Glycolipids from seaweeds and their potential biotechnological applications

Marine macroalgae, or seaweeds, are a formidable source of natural compounds with diverse biological activities. In the last five decades it has been estimated that more than 3000 natural compounds were discovered from these organisms. The great majority of the published works have focused on terpenoids. In comparison, glycolipids are a neglected class of macroalgal secondary metabolites therefore remaining as a largely unknown reservoir of molecular diversity. Nevertheless, the interest regarding these compounds has been growing fast in the last decades as activities of ecological or pharmaceutical interest have been highlighted. This paper will review recent work regarding isolation and structural characterization of glycolipids from seaweeds and their prospective biological activities.


INTRODUCTION
For the last five decades, both chemists and biologists have carried out an intense effort regarding marine natural products (Bhakuni and Rawat, 2005;Blunt et al., 2014). Such dedication has resulted in the discovery of more than 20,000 compounds from marine microorganisms, invertebrates, and macroalgae (Hu et al., 2011). Marine organisms appear then as a formidable source of natural products. While products of primary metabolism like amino acids, carbohydrates and proteins, are vital for maintaining life processes, others such as alkaloids, phenolics, steroids, terpenoids, are secondary metabolites that have ecological, toxicological, and pharmacological significance (Maschek and Baker, 2008). Bioactivities such as antiherbivory, antifoulant, antifungal, antitumor, antimicrobial, and antiparasitic effects have been highlighted for marine natural products (Noda et al., 1990;Deal et al., 2003;Bhadury and Wright, 2004;Cheung et al., 2014).
With an estimation of more than 30,000 species identified around the world, marine macroalgae, or seaweeds, constitute a huge source of natural compounds with diverse biological activities (Guiry, 2012). In the last 50 years it has been estimated that more than 3000 natural compounds were discovered from these organisms (Leal et al., 2013). The great majority of the published works have focused on terpenoids, phenolic compounds, or polysaccharides. Glycolipids represent a less studied class of metabolites with recently growing interest. Seaweeds biosynthesize three major types of glycolipids: monogalactosyldiacylglycerides (MGDGs), digalactosyldiacylglycerides (DGDGs), and sulfoquinovosyldiacylglycerides (SQDGs) (Figure 1).
These glycoglycerolipids are present in chloroplasts of eukaryotic algae where MGDGs and DGDGs are the most abundant lipids of the thylakoid membrane and appear to play a crucial role in photosynthesis (Hölzl and Dörmann, 2007).
This work will present a concise review of studies from the last 15 years regarding the isolation and structural characterization of bioactive glycolipids from marine macroalgae.

OCHROPHYTA
The MGDG isolated from Petalonia binghamiae was characterized as a potent inhibitor of the activities of mammalian DNA polymerase α . SQDG and DGDG from the Japanese macroalga Sargassum horneri were found to induce apoptosis of the human colon carcinoma Caco-2 cell when associate with sodium butyrate (Hossain et al., 2005).
The acetone fraction of Sargassum thunbergii extract exhibited moderate antifungal effects on Candida albicans. and 1-O-myristoyl-2-O-oleoyl-3-O-(α-glucopyranosyl)-glycerol and showed fibrinolytic activity in the reaction system of single chain urokinase-type plasminogen activator and plasminogen (Wu et al., 2009). The dichloromethane-methanol (7/3) extract of Lobophora variegata from the Yucatan coast (Mexico) demonstrated activity against the protozoa Trichomonas vaginalis, with an IC 50 value of 3.2 μg/ml (Cantillo-Ciau et al., 2010). Further fractionation of that extract was undergone and led to a chloroform fraction that showed activity against the protozoa T. vaginalis, Giardia intestinalis, and Entamoeba histolytica, with good selectivity (SI > 10). Purification of this fraction allowed the isolation of three SQDGs: the major compound 1 Plouguerné et al. (2010) isolated MGDGs in a fraction obtained from Sargassum muticum collected from the coast of Britanny (France), that inhibited the bacteria Shewanella putrefaciens and Polaribacter irgensii and the fungi Halosphaeriopsis mediosetigera, Asteromyces cruciatus, Lulworthia uniseptata, Monodictys pelagica, all involved in marine microfouling. The inhibitory activity was reported for a concentration of 0.75 mg/l.
The crude ethyl acetate extract of Fucus evanescens, collected on the Arctic coast of Ungava Bay, Nunavik (Canada), showed strong antibacterial activity (≥4 log 10 colony-forming units (cfu) against Hemophilus influenza, Legionella pneumophila, Propionibacterium acnes, and Streptococcus pyogenes, when tested at 100 μg/ml. This glycolipid rich extract also inhibited by 3 log 10 cfu the bacteria Clostridium difficile and Staphylococcus aureus (Amiguet et al., 2011). Further purification of the glycolipid rich extract led to the isolation and identification of the main compound as the MGDG 2 , 3 -propyl dilinolenate-β-Dgalactopyranoside.
El Baz et al. (2013) investigated the structures and biological activities of sulfolipids from the Mediterranean macroalgae Dilophus fasciola and Taonia atomaria. The authors highlighted antibacterial and antiviral activities from sulfolipids extracts. The major compounds were identified as SQDG and SQMG (sulfoquinovosylmonoacylglyceride).
SGDGs were identified in fractions obtained after the purification of the organic extract of the Brazilian macroalga Sargassum vulgare. The main SQDG responsible for the anti-HSV1 and anti-HSV2 activities highlighted was characterize as 1, (Plouguerné et al., 2013). The structure of the SQDG was determined using Electrospray Ionization Mass Spectrometry (ESI-MS) (Figure 2A) and the configuration of the anomeric carbon was confirmed by 1 H and 13 C Nuclear Magnetic Resonance (NMR) analysis, based on Heteronuclear Single Quantum Coherence (HSQC) fingerprints ( Figure 2B).  Ohta et al. (1998)  The MGDG lithonoside isolated from the cytotoxic hexanesoluble extract of the Fijian coralline macroalga Hydrolithon reinboldii demonstrated moderate growth inhibitory activity against cancer cell lines with a mean IC 50 value of 19.8 μM (Jiang et al., 2008).
El Baz et al. (2013) investigated the structures and biological activities of sulfolipids from Laurencia papillosa and Galaxaura cylindrica collected from the Red Sea. The authors highlighted antibacterial and antiviral activities from sulfolipids extracts. The major compounds were identified as SQDG and SQMG. Anti-inflammatory activity was highlighted for two SQDGs isolated from Palmaria palmata (Banskota et al., 2014).

CHLOROPHYTA
MGDGs capsofulvesin A and B, along with the MGMG capsofulvesin C, isolated from Capsosiphon fulvescens collected from the southern coastal area of Wando (Korea), demonstrated cholinesterase inhibitory activity (Fang et al., 2012). Islam et al. (2014) revealed for the first time the aldose reductase inhibitory activity of the capsofulvesin A and capsofulvesin B.
Such results highlighted the potential health benefits of C. fulvescens in improving neurotransmission as well as in preventing diabetic complications.
El Baz et al. (2013) investigated the structures and biological activities of sulfolipids from the Mediterranean macroalga Ulva fasciata. The authors highlighted antibacterial and antiviral activities from sulfolipids extracts, and the major compounds were identified as SQDG and SQMG.

CONCLUSION
Among the three phyla of marine macroalgae, Ochrophyta appears as the main source of recently reported bioactive glycolipids, followed by Rhodophyta and Chlorophyta. Within Ochrophyta, the order Fucales was the most reported, followed by Dictyotales.
Among bioactive glycolipids isolated from marine macroalgae, SQDGs, and MGDGs dominated the reports for the last 15 years. Khotimchenko (2003) studied the distribution of glyceroglycolipids in marine macroalgae and highlighted the predominance of SQDG as a characteristic of brown seaweeds from the order Fucales. It would then be logical to expect such order as a major source of bioactive SQDGs. However, according to the recent literature, bioactive SQDGs were more abundant in Dictyotales.
The most reported biological activities for glycolipids from seaweeds were antibacterial, antitumor, and antiviral activities, enhancing the pharmacological potential of these compounds. Less reported, but of significant interest, is the ecological role that may assume glycolipids biosynthetized by seaweeds. Antifouling, and antiherbivory activities were already reported for glycolipids from Sargassum muticum and Fucus vesiculosus, respectively (Deal et al., 2003;Plouguerné et al., 2010).
Concerning the mechanism of action of glycolipids, it remains until today relatively obscure. Recent studies about synthesis and structural modification of glycoglycerolipids helped to understand the structure-activity relationship. On general point of view, the bioactivities of glycoglycerolipids are related to the sugar moiety, the position of the glycerol linkage to the sugar, the length and location of the acyl chain, and the anomeric configuration of the sugar (Zhang et al., 2014). Polyunsaturated fatty acids (PUFAs) are biologically active compounds, which are abundant components of macrophytic glycolipids (Khotimchenko, 1993a(Khotimchenko, ,b, 2003Sanina et al., 2000Sanina et al., , 2004Raposo et al., 2013). Therefore, it has been hypothesize that fatty acids may be responsible for biological activities highlighted from macrophytic glycolipids (Tsai and Pan, 2012). As the distribution of PUFAs in glycolipids from seaweeds depends from their taxonomic position, it may then be logical to expect some taxa to be more active than others (Sanina et al., 2004). Among glycolipids, the degree of saturation of fatty acid increased in the lines of MGDG->DGDG->SQDG. Concerning SQDGs, the antiviral activity may be related to the presence of the sulfonate group (Plouguerné et al., 2013). The sulfate moiety along with the nature of the fatty acids of the SQDGs are also important for the inhibition of DNA polymerase α and β (Mizushina et al., 1996(Mizushina et al., , 1997Hanashima et al., 2000Hanashima et al., , 2001.
Glycoglycerolipids from seaweeds are compounds with both biotechnological potential and ecological interest. Further studies are needed to extend knowledge concerning the mechanism of action of these molecules as well as their distribution between macroalgal species.