Synthesis and biological activity of cyclolinopeptide A analogues modified with γ(3)-bis(homophenylalanine)

The Immunosuppressive Properties of Cyclo-[D-Pro-Pro-β3-HoPhe-Phe-] Tetrapeptide Selected from Stereochemical Variants of Cyclo-[Pro-Pro-β3-HoPhe-Phe-] Peptide

The anti-inflammatory, antiviral, and anti-cancer properties, as well as the mechanism of action of cyclo-[Pro-Pro-β3-HoPhe-Phe-] tetrapeptide (denoted as 4B8M), were recently described. The aim of this work was to synthesize and evaluate the immunosuppressive actions of the stereochemical variants of 4B8M by sequential substitution of L-amino acids by D-amino acids (a series of peptides denoted as P01-P07) using parent 4B8M as a reference compound. In addition, diverse available bioinformatics tools using machine learning and artificial intelligence were tested to find the bio-pharmacokinetic and polypharmacological attributes of analyzed stereomers. All peptides were non-toxic to human peripheral blood mononuclear cells (PBMCs) and only cyclo-[D-Pro-Pro-β3-HoPhe-Phe-] peptide (P03) was capable of inhibiting mitogen-induced PBMC proliferation. The peptides inhibited the lipopolysaccharide (LPS)-induced production of tumor necrosis factor-alpha (TNF-α) to various degrees, with P04 (cyclo-[Pro-Pro-D-β3-HoPhe-Phe-]) and P03 being the most potent. For further in vivo studies, P03 was selected because it had the combined properties of inhibiting cell proliferation and TNF-α production. P03 demonstrated a comparable ability to 4B8M in the inhibition of auricle edema and lymph node cell number and in the normalization of a distorted blood cell composition in contact sensitivity to the oxazolone mouse model. In the mouse model of carrageenan-induced inflammation of the air pouch, P03 exhibited a similar inhibition of the cell number in the air pouches as 4B8M, but its inhibitory effects on the percentage of neutrophils and eosinophils in the air pouches and blood, as well as on mastocyte degranulation in the air pouches, were stronger in comparison to 4B8M. Lastly, in a mouse model of dextran sulfate-induced colitis, similar effects to 4B8M regarding thymocyte number restoration and normalization of the blood cell pictures by P03 were observed. In summary, depending on either experimental findings or in silico predictions, P03 demonstrated comparable, or even better, anti-inflammatory and bio-pharmacokinetic properties to 4B8M and may be considered as a potential therapeutic. The possibility of P00 and P03 identification by circular dichroism measurements was tested by quantum-chemical calculations.

Effects of Modifications on the Immunosuppressive Properties of Cyclolinopeptide A and Its Analogs in Animal Experimental Models

The consequences of manipulations in structure and amino acid composition of native cyclolinopeptide A (CLA) from linen seeds, and its linear precursor on their biological activities and mechanisms of action, are reviewed. The modifications included truncation of the peptide chain, replacement of amino acid residues with proteinogenic or non-proteinogenic ones, modifications of peptide bond, and others. The studies revealed changes in the immunosuppressive potency of these analogs investigated in a number of in vitro and in vivo experimental models, predominantly in rodents, as well as differences in their postulated mechanism of action. The modified peptides were compared with cyclosporine A and parent CLA. Some of the synthesized and investigated peptides show potential therapeutic usefulness.

Natural cyclic polypeptides as vital phytochemical constituents from seeds of selected medicinal plants

Cyclopolypeptides are among the most predominant biomolecules in nature, especially those derived from plant seeds. This category of compounds has gained extraordinary attention due to remarkable variety of structures and valuable biofunctions. These congeners display enormous variation in terms of both structure and function and are the most significant biomolecules due to their widespread bioproperties. The estrogenic activity, immunosuppressive activity, cytotoxicity, vasorelaxant activity, and other properties possessed by cyclic peptides from seeds of plants make these congeners attractive leads for the drug discovery process. The current study covers the important structural features, structure-activity relationship, synthesis methods, and bioproperties of plant seeds-originated bioactive peptides from Vaccaria segetalis, Linum usitatissimum, and Goniothalamus leiocarpus, which may prove vital for the development of novel therapeutics based on a peptide skeleton.

4-Methylpseudoproline analogues of cyclolinopeptide A: Synthesis, structural analysis and evaluation of their suppressive effects in selected immunological assays

The synthesis of new analogues of cyclolinopeptide A (CLA) and their linear precursors modified with (R)- and (S)-4-methylpseudoproline in the Pro³-Pro⁴ fragment are presented. The peptides were tested in comparison with cyclosporine A (CsA) in concanavalin A (Con A) and pokeweed mitogen (PWM)-induced mouse splenocyte proliferation and in secondary humoral immune response in vitro to sheep erythrocytes (SRBC). Their effects on expression of selected signaling molecules in the Jurkat T cell line were also determined. In addition, the structural features of the peptides, applying nuclear magnetic resonance and circular dichroism, were analyzed. The results showed that only peptides 7 and 8 modified with (R)-4-methylpseudoproline residue (c(Leu¹-Val²-(R)-(αMe)Ser(ΨPro)³-Pro⁴-Phe⁵-Phe⁶-Leu⁷-Ile⁸-Ile⁹) and c(Leu¹-Val²-Pro³-(R)-(αMe)Ser(ΨPro)⁴-Phe⁵-Phe⁶-Leu⁷-Ile⁸-Ile⁹), respectively) strongly suppressed mitogen-induced splenocyte proliferation and the humoral immune response, with peptide 8 being more potent. Likewise, peptide 8 more strongly elevated expression of Fas, a proapoptotic signaling molecule in Jurkat cells. We postulate that the increased biological activity of peptide 8, compared to the parent molecule and other studied peptides, resulted from its more flexible structure, found on the basis of both CD and NMR studies. CD and NMR spectra showed that replacement of Pro³ by (R)-(αMe)Ser(¬Pro) caused much greater conformational changes than the same replacement of the Pro⁴ residue. Such a modification could lead to increased conformational freedom of peptide 8, resulting in a greater ability to adopt a more compact structure, better suited to its putative receptor. In conclusion, peptide 8 is a potent immune suppressor which may find application in controlling immune disorders.

Synthesis and biological activity of cyclolinopeptide A analogues modified with γ4-bis(homo-phenylalanine)

Cyclolinopeptide A (CLA), an immunosuppressive nonapeptide derived from linen seeds, was modified with S or R-γ⁴-bis(homo-phenylalanine) in positions 3 or 4, or both 3 and 4. These modifications changed the flexibility of new analogues and distribution of intramolecular hydrogen bonds. Analogues 11 c(Pro¹-Pro²-Phe³-S-γ⁴-hhPhe⁴-Leu⁵-Ile⁶-Ile⁷-Leu⁸-Val⁹), 13 c(Pro¹-Pro²-S-γ⁴-hhPhe³-R-γ⁴-hhPhe⁴-Leu⁵-Ile⁶-Ile⁷-Leu⁸-Val⁹) and 15 c(Pro¹-Pro²-R-γ⁴-hhPhe³-Phe⁴-Leu⁵-Ile⁶-Ile⁷-Leu⁸-Val⁹) existed as a mixture of stable cis/trans isomers of Pro-Pro peptide bond. The comparison of the relative spatial orientations in crystal state of the two carbonyl groups, neighboring γ-amino acids, revealed conformational similarities to α-peptides. The addition of two -CH₂- groups in γ-amino acids led to a more rigid conformation, although a more flexible one was expected. A significant difference in the relative orientation of the carbonyl groups was found for cyclic γ-peptides with a dominance of an antiparallel arrangement. As carbonyl groups may be engaged in the interactions with plausible receptors through hydrogen bonds, a similar biological activity of the modified peptides was expected. Our biological studies showed that certain cyclic, but not the corresponding linear peptides, lowered the viability of peripheral blood mononuclear cells (PBMC) at 100μg/mL concentration. The proliferation of PBMC induced by phytohemagglutinin A (PHA) was strongly inhibited by cyclic peptides only, in a dose-dependant manner. On the other hand, lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNF-α) production in whole blood cell cultures was inhibited by both linear and cyclic peptides. Peptide 15 c(Pro¹-Pro²-R-γ⁴-hhPhe³-Phe⁴-Leu⁵-Ile⁶-Ile⁷-Leu⁸-Val⁹) blocked the expression of caspase-3, inhibited the expression of caspases-8 and -9 in 24h culture of Jurkat cells, and caused DNA fragmentation in these cells, as an indicator of apoptosis. Thus, we revealed a new mechanism of immunosuppressive action of a nonapeptide.