Stimulation of mono (ADP-ribosyl)ation by reduced extracellular calcium levels in human fibroblasts

MIBG, an inhibitor of arginine-dependent mono(ADP-ribosyl)ation, prevents differentiation of L6 skeletal myoblasts by inhibiting expression of myogenin and p21cip1

The development of skeletal muscle is controlled by a highly synchronized series of cellular events, and various signals from both inside and outside the cell play a role in the switch from multipotential mesodermal stem cells to muscle fibers. Meta-iodobenzylguanidine (MIBG), an inhibitor of mono(ADP-ribosyl)ation, has been shown to prevent terminal differentiation of skeletal myoblasts; however, its mechanism of action has not been established. We recently reported that MIBG is capable of preventing phenotypic modulation of smooth muscle cells by interfering with specific trans-acting factors [L. Yau, B. Litchie, S. Thomas, B. Storie, N. Yurkova, P. Zahradka, Endogenous mono-ADP-ribosylation mediates smooth muscle cell proliferation and migration via protein kinase N-dependent induction of c-fos expression. Eur. J. Biochem. 270 (2003) 101-110.]. We therefore examined the effect of MIBG on select myogenic regulatory factors known to control terminal differentiation. It was confirmed that MIBG, but not inhibitors of poly-ADP-ribose polymerase (3-aminobenzamide, PD128763), inhibits fusion of L6 skeletal myoblasts in a concentration-dependent manner. Moreover, inhibition by MIBG correlated with a failure to induce expression of myogenin and p21(cip1), while levels of MyoD and MEF2 were unaffected. Time-of-addition studies revealed that MIBG also affected a late event possibly linked to cell fusion. Finally, arginine-dependent mono(ADP-ribosyl)transferase activity increased over the first 24 h of the differentiation period. These data support a role for arginine-dependent mono(ADP-ribosyl)transferase as an essential positive regulator of differentiation in skeletal muscle cells that operates by modulating the expression of specific myogenic factors.

Regulatory role of arginine-specific mono(ADP-ribosyl)transferase in muscle cells

Earlier we demonstrated that meta-iodobenzylguanidine (MIBG), a specific inhibitor of arginine mono-ADP-ribosylation blocks proliferation and differentiation of chick skeletal myogenic cells in culture (Exp. Cell Res., 1992, 201:33-42). Membrane fractions from 4-day, myotube cultures of embryonic chick muscle cells were incubated with 32P-NAD+. Several proteins were labeled, but labeling of two hands of about 53 and 36 kDa appeared to be due to arginyl ADP-ribosylation. Immunoprecipitation with D3 monoclonal antibody to the intermediate filament protein desmin, SDS-PAGE and autoradiography demonstrated that the 53 kDa band contained desmin, and that this desmin is ADP-ribosylated by the endogenous arginine-specific mono(ADP-ribosyl)transferase (Exp. Cell Res., 1996, in press). Desmin is the muscle-specific intermediate filament protein, and it appears to be one of the first muscle-specific proteins expressed during terminal myogenic differentiation. We have examined whether desmin can be ADP-ribosylated in muscle cells by use of polyclonal antibodies for ADP-ribosylated arginyl residues. We have found that soluble desmin is present in 5-6 day myogenic cell cultures and that this desmin contains ADP-ribose, demonstrating that desmin is ADP-ribosylated in skeletal muscle cells. We also found that purified avian desmin contains antigenic material that reacts with these antibodies. In both cases, NaCl had no effect on the reactivity, but NH2OH did, which is consistent with an arginine-ADPR linkage. In summary, these results suggest that ADP-ribosylation is an important regulatory mechanism in differentiating muscle cells, and that the intermediate filament protein desmin is an important substrate for modification in muscle cells.

Vertebrate mono-ADP-ribosyltransferases

Mono-ADP-ribosylation appears to be a reversible modification of proteins, which occurs in many eukaryotic and prokaryotic organisms. Multiple forms of arginine-specific ADP-ribosyltransferases have been purified and characterized from avian erythrocytes, chicken polymorphonuclear leukocytes and mammalian skeletal muscle. The avian transferases have similar molecular weights of approximately 28 kDa, but differ in physical, regulatory and kinetic properties and subcellular localization. Recently, a 38-kDa rabbit skeletal muscle ADP-ribosyltransferase was purified and cloned. The deduced amino acid sequence contained hydrophobic amino and carboxy termini, consistent with known signal sequences of glycosylphosphatidylinositol (GPI)-anchored proteins. This arginine-specific transferase was present on the surface of mouse myotubes and of NMU cells transfected with the cDNA and was released with phosphatidylinositol-specific phospholipase C. Arginine-specific ADP-ribosyltransferases thus appear to exhibit considerable diversity in their structure, cellular localization, regulation and physiological role.

Nitric oxide stimulates the ADP-ribosylation of a 41-kDa cytosolic protein in Dictyostelium discoideum

Nitric oxide-releasing compounds were shown to activate an ADP-ribosyltransferase activity in the cytosol of Dictyostelium discoideum. The enzyme ADP-ribosylated a cytosolic protein of approximately 41 kDa, p41. Neither cGMP nor GTP and its analogues affected this ADP-ribosylation. p41 differs from other substrates ADP-ribosylated by cholera, pertussis, or diphtheria toxins. Treatment of ADP-ribosylated p41 with snake venom phosphodiesterase released adenosine 5'-monophosphate, indicating a mono-ADP-ribose-protein linkage. This linkage was stable to neutral hydroxylamine but was sensitive to mercury ions and iodomethane, suggesting an attachment to a cysteine residue. Treatment of intact cells with nitric oxide-releasing compounds appeared to stimulate the ADP-ribosylation of p41 and this modification was reversible.

Effect of an arginine-specific ADP-ribosyltransferase inhibitor on differentiation of embryonic chick skeletal muscle cells in culture

Primary cultures of embryonic chick skeletal myogenic cells were used as an experimental model to examine the possible role of mono(ADP-ribosyl)ation reactions in myogenic differentiation. Initial studies demonstrated arginine-specific mono(ADP-ribosyl)transferase activity in the myogenic cell cultures. We then examined the effect of a novel inhibitor of cellular arginine-specific mono(ADP-ribosyl)transferases, meta-iodobenzylguanidine (MIBG), on differentiation of cultured embryonic chick skeletal myoblasts. MIBG reversibly inhibited both proliferation and differentiation of embryonic chick myoblasts grown in culture. Micromolar (15-60 microM) concentrations of MIBG blocked myoblast fusion, the differentiation-specific increase in creatine phosphokinase activity, and both DNA and protein accumulation in myogenic cell cultures. Meta-iodobenzylamine, an analog of MIBG missing the guanidine group, had no effect. Low concentrations of methylglyoxal bis-guanylhydrazone, a substrate for cholera toxin with a higher Km than MIBG, also had no effect, but higher concentrations reversibly inhibited fusion. These findings suggest a possible role for mono(ADP-ribosyl)ation reactions in myogenesis. In addition, the total arginine-specific mono(ADP-ribosyl)transferase activity increased with differentiation in the myogenic cell cultures, and this increase was also blocked by MIBG treatment. Because high levels of activity were found in the membrane fraction derived from later, myotube cultures, the membrane fraction from 96-h cultures was incubated with [32P]NAD+ and subjected to electrophoresis and autoradiography. Three proteins, migrating at 21, 20, and 17 kDa, that were ADP-ribosylated in the absence, but not the presence, of MIBG were identified. These proteins may be endogenous substrates for this enzyme.

Decline of poly(ADP-ribosyl)ation during in vitro senescence in human diploid fibroblasts

Poly(ADP-ribose) polymerase activity was determined at various times during the in vitro life span of two human diploid fibroblast-like cell lines of different donor ages. The cell lines differed in their ability to transfer ADP-ribose, with cells from an embryonic donor exhibiting 2 to 3 times the activity found in cells obtained from a newborn donor. The activity in both cell lines decreased by 30-60% as the cells moved through their in vitro life spans. The decline could not be attributed to increases in glycohydrolase or the leakage of polymerase from older cell preparations. Enzyme activation with DNase I indicated that similar levels of enzyme were present in both cell lines at all in vitro ages. These results indicate that although poly(ADP-ribosyl)ation is inversely related to donor age as well as in vitro age the decrease is in response to other factors which change with increasing age.

Utilization of 2'-deoxynad for ADP-ribose transfer reactions

2'-deoxyNAD was examined as a substrate for both mono(ADP-ribosyl)ation and poly(ADP-ribosyl)ation reactions. 2'-deoxyNAD is a substrate for the diphtheria toxin-catalyzed mono(ADP-ribosyl)ation of elongation factor-2, inactivating its function to enhance protein synthesis. On the other hand, 2'-deoxyNAD is a poor substrate for poly(ADP-ribose) polymerase. 2'-deoxyNAD was not synthesized intracellularly from deoxyATP, even when deoxyATP content was markedly increased by incubation of cells with deoxyadenosine and an adenosine deaminase inhibitor. 2'-deoxyNAD, because of its specificity, could be a quite useful reagent for the investigation of cellular mono(ADP-ribosyl)ation reactions.