Signal-induced site-specific phosphorylation targets I kappa B alpha to the ubiquitin-proteasome pathway

The transcription factor NF-kappa B is sequestered in the cytoplasm by the inhibitor protein I kappa B alpha. Extracellular inducers of NF-kappa B activate signal transduction pathways that result in the phosphorylation and subsequent degradation of I kappa B alpha. At present, the link between phosphorylation of I kappa B alpha and its degradation is not understood. In this report we provide evidence that phosphorylation of serine residues 32 and 36 of I kappa B alpha targets the protein to the ubiquitin-proteasome pathway. I kappa B alpha is ubiquitinated in vivo and in vitro following phosphorylation, and mutations that abolish phosphorylation and degradation of I kappa B alpha in vivo prevent ubiquitination in vitro. Ubiquitinated I kappa B alpha remains associated with NF-kappa B, and the bound I kappa B alpha is degraded by the 26S proteasome. Thus, ubiquitination provides a mechanistic link between phosphorylation and degradation of I kappa B alpha.

Estrogen receptor alpha mediates the nongenomic activation of endothelial nitric oxide synthase by estrogen

Estrogen is an important vasoprotective molecule that causes the rapid dilation of blood vessels by activating endothelial nitric oxide synthase (eNOS) through an unknown mechanism. In studies of intact ovine endothelial cells, 17beta-estradiol (E2) caused acute (five-minute) activation of eNOS that was unaffected by actinomycin D but was fully inhibited by concomitant acute treatment with specific estrogen receptor (ER) antagonists. Overexpression of the known transcription factor ERalpha led to marked enhancement of the acute response to E2, and this was blocked by ER antagonists, was specific to E2, and required the ERalpha hormone-binding domain. In addition, the acute response of eNOS to E2 was reconstituted in COS-7 cells cotransfected with wild-type ERalpha and eNOS, but not by transfection with eNOS alone. Furthermore, the inhibition of tyrosine kinases or mitogen-activated protein (MAP) kinase kinase prevented the activation of eNOS by E2, and E2 caused rapid ER-dependent activation of MAP kinase. These findings demonstrate that the short-term effects of estrogen central to cardiovascular physiology are mediated by ERalpha functioning in a novel, nongenomic manner to activate eNOS via MAP kinase-dependent mechanisms.

neurogenin1 is essential for the determination of neuronal precursors for proximal cranial sensory ganglia

The NEUROGENINS (NGNs) are neural-specific basic helix-loop-helix (bHLH) transcription factors. Mouse embryos lacking ngn1 fail to generate the proximal subset of cranial sensory neurons. ngn1 is required for the activation of a cascade of downstream bHLH factors, including NeuroD, MATH3, and NSCL1. ngn1 is expressed by placodal ectodermal cells and acts prior to neuroblast delamination. Moreover, NGN1 positively regulates the Delta homolog DLL1 and can be negatively regulated by Notch signaling. Thus, ngn1 functions similarly to the proneural genes in Drosophila. However, the initial pattern of ngn1 expression appears to be Notch independent. Taken together with the fact that ectopic ngn1 expression can convert ectodermal cells to neurons in Xenopus (Ma et al., 1996), these data and those of Fode et al. (1998 [this issue of Neuron]) identify ngns as vertebrate neuronal determination genes, analogous to myoD and myf5 in myogenesis.

A family of proteins that inhibit signalling through tyrosine kinase receptors

Phosphotyrosine phosphatases are critical negative or positive regulators in the intracellular signalling pathways that result in growth-factor-specific cell responses such as mitosis, differentiation, migration, survival, transformation or death. The SH2-domain-containing phosphotyrosine phosphatase SHP-2 is a positive signal transducer for several receptor tyrosine kinases (RTKs) and cytokine receptors. To investigate its mechanism of action we purified a tyrosine-phosphorylated glycoprotein which in different cell types associates tightly with SHP-2 and appears to serve as its substrate. Peptide sequencing in conjunction with complementary DNA cloning revealed a new gene family of at least fifteen members designated signal-regulatory proteins (SIRPs). They consist of two subtypes distinguished by the presence or absence of a cytoplasmic SHP-2-binding domain. The transmembrane polypeptide SIRP alpha1 is a substrate of activated RTKs and in its tyrosine-phosphorylated form binds SHP-2 through SH2 interactions and acts as its substrate. It also binds SHP-1 and Grb2 in vitro and has negative regulatory effects on cellular responses induced by growth factors, oncogenes or insulin. Our findings indicate that proteins belonging to the SIRP family generally regulate signals defining different physiological and pathological processes.

Active oxygen species in the induction of plant systemic acquired resistance by salicylic acid

A complementary DNA encoding a salicylic acid (SA)-binding protein has been cloned. Its properties suggest involvement in SA-mediated induction of systemic acquired resistance (SAR) in plants. The sequence of the protein is similar to that of catalases and the protein exhibits catalase activity. Salicylic acid specifically inhibited the catalase activity in vitro and induced an increase in H2O2 concentrations in vivo. H2O2 or compounds, such as SA, that inhibit catalases or enhance the generation of H2O2, induced expression of defense-related genes associated with SAR. Thus, the action of SA in SAR is likely mediated by elevated amounts of H2O2.

Analysis of oncogenic signaling networks in glioblastoma identifies ASPM as a molecular target

Glioblastoma is the most common primary malignant brain tumor of adults and one of the most lethal of all cancers. Patients with this disease have a median survival of 15 months from the time of diagnosis despite surgery, radiation, and chemotherapy. New treatment approaches are needed. Recent works suggest that glioblastoma patients may benefit from molecularly targeted therapies. Here, we address the compelling need for identification of new molecular targets. Leveraging global gene expression data from two independent sets of clinical tumor samples (n = 55 and n = 65), we identify a gene coexpression module in glioblastoma that is also present in breast cancer and significantly overlaps with the "metasignature" for undifferentiated cancer. Studies in an isogenic model system demonstrate that this module is downstream of the mutant epidermal growth factor receptor, EGFRvIII, and that it can be inhibited by the epidermal growth factor receptor tyrosine kinase inhibitor Erlotinib. We identify ASPM (abnormal spindle-like microcephaly associated) as a key gene within this module and demonstrate its overexpression in glioblastoma relative to normal brain (or body tissues). Finally, we show that ASPM inhibition by siRNA-mediated knockdown inhibits tumor cell proliferation and neural stem cell proliferation, supporting ASPM as a potential molecular target in glioblastoma. Our weighted gene coexpression network analysis provides a blueprint for leveraging genomic data to identify key control networks and molecular targets for glioblastoma, and the principle eluted from our work can be applied to other cancers.

Fusion between a novel Krüppel-like zinc finger gene and the retinoic acid receptor-alpha locus due to a variant t(11;17) translocation associated with acute promyelocytic leukaemia

We have identified a unique case of acute promyelocytic leukaemia (APL) with a t(11;17) reciprocal chromosomal translocation involving the retinoic acid receptor alpha (RAR alpha) and a previously uncharacterized zinc finger gene. As a result of this translocation, mRNAs containing the coding sequences of the new gene, fused in-frame either upstream of the RAR alpha B region or downstream from the unique A1 and A2 regions of the two major RAR alpha isoforms, are expressed from the rearranged alleles. The above gene, which we have termed PLZF (for promyelocytic leukaemia zinc finger), encodes a potential transcription factor containing nine zinc finger motifs related to the Drosophila gap gene Krüppel and is expressed as at least two isoforms which differ in the sequences encoding the N-terminal region of the protein. Within the haematopoietic system the PLZF mRNAs were detected in the bone marrow, early myeloid cell lines and peripheral blood mononuclear cells, but not in lymphoid cell lines or tissues. In addition, the PLZF mRNA levels were down-regulated in NB-4 and HL-60 promyelocytic cell lines in response to retinoic acid-induced granulocytic differentiation and were very low in mature granulocytes. Our results demonstrate for the first time the association of a variant chromosomal translocation involving the RAR alpha gene with APL, further implicating the RAR alpha in leukaemogenesis and also suggesting an important role for PLZF as well as retinoic acid and its receptors in myeloid maturation.

Mice deficient in methylenetetrahydrofolate reductase exhibit hyperhomocysteinemia and decreased methylation capacity, with neuropathology and aortic lipid deposition

Hyperhomocysteinemia, a risk factor for cardiovascular disease, is caused by nutritional and/or genetic disruptions in homocysteine metabolism. The most common genetic cause of hyperhomocysteinemia is the 677C-->T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene. This variant, with mild enzymatic deficiency, is associated with an increased risk for neural tube defects and pregnancy complications and with a decreased risk for colon cancer and leukemia. Although many studies have reported that this variant is also a risk factor for vascular disease, this area of investigation is still controversial. Severe MTHFR deficiency results in homocystinuria, an inborn error of metabolism with neurological and vascular complications. To investigate the in vivo pathogenetic mechanisms of MTHFR deficiency, we generated mice with a knockout of MTHFR: Plasma total homocysteine levels in heterozygous and homozygous knockout mice are 1.6- and 10-fold higher than those in wild-type littermates, respectively. Both heterozygous and homozygous knockouts have either significantly decreased S-adenosylmethionine levels or significantly increased S-adenosylhomocysteine levels, or both, with global DNA hypomethylation. The heterozygous knockout mice appear normal, whereas the homozygotes are smaller and show developmental retardation with cerebellar pathology. Abnormal lipid deposition in the proximal portion of the aorta was observed in older heterozygotes and homozygotes, alluding to an atherogenic effect of hyperhomocysteinemia in these mice.

Signal-induced degradation of I kappa B alpha requires site-specific ubiquitination

The inhibitor protein I kappa B alpha controls the nuclear import of the transcription factor NF-kappa B. The inhibitory activity of I kappa B alpha is regulated from the cytoplasmic compartment by signal-induced proteolysis. Previous studies have shown that signal-dependent phosphorylation of serine residues 32 and 36 targets I kappa B alpha to the ubiquitin-proteasome pathway. Here we provide evidence that lysine residues 21 and 22 serve as the primary sites for signal-induced ubiquitination of I kappa B alpha. Conservative Lys-->Arg substitutions at both Lys-21 and Lys-22 produce dominant-negative mutants of I kappa B alpha in vivo. These constitutive inhibitors are appropriately phosphorylated but fail to release NF-kappa B in response to multiple inducers, including viral proteins, cytokines, and agents that mimic antigenic stimulation through the T-cell receptor. Moreover, these Lys-->Arg mutations prevent signal-dependent degradation of I kappa B alpha in vivo and ubiquitin conjugation in vitro. We conclude that site-specific ubiquitination of phosphorylated I kappa B alpha at Lys-21 and/or Lys-22 is an obligatory step in the activation of NF-kappa B.

Platelet glycoprotein ibalpha is a counterreceptor for the leukocyte integrin Mac-1 (CD11b/CD18)

The firm adhesion and transplatelet migration of leukocytes on vascular thrombus are both dependent on the interaction of the leukocyte integrin, Mac-1, and a heretofore unknown platelet counterreceptor. Here, we identify the platelet counterreceptor as glycoprotein (GP) Ibalpha, a component of the GP Ib-IX-V complex, the platelet von Willebrand factor (vWf) receptor. THP-1 monocytic cells and transfected cells that express Mac-1 adhered to GP Ibalpha-coated wells. Inhibition studies with monoclonal antibodies or receptor ligands showed that the interaction involves the Mac-1 I domain (homologous to the vWf A1 domain), and the GP Ibalpha leucine-rich repeat and COOH-terminal flanking regions. The specificity of the interaction was confirmed by the finding that neutrophils from wild-type mice, but not from Mac-1-deficient mice, bound to purified GP Ibalpha and to adherent platelets, the latter adhesion being inhibited by pretreatment of the platelets with mocarhagin, a protease that specifically cleaves GP Ibalpha. Finally, immobilized GP Ibalpha supported the rolling and firm adhesion of THP-1 cells under conditions of flow. These observations provide a molecular target for disrupting leukocyte-platelet complexes that promote vascular inflammation in thrombosis, atherosclerosis, and angioplasty-related restenosis.

All other things being equal: acquisition and transfer of the control of variables strategy

The ability to design unconfounded experiments and make valid inferences from their outcomes is an essential skill in scientific reasoning. The present study addressed an important issue in scientific reasoning and cognitive development: how children acquire a domain-general processing strategy (Control of Variables Strategy or CVS) and generalize it across various contexts. Seven- to 10-year-olds (N = 87) designed and evaluated experiments and made inferences from the experimental outcomes. When provided with explicit training within domains, combined with probe questions, children were able to learn and transfer the basic strategy for designing unconfounded experiments. Providing probes without direct instruction, however, did not improve children's ability to design unconfounded experiments and make valid inferences. Direct instruction on CVS not only improved the use of CVS, but also facilitated conceptual change in the domain because the application of CVS led to unconfounded, informative tests of domain-specific concepts. With age, children increasingly improved their ability to transfer learned strategies to remote situations. A trial-by-trial assessment of children's strategy use also allowed the examination of the source, rate, path, and breadth of strategy change.

Effect of trends on detrended fluctuation analysis

Detrended fluctuation analysis (DFA) is a scaling analysis method used to estimate long-range power-law correlation exponents in noisy signals. Many noisy signals in real systems display trends, so that the scaling results obtained from the DFA method become difficult to analyze. We systematically study the effects of three types of trends--linear, periodic, and power-law trends, and offer examples where these trends are likely to occur in real data. We compare the difference between the scaling results for artificially generated correlated noise and correlated noise with a trend, and study how trends lead to the appearance of crossovers in the scaling behavior. We find that crossovers result from the competition between the scaling of the noise and the "apparent" scaling of the trend. We study how the characteristics of these crossovers depend on (i) the slope of the linear trend; (ii) the amplitude and period of the periodic trend; (iii) the amplitude and power of the power-law trend, and (iv) the length as well as the correlation properties of the noise. Surprisingly, we find that the crossovers in the scaling of noisy signals with trends also follow scaling laws--i.e., long-range power-law dependence of the position of the crossover on the parameters of the trends. We show that the DFA result of noise with a trend can be exactly determined by the superposition of the separate results of the DFA on the noise and on the trend, assuming that the noise and the trend are not correlated. If this superposition rule is not followed, this is an indication that the noise and the superposed trend are not independent, so that removing the trend could lead to changes in the correlation properties of the noise. In addition, we show how to use DFA appropriately to minimize the effects of trends, how to recognize if a crossover indicates indeed a transition from one type to a different type of underlying correlation, or if the crossover is due to a trend without any transition in the dynamical properties of the noise.

The cardiac homeobox gene Csx/Nkx2.5 lies genetically upstream of multiple genes essential for heart development

Csx/Nkx2.5 is a vertebrate homeobox gene with a sequence homology to the Drosophila tinman, which is required for the dorsal mesoderm specification. Recently, heterozygous mutations of this gene were found to cause human congenital heart disease (Schott, J.-J., Benson, D. W., Basson, C. T., Pease, W., Silberbach, G. M., Moak, J. P., Maron, B. J., Seidman, C. E. and Seidman, J. G. (1998) Science 281, 108–111). To investigate the functions of Csx/Nkx2.5 in cardiac and extracardiac development in the vertebrate, we have generated and analyzed mutant mice completely null for Csx/Nkx2.5. Homozygous null embryos showed arrest of cardiac development after looping and poor development of blood vessels. Moreover, there were severe defects in vascular formation and hematopoiesis in the mutant yolk sac. Interestingly, TUNEL staining and PCNA staining showed neither enhanced apoptosis nor reduced cell proliferation in the mutant myocardium. In situ hybridization studies demonstrated that, among 20 candidate genes examined, expression of ANF, BNP, MLC2V, N-myc, MEF2C, HAND1 and Msx2 was disturbed in the mutant heart. Moreover, in the heart of adult chimeric mice generated from Csx/Nkx2.5 null ES cells, there were almost no ES cell-derived cardiac myocytes, while there were substantial contributions of Csx /Nkx2.5-deficient cells in other organs. Whole-mount β-gal staining of chimeric embryos showed that more than 20% contribution of Csx/Nkx2. 5-deficient cells in the heart arrested cardiac development. These results indicate that (1) the complete null mutation of Csx/Nkx2.5 did not abolish initial heart looping, (2) there was no enhanced apoptosis or defective cell cycle entry in Csx/Nkx2.5 null cardiac myocytes, (3) Csx/Nkx2.5 regulates expression of several essential transcription factors in the developing heart, (4) Csx/Nkx2.5 is required for later differentiation of cardiac myocytes, (5) Csx/Nkx2. 5 null cells exert dominant interfering effects on cardiac development, and (6) there were severe defects in yolk sac angiogenesis and hematopoiesis in the Csx/Nkx2.5 null embryos.

Obesity, body size, and risk of postmenopausal breast cancer: the Women's Health Initiative (United States)

OBJECTIVE

Body size is an important modifiable risk factor for breast cancer. Although obesity has generally been found to be associated with increased risk for postmenopausal breast cancer, there remain questions concerning the role of body fat distribution, lifetime weight history, and effects within specific subgroups of women.

METHODS

We assessed the relationship of several anthropometric measures and risk of postmenopausal breast cancer in 85,917 women aged 50-79 at entry in the Women's Health Initiative Observational Study. Women were enrolled during 1993-1998 at 40 clinics in the US and 1030 developed invasive breast cancer by April 2000. Upon entry, trained clinical center staff measured each woman's height, weight, and waist and hip circumference.

RESULTS

Anthropometric factors were not associated with breast cancer among women who had ever used hormone replacement therapy (HRT). Among HRT non-users, heavier women (baseline body mass index (BMI) >31.1) had an elevated risk of postmenopausal breast cancer (relative risk (RR) = 2.52; 95% confidence interval (CI) = 1.62-3.93), compared to slimmer women (baseline BMI < 22.6). The elevation in risk associated with increasing BMI appeared to be most pronounced among younger postmenopausal women. Change in BMI since age 18, maximum BMI, and weight were also associated with breast cancer in HRT non-users. While both waist and hip circumference were associated with breast cancer risk, their ratio, a measure of fat distribution, was not (RR = 1.33; 95% CI = 0.88-2.01).

CONCLUSIONS

Our study confirms previously reported findings that generalized obesity is an important risk factor for postmenopausal breast cancer, but only among women who have never taken HRT. Lifetime weight gain is also a strong predictor of breast cancer. Waist to hip ratio, a measure of weight distribution, does not appear to be related to postmenopausal breast cancer risk.

Serum cholesterol concentration and coronary heart disease in population with low cholesterol concentrations

OBJECTIVE

To examine the relation between serum cholesterol concentration and mortality (from coronary heart disease and from other causes) below the range of cholesterol values generally seen in Western populations.

DESIGN

Prospective observational study based on 8-13 years of follow up of subjects in a population with low cholesterol concentrations.

SETTING

Urban Shanghai, China.

SUBJECTS

9021 Chinese men and women aged 35-64 at baseline.

MAIN OUTCOME MEASURE

Death from coronary heart disease and other causes.

RESULTS

The average serum cholesterol concentration was 4.2 mmol/l at baseline examination, and only 43 (7%) of the deaths that occurred during 8-13 years of follow up were attributed to coronary heart disease. There was a strongly positive, and apparently independent, relation between serum cholesterol concentration and death from coronary heart disease (z = 3.47, p less than 0.001), and within the range of usual serum cholesterol concentration studied (3.8-4.7 mmol/l) there was no evidence of any threshold. After appropriate adjustment for the regression dilution bias, a 4 (SD 1)% difference in usual cholesterol concentration was associated with a 21 (SD 6)% (95% confidence interval 9% to 35%) difference in mortality from coronary heart disease. There was no significant relation between serum cholesterol concentration and death from stroke or all types of cancer. The 79 deaths due to liver cancer or other chronic liver disease were inversely related to cholesterol concentration at baseline.

CONCLUSION

Blood cholesterol concentration was directly related to mortality from coronary heart disease even in those with what was, by Western standards, a "low" cholesterol concentration. There was no good evidence of an adverse effect of cholesterol on other causes of death.

The earliest angiosperms: evidence from mitochondrial, plastid and nuclear genomes

Angiosperms have dominated the Earth's vegetation since the mid-Cretaceous (90 million years ago), providing much of our food, fibre, medicine and timber, yet their origin and early evolution have remained enigmatic for over a century. One part of the enigma lies in the difficulty of identifying the earliest angiosperms; the other involves the uncertainty regarding the sister group of angiosperms among extant and fossil gymnosperms. Here we report a phylogenetic analysis of DNA sequences of five mitochondrial, plastid and nuclear genes (total aligned length 8,733 base pairs), from all basal angiosperm and gymnosperm lineages (105 species, 103 genera and 63 families). Our study demonstrates that Amborella, Nymphaeales and Illiciales-Trimeniaceae-Austrobaileya represent the first stage of angiosperm evolution, with Amborella being sister to all other angiosperms. We also show that Gnetales are related to the conifers and are not sister to the angiosperms, thus refuting the Anthophyte Hypothesis. These results have far-reaching implications for our understanding of diversification, adaptation, genome evolution and development of the angiosperms.

Phase-resolved optical coherence tomography and optical Doppler tomography for imaging blood flow in human skin with fast scanning speed and high velocity sensitivity

We have developed a novel phase-resolved optical coherence tomography (OCT) and optical Doppler tomography (ODT) system that uses phase information derived from a Hilbert transformation to image blood flow in human skin with fast scanning speed and high velocity sensitivity. Using the phase change between sequential scans to construct flow-velocity imaging, this technique decouples spatial resolution and velocity sensitivity in flow images and increases imaging speed by more than 2 orders of magnitude without compromising spatial resolution or velocity sensitivity. The minimum flow velocity that can be detected with an axial-line scanning speed of 400 Hz and an average phase change over eight sequential scans is as low as 10 microm/s, while a spatial resolution of 10 microm is maintained. Using this technique, we present what are to our knowledge the first phase-resolved OCT/ODT images of blood flow in human skin.

Phosphotyrosyl peptides block Stat3-mediated DNA binding activity, gene regulation, and cell transformation

Signal transducers and activators of transcription (STATs) comprise a family of cytoplasmic signaling proteins that participates in normal cellular responses to cytokines and growth factors. Frequently, however, constitutive activation of certain STAT family members, particularly Stat3, has accompanied a wide variety of human malignancies. To identify small molecule inhibitors of Stat3, we investigated the ability of the Stat3 SH2 domain-binding peptide, PY*LKTK (where Y* represents phosphotyrosine), to disrupt Stat3 activity in vitro. The presence of PY*LKTK, but not PYLKTK or PFLKTK, in nuclear extracts results in significant reduction in the levels of DNA binding activities of Stat3, to a lesser extent of Stat1, and with no effect on that of Stat5. Analyses of alanine scanning mutagenesis and deletion derivatives of PY*LKTK reveal that the Leu residue at the Y+1 position and a substituent at the Y-1 position (but not necessarily Pro) are essential for the disruption of active Stat3, thereby mapping the minimum active sequence to the tripeptide, XY*L. Studies involving bead-coupled PY*LKTK peptide demonstrate that this phosphopeptide directly complexes with Stat3 monomers in vitro, suggesting that PY*LKTK disrupts Stat3:Stat3 dimers. As evidence for the functional importance of peptide-directed inhibition of Stat3, PY*LKTK-mts (mts, membrane translocating sequence) selectively inhibits constitutive and ligand-induced Stat3 activation in vivo. Furthermore, PY*LKTK-mts suppresses transformation by the Src oncoprotein, which has been shown previously to require constitutive Stat3 activation. Altogether, we have identified a minimal peptide that inhibits Stat3 signaling and provides the conceptual basis for use of this peptide as a lead for novel peptidomimetic drug design.

The influence of cholesterol on phospholipid membrane curvature and bending elasticity

The behavior of dioleoylphosphatidylethanolamine (DOPE)/cholesterol/tetradecane and dioleoylphosphatidylcholine (DOPC)/cholesterol/tetradecane were examined using x-ray diffraction and the osmotic stress method. DOPE/tetradecane, with or without cholesterol, forms inverted hexagonal (HII) phases in excess water. DOPC/tetradecane forms lamellar phases without cholesterol at lower temperatures. With tetradecane, as little as 5 mol% cholesterol in DOPC induced the formation of HII phases of very large dimension. Increasing levels of cholesterol result in a systematic decrease in the HII lattice dimension for both DOPE and DOPC in excess water. Using osmotic pressure to control hydration, we applied a recent prescription to estimate the intrinsic curvature and bending modulus of the HII monolayers. The radii of the intrinsic curvature, RPO, at a pivotal plane of constant area within the monolayer were determined to be 29.4 A for DOPE/tetradecane at 22 degrees C, decreasing to 27 A at 30 mol% cholesterol. For DOPC/tetradecane at 32 degrees C, RPO decreased from 62.5 A to 40 A as its cholesterol content increased from 30 to 50 mol%. These data yielded an estimate of the intrinsic radius of curvature for pure DOPC of 87.3 A. The bending moduli kc of DOPE/tetradecane and DOPC/tetradecane, each with 30 mol% cholesterol, are 15 and 9 kT, respectively. Tetradecane itself was shown to have little effect on the bending modulus in the cases of DOPE and cholesterol/DOPE. Surprisingly, cholesterol effected only a modest increase in the kc of these monolayers, which is much smaller than estimated from its effect on the area compressibility modulus in bilayers. We discuss possible reasons for this difference.