Cholesterol is an amphipathic lipid and as such is an essential structural component of all cell membranes and of the outer layer of plasma lipoproteins. It is present in tissues and in plasma lipoprotein either as free cholesterol or, combined with a long-chain fatty acid, as cholesteryl ester. It is synthesized in many tissues from acetyl-CoA and is ultimately eliminated from the body in the bile as cholesterol or bile salts 11. Lipoprotein transports free cholesterol in the circulation, where it readily equilibrates cholesterol in other lipoproteins and in membranes. Cholesteryl ester is a stored form of cholesterol found in most tissues. It is transported as cargo in the hydrophobic core of lipoproteins 12. So, the fact that cholesterol in a hydrophobic molecule, which resides in lipoproteins and cell membranes, raises two questions: (i) how does the cell sense the level of cholesterol? (ii) how in this cholesterol specific signaling transmitted to the nucleus for the regulation of various genes?

Recent studies directed to resolve these questions, led to the discovery of a novel cell surface cholesterol-sensor designated as receptor-Ck which was not only shown to be ubiquitously present in various human organs but also [through its signaling pathway] regulated various genes involved in cholesterol homeostasis [HMG CoA synthase; HMG CoA reductase; apo 'B'- specific LDL – receptor] cell growth [cyclin 'D'; C – fos; C-myc; p27 etc]; cell death [Bc1-2] through a 47 KDa transcription factor [derived from the cleavage of 125 KDa SREBP] having affinity for genomic sterol regulatory element [SRE] sequence as well as through other transcription factors 1314151617.

Entamoeba histolytica parasitizes the gastro-intestinal tract of humans and is a major cause of morbidity and mortality in tropical and subtropical countries 18. Although several successive life cycle stages of E. histolytica have been documented, briefly, it can be classified into two main morphologic forms, i.e. trophozoites and cysts. In E. histolytica, excystation occur in the small intestine, where four amoebae are released from the mature quadri nucleate cyst. Trophozoites dwell in the colon, where they multiply and encyst typically producing four nucleated cyst 19. Cholesterol has been reported to be a growth – promoting factor of E. histolytica 20. The virulence of a virulent strain could be revived in vitro by adding cholesterol to medium or in vivo by feeding cholesterol to experimental animals or the host 212223. Cholesterol is thought to act as an irritant to mucus membrane and thereby helps the amoebae to establish and colonize at the injured site and this enhances the virulence 24. In-vitro study has shown that substitution of lipid/cholesterol instead of serum, supported vigorous growth of E. histolytica in axenic culture 25.

Recently, Laughlin et al 26 have shown that the disruption of cholesterol rich raft-like membrane domains in Entamoeba with the cholesterol-binding agent filipin and methyl-β-cyclodextrin inhibit several important virulence functions, fluid phase pinocytosis and adhesion to host cell monolayers. However, disruption of raft-like domains did not inhibit constitutive secretion of cysteine proteases, another important virulent function of Entamoeba. Cysteine proteinases in Entamoeba are likely to be associated with tissue invasion and pathogenesis 27. Andra et al 28, revealed that the lipid composition of amoebic membranes prevents binding of the cytolytic molecules and that both the phospholipid ingredients and the high content of cholesterol contributes to the protection of the toxin-producing cell.

More recently the authors have assessed the impact of lipid parameter in patients infected with E. histolytica and E. dispar. The result showed significant lower levels of lipid profile [total cholesterol, HDL & LDL] in E. histolytica and E. dispar cyst passers and ALA patients as compared to healthy controls. Also, cyst passers had lower levels of cholesterol than ALA cases 29.

Giardiasis is the most common waterborne disease in human, which is caused by an enteric flagellated protozoan Giardia lamblia. Giardia is widespread with children being the most vulnerable 30. Giardia exists in two morphologic forms: trophozoite and cysts. In G. lamblia, excystation is accomplished in two steps first by limiting the acidic conditions present in the stomach and secondly by the protease-rich and slightly alkaline small intestine. Encystation, in turn, can be induced in vitro by starving the trophozoites of cholesterol, either by using lipoprotein-deficient serum or augmenting the bile concentration in the culture medium 3132. Membrane biogenesis in Giardia requires cholesterol 3334. Because Giardia is unable to synthesize cholesterol 33 it obtains the same from upper small intestine, which is rich in biliary and dietary cholesterol 3536. Effects of bile salts on encystations are directly related to the uptake of cholesterol by the trophozoites 37.

In vitro, it was observed that replacing bovine serum with a lipoprotein cholesterol (LPC) solution and bovine serum albumin in pre-encystation and excystation media, stimulated G.1amblia encystations and encystation specific secretory vesicles (ESV) formation 38. It is established that cholesterol-dependent down-regulation of encystations-specific CWP-1 gene expression has contributed to the inhibition of Giardia encystation process 3137. Previously, we have reported that receptor Ck dependent signaling is responsible for the regulation of Giardia encystations process by cholesterol 39. Recently we have observed that patients infected with G. lamblia showed lower levels of lipid parameters (total cholesterol, HDLc and LDLc) as compared to control healthy group 29.

Trichomonas vaginalis is a sexually transmitted protozoan parasite, adheres to the vaginal epithelium, causing vaginitis and other complications in women 40. It is an anaerobic protozoan flagellate, which lacks mitochondria and peroxisomes, but has a specialized double-membrane-bounded organelle called the hydrogenosome, which is involved in metabolic processes that extend glycolysis 41. In vitro study has shown that when serum was replaced by bovine serum albumin and cholesterol, it resulted in good growth 42.

Plasmodium falciparum is the most pathogenic species causing human malaria. Erythrocytes become infected following attachment and invasion by merozoite. Various phases of parasite development can be observed during the 48 h – erythrocytic cycle i.e. rings, trophozoites and schizonts. In vitro the parasite grows in 5–10% human serum in an atmosphere of low oxygen 43. Very little is known about mechanism involved in lipid changes related to malaria. Hypercholesterolemia and hypertriglyceridemia was observed in both uncomplicated and complicated malaria 444546, whereas Kittl et al 47, have shown no correlation between severity of malaria attacks and extent of HDL – cholesterol decrease. Human serum HDL is necessary for P. falciparum in in vitro culture. However, it has been reported that HDL can be toxic for the parasite at high concentrations 48.

Recently, Imrie et al 49, have also reported that in the absence of serum, HDL in low concentration (0.75 mg/ml) supported growth of P. falciparum in vitro, whereas at high concentration (3 mg/ml), it was toxic to the parasite. Recent findings, however, would suggest that the plasmodium genome contains genes encoding enzymes of phospholipids metabolism, allowing de novo synthesis of phosphatidyl choline via the knneddy pathway and necessitating only the uptake of the small choline molecule 50. In addition, the genome of P. falciparum contains genes similar to those for type II fatty acid synthesis pathway. The protein products of these genes are located within the apicoplast and allow for the production of fatty acids, some of which are unique to the parasite 50. Thus the parasite may be able to meet many of its lipid requirements from its own biosynthetic pathways, although some extracellular lipids are necessary for in vitro growth. It has also been seen that plasma membrane cholesterol plays a role in the pathogenesis of immune evasion and clinical manifestations of falciparum malaria 51.

There are few studies which suggest that there are changes in lipid plasma or serum levels in-vivo after infection. But, no significant changes were seen in the plasma cholesterol during and after infection of malaria 52. However, low levels of the cholesterol in patients infected with malaria as compared to normal healthy controls have been reported 53. In another study changes in plasma lipoprotein was seen in acute malaria resulting decreased levels of HDL and LDL and moderately increased triglycerides 54. In malaria endemic areas, when plasma levels of cholesterol, triglcerides, HDLc and LDLc were analysed in children infected with P. falciparum, investigators have found significantly low levels of lipid profile 55. Brotons et al 56 reviewed that population studies on common lipid parameters and observed that cholesterol values are lower in Africa, where, malaria is endemic, than in many other parts of the world.

Toxoplasma gondii, an apicomplexan protozoan parasite, is an important pathogen of humans and animals. It is widely distributed with a very high prevalence in many regions, and can cause serious infections in immuno-compromised patients (particularly AIDS patients) and in the developing fetus 57. Host cell cholesterol is implicated in the entry and replication of an increasing number of intracellular microbial pathogens. However, recently new mechanism has been described by which host cholesterol specifically controls entry of an intracellular pathogen. Briefly, the parasitophorus vacuole membrane (PVM) surrounding T. gondii contains cholesterol. At the time of cell entry host plasma membrane cholesterol is incorporated into the forming PVM during invasion, through a caveolae independent mechanism. Depleting host cell plasma membrane cholesterol blocks parasite internalization by reducing the release of rhoptry proteins that are necessary for invasion 58.

In another study, Coppens et al 9 had shown that T. gondii exploits host low density lipoprotein receptor-mediated endocytosis for cholesterol acquisition. Whereas, acyl-CoA: cholesterol acyl transferase (ACAT) and cholesterol esters play a crucial role in the optimal replication of T. gondii 59. These studies indicate the cholesterol does have a role in the pathogenesis of Toxoplasmosis.

Heterogeneous distribution of membrane cholesterol at the attachment site of Cryptosporidium muris to host cells has already been investigated. Although many filipin-cholesterol complexes were observed on the plasma membrane of host cells and parasites, a line showing no complexes was evident at the above two membrane junctures. These observations indicate that parasitic infection of C. muris altered the organization of membrane cholesterol 60. The exact role of cholesterol in pathogenesis/virulence of parasite needs to be determined.

Leishmania is an obligate intracellular parasite that infects macrophages of the vertebrate host, resulting in visceral, cutaneous and mucocutaneous leishmaniasis in humans. Recently Pucadyil 61 reported that plasma membrane cholesterol is required for efficient attachment and internalization of the parasite in macrophages, leading to Leishmania donovani infection. Rodrigues et al 62 have shown that when amastigotes and promastigotes forms of L. amzonensis are incubated with 22,26-azasterol, which is a delta (24(25))-sterol methyltransferase (SMT) inhibitor, it results in arrest of growth. They also observed that alteration occur in the lipid composition of the parasite membrane, resulting in loss of viability. The study suggests the use of 24-SMT inhibitor could be used as selective antileishmanial agent. Dietz et al 63 have reported successful treatment of Brazilian kala-azar using lipid encapsulated amphotericin B. Thus, a level of cholesterol in the patients infected with leishmaniasis has to be determined.

African trypanosomes are lipid auxotrophs that live in the bloodstream of their human and animal hosts and are unable to synthesize cholesterol but appear to bind and take up plasma low-density lipoproteins (LDL) from their host 64. Whether cholesterol homeostasis of this unicellular parasite also requires interactions with host high-density lipoprotein (HDL) particles is unknown. Trypanosomes require lipoproteins to multiply under axenic culture conditions 65. Recently, Green et al 66 reported that HDL, LDL, and trypanosome lytic factor (TLF1) were bound and taken up by a lipoprotein scavenger receptor, which may constitute the parasite's major pathway mediating the uptake of essential lipids. Frequent turnover of variable surface glycoproteins is a well established method of immune evasion by Trypanosomes. Does it involve the role of HDL, LDL, and TLF1 in getting eliminated by attaching to the surface receptors of Trypanosomes?

It has been shown that Schistosome infection could be counteracting the effects of an atherogenic diet by modulating host lipid metabolism and inducing a reduction in blood total cholesterol concentration 67. Little information is available on the lipid changes caused by S. mansoni reinfection. Popiel et al 68, studied the metabolism of cholesterol uptake by paired and unpaired worms of S. mansoni during pairing, the results showed labeled worms lost upto 65% of their cholesterol. This suggests that normal cholesterol transfer in worm pairs is bi-directional and that it is facilitated by physical contact between juxtaposed membranes. Cholesterol exchange in schistosome worm pairs may be partly or wholly consequences of normal tegumental turnover of the molecule.

In another study Silveira et al 69 investigated the transfer of cholesterol and its metabolites between adult male and female worms of S. mansoni. They found that the adult male and female worms of S. mansoni are able to incorporate cholesterol and convert it into several metabolites. On the other hand, Schistosomula cannot convert the incorporated cholesterol. However, a significant reduction in levels of serum lipid profile was observed in mice infected with S. mansoni 70. These changes might be attributed to several metabolites released by S. mansoni, which affect the host hepatic tissue resulting in decreased synthesis of these parameters and their release into the circulation. The ability of Schistosomula to convert cholesterol into its metabolites, shows that it is a property acquired, used by adults only and is specifically active during pairing. What is the exact role of cholesterol during pairing needs to be investigated. What happens to cholesterol levels in serum of patients also needs to be studied.

In-vivo study has shown the decreased serum lipid levels in the Shipibo population (Peru), showed a significant inverse correlation between worm egg excretion and HDL levels in hookworm, Strongyloides and Trichuris infected patients but not in Ascaris infected cases 71. The mechanisms underlying the observed association between intestinal worm load and HDL reduction are not completely understood and may include reduced HDL synthesis in the gut wall due to inflammatory toxic irritation. Hence, cholesterol may have a role in pathogenesis by helping the larvae to survive in the host tissue.

During larval ascariasis the metabolism of lipids is significantly disturbed. Decreased levels of total cholesterol, HDL cholesterol and triglycerides were observed in guinea pigs. The changes are due to the break in liver function and, presumably, changes in hormone secretion, which are provoked by the presence of the parasite 72. It has also been seen that cholesterol enhanced larval survival and the yield and growth of L4 of A. summ larvae when added to RPMI-1640 culture medium 73.

In vitro A. duodenalis grown axenically in medium supplemented with cholesterol gives good growth 74. Phospholipid/cholesterol ratio in liver plasma membrane of the infected golden hamsters with A. ceylanicum group was significantly reduced, which suggest that both the structural and functional organization of membrane may be a biochemical basis of the hepatotoxic effects 75.

Decreased levels of lipid contents were observed in liver of Mastomys natalensis during the development of B. malayi infection 76. The lipid peroxide formation was enhanced in liver during the development of filarial infection. The studies in human beings are lacking.

Balb/c mice infected with Hymenolepis microstoma significantly affected the lipid metabolism 77. The changes were in relation to nutritional interactions between host and parasite and the possible effect on host hormone levels. Johnson et al 78 studied on the mechanisms of cholesterol uptake by the rat tapeworm H. diminuta. The results support the hypothesis that the tapeworm absorbs cholesterol by a specific carrier-mediated process. Cholesterol uptake is reduced when the capacity of the micellar phase of the medium is increased, suggesting that uptake involves the intermediate partitioning of sterol from micelles into the aqueous phase of the medium 79.