Comparison of protein acetyltransferase action of CRTAase with the prototypes of HAT

Radiosensitization of calreticulin-overexpressing human glioma cell line by the polyphenolic acetate 7, 8-diacetoxy-4-methylcoumarin

BACKGROUND

Calreticulin (CRT), an endoplasmic reticulum-resident protein generally overexpressed in cancer cells, is associated with radiation resistance. CRT shows higher transacetylase activity, as shown by us earlier, in the presence of the polyphenolic acetates (like 7, 8-diacetoxy-4-methylcoumarin, DAMC) and modifies the activity of a number of proteins, thereby influencing cell signaling.

AIM

To investigate the relationship between CRT expression and radiation response in a human glioma cell line and to evaluate the radiomodifying effects of DAMC.

METHODS AND RESULTS

Studies were carried out in an established human glioma cell line (BMG-1) and its isogenic clone overexpressing CRT (CROE, CRT-overexpressing cells) by analyzing clonogenic survival, cell proliferation, micronuclei analysis, and protein levels by Western blotting as parameters of responses. CRT overexpression conferred resistance against radiation-induced cell death in CROE cells (D₃₇  = 7.35 Gy, D₁₀  = 12.6 Gy and D₀  = 7.25 Gy) as compared to BMG-1 cells (D₃₇  = 5.70 Gy, D₁₀  = 9.2 Gy and D₀  = 5.6 Gy). A lower level of radiation-induced micronuclei formation observed in CROE cells suggested that reduced induction and/or enhanced DNA repair partly contributed to the enhanced radioresistance. Consistent with this suggestion, we noted that CRT-mediated radioresistance was coupled with enhanced grp78 level and reduced P53 activation-mediated prodeath signaling, while no changes were noted in acetylation of histone H4. DAMC-enhanced radiation-induced delayed (secondary) apoptosis, which was higher in CROE cells.

CONCLUSION

CRT overexpression confers resistance against radiation-induced death of human glioma cells, which can be overcome by the polyphenolic acetate DAMC.

Site-directed mutagenesis in the P-domain of calreticulin transacylase identifies Lys-207 as the active site residue

In silico-docking studies from previous work have suggested that Lys-206 and lys-207 of calreticulin (CR) play a pivotal key role in its well-established transacetylation activity. To experimentally validate this prediction, we introduced three mutations at lysine residues of P-domain of CR: K → A, P ^mut-1 (K -206, -209), P ^mut-2 (K -206, -207) and P ^mut-3 (K -207, -209) and analyzed their immunoreactivity and acetylation potential. The clones of wild-type P-domain (P ^wt ) and three mutated P-domain (P ^mut-1, P ^mut-2 and P ^mut-3) were expressed in pTrcHis C vector and the recombinant P ^wt , P ^mut-1 , P ^mut-2 and P ^mut-3 proteins were purified by Ni-NTA affinity chromatography. Screening of the transacylase activity (TAase) by the Glutathione S Transferase (GST) assay revealed that the TAase activity was associated with the P ^wt and P ^mut-1 while P ^mut-2 and P ^mut-3 did not show any activity. The immune-reactivity to anti-lysine antibody was also retained only by the P ^mut-1 in which the Lys-207 was intact. Retention of the TAase activity and immunoreactivity of P ^mut-1 with mutations introduced at Lys-206, Lys-209, while its loss with a mutation at Lys-207 residue indicated that lysine-207 of P-domain constitutes the active site residue controlling TAase activity.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02659-1.

Non-Enzymatic Protein Acetylation by 7-Acetoxy-4-Methylcoumarin: Implications in Protein Biochemistry

BACKGROUND

The semi-synthetic acetoxycoumarins are known to acetylate proteins using novel enzymatic Calreticulin Transacetylase (CRTAase) system in cells. However, the nonenzymatic protein acetylation by polyphenolic acetates is not known.

OBJECTIVE

To investigate the ability of 7-acetoxy-4-methyl coumarin (7-AMC) to acetylate proteins non-enzymatically in the test tube.

METHODS

We incubated 7-AMC with BSA and analyzed the protein acetylation using Western blot technique. Further, BSA induced biophysical changes in the spectroscopic properties of 7-AMC was analyzed using Fluorescence spectroscopy.

RESULTS

Using pan anti-acetyl lysine antibody, herein we demonstrate that 7-AMC acetylates Bovine Serum Albumin (BSA) in time and concentration dependent manner in the absence of any enzyme. 7-AMC is a relatively less fluorescent molecule compared to the parental compound, 7- hydroxy-4-methylcoumarin (7-HMC), however the fluorescence of 7-AMC increased by two fold on incubation with BSA, depending on the time of incubation and concentration of BSA. Analysis of the reaction mixture of 7-AMC and BSA after filtration revealed that the increased fluorescence is associated with the compound of lower molecular weight in the filtrate and not residual BSA, suggesting that the less fluorescent 7-AMC undergoes self-hydrolysis in the presence of protein to give highly fluorescent parental molecule 7-HMC and acetate ion in polar solvent (phosphate buffered saline, PBS). The protein augmented conversion of 7-AMC to 7-HMC was found to be linearly related to the protein concentration.

CONCLUSION

Thus protein acetylation induced by 7-AMC could also be non-enzymatic in nature and this molecule can be exploited for quantification of proteins.

Calreticulin stabilizes F-actin by acetylating actin and protects microvascular endothelial cells against microwave radiation

AIMS

Calreticulin (CRT) is a multifunctional protein that protects endothelial cells by alleviating actin cytoskeleton injury, but the underlying mechanism remains unclear. CRT was recently identified as a novel acyltransferase; acetylation at the N-terminus of actin monomers strengthens actin polymerization. This study was undertaken to determine whether CRT protects human microvascular endothelial cells (HMECs) against microwave radiation through actin acetylation.

MATERIALS AND METHODS

We prepared a eukaryotic-derived recombinant CRT and incubated the HMECs with it prior to microwave exposure. We then assessed cell injury and endothelial function, detected actin polymerization and acetylation after HMECs exposure to S-band high-power microwaves. Coimmunoprecipitation, pull-down, and ex vitro acetylation reaction were performed to determine whether actin is a novel substrate of CRT acyltransferase. Finally, we employed the mutant experiments to demonstrate the acetylation sites contributing to CRT acetyltransferase activity.

KEY FINDINGS

Microwave radiation induced severe cell injury and endothelial contact dysfunction, reduced the polymerization of actin filaments, and destroyed the actin arrangement, ultimately reducing acetylated actin expression. CRT treatment upregulated actin acetylation levels, promoted polymerization, and facilitated thicker and longer F-actin stress fibre formation. Pre-incubation with CRT rescued microwave-induced cell injury, decreased actin acetylation, and rendered the actin cytoskeleton radiation-retardant. The level of acetyl-actin was positively correlated with actin polymerization. Actin was identified as a novel substrate of CRT, being acetylated mainly through the CRT P-domain at lys-206 and -207.

SIGNIFICANCE

This work provides a better understanding of the underlying mechanism of CRT-induced cytoprotection, and suggests a novel therapeutic target for microwave radiation-related diseases with endothelial dysfunction.