Naturally occurring cell adhesion inhibitors

This paper reviews naturally occurring cell adhesion inhibitors derived from a plant, microbial and marine origin. Plant-derived inhibitors are classified according to a type of structure. Microbially and marine-derived inhibitors were described according to age. In addition, effects of inhibitors on cell proliferation and that of standards on cell adhesion are listed as much as possible.


Introduction
Cell adhesion molecules (CAMs) such as intercellular adhesion molecule 1 (ICAM-1, CD54 and immunoglobulin), vascular cell adhesion molecule 1 (VCAM-1) and E-selectin (CD62E) are critical in the regulation of immune response and inflammation. The extracellular interactions between specific CAMs expressing on the endothelium and leukocytes mediate leukocyte entry into tissues, T cell proliferation, and antigen presentation [1][2][3][4]. The key event in autoimmune disease is the migration of leukocytes to the disease site. Agents that inhibit leukocyte adhesion, transmigration and expression of related CAMs represent therapeutic potential as immunosuppressives and anti-inflammatory drugs. The major adhesive force for lymphocyte extravasation from the blood stream into tissue site is the protein-protein interaction of the adhesion molecules lymphocyte function-associated molecule 1 (LFA-1, CD11a/ CD18 and β2 integrin) and its endothelial counter-receptor ICAM-1 [5,6]. Monoclonal antibodies to ICAM-1 have been shown to inhibit lymphocyte transendothelial migration and have yielded very promising results in clinical trials for rheumatoid arthritis and organ transplantation [7,8]. Therefore, the search for specific inhibitors of integrinmediated cell adhesion with a small molecule in expectation of anti-inflammatory and anti-metastatic drugs started in the 1990s.
Generally, a cell adhesion inhibitor is categorized as target for cell-cell adhesion and for expression of cell adhesion molecules. Though certain small molecules such as flavonoids [9,10] and others [11,12] affecting expression of cell adhesion molecules are known, specific inhibitors for cell-cell contact are limited here in the review, as possible.
So far, some cell adhesion inhibitors based on synthetic methods and computer-aided drug design have been developed [13,14]; however, natural products can still make unexpected structural discovery possible and they are believed to be a reservoir of resources for new types of drugs. This review introduces cell adhesion inhibitors focusing on those of naturally occurring plant, microbial and marine origin.
Andrographolide (9), an ent-labdane diterpenoid lactone isolated from the Chinese official herbal Andrographis paniculata (Acanthaceae), has been reported to have anticancer activity [18][19][20]. Jiang and co-workers reported that 9 inhibited the adhesion of gastric cancer cells with a highly expressing level of sialyl Lewis X (SLe X ) to the TNFα-stimulated human endothelial cells by blocking E-selectin expression in a dose-dependent manner, in a concentration range of 1-10 µM [21].
Touihri-Barakati and co-workers reported that cucurbitacin B (10) from the leaves of Tunisian Ecballium elaterium (Cucurbitaceae) showed anti-integrin activity on human glioblastoma U87 cells, without being cytotoxic at concentrations up to 500 nM [23].

Alkaloid
Piperine (28) and ethyl 3′,4′,5′-trimethoxycinnamate (34; the structure is shown in a section of other compounds described below) from the combined hexane and chloroform extracts of Piper longum (Piperaceae) were isolated as potent inhibitors of cell adhesion molecules on HUVECs [29]. Both 28 and 34 inhibited the TNF-α-induced expression of ICAM-1 at IC 50 values of 45 and 25 µg/mL, respectively. In further study, 34 significantly blocked the adhesion of neutrophils to endothelium in a concentration-dependent manner. Compound 34 also significantly inhibited TNF-α-induced expression of VCAM-1 and E-selectin at 50 µg/mL. To elucidate its structure-activity relationship, effects of synthesized analogues of 34 and its thio, thiono analogues, and synthesized 7-hydroxy-4-methylcoumarin derivatives on cell adhesion molecules were studied [30,31].
Lee and co-workers reported four quinolone alkaloids (29-32) isolated from the methanol extracts of Evodiae fructus, as the specific inhibitor on the binding of LFA-1 and ICAM-1 [32]. Evodiae fructus is natural medicine originated from Evodia rutaecarpa (Juss.) Benth. (Rutaceae), which has been used for treatment of gastrointestinal disorders and headaches, as an analgesic and antiemetic, and for amenorrhea in Korea.
Among four quinolone alkaloids (29)(30)(31)(32), cell adhesion inhibitory activity was suggested to be positively influenced by the presence of a double bond and an increase in aliphatic side chain length.

Other compounds
Lee and co-workers found an inhibitory effect of methanol extract of Rheum undulatum (Polygonaceae) rhizomes on cell adhesion in search for anti-inflammatory or anti-metastasis agents, and isolated six stilbenes from the by bioactivity-guided fractionation. Six stilbenes were identified as desoxyrhapontigenin (35) Table 1). Compounds 36, 37 and 38 also had an inhibitory effect on direct binding between sVCAM-1 and VLA-4 of THP-1 cells [34].
Sparstolonin B (39) significantly suppressed the adhesion of THP-1 cells to LPS-activated HUVECs at a concentration of 100 µM. The inhibitory effect of 39 on LPS-induced phosphorylation of extracellular signal-regulated kinase (Erk1/2) and serine/threonine kinase (Akt, protein kinase B) was also reported.

Plant extract, etc.
Effects of crude plant extract, snake venom and other naturally occurring fatty acid derivatives on cell adhesion molecules are also reported. As they are not isolated as pure components, only references are shown [36][37][38][39][40][41][42][43][44][45]. It was suggested that they prevent the cell-cell adhesion by blocking the binding of SLe x to ELAM-1. However, MSs showed no effect on the adhesion of sialyl Lewis A (SLe a )-expressing HL-60 cells to HUVECs. Furthermore, pretreatment of HL-60 cells, not HUVECs, with MSs caused inhibition of the adhesion of HL-60 to HUVECs. These findings indicated that MSs specifically bound to SLe x on HL-60 cells to block the cell-cell adhesion [55]. MSs proved to be effective in several in vivo models. In the mouse model of B16/BL6 melanoma lung metastasis, MS-B (41) caused a dose-dependent decrease in lung metastatic nodules without any toxic effect including body weight loss in the range of 5-20 mg/kg. Furthermore, its efficacy in combination therapy with anti-cancer drugs was demonstrated. Combined therapy of MS-B (41) and cisplatin (CDDP) induced remarkable lung metastasis inhibition without adverse effects of CDDP to the host [56,57].

Cell adhesion inhibitors of microbial origin
The total synthesis of MS-A (40) has been reported by several groups. A novel total synthesis have been accomplished by the group of the Kitasato Institute [58,59]. The combinatorial synthesis of a 122-member MS library including MSs A (40), C (42), E (44) and F (45) has been achieved based on a unique strategy for a three-component coupling utilizing a palladium-catalyzed chemoselective 1 3 carbonylation and an unprecedented macrolactonization on a polymer support [60]. Synthetic approaches to MS derivatives, based on medicinal chemistry, were reviewed [61]. At present, MS-A (40) is commercially available as a reagent.
HUN-7293 (54) almost inhibited cell adhesion between the human Burkitt's lymphoma B (BL 2) cell and TNF-αstimulated HMEC-1 at a concentration of 20 nM. Total synthesis of 54 had done and evaluation of synthetic analogues as inhibitors of VCAM-1 expression was further reported [63,64].
Both 55 and 56 inhibited homotypic aggregation of Epstein-Barr virus (EBV)-immortalised B cell lymphoblastoid line (JY cell) from 1.5 µg/mL in a dose dependent manner. A complete inhibition was observed at 6.25 µg/mL.
The toxicity (IC 50 ) of 55 and 56 against JY cell was 15.2 µg/mL. Compounds 55 and 56 also inhibited SKW-3 adhesion to soluble ICAM-1 in a dose-dependent manner with an IC 50 of 18.8 and 25.0 µg/mL, respectively. The cell toxicity (IC 50 ) of adxanthromycins against SKW-3 was 110.0 µg/mL. In the cell-free receptor binding assay, both 55 and 56 showed weak inhibition with an IC 50 of 760 µg/ mL. They were reported as the first example of inhibitors of ICAM-l/LFA-1-mediated adhesion molecule isolated from microbial sources.
Members of the efomycine family from Streptomyces BS1261 were found to inhibit leukocyte adhesion, from a screening library of 20,000 natural compounds [71]. Random screening of chemical libraries identified the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor lovastatin (66), a drug clinically used for lowering cholesterol levels, as an inhibitor of the LFA-1/ ICAM-1 interaction [73]. Lovastatin (66) showed binding inhibition of recombinant ICAM-1 to purified LFA-1 with an IC 50 value of 2.1 µM. Inhibitory effects of statin-derived compounds on the binding were also shown with a range of IC 50 0.04-14 µM. The biological relevance of LFA-1 inhibition by statins with respect to the overall benefit of this drug class was reviewed [74].
Structurally related to known naturally occurring cyclic heptadepsipeptides, HUN-7293 (54), named heptadepsin (67), was isolated from the culture broth of Paenibacillus sp. [75]. Compound 67 inhibited LPS-stimulated adhesion between HUVECs and HL-60 cells with an IC 50 value of 0.92 µg/mL, without showing any cytotoxicity up to 30 µg/ mL. Compound 67 also inhibited cellular adhesion induced by lipid A, the active component of LPS, but it did not inhibit TNF-α-or IL-1β-induced cell adhesion. Heptadepsin (67) was shown to inactivate LPS by direct interaction with LPS and lipid A from the results of surface plasmon resonance analysis.

Cell adhesion inhibitors derived from a marine organism
In the screening for P-selectin inhibitors, sulfonoquinovosyl dipalmitoyl glyceride (78) and phosphatidylglycerol (79) were isolated from the 85% EtOH extract of the marine alga Dictyochloris fragrans. Both 78 and 79 inhibited P-selectin binding to sulfatides in the P-selectin-IgG ELISA assay, with IC 50 values of 5 and 1 µM, respectively [80].
The inhibitory effect of 78 on HL-60 cell adhesion to immobilized P-selectin receptor globulin (Rg) was only shown with an IC 50 value of 40 µM. Compound 78 was further shown for its ability to inhibit (24%) the P-selectindependent binding of activated platelets to HL-60 cells.  Table 2). The pharmacological activity and structure-activity relationships of selected marine algal metabolites are described [81].
Basically, all inhibitors covered this time were found using a cell-based assay for cell-cell adhesion or cell-soluble cell adhesion molecule. In a different way from the cellbased assay, the pharmacophore of sLe x , recognized by a family of selectin, was used to search a three-dimensional database of chemical structures. As result of a search for a binding inhibitor between selectins and sLe x , glycyrrhizin (structure was not shown), a saponin and sweet-tasting constituent of Glycyrrhiza glabra (liquorice, Fabaceae) root, was matched as a pharmacophore [82].