Stabilization of conformationally dynamic helices by covalently attached acyl chains

Effect of Hydrophobicity on the Anticancer Activity of Fatty-Acyl-Conjugated CM4 in Breast Cancer Cells

Antimicrobial peptides (AMPs) are important anticancer resources, and exploring AMP conjugates as highly effective and selective anticancer agents would represent new progress in cancer treatment. In this study, we synthesized C4-C16 fatty-acyl-conjugated AMP CM4 and investigated its physiochemical properties and cytotoxicity activity in breast cancer cells. Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and reversed-phase high-performance liquid chromatography (RP-HPLC) showed that long-chain fatty acyl (≥C12) conjugation prevented N-acyl-CM4 from trypsin hydrolysis. RP-HPLC and circular dichroism (CD) spectra showed that the hydrophobicity and helical content of N-acyl-CM4 increased with the acyl length. The acyl chain length was positively related to the cytotoxicity of C8-C16 conjugates, and C12-C16 fatty acyl conjugates exhibited significant cytotoxicity against MX-1, MCF-7, and MDA-MB-231 cells, with IC₅₀ values <8 μM. Flow cytometry and confocal laser scanning microscopy results showed that N-acylated conjugation significantly increased the membrane affinity in breast cancer cells, and C12-C16 acyl conjugates were capable of translocating to the intracellular space, thereby targeting mitochondria and inducing apoptosis. N-acyl-CM4 showed low cytotoxicity against normal mammalian cells and erythrocytes, especially ≤C12 fatty acyl conjugates, exhibiting selective cytotoxicity to breast cancer cells. The current work indicated that increasing hydrophobicity by attaching long fatty acyl (≥C12) to AMPs may be an effective method to improve the anticancer activity, together with selectivity and resistance to trypsin hydrolysis. This finding provides a good strategy to develop AMPs as effective anticancer agents in the future.

Effects of truncation of the peptide chain on the secondary structure and bioactivities of palmitoylated anoplin

Anoplin (GLLKRIKTLL-NH₂) is of current interest due to its short sequence and specificity towards bacteria. Recent studies on anoplin have shown that truncation and acylation compromises its antimicrobial activity and specificity, respectively. In this study, truncated analogues (pal-ano-9 to pal-ano-5) of palmitoylated anoplin (pal-anoplin) were synthesized to determine the effects of C-truncation on its bioactivities. Moreover, secondary structure of each analogue using circular dichroism (CD) spectroscopy was determined to correlate with bioactivities. Interestingly, pal-anoplin, pal-ano-9 and pal-ano-6 were helical in water, unlike anoplin. In contrast, pal-ano-8, pal-ano-7 and pal-ano-5, with polar amino acid residues at the C-terminus, were random coil in water. Nevertheless, all the peptides folded into helical structures in 30% trifluoroethanol/water (TFE/H₂O) except for the shortest analogue pal-ano-5. Hydrophobicity played a significant role in the enhancement of activity against bacteria E. coli and S. aureus as all lipopeptides including the random coil pal-ano-5 were more active than the parent anoplin. Meanwhile, the greatest improvement in activity against the fungus C. albicans was observed for pal-anoplin analogues (pal-ano-9 and pal-ano-6) that were helical in water. Although, hydrophobicity is a major factor in the secondary structure and antimicrobial activity, it appears that the nature of amino acids at the C-terminus also influence folding of lipopeptides in water and its antifungal activity. Moreover, the hemolytic activity of the analogues was found to correlate with hydrophobicity, except for the least hemolytic, pal-ano-5. Since most of the analogues are more potent and shorter than anoplin, they are promising drug candidates for further development.

Position and length of fatty acids strongly affect receptor selectivity pattern of human pancreatic polypeptide analogues

Pancreatic polypeptide (PP) is a satiety-inducing gut hormone targeting predominantly the Y4 receptor within the neuropeptide Y multiligand/multireceptor family. Palmitoylated PP-based ligands have already been reported to exert prolonged satiety-inducing effects in animal models. Here, we suggest that other lipidation sites and different fatty acid chain lengths may affect receptor selectivity and metabolic stability. Activity tests revealed significantly enhanced potency of long fatty acid conjugates on all four Y receptors with a preference of position 22 over 30 at Y1 , Y2 and Y5 receptors. Improved Y receptor selectivity was observed for two short fatty acid analogues. Moreover, [K(30)(E-Prop)]hPP2-36 (15) displayed enhanced stability in blood plasma and liver homogenates. Thus, short chain lipidation of hPP at key residue 30 is a promising approach for anti-obesity therapy because of maintained selectivity and a sixfold increased plasma half-life.

Peptide lipidation stabilizes structure to enhance biological function

Medicines that decrease body weight and restore nutrient tolerance could improve human diabetes and obesity treatment outcomes. We developed lipid-acylated glucagon analogs that are co-agonists for the glucagon and glucagon-like peptide 1 receptors, and stimulate weight loss and plasma glucose lowering in pre-diabetic obese mice. Our studies identified lipid acylation (lipidation) can increase and balance in vitro potencies of select glucagon analogs for the two aforementioned receptors in a lipidation site-dependent manner. A general capacity for lipidation to enhance the secondary structure of glucagon analogs was recognized, and the energetics of this effect quantified. The molecular structure of a lipid-acylated glucagon analog in water was also characterized. These results support that lipidation can modify biological activity through thermodynamically-favorable intramolecular interactions which stabilize structure. This establishes use of lipidation to achieve specific pharmacology and implicates similar endogenous post-translational modifications as physiological tools capable of refining biological action in means previously underappreciated.

Helix-destabilizing, beta-branched, and polar residues in the baboon reovirus p15 transmembrane domain influence the modularity of FAST proteins

The fusogenic reoviruses induce syncytium formation using the fusion-associated small transmembrane (FAST) proteins. A recent study indicated the p14 FAST protein transmembrane domain (TMD) can be functionally replaced by the TMDs of the other FAST proteins but not by heterologous TMDs, suggesting that the FAST protein TMDs are modular fusion units. We now show that the p15 FAST protein is also a modular fusogen, as indicated by the functional replacement of the p15 ectodomain with the corresponding domain from the p14 FAST protein. Paradoxically, the p15 TMD is not interchangeable with the TMDs of the other FAST proteins, implying that unique attributes of the p15 TMD are required when this fusion module is functioning in the context of the p15 ecto- and/or endodomain. A series of point substitutions, truncations, and reextensions were created in the p15 TMD to define features that are specific to the functioning of the p15 TMD. Removal of only one or two residues from the N terminus or four residues from the C terminus of the p15 TMD eliminated membrane fusion activity, and there was a direct correlation between the fusion-promoting function of the p15 TMD and the presence of N-terminal, hydrophobic β-branched residues. Substitution of the glycine residues and triserine motif present in the p15 TMD also impaired or eliminated the fusion-promoting activity of the p15 TMD. The ability of the p15 TMD to function in an ecto- and endodomain-specific context is therefore influenced by stringent sequence requirements that reflect the importance of TMD polar residues and helix-destabilizing residues.

Alterations of the C-terminal end do not affect in vitro or in vivo activity of surfactant protein C analogs

The secondary structure, orientation and hydrogen/deuterium exchange of SP-C33, a surfactant protein C analog, in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/egg phosphatidylglycerol (8:2, wt./wt.) bilayers, was studied by attenuated total reflection Fourier transform infrared spectroscopy. This showed a transmembrane α-helix, in which about 55% of the amide hydrogens do not exchange for up to 20 h. Moreover, C-terminally modified SP-C33, either truncated after position 30, or having the methionine at position 31 exchanged for either lysine or isoleucine, showed the same secondary structure and orientation. The different peptides, suspended in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol 68:31 (wt./wt.), were tested for surfactant activity in vitro in a captive bubble surfactometer and in vivo in an animal model of respiratory distress syndrome using premature rabbit fetuses. All preparations showed similar surface activity in the captive bubble surfactometer. Also, in the rabbit model, all preparations performed equally well and significantly better than non-treated controls, both regarding tidal volumes and lung gas volumes. Thus, truncation or residue replacements in the C-terminal part of SP-C33 do not seem to affect membrane association or surfactant activity.

Properties of double-stranded oligonucleotides modified with lipophilic substituents

We have synthesized a series of short, self-complementary oligonucleotide sequences modified at their 5'- and/or 3'- termini with a lipophilic dodecane (C12); these systems serve as models to assess the biophysical properties of double-stranded DNA (dsDNA) equipped with potentially stabilizing lipophilic substituents. Addition of C12 to the 5'-termini of self-complementary 10 nucleotide sequences increased their duplex melting temperatures (T(m)) by approximately 4-8 degrees C over their corresponding unmodified sequences. C12 functionalities added to both the 3'- and 5'-termini increased T(m) values by approximately 10-12 degrees C. The observed increases in T(m) correlated with greater duplex stabilities as determined by the free energy values (DeltaG) derived from T(m) plots. There is a greater degree of stabilization when C12 is positioned with a C.G base pair at the termini, and the stabilizing effect of lipophilic groups far exceeds the effect seen in adding an additional base pair to both ends of DNA. Stable, short dsDNA sequences are of potential interest in the development of transcription factor decoy oligonucleotides as possible therapeutic agents and/or biological tools. These results suggest that the stability of short dsDNA sequences are improved by lipophilic substituents and can be used as the basis for the design of dsDNAs with improved biological stabilities and function under physiological conditions.