Katarzyna Gach

Natural and synthetic α-methylenelactones and α-methylenelactams with anticancer potential.

α-Methylene-γ- and δ-lactones, as well as α-methylene-γ- and δ-lactams, are plant-derived compounds often used in traditional medicine for the treatment of inflammatory diseases. In recent years, the anticancer properties of these compounds and the molecular mechanisms of their action have been studied extensively. In the search for modern anticancer drugs, various synthetic analogs of α-methylene-γ- and δ-lactones and lactams have been synthesized and tested for their cytotoxic activity. In this review, we give a brief description of the occurrence and biological activity of such compounds isolated from plants and their diverse synthetic analogs.

The role of morphine in regulation of cancer cell growth

Morphine is considered the “gold standard” for relieving pain and is currently one of the most effective drugs available clinically for the management of severe pain associated with cancer. In addition to its use in the treatment of pain, morphine appears to be important in the regulation of neoplastic tissue. Although morphine acts directly on the central nervous system to relieve pain, its activities on peripheral tissues are responsible for many of the secondary complications. Therefore, understanding the impact, other than pain control, of morphine on cancer treatment is extremely important. The effect of morphine on tumor growth is still contradictory, as both growth-promoting and growth-inhibiting effects have been observed. Accumulating evidence suggests that morphine can affect proliferation and migration of tumor cells as well as angiogenesis. Various signaling pathways have been suggested to be involved in these extra-analgesic effects of morphine. Suppression of immune system by morphine is an additional complication. This review provides an update on the influence of morphine on the growth and migration potential of tumor cells.

Enzymatic degradation of endomorphins.

Centrally acting plant opiates, such as morphine, are the most frequently used analgesics for the relief of severe pain, even though their undesired side effects are serious limitation to their usefulness. The search for new therapeutics that could replace morphine has been mainly focused on the development of peptide analogs or peptidomimetics with high selectivity for one receptor type and high bioavailability, that is good blood-brain barrier permeability and enzymatic stability. Drugs, in order to be effective, must be able to reach the target tissue and to remain metabolically stable to produce the desired effects. The study of naturally occurring peptides provides a rational and powerful approach in the design of peptide therapeutics. Endogenous opioid peptides, endomorphin-1 and endomorphin-2, are two potent and highly selective mu-opioid receptor agonists, discovered only a decade ago, which display potent analgesic activity. However, extensive studies on the possible use of endomorphins as analgesics instead of morphine met with failure due to their instability. This review deals with the recent investigations that allowed determine degradation pathways of endomorphins in vitro and in vivo and propose modifications that will lead to more stable analogs.

The influence of opioids on matrix metalloproteinase-2 and -9 secretion and mRNA levels in MCF-7 breast cancer cell line.

Matrix metalloproteinases (MMPs) are proteolytic enzymes involved in degradation of extracellular matrix, a process that initiates uncontrolled spread of proliferating cancer cells and therefore plays a crucial role in cancer invasion and metastasis. Compounds able to modulate MMP activity may become important tools in cancer research. In the present study we examined the effect of two μ-selective opioids, morphine and endomorphin-2 (EM-2) on the production of MMP-2 and MMP-9 in MCF-7 cells. We report that both opioids time- and concentration-dependently inhibited the expression and secretion of these MMPs. The observed effect was not reversed by naloxone (Nal). Further experiments showed that morphine and EM-2 decreased endothelial nitric oxide synthase (eNOS) mRNA level and nitric oxide (NO) secretion in MCF-7 cells. These findings indicate that attenuation of MMP secretion by opioids was not mediated by opioid receptors but was under the control of nitric oxide system.

The Influence of Opioids on Urokinase Plasminogen Activator on Protein and mRNA Level in MCF‐7 Breast Cancer Cell Line

Urokinase plasminogen activator plays a key role in tumor‐associated processes, increasing cancer cell invasion and metastasis, and is therefore used as a marker in cancer prognosis. In this study, we have determined the effect of μ‐opioid receptor agonists and antagonists on the urokinase plasminogen activator secretion in MCF‐7 cell line. It was shown that μ‐opioid receptor agonists, such as morphine and endomorphins, greatly stimulate urokinase plasminogen activator secretion, while naloxone and MOR‐selective antagonists elicit the opposite effect. The same tendency was observed also on the urokinase plasminogen activator mRNA level. However, neither agonists nor antagonists had any effect on proliferation of MCF‐7 cells. The findings reported in this study may be useful in designing further experiments aimed at elucidating the role of the opioid system in cancer cells.

The role of oxidative stress in anticancer activity of sesquiterpene lactones

Sesquiterpene lactones (SLs) are plant-derived compounds that are abundant in plants of the Asteraceae family and posses a broad spectrum of biological activities, ranging from anti-inflammatory, phytotoxic, antibacterial, and antifungal to cytotoxic/anticancer. In recent years, anticancer properties of these compounds and molecular mechanisms of their action have been studied extensively on numerous cell lines and also on experimental animals. SLs have been shown to disrupt cellular redox balance and induce oxidative stress in cancer cells. Oxidative stress is associated with increased production of reactive oxygen species (ROS) which in turn can promote many aspects of cancer development and progression. On the other hand, ROS, which initiate apoptosis via the mitochondrial-dependent pathway, can also be used to kill cancer cells, if they can be generated in cancer. One of the most important regulators of the redox equilibrium in the cells is reduced glutathione (GSH). In cancer cells, GSH levels are higher than in normal cells. Therefore, SL can induce apoptosis of cancer cells by decreasing intracellular GSH levels. The use of SL which can affect intracellular redox signaling pathways can be considered an interesting approach for cancer treatment. In this review, we give a brief description of the mechanisms and pathways involved in oxidative stress-induced anticancer activity of SL.

Functional Characterization of Opioid Receptor Ligands by Aequorin Luminescence-Based Calcium Assay

A functional assay, based on aequorin-derived luminescence triggered by receptor-mediated changes in intracellular calcium levels, was used to examine relative potency and efficacy of the μ-opioid agonists endomorphin-1, endomorphin-2, morphiceptin, and their position 3-substituted analogs, as well as the δ-agonist deltorphin-II. The results of the aequorin assay, performed on recombinant cell lines, were compared with those obtained in the functional assay on isolated tissue preparations (guinea pig ileum and mouse vas deferens). A range of nine opioid peptide ligands produced a similar rank order of potency for the μ- and δ-opioid receptor agonists in both functional assays. The highest potency at the μ-receptor was observed for endomorphin-1, endomorphin-2, and [d-1-Nal3]morphiceptin, whereas deltorphin-II was the most potent δ-receptor agonist. In the aequorin assay, the μ- and δ-agonist-triggered luminescence was inhibited by the opioid antagonists naloxone and naltrindole, respectively. We can conclude that the use of the aequorin assay for new μ- and δ-receptor-selective opioid analogs gives pharmacologically relevant data and allows high-throughput compound screening, which does not involve radioactivity or animal tissues. This is the first study that validates the application of this assay in the screening of opioid analogs.

Synthesis and Biological Activity of Endomorphin‐2 Analogs Incorporating Piperidine‐2‐, 3‐ or 4‐Carboxylic Acids Instead of Proline in Position 2

Novel endomorphin‐2 (EM‐2) analogs have been synthesized, incorporating unnatural amino acids with six‐membered heterocyclic rings, such as piperidine‐2‐, 3‐ and 4‐carboxylic acids (Pip, Nip and Inp, respectively) instead of Pro in position 2. [(R)‐Nip2]EM‐2 displayed an extremely high affinity for the μ‐opioid receptor with IC50 = 0.04 ± 0.01 nm in comparison with IC50 = 0.69 ± 0.03 nm for EM‐2. This analog was also very potent in the aequorin luminescence‐based functional calcium assay and showed significantly enhanced stability in rat brain homogenate.

Cyclization in opioid peptides.

Endogenous opioid peptides have been studied extensively as potential therapeutics for the treatment of pain. The major problems of using natural opioid peptides as drug candidates are their poor receptor specificity, metabolic instability and inability to reach the brain after systemic administration. A lot of synthetic efforts have been made to opioid analogs with improved pharmacological properties. One important structural modification leading to such analogs is cyclization of linear sequences. Intramolecular cyclization has been shown to improve biological properties of various bioactive peptides. Cyclization reduces conformational freedom responsible for the simultaneous activation of two or more receptors, increases metabolic stability and lipophilicity which may result in a longer half-life and easier penetration across biological membranes. This review deals with various strategies that have been employed to synthesize cyclic analogs of opioid peptides. Discussed are such bridging bonds as amide and amine linkages, sulfur-containing bonds, including monosulfide, disulfide and dithioether bridges, bismethylene bonds, monosulfide bridges of lanthionine and, finally, carbonyl and guanidine linkages. Opioid affinities and activities of cyclic analogs are given and compared with linear opioid peptides. Analgesic activities of analogs evaluated in the in vivo pain tests are also discussed.

Comparison of Anti‐Invasive Activity of Parthenolide and 3‐Isopropyl‐2‐Methyl‐4‐Methyleneisoxazolidin‐5‐One (MZ‐6) – A New Compound with α‐Methylene‐γ‐Lactone Motif – on Two Breast Cancer Cell Lines

The biological activities of parthenolide, a sesquiterpene lactone isolated from feverfew, have been attributed to the presence of the α‐methylene‐γ‐lactone skeleton. The lactone skeleton can react via the Michael type addition with sulfhydryl groups of enzymes and other functional proteins, interfering with key biological processes in the cell. In the present study, we describe an efficient method of preparation of 3‐isopropyl‐2‐methyl‐4‐methyleneisoxazolidin‐5‐one (MZ‐6), a synthetic compound with α‐methylene‐γ‐lactone ring, as in parthenolide, additionally modified by introduction of a nitrogen atom. Furthermore, we investigated the cytotoxic activity and anti‐metastatic potential of MZ‐6 in comparison with parthenolide. Both compounds showed considerable cytotoxicity against breast cancer MCF‐7 and MDA‐MB‐231 adenocarcinoma cells in vitro and were then evaluated for their anti‐metastatic potential. The experimental results showed that MZ‐6 and parthenolide suppressed, to a similar degree, migration of MCF‐7, but not more aggressive MDA‐MB‐231 cells. In both cell lines, tested compounds down‐regulated mRNA and protein levels of metalloproteinase‐9 and urokinase plasminogen activator, the key proteases involved in the degradation of extracellular matrix and dissemination of cancer cells. The obtained results indicate that simple analogs of α‐methylene‐γ‐lactones can be good substitutes for more complex structures isolated from plants.