Membrane Lipid Content Research Articles

Corpuscular oxidation in newborn crossbred calves naturally infected with Theileria annulata

Erythrocytic lipid peroxidation has been implicated as a cause of anemia in Theileria annulata infection in cattle. The present study aimed to evaluate oxidative damage of membrane lipids and proteins in addition to hemoglobin (Hb) as three criterions of erythrocyte oxidation and their relation to erythrocyte deformability and anemia of newborn crossbred calves (Friesian×Egyptian Balady breed) naturally infected with T. annulata. Twenty-five T. annulata-infected calves (aged 20–30 days) along with 15 age matched healthy controls were used. Percentage of parasitemia varied from 12% to 63% (34.76±3.05%). In comparison to controls, infected calves showed increased levels (P<0.001) of lipid peroxidation (malondialdehyde; MDA, 52%) and protein oxidation (protein carbonyls; PCs, 132%) in erythrocyte membrane as well as increased values of Hb oxidation (methemoglobin; MetHb, 186%), corpuscular osmotic fragility (15.1%) and hemolysis (free Hb; 195.5%). Parasitemia was positively correlated with MDA (r=0.41, P=0.039), PCs (r=0.45, P=0.023) and MetHb (r=0.40, P=0.042). Also, percent of erythrocytic deformability (echinocytosis) was positively correlated with MDA (r=0.49, P=0.013) and PCs (r=0.63, P<0.001). On the other hand, erythrocytic packed cell volume was negatively correlated with MDA (r=−0.44, P=0.028), PCs (r=−0.72, P<0.001) and MetHb (r=−0.42, P=0.037). In conclusion, T. annulata infection is associated with a parasitic burden-dependant oxidative damage to the erythrocyte membrane protein and lipid contents in addition to Hb. This oxidative damage is linked to the morphological changes of the erythrocyte and may act as mechanisms contribute to pathogenesis of anemia in T. annulata infection in newborn calves.

Finding knizocytes in a peripheral blood smear

A 61-year-old female with unremarkable medical history and no evidence of chronic alcohol abuse presented with acute liver failure secondary to massive paracetamol overdose. Laboratory tests indicated: Hb 11.5 g/dL (reference value, RV ≥ 12), WBC 13.8 × 109/L (RV ≤ 10), platelets 45 × 109/L (RV ≥ 150), bilirubin 238 μmol/L (RV ≤ 25), GOT/ASAT 210 UI/L (VR < 30), GPT/ALAT 103 UI/L(RV < 40), alcaline phosphatase 212 UI/L, RV < 120), cholesterol 3.2 mmol/L (RV ≤ 3.9), and triglycerids 6.5 mmol/L (RV ≤1.7). The plasma did not show visible hemolysis. Blood smear (Image 1, May Grünwald Giemsa, ×1000) demonstrated numerous knizocytes in a background of moderate poïkilocytosis. Mosaic image of knizocytes (May-Grünwald-Giemsa 1000×). Knizocytes are triconcave RBCs with a “ridge,” a “bridge” separating the three concavities (in scanning electron micrograph), or a strip of hemoglobin crossing the clear central area (in standard staining). They are very rarely observed in routine practice even if their percentage was estimated once 0.6% ± 0.5 in healthy controls (min-max values: 0–2.5%) [1]. By contrast, they are regularly seen in newborns where they are considered as relatively young RBCs with impaired membrane deformability usually among stomatocytes, spherocytes, and erythrocytes with spicules and protrusions (echinocytes/acanthocytes) [2]. In adults, they are mainly observed in the context of anomalies of the cholesterol metabolism related to any acute liver dysfunction. As the RBCs morphology is influenced by the membrane lipid content and as RBCs exchange continuously lipids with plasma [3], change in circulating lipids levels associated with acute liver disease can alter the RBCs shape, in a delay shorter than their lifetime, as recently reported in this journal [4] or by others [5]. More chronically, knizocytes and/or target cells are observed in patients with familial lecithin/cholesterol acyltransferase deficiency, which induce a decrease in the membrane deformability and as a consequence a higher RBCs fragility [4, 6]. Knizocytes are also frequently observed in chronic liver diseases as chronic hepatitis or cirrhosis (alcoholic or postviral causes) where they can account for up to 15% of the red cells [7]. Alcohol abuse induce a variety of changes in the RBC morphology, including typically round macrocytes, stomatocytes but also knizocytes related to the presence of acetaldehyde-derived epitopes both on the cell membrane and inside the RBCs [8]. Although nonspecific, observation of knizocytes on a blood smear should prompt an assessment of liver function and lipid parameters.

Increase in unsaturated fatty acids in membrane lipids of Suaeda salsa L. enhances protection of photosystem II under high salinity

In order to examine the possible role of unsaturated fatty acids in photosynthesis of halophytes under high salinity, the effect of salinity on plant growth, chlorophyll (Chl) content, photochemical efficiency of PSII, membrane lipid content and fatty acids composition of a C3 euhalophyte Suaeda salsa L. was investigated. Salt stress induced a slight increase of the maximal photochemical efficiency of PSII (Fv/Fm), actual PSII efficiency (&Phi;PSII), Chl a content and Chl a/b ratio. The unsaturated fatty acid content also increased under salt stress. The proportion of MGDG, DGDG, SQDG, and PC decreased, while the proportion of PG increased from 10.9% to 26.9% under salt stress. These results suggest that S. salsa displays high resistance to photoinhibition under salt stress and that increased concentration of unsaturated fatty acids in membrane lipids of S. salsa enhances the tolerance of photosystem II to salt stress.

The role of exogenous lipids in lycopene synthesis in the mucoraceous fungus Blakeslea trispora

The addition of plant oils to the growth medium stimulated growth and lipid synthesis in the fungus Blakeslea trispora. However, only oils with high content of linoleic and especially linolenic acid enhanced lycopene formation. The increase in lycopene formation was accompanied by accumulation in the neutral lipid fraction of the fatty acids prevailing in plant oils. In contrast, the influence of exogenous lipids on the fatty acid composition of bulk fungal phospholipids was insignificant. Nonetheless, a marked increase in the content of membrane lipids and of their phosphatidylethanolamine content was revealed. Presumably, the main mechanism of stimulation of lycopene formation by the oils involves an increase in the solubility of lycopene in the triacylglycerols of the lipid bodies, which is due to an increase in the desaturation degree of their fatty acids. The predominance of linoleic and especially of linolenic fatty acid in plant oils is regarded as a criterion for selecting the oil species for the purpose of intensifying lycopene synthesis.

Efficient Light Harvesting by Photosystem II Requires an Optimized Protein Packing Density in Grana Thylakoids

A recently developed technique for dilution of the naturally high protein packing density in isolated grana membranes was applied to study the dependence of the light harvesting efficiency of photosystem (PS) II on macromolecular crowding. Slight dilution of the protein packing from 80% area fraction to the value found in intact grana thylakoids (70%) leads to an improved functionality of PSII (increased antenna size, enhanced connectivity between reaction centers). Further dilution induces a functional disconnection of light-harvesting complex (LHC) II from PSII. It is concluded that efficient light harvesting by PSII requires an optimal protein packing density in grana membranes that is close to 70%. We hypothesize that the decreased efficiency in overcrowded isolated grana thylakoids is caused by excited state quenching in LHCII, which has previously been correlated with neoxanthin distortion. Resonance Raman spectroscopy confirms this increase in neoxanthin distortion in overcrowded grana as compared with intact thylakoids. Furthermore, analysis of the changes in the antenna size in highly diluted membranes indicates a lipid-induced dissociation of up to two trimeric LHCII from PSII, leaving one trimer connected. This observation supports a hierarchy of LHCII-binding sites on PSII.

Changes in intact membrane lipid content of archaeal cells as an indication of metabolic status

Archaea build their cellular membrane with glycerol dialkyl glycerol tetraether (GDGT) core lipids, bound to polar head groups to form intact GDGTs. These lipids are commonly used as indicators of archaeal biomass in the environment. Here, we measured both intact and core GDGTs and archaeal 16S rRNA gene copies (a proxy for cell abundance) in order to determine cellular GDGT content in a coastal fjord at different locations, seasons and depths. We found that the abundance of both intact and core GDGTs relative to 16S rRNA gene copy numbers varied by two orders of magnitude, were generally higher in surface than deeper water and were elevated during periods associated with seasonal phytoplankton blooms. Core and intact GDGTs followed similar trends, but the ratio of core GDGT/16S rRNA gene copy number reached a maximum in October, following a peak in the ratio of intact GDGT/16S rRNA gene copy number that occurred in August. Our data suggest that spatial and temporal variability in these ratios must be considered when attempting to quantify cell numbers in the environment using either GDGTs or 16S rRNA genes. We used optimized extraction methods for both GDGTs and DNA. However, there is still potential for bias in qPCR methodology that could influence the GDGT/16S rRNA gene copy ratio. The apparent variability in cellular membrane lipid concentration could serve to evaluate the status of archaeal populations as it relates to: (i) varying cell size resulting from changes in either community structure or nutritional status, (ii) population turnover times and (iii) changing environmental factors that affect the lifetime of intact GDGTs after cell death.

Lipids, Proteins, and Their Interplay in the Dynamics of Temperature-Stressed Membranes of a Cyanobacterium, Synechocystis PCC 6803

Proper responses to low- and high-temperature stresses are essential for the survival of many organisms. It has been established that at low-temperature stress the sufficient microviscosity of the lipids is decisive in this respect. In many organisms, adapting the level of lipid unsaturation to the low growth temperature regulates this feature. At high-temperature stresses, however, there are no unequivocal results concerning the role of the lipids. In these temperature ranges, the lipids are all disordered and fluid and their physical parameters change slowly with increasing temperatures, while biological organisms give characteristic stress responses in rather narrow temperature ranges. Therefore, one may speculate that other membrane parameters/components, which change sharply in the range of the high-temperature stress, may give a signal to initiate the general response of the cells. For such a role, proteins are the trivial candidates. To reveal the role of the lipids and the proteins in these processes, we used a genetically engineered system, based on a cyanobacterium, Synechocystis PCC 6803. In the wild-type cells of this bacterium, by altering the growth temperature, the polyunsaturated lipid content of the cell membranes can be varied considerably (as required by the homeoviscous adaptation principle). In the case of desA(-)/desD(-) mutant cells, which can contain only monounsaturated fatty acyl chains in their lipids, homeoviscous adaptation of the lipids is not possible. Since desA(-)/desD(-) mutation affects only the lipids, additional perturbations (e.g., altered protein content) should minimally disturb the comparison of the lipid behaviors in wild-type and mutant cells. Infrared spectra of thylakoid and cytoplasmic membranes isolated from wild-type and mutant cells were recorded in 3 degrees C steps between 8 and 92 degrees C. By analyzing the rates of protein structural changes, hydrogen-deuterium exchange, in-membrane lipid disorder, and water-membrane interfacial order/hydration as functions of the temperature, we conclude that (i) the gel-to-liquid crystalline phase transition of the lipids correlates with the growth temperature in the wild-type cells but not in the desA(-)/desD(-) mutants, (ii) over the physiological temperature range, both protein and lipid dynamics are regulating/regulated, providing remarkably constant dynamics for both the thylakoid and cytoplasmic membrane, (iii) in the high-temperature stress region, protein structure and dynamics are changing sharply without any correlation with growth temperature and/or mutation, i.e., membrane protein stability does not seem to depend on the lipid composition of the membrane (this finding points to the possible primacy of proteins as initiators/targets of heat-shock alarms), and (iv) there are substantial differences between the dynamics of the proteins of the thylakoid and cytoplasmic membranes, reflecting their different protein complexes and lipid-to-protein ratios.

Bax distribution into mitochondrial detergent‐resistant microdomains is related to ceramide and cholesterol content in postischemic hearts

*Diverse changes have been described in mitochondria of apoptotic cells: the phospholipid content is modified, ceramide and GD3 concentrations increase, the cristae structure is modified, and nonresident proteins are recruited into the mitochondrial membranes. In particular, Bax, a Bcl-2 family member protein, moves from the cytosol to the mitochondria, inducing cytochrome c release. Modifications of the content and distribution of specific lipids in the mitochondrial membranes, along with the well-known participation of the mitochondrial permeability transition pore in triggering apoptosis, led us to propose that lipid microdomains in mitochondria could coexist as structural elements with some of the mitochondrial permeability transition pore-forming proteins and with members of the Bcl-2 family. In this work, we demonstrated that Bax was associated preferentially with mitochondrial detergent-resistant membranes (mDRMs) in reperfused rat hearts, a well-known apoptotic model. Bax insertion into mDRMs correlated with cytochrome c release from such mitochondria. Bax location in mDRMs was associated with both the voltage-dependent anion channel and the adenine nucleotide translocator, two mitochondrial permeability transition pore-forming proteins. Interestingly, the voltage-dependent anion channel was more abundant in the mDRM fraction than in the Triton X-100-soluble fraction. Ceramide and cholesterol contents were higher in mDRMs from reperfused hearts. Our results suggest that membrane microenvironments enriched in cholesterol and ceramide in mitochondria favor Bax translocation to this organelle, fostering propagation of the apoptotic cascade.

Zinc prevents indomethacin-induced renal damage in rats by ameliorating oxidative stress and mitochondrial dysfunction

The clinical utility of non-steroidal anti-inflammatory drugs (NSAIDs) is limited by their gastrointestinal and renal toxicities. Indomethacin (an NSAID commonly used in toxicity studies) has been shown to induce significant oxidative stress and mitochondrial dysfunction in the rat kidney. The current study was designed to assess the potential of zinc, a known antioxidant, to protect the kidney against these drug-induced effects. Male Wistar rats were pre-treated with zinc sulphate (50 mg/kg) and dosed with indomethacin (20 mg/kg) by oral gavage. Rats were sacrificed 24 h after the dose of indomethacin. Parameters of oxidative stress, mitochondrial function and lipid content of the mitochondrial membranes were measured in the kidneys of these animals. It was found that zinc significantly attenuated indomethacin-induced oxidative stress, mitochondrial dysfunction and changes in the lipids in mitochondrial membranes in the kidney. The content of metallothionein, a cysteine-rich zinc-binding protein, was also determined in the tissue. There was no significant induction of metallothionein in the kidney in zinc-treated animals. Estimation of serum creatinine showed that zinc seemed to hasten functional recovery of the kidney following indomethacin administration. We conclude that pretreatment with zinc is effective in protecting against indomethacin-induced changes in the rat kidney. This protective effect does not appear to be mediated by metallothionein.

The essential role of lipids in Alzheimer's disease

In the absence of efficient diagnostic and therapeutic tools, Alzheimer's disease (AD) is a major public health concern due to longer life expectancy in the Western countries. Although the precise cause of AD is still unknown, soluble β-amyloid (Aβ) oligomers are considered the proximate effectors of the synaptic injury and neuronal death occurring in the early stages of AD. Aβ oligomers may directly interact with the synaptic membrane, leading to impairment of synaptic functions and subsequent signalling pathways triggering neurodegeneration. Therefore, membrane structure and lipid status should be considered determinant factors in Aβ-oligomer-induced synaptic and cell injuries, and therefore AD progression. Numerous epidemiological studies have highlighted close relationships between AD incidence and dietary patterns. Among the nutritional factors involved, lipids significantly influence AD pathogenesis. It is likely that maintenance of adequate membrane lipid content could prevent the production of Aβ peptide as well as its deleterious effects upon its interaction with synaptic membrane, thereby protecting neurons from Aβ-induced neurodegeneration. As major constituents of neuronal lipids, n-3 polyunsaturated fatty acids are of particular interest in the prevention of AD valuable diet ingredients whose neuroprotective properties could be essential for designing preventive nutrition-based strategies. In this review, we discuss the functional relevance of neuronal membrane features with respect to susceptibility to Aβ oligomers and AD pathogenesis, as well as the prospective capacities of lipids to prevent or to delay the disease.

Amifostine, a Radioprotectant Agent, Protects Rat Hepatic Microsomal Membranes Against Ionizing Radiation Induced Damage

In the present study, the protective effect of amifostine (WR-2721), which is the only approved radioprotective agent by the Food and Drug Administration (FDA), was investigated against the deleterious effects of ionizing radiation on rat liver microsomal membranes at molecular level. To achieve this, Sprague-Dawley rats, which were administered amifostine or not, were whole-body irradiated using Cobalt-60 irradiator at a single dose of 800 cGy, decapitated after 24 h and the microsomal membranes isolated from the livers of these rats were analyzed using FTIR spectroscopy. The results revealed that ionizing radiation caused a significant increase in the concentration of lipids whereas amifostine treatment restored the lipid content of microsomal membranes to control values. In addition, the significant increase in lipid order and a significant decrease in membrane dynamics resulting from ionizing radiation were prevented by amifostine. While ionizing radiation caused a significant decrease in the lipid to protein ratio, amifostine injection before radiation, maintained this ratio as in the control group. Furthermore, ionizing radiation-induced variations in protein secondary structure were restored by amifostine. In conclusion, the data obtained in this study indicate that amifostine administration to the rats prior to whole body irradiation protects liver microsomal membranes against the radiation induced damages. Supported by TUBITAK, (SBAG-2939) and by the METU (BAP-2006-07-02-0001).

Phosphatidylethanolamine Synthesis Is Required for Optimal Virulence of Brucella abortus

The Brucella cell envelope contains the zwitterionic phospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Synthesis of PC occurs exclusively via the PC synthase pathway, implying that the pathogen depends on the choline synthesized by the host cell to form PC. Notably, PC is necessary to sustain a chronic infection process, which suggests that the membrane lipid content is relevant for Brucella virulence. In this study we investigated the first step of PE biosynthesis in B. abortus, which is catalyzed by phosphatidylserine synthase (PssA). Disruption of pssA abrogated the synthesis of PE without affecting the growth in rich complex medium. In minimal medium, however, the mutant required choline supplementation for growth, suggesting that at least PE or PC is necessary for Brucella viability. The absence of PE altered cell surface properties, but most importantly, it impaired several virulence traits of B. abortus, such as intracellular survival in both macrophages and HeLa cells, the maturation of the replicative Brucella-containing vacuole, and mouse colonization. These results suggest that membrane phospholipid composition is critical for the interaction of B. abortus with the host cell.

A defect of sphingolipid metabolism modifies the properties of normal appearing white matter in multiple sclerosis

Maintaining the appropriate complement and content of lipids in cellular membranes is critical for normal neural function. Accumulating evidence suggests that even subtle perturbations in the lipid content of neurons and myelin can disrupt their function and may contribute to myelin and axonal degradation. In this study, we determined the composition and quantified the content of lipids and sterols in normal appearing white matter (NAWM) and normal appearing grey matter (NAGM) from control and multiple sclerosis brain tissues by electrospray ionization tandem mass spectrometry. Our results suggest that in active-multiple sclerosis, there is a shift in the lipid composition of NAWM and NAGM to a higher phospholipid and lower sphingolipid content. We found that this disturbance in lipid composition was reduced in NAGM but not in NAWM of inactive-multiple sclerosis. The pattern of disturbance in lipid composition suggests a metabolic defect that causes sphingolipids to be shuttled to phospholipid production. Modelling the biophysical consequence of this change in lipid composition of NAWM indicated an increase in the repulsive force between opposing bilayers that could explain decompaction and disruption of myelin structure.

Atrazine‐induced alterations in rat erythrocyte membranes: Ameliorating effect of vitamin E

Erythrocytes are a convenient model to understand oxidative damage to the membranes induced by various xenobiotics. The objective of the present study was to investigate the propensity of atrazine to induce oxidative stress and its possible attenuation by vitamin E. Experimental animals were orally administered atrazine (300 mg kg(-1) body weight, daily) and vitamin E (100 mg kg(-1) body weight, daily) for a period of 7, 14, and 21 days. Erythrocyte membranes were prepared and analyzed for acetylcholinesterase (AChE) activity, lipid peroxidation (LPO), and lipid composition. Susceptibility of erythrocytes to atrazine exposure was further investigated in terms of morphological alterations by scanning electron microscopy (SEM). Results indicate that atrazine exposure caused a significant inhibition of AChE activity and induction of oxidative stress in terms of increased malondialdehyde (MDA) levels. Atrazine treatment significantly decreased total lipid, cholesterol, and phospholipid content of erythrocyte membranes. SEM revealed varying degrees of distortion depending on duration of atrazine exposure. However, administration of vitamin E ameliorated the oxidative stress and changes in the erythrocyte membranes induced by atrazine.

Effects of chronic proline administration on lipid contents of rat brain

In the present work we investigated the effects of chronic proline administration on ganglioside, cholesterol and phospholipid total contents, as well as on ganglioside profile in cerebral cortex, hippocampus, hypothalamus and cerebellum of rats. We also evaluated the ganglioside content and profile in detergent-soluble and resistant microdomains isolated from synaptic membranes obtained from cerebral cortex. Proline solution (hyperprolinemic) or saline (control) were subcutaneously administered to rats from 6th to 28th post-natal day, according to body weight. Twelve hours after the last injection, the animals were sacrificed by decapitation without anaesthesia. Brain structures were homogenized with chloroform:methanol for lipid extraction. Synaptic membranes were obtained by differential centrifugation and detergent-soluble and resistant microdomains were isolated by cold Triton X-100 treatment. Results showed that rats subjected to chronic proline treatment presented a significant increase of ganglioside content in cortex and hippocampus, while this membrane lipid content was not altered in hypothalamus and cerebellum. Besides, phospholipid and cholesterol contents were not modified in all structures studied. On the other hand, ganglioside content decreased in detergent-soluble and resistant microdomains isolated from synaptic membrane obtained from hyperprolinemic cortex. Although ganglioside profiles were apparently not modified, the individual absolute quantities were altered in cortex and hippocampus total lipid extract and membrane microdomains. Our findings suggest that chronic proline treatment affects in a distinct manner different cerebral regions concerning the lipid composition of the cell membranes, reflecting on its distribution in the cortex membrane microdomains. Among these phenomena consequences, distinct modulations in synaptic transmission may be suggested which might contribute to the impairment in cognition and/or other neurological dysfunctions found in hyperprolinemia type II patients.

Dependence of erythrocyte hemolysis under gramicidin S action on their membrane lipid content

Dependence of erythrocyte hemolysis under gramicidin S action on their membrane lipid content

Oxidative stress alters membrane sulfhydryl status, lipid and phospholipid contents of crossbred cattle bull spermatozoa

Ferrous ascorbate (FeAA: FeSO 4 + ascorbic acid) has been used in the past by different investigators to induce oxidative stress. The optimum dose of FeAA for inducing oxidative stress by affecting thiols [total thiols (TSH), glutathione reduced (GSH), glutathione oxidized (GSSG), redox ratio (GSH/GSSG)], total lipids and phospholipids has been ascertained in the local crossbred cattle bull spermatozoa. The fractions of spermatozoa suspended in 2.9% sodium citrate were subjected to three doses of FeAA (100 μM:500 μM, 150 μM:750 μM, 200 μM:1000 μM; FeSO 4:ascorbic acid), and were assessed for various parameters. On increasing the concentration of FeAA, a gradual decrease in TSH, GSH, GSH/GSSG, lipid and phospholipid levels, but increase in GSSG content were observed. It is concluded that thiol groups play an important role in antioxidation and detoxification of ROS as well as maintaining intracellular redox status. Thiol groups, thus, serve as defense mechanisms of sperm cells to fight against oxidative stress. In addition, all doses of FeAA cause leakage of lipids and phospholipids from the bull sperm membranes.

Functional expression and microdomain localization of prestin in cultured cells

Prestin is an essential component of the molecular motor of cochlear outer hair cells that contribute to frequency selectivity and sensitivity of mammalian hearing. A model system to study prestin employs its transfection into cultured HEK 293 cells. Our goal was to characterize prestin's trafficking pathway and localization in the plasma membrane. We used immuno-colocalization of prestin with intracellular and plasma membrane markers and sucrose density fractionation to analyze prestin in membrane compartments. Voltage clamping was used to measure nonlinear capacitance (NLC), prestin's electrical signature. Prestin targets to the membrane by 24 hours post-transfection when NLC is measurable. Prestin then concentrates into membrane foci that colocalize and fractionate with membrane microdomains. Depleting membrane cholesterol content altered prestin localization and NLC. Prestin activity in HEK 293 cells results from expression in the plasma membrane and altering membrane lipid content affects prestin localization and activity.

The proteome of the heterocyst cell wall in Anabaena sp. PCC 7120

Anabaena sp. PCC 7120 is a filamentous cyanobacterium that serves as a model to analyze prokaryotic cell differentiation, evolutionary development of plastids, and the regulation of nitrogen fixation. The cell wall is the cellular structure in contact with the surrounding medium. To understand the dynamics of the cell wall proteome during cell differentiation, the cell wall from Anabaena heterocysts was enriched and analyzed. In line with the recently proposed continuity of the outer membrane along the Anabaena filament, most of the proteins identified in the heterocyst cell-wall fraction are also present in the cell wall of vegetative cells, even though the lipid content of both membranes is different.

Transit of hormonal and EGF receptor-dependent signals through cholesterol-rich membranes

The functional consequences of changes in membrane lipid composition that coincide with malignant growth are poorly understood. Sufficient data have been acquired from studies of lipid binding proteins, post-translational modifications of signaling proteins, and biochemical inhibition of lipidogenic pathways to indicate that growth and survival pathways might be substantially re-directed by alterations in the lipid content of membranes. Cholesterol and glycosphingolipids segregate into membrane patches that exhibit a liquid-ordered state in comparison to membrane domains containing relatively lower amounts of these classes of lipids. These “lipid raft” structures, which may vary in size and stability in different cell types, both accumulate and exclude signaling proteins and have been implicated in signal transduction through a number of cancer-relevant pathways. In prostate cancer cells, signaling from epidermal growth factor receptor (EGFR) to the serine-threonine kinase Akt1, as well as from IL-6 to STAT3, have been demonstrated to be influenced by experimental interventions that target cholesterol homeostasis. The recent finding that classical steroid hormone receptors also reside in these microdomains, and thus may function within these structures in a signaling capacity independent of their role as nuclear factors, suggests a novel means of cross-talk between receptor tyrosine kinase-derived and steroidogenic signals. Potential points of intersection between components of the EGFR family of receptor tyrosine kinases and androgen receptor signaling pathways, which may be sensitive to disruptions in cholesterol metabolism, are discussed. Understanding the manner in which these pathways converge within cholesterol-rich membranes may present new avenues for therapeutic intervention in hormone-dependent cancers.