited protonated population and leading to a low efficiency of excited-state proton transfer to the deprotonated species. ECFP displays a multiexponential fluorescence decay with a major contributing component of 3.2 ns. The time-resolved fluorescence data obtained upon excitation at 420 nm of Ca(2+)-free and Ca(2+)-bound YC3.1 cameleon constructs point to the existence of different conformations of calmodulin dependent on Ca(2+) binding. Whereas steady-state data show only an increase in the efficiency of energy transfer upon Ca(2+) binding, the time-resolved data demonstrate the existence of three distinct conformations/populations within the investigated sample. Although the mechanism of the interconversion between the different conformations and the extent of interconversion are still unclear, the time-resolved fluorescence data offer an estimation of the rate constants, of the efficiency of the energy transfer, and of the donor-acceptor distances in the Ca(2+)-free and Ca(2+)-bound YC3.1 samples.

A pilot study on the effect of radiation on calmodulin in rat submandibular salivary glands.

Xerostomia and loss of salivary gland secretion is one of the most common complications of the radiation treatment of head-and-neck malignancies. The secretory mechanism in the salivary glands can be modulated by the concentration of intracellular Ca2+. Calmodulin is a calcium-binding protein that is widely distributed in nature and is involved in regulating intracellular calcium. In this study the effect of radiation on the concentration of calmodulin in rat salivary glands was investigated. Fourteen rats were divided into three groups: R1 (n = 4) and R2 (n = 5) received a single dose of 15 Gy and group C (n = 5) received no radiation. R1 and R2 animals were killed at weeks 2 and 10 post-irradiation, respectively. The submandibular glands were removed, homogenized and their total calmodulin was determined. The mean calmodulin concentrations were 6.4+/-1.1 microg/gland for controls, 14.1+/-3.7 microg/gland for R1 and 68.2+/-14.4 microg/gland for R2. Kruskal-Wallis ANOVA revealed a significant increase in the concentration of calmodulin following irradiation (p = 0.003). The relationship between this increase and the loss of salivary gland function is not yet clear.

[The content of immunoreactive calmodulin in the blood cells of irradiated animals]. / Soderzhanie immunoreaktivnogo kal'modulina v kletkakh krovi obluchennykh zhivotnykh.

The immunoreactive calmodulin content was shown to increase in the irradiated platelets of rats and cows. Total Ca content did not vary significantly in platelets of exposed animals although there was a tendency toward its increase. At the same time, total Ca content in lymphocytes of irradiated cows increased while the immunoreactive calmodulin level remained unchanged.

Peptide cross-linking to calmodulin: attachment of [Tyr8]substance P.

Ultraviolet irradiation of calmodulin in the presence of calcium results in either the intramolecular cross-linking of Tyr99 and Tyr138 [Malencik, D.A., & Anderson, S.R. (1986) Biochemistry 25, 709] or, when [Tyr8]substance P is bound, the generation of peptide-calmodulin adducts. The latter consist of two chromatographically distinct fractions, one of which was purified to homogeneity with phenylagarose, DEAE-Sepharose, and reverse-phase chromatography. Chemical characterization shows that the purified conjugate contains 1 mol/mol of peptide covalently attached to Tyr138 of calmodulin. The fluorescence intensity and anisotropy of the dityrosine moiety demonstrate that this novel derivative undergoes interactions with calcium, smooth muscle myosin light chain kinase, and phenylagarose which are similar to those of unmodified calmodulin.

Differential inhibition of calmodulin-sensitive phosphodiesterase and Ca++-adenosine triphosphatase by chlorpromazine-linked calmodulin.

Upon irradiation with UV light, chlorpromazine binds irreversibly to calmodulin and inactivates it. To determine whether this chlorpromazine-calmodulin (CPZ-CaM) complex can inhibit the actions of native calmodulin, we examined its effects on the activity of calmodulin-sensitive cyclic nucleotide phosphodiesterase from rat brain and on the Ca++-adenosine triphosphatase (ATPase) of human erythrocyte membranes. The CPZ-CaM complex was prepared by irradiating purified bovine brain calmodulin in the presence of chlorpromazine and Ca++. The sample was then dialyzed extensively to remove reversibly bound chlorpromazine and then assayed for its ability to activate calmodulin-sensitive phosphodiesterase and Ca++-ATPase, and for its ability to block the stimulatory effects of native calmodulin on these enzymes. The CPZ-CaM complex had no effect on the basal activity of either enzyme; it neither activated nor inhibited the enzymes when assayed in the absence of calmodulin. However, it affected differentially the activation of the two enzymes by native calmodulin. The CPZ-CaM complex totally inhibited calmodulin-stimulated phosphodiesterase but had no effect on the activation of the ATPase by calmodulin. Other studies showed that CPZ-CaM increased the activation constant (Ka) for the interaction of calmodulin with phosphodiesterase but did not affect the maximal activation (Vmax) of the enzyme by calmodulin. Neither calmodulin nor CPZ-CaM altered the Km for the interaction between phosphodiesterase and cyclic AMP. These results suggest that CPZ-CaM inhibits the calmodulin-induced activation of phosphodiesterase by competing with calmodulin for regulatory sites on the enzyme and not by interacting with calmodulin itself or by blocking the interaction of cyclic AMP with the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of calmodulin with iodothyronines: effect of iodothyronines on the calmodulin activation of cyclic AMP phosphodiesterase.

The interaction between calmodulin and iodothyronines and the effect of iodothyronines on the calmodulin activation of cyclic AMP phosphodiesterase were investigated. Binding of [L-125I]triiodothyronine to calmodulin from pig brain, studied by equilibrium dialysis, was dependent on Ca2+, was saturable and reversible, with an apparent Kd of 2.79 microM and binding capacity of 0.5 nmol/20 micrograms of calmodulin L- and D-thyroxine, D-triiodothyronine and tetrac displaced [L-125I]triiodothyronine at concentrations of 8-10 microM; triac, 3,3'-diiodothyronine and reverse-triiodothyronine were weak displacers. In the presence of the antipsychotic drug trifluoperazine, binding decreased in a dose-related manner. Ultraviolet irradiation of calmodulin in the presence of trifluoperazine reduced the binding of [L-125I]triiodothyronine to calmodulin irreversibly. Calmodulin activation of cyclic AMP phosphodiesterase decreased when iodothyronines were bound to calmodulin; the calmodulin-L-triiodothyronine complex was the most active among the stereoisomers of thyroxine and triiodothyronine. These results suggest that, when triiodothyronine was bound to Ca2+-calmodulin, the activation of cyclic AMP phosphodiesterase by the latter is suppressed.

Dityrosine formation in calmodulin.

Ultraviolet (280-nm) irradiation of bovine brain calmodulin results in calcium-dependent changes in its fluorescence emission spectrum. These consist of a decline in the intrinsic tyrosine fluorescence of the protein and the appearance of a new emission maximum at 400 nm. Chromatography of irradiated calmodulin, using Ultrogel AcA 54 and phenyl-agarose columns, yields several distinctive fractions. One of these, representing 2.8% of the total recovered protein and 53% of the total fluorescence emission at 400 nm, was selected for detailed characterization. Analyses performed on acid hydrolysates reveal the presence of dityrosine, a derivative of tyrosine known for its fluorescence near 400 nm, at the level of 0.59-0.89 mol per 16,700 g of protein. Sodium dodecyl sulfate gel electrophoresis experiments demonstrate two components of apparent molecular weights 14,000 (80%) and 16,000 (20%). Observations on the effects of UV irradiation on the thrombic fragments of calmodulin and on related calcium binding proteins (rabbit skeletal muscle troponin C, bovine cardiac troponin C, and parvalbumin) support the interpretation that dityrosine formation in calmodulin results from the intramolecular cross-linking of Tyr-99 and Tyr-138. The dityrosine-containing photoproduct of calmodulin is unable to stimulate the p-nitrophenyl phosphatase activity of calcineurin under standard assay conditions. Fluorescence titrations show a generally weakened interaction with calcium ion occurring in two stages. The pKa of the derivative is considerably higher than that of free dityrosine and is calcium dependent, decreasing from 7.88 to 7.59 on the addition of 3 mM CaCl2. Smooth muscle myosin light chain kinase binds the derivative about 280-fold less effectively than it binds native calmodulin. Of several metal ions tested, only Cd2+ approaches Ca2+ in its ability to promote the appearance of the 400-nm emission band during UV irradiation of calmodulin. Mn2+ and Cu2+ appear to inhibit dityrosine formation. Ascorbic acid, dithiothreitol, and glutathione are also inhibitory.

Calmodulin and the target size of the (Ca2+ + Mg2+)-ATPase of human red-cell ghosts.

An average target size of 251 kDa has been obtained for the (Ca2+ + Mg2+)-ATPase of calmodulin-depleted erythrocyte ghosts by radiation inactivation with 16 MeV electrons. This is close to twice the size of the purified calcium-pump polypeptide. When calmodulin was included during the ATPase assay, a component of about 1 MDa appeared in addition to the activated dimer.