Metal-ion affinity and specificity in EF-hand proteins: coordination geometry and domain plasticity in parvalbumin

BACKGROUND

The EF-hand family is a large set of Ca(2+)-binding proteins that contain characteristic helix-loop-helix binding motifs that are highly conserved in sequence. Members of this family include parvalbumin and many prominent regulatory proteins such as calmodulin and troponin C. EF-hand proteins are involved in a variety of physiological processes including cell-cycle regulation, second messenger production, muscle contraction, microtubule organization and vision.

RESULTS

We have determined the structures of parvalbumin mutants designed to explore the role of the last coordinating residue of the Ca(2+)-binding loop. An E101D substitution has been made in the parvalbumin EF site. The substitution decreases the Ca(2+)-binding affinity 100-fold and increases the Mg(2+)-binding affinity 10-fold. Both the Ca(2+)- and Mg(2+)-bound structures have been determined, and a structural basis has been proposed for the metal-ion-binding properties.

CONCLUSIONS

The E101D mutation does not affect the Mg(2+) coordination geometry of the binding loop, but it does pull the F helix 1.1 A towards the loop. The E101D-Ca(2+) structure reveals that this mutant cannot obtain the sevenfold coordination preferred by Ca(2+), presumably because of strain limits imposed by tertiary structure. Analysis of these results relative to previously reported structural information supports a model wherein the characteristics of the last coordinating residue and the plasticity of the Ca(2+)-binding loop delimit the allowable geometries for the coordinating sphere.

The role of the NH(2)- and COOH-terminal domains of the inhibitory region of troponin I in the regulation of skeletal muscle contraction

The role of the inhibitory region of troponin (Tn) I in the regulation of skeletal muscle contraction was studied with three deletion mutants of its inhibitory region: 1) complete (TnI-(Delta96-116)), 2) the COOH-terminal domain (TnI-(Delta105-115)), and 3) the NH(2)-terminal domain (TnI-(Delta95-106)). Measurements of Ca(2+)-regulated force and relaxation were performed in skinned skeletal muscle fibers whose endogenous TnI (along with TnT and TnC) was displaced with high concentrations of added troponin T. Reconstitution of the Tn-displaced fibers with a TnI.TnC complex restored the Ca(2+) sensitivity of force; however, the levels of relaxation and force development varied. Relaxation of the fibers (pCa 8) was drastically impaired with two of the inhibitory region deletion mutants, TnI-(Delta96-116).TnC and TnI-(Delta105-115).TnC. The TnI-(Delta95-106).TnC mutant retained approximately 55% relaxation when reconstituted in the Tn-displaced fibers. Activation in skinned skeletal muscle fibers was enhanced with all TnI mutants compared with wild-type TnI. Interestingly, all three mutants of TnI increased the Ca(2+) sensitivity of contraction. None of the TnI deletion mutants, when reconstituted into Tn, could inhibit actin-tropomyosin-activated myosin ATPase in the absence of Ca(2+), and two of them (TnI-(Delta96-116) and TnI-(Delta105-115)) gave significant activation in the absence of Ca(2+). These results suggest that the COOH terminus of the inhibitory region of TnI (residues 105-115) is much more critical for the biological activity of TnI than the NH(2)-terminal region, consisting of residues 95-106. Presumably, the COOH-terminal domain of the inhibitory region of TnI is a part of the Ca(2+)-sensitive molecular switch during muscle contraction.

Hormone replacement therapy and improved survival among postmenopausal women diagnosed with colon cancer (USA)

OBJECTIVES

Hormone replacement therapy (HRT) has been inversely associated with colon cancer incidence in several epidemiologic studies. In this study we used data from a population-based incident case-control study of colon cancer to evaluate the role of HRT use in survival after diagnosis with colon cancer.

METHODS

Data from 815 postmenopausal women living in Utah, California, and Minnesota diagnosed between 1 September 1991 and 30 September 1994 were used to examine associations between HRT and survival.

RESULTS

After adjusting for age at time of diagnosis, stage of disease at time of diagnosis, study center, and body mass index (BMI), we observed that women who had ever used HRT had a 30% lesser probability of dying of any cause and a 40% lower probability of dying from colon cancer specifically during the follow-up period. Further evaluation by years of HRT use showed that those who had used HRT for 4 or more years had the lowest risk of dying of colon cancer (hazard rate ratio 0.5, 95% confidence interval 0.3-0.9). Evaluation of other lifestyle variables with HRT use did not show significant confounding or effect modification.

CONCLUSIONS

These findings suggest that HRT use may improve short-term survival after diagnosis with colon cancer; there is no suggestion that HRT use is detrimental to survival.

The shape of age-incidence curves of female breast cancer by hormone-receptor status

OBJECTIVES

Substantial decline of ovarian hormones at menopause plays an important role in breast cancer etiology. Hormones must bind to specific receptors to elicit biological responses, however. We therefore hypothesized and examined whether the age-specific risk of breast cancer, especially its change at menopause, differs by estrogen and progesterone receptor (ER/PR) status.

METHODS

Age-specific incidence rates, stratified by ER/PR status, were estimated by multiplying the age-specific ER/PR distribution among 3359 cases in the Danish Breast Cancer Cooperative Group by Danish national age-specific incidence rates. International variations in the age-incidence curve were also reviewed in relation to the hypothesis.

RESULTS

The incidence of ER +/PR + subtype (62.9% of all cases) increased with age continually, with a sudden decrease in the rate of increase around age 44. The incidence of ER-/PR- subtype (17.6%) increased with age prior to about age 50 but remained unchanged subsequently. The incidence of ER+ /PR- subtype (13.9%) increased rapidly during the menopausal period but only slightly afterwards. The incidence of ER-/PR+ subtype (5.6%) increased until about age 43 and decreased subsequently. The international comparison revealed Western women, particularly the elderly, might be at substantially higher risk for ER+ /PR+ subtype compared to Japanese women.

CONCLUSION

Age-specific risk of breast cancer differs by ER/PR status. The large international variation of breast cancer incidence rates may be explained largely by the risk difference for ER+ /PR+ subtype.

Optical spectroscopic characterization of single tryptophan mutants of chicken skeletal troponin C: evidence for interdomain interaction

The effects of metal ion binding on the optical spectroscopic properties and temperature stability of two single tryptophan mutants of chicken skeletal TnC, F78W and F154W, have been examined. The absence of tyrosine and other tryptophan residues allowed the unambiguous assignment of the spectral signal from the introduced Trp residue. Changes in the molar ellipticity values in the far-UV CD spectra of the mutant proteins on metal ion binding were similar to those of wild-type TnC suggesting that the introduction of the Trp residue had no effect on the total secondary structure content. The fluorescence and near-UV absorbance data reveal that, in the apo state, Trp-78 is buried while Trp-154 is exposed to solvent. Additionally, the highly resolved (1)L(b) band of Trp-78 seen in the near-UV absorbance and CD spectra of the apo state of F78W suggest that this residue is likely in a rigid molecular environment. In the calcium-saturated state, Trp-154 becomes buried while the solvent accessibility of Trp-78 increases. The fluorescence emission and near-UV CD of Trp-78 in the N-terminal domain were sensitive to calcium binding at the C-terminal domain sites. Measurements of the temperature stability reveal that events occurring in the N-terminal domain affect the stability of the C-terminal domain and vice versa. This, coupled with the titration data, strongly suggests that there are interactions between the N- and C-terminal domains of TnC.

Parental occupational exposure to hydrocarbons and risk of acute lymphocytic leukemia in offspring

Parental exposure to hydrocarbons at work has been suggested to increase the risk of childhood leukemia. Evidence, however, is not entirely consistent. Very few studies have evaluated the potential parental occupational hazards by exposure time windows. The Children's Cancer Group recently completed a large-scale case-control study involving 1842 acute lymphocytic leukemia (ALL) cases and 1986 matched controls. The study examined the association of self-reported occupational exposure to various hydrocarbons among parents with risk of childhood ALL by exposure time window, immunophenotype of ALL, and age at diagnosis. We found that maternal exposure to solvents [odds ratio (OR), 1.8; 95% confidence interval (CI), 1.3-2.5] and paints or thinners (OR, 1.6; 95% CI, 1.2-2.2) during the preconception period (OR, 1.6; 95% CI, 1.1-2.3) and during pregnancy (OR, 1.7; 95% CI, 1.2-2.3) and to plastic materials during the postnatal period (OR, 2.2; 95% CI, 1.0-4.7) were related to an increased risk of childhood ALL. A positive association between ALL and paternal exposure to plastic materials during the preconception period was also found (OR, 1.4; 95% CI, 1.0-1.9). The ALL risk associated with parental exposures to hydrocarbons did not vary greatly with immunophenotype of ALL. These results suggest that the effect of parental occupational exposure to hydrocarbons on offspring may depend on the type of hydrocarbon and the timing of the exposure.

Colorectal adenomas and the C677T MTHFR polymorphism: evidence for gene-environment interaction?

5,10-Methylene-tetrahydrofolate reductase (MTHFR), an enzyme in folate metabolism, may play a role in the etiology of colorectal adenomas via effects on DNA methylation and nucleotide synthesis. We investigated the association between a common polymorphism (C677T, reduced MTHFR activity) and colorectal adenomas within the Minnesota CPRU case-control study. Cases (n = 527) were diagnosed with colonoscopically confirmed adenomas; controls (n = 645) were derived from the same gastroenterology practice and were polyp free at colonoscopy. Dietary intakes were obtained from a self-administered food-frequency questionnaire prior to colonoscopy. Age- and sex-adjusted odds ratios (ORs) and 95% confidence intervals for the MTHFR genotype were 0.9 (0.7-1.2; CT versus CC wild-type) and 0.8 (0.6-1.3; TT versus CC). The associations between dietary intakes of folate, vitamin B12, vitamin B6, or methionine and risk of adenomas showed consistent patterns dependent upon MTHFR genotype. Individuals with the TT genotype and intakes of any of these nutrients in the lowest tertile were at elevated risk for adenomas (about 2-3-fold when compared with TT genotype with high intakes). These trends were more pronounced among individuals over age 60, resulting in a 3-6-fold increase for low intakes of folate, B12, and B6. An increased risk with increasing alcohol consumption was observed only among those with the CC genotype (P-trend = 0.005); among those with the TT genotype, those with moderate alcohol consumption were at lowest risk (P for interaction P = 0.02). In conclusion, nutrients involved in the MTHFR metabolic pathway may modify the relationship between the MTHFR C677T polymorphism and colorectal adenomas. Low intakes of folate, vitamin B12, and vitamin B6 increase risk among those (particularly the elderly) with the MTHFR TT genotype.

Urinary isoflavonoid and lignan excretion on a Western diet: relation to soy, vegetable, and fruit intake

Dietary isoflavone and lignan phytoestrogens are potential chemopreventive agents. This has led to a need to monitor exposure to these compounds in human populations and to determine which components of a mixed diet contribute to the exposure. Typically, urinary isoflavonoid excretion is associated with soy consumption and that of lignans is associated with whole grains. However, other plant foods are known to contain phytoestrogen precursors. The purpose of this study was to examine the association between urinary isoflavonoid and lignan excretion and intakes of vegetables and fruits (V&F). Isoflavonoids (genistein, daidzein, O-desmethylangolensin, and equol) and lignans (enterolactone, enterodiol, and matairesinol) were measured in urine collected for 3 days from 49 male and 49 female volunteers (age, 18-37 years) reporting a wide range of habitual V&F intakes. Dietary intakes were assessed using 5-day diet records and a food frequency questionnaire. V&F groupings (total V&F, total V, total F, soyfoods, and V&F grouped by botanical families) were used to assess the relationship between V&F intake and urinary isoflavonoid and lignan excretion. Pearson correlations were performed. Intake of soyfoods was correlated significantly with urinary genistein (r = 0.40; P = 0.0001), O-desmethylangolensin (r = 0.37; P = 0.0002), daidzein (r = 034; P = 0.0007), and the sum of isoflavonoids (r = 0.39; P = 0.0001). There was no association between equol excretion and soy intake or between the isoflavonoids and any other V&F groupings. In addition, isoflavonoid excretion was correlated positively with intake of high-fat and processed meats, particularly among men who did not consume soy. This suggests that, even in the United States, on a Western diet, soyfoods are the primary contributors to isoflavone intake; however, additional "hidden sources" of soy may also contribute to exposure. In contrast, a variety of fiber-containing foods contributed to lignan excretion; the sum of the urinary lignans, enterodiol, enterolactone, and matairesinol, was associated with intake of total F (r = 0.27; P = 0.008), total V&F (r = 0.25; P = 0.01), soyfoods (r = 0.28; P = 0.006), and dietary fiber (r = 0.36; P = 0.0003). Overall, urinary phytoestrogens (isoflavonoids + lignans) were significantly higher in "high" compared with "low" V&F consumers. Compared with the "low" V&F group, the "high" group consumed diets that were, on average, higher in fiber and carbohydrate and soyfoods and lower in fat; thus, the urinary phytoestrogens may also be a useful marker of healthier dietary patterns.

Alcohol consumption and non-Hodgkin lymphoma in a cohort of older women

We investigated the relation of alcohol consumption to risk of non-Hodgkin's lymphoma (NHL) in a cohort of 35 156 Iowa women aged 55-69 years who participated in the Iowa Women's Health Study in 1986. Alcohol consumption at baseline was obtained using a mailed questionnaire. During the 9-year follow-up period, 143 incident cases of NHL were identified. Higher alcohol consumption was significantly associated with a decreased risk of NHL (P-trend = 0.03). Compared to non-drinkers, multivariate-adjusted relative risks (RRs) were decreased for women with intake of < or = 3.4 g day(-1) (RR = 0.78; 95% confidence interval (CI) 0.51-1.21) and > 3.4 g day(-1) (RR = 0.59; 0.36-0.97). The inverse association could not be attributed to one particular type of alcoholic beverage, although red wine (RR = 0.21 for > 2 glasses per month vs non-drinker; 0.05-0.86; P-trend = 0.02) has the most distinct effect. The apparent protective effect was universal regardless of specific NHL grade or Working Formulation subtype, but was most pronounced for nodal NHL (RR = 0.48; 0.26-0.90; P-trend = 0.01) and low-grade NHL (RR = 0.52; 0.21-1.26; P-trend = 0.05). These data suggest that moderate alcohol consumption is inversely associated with the risk of NHL in older women and the amount of alcohol consumed, rather than the type of alcoholic beverages, appears to be the main effect determinant.

Rotational and translational motion of troponin C

Time resolved fluorescence anisotropy and sedimentation velocity has been used to study the rotational and translational hydrodynamic behavior of two mutants of chicken skeletal troponin C bearing a single tryptophan residue at position 78 or 154 in the metal-free-, metal-bound-, and troponin I peptide (residues 96-116 of troponin I)-ligated states. The fluorescence anisotropy data of both mutants were adequately described by two rotational correlation times, and these are compared with the theoretically expected values based on the rotational diffusion of an idealized dumbbell. These data imply that the motion of the N- and C-terminal domains of troponin C are independent. They also suggest that in the metal-free, calcium-saturated and calcium-saturated troponin I peptide-bound states, troponin C is elongated, having an axial ratio of 4-5. Calcium or magnesium binding to the high affinity sites alone reduces the axial ratio to approximately 3. However, with calcium bound to sites III and IV and in the presence of a 1:1 molar ratio of the troponin I peptide, troponin C is approximately spherical. The metal ion and troponin I peptide-induced length changes in troponin C may play a role in the mechanism by which the regulatory function of troponin C is effected.

Colorectal cancer: molecules and populations

The epidemiology and molecular biology of colorectal cancer are reviewed with a view to understanding their interrelationship. Risk factors for colorectal neoplasia include a positive family history, meat consumption, smoking, and alcohol consumption. Important inverse associations exist with vegetables, nonsteroidal anti-inflammatory drugs (NSAIDs), hormone replacement therapy, and physical activity. There are several molecular pathways to colorectal cancer, especially the APC (adenomatous polyposis coli)-beta-catenin-Tcf (T-cell factor; a transcriptional activator) pathway and the pathway involving abnormalities of DNA mismatch repair. These are important, both in inherited syndromes (familial adenomatous polyposis [FAP] and hereditary nonpolyposis colorectal cancer [HNPCC], respectively) and in sporadic cancers. Other less well defined pathways exist. Expression of key genes in any of these pathways may be lost by inherited or acquired mutation or by hypermethylation. The roles of several of the environmental exposures in the molecular pathways either are established (e.g., inhibition of cyclooxygenase-2 by NSAIDs) or are suggested (e.g., meat and tobacco smoke as sources of specific blood-borne carcinogens; vegetables as a source of folate, antioxidants, and inducers of detoxifying enzymes). The roles of other factors (e.g., physical activity) remain obscure even when the epidemiology is quite consistent. There is also evidence that some metabolic pathways, e.g., those involving folate and heterocyclic amines, may be modified by polymorphisms in relevant genes, e.g., MTHFR (methylenetetrahydrofolate reductase) and NAT1 (N-acetyltransferase 1) and NAT2. There is at least some evidence that the general host metabolic state can provide a milieu that enhances or reduces the likelihood of cancer progression. Understanding the roles of environmental exposures and host susceptibilities in molecular pathways has implications for screening, treatment, surveillance, and prevention.

Characterisation of a mutant of barnacle troponin C lacking Ca2+-binding sites at positions II and IV

This study investigates a mutant barnacle troponin C (TnC) protein (BTnC2-4-) in which the Ca2+-binding sites (sites II and IV) have been rendered non-functional. Eliminating Ca2+ binding at both Ca2+-binding sites of barnacle TnC did not prevent the incorporation of BTnC2-4- into TnC-depleted myofibrillar bundles, although, as expected, the mutant was not able to effect muscle regulation. We conclude that the Mg2+ involved in stabilising the interaction between TnC and TnI in the barnacle must bind at a separate location to the Ca2+-binding sites. Competition experiments between BTnC2-4- and wild-type barnacle TnC (BTnCWT) or the native isoform BTnC2 indicate that BTnC2-4- has an approximately fourfold higher affinity for barnacle TnI than BTnCWT but a lower affinity for TnI compared to BTnC2. These results indicate that disabling sites II and IV changes the affinity of BTnC2-4- for TnC-denuded barnacle myofibrils, altering the stability of the bond formed between TnC and the thin filament.

UDP-glucuronosyltransferase (UGT1A1*28 and UGT1A6*2) polymorphisms in Caucasians and Asians: relationships to serum bilirubin concentrations

Polymorphisms that alter UDP-glucuronosyltransferase (UGT) activities have been identified. Mutations in the promoter of the UGT1A1 gene (UGT1A1*28), resulting in 5, 7 or 8, instead of 6 thymine-adenine (TA) repeats, alter bilirubin conjugation. Two missense mutations on one allele of UGT1A6 (UGT1A6*2) result in T181A and R184S amino acid substitutions and reduced activity against phenolics, such as 4-nitrophenol, 4-hydroxycoumarin and butylated hydroxy anisole. We determined the frequency of these polymorphisms in 245 healthy men and women, aged 20-40 years and examined the relationship between TA repeat number and serum bilirubin concentrations in a subset of 24 Asians and 169 Caucasians. The frequencies of the UGT1A1*28 genotypes were 0.537, 0.348, 0.098, 0.008 and 0.008 for promoter TA repeats 6/6, 6/7, 7/7, 5/6 and 6/8, respectively. Both allele and genotype frequencies varied by race (P < 0.02), with 11% of the Caucasians and none of the Asians having the 7/7 genotype. Within both ethnic groups, serum bilirubin increased with increased numbers of UGT1A1 promoter TA repeats (P = 0.0001). However, a strong ethnic group-by-UGT1A1 genotype interaction suggests that additional ethnic differences in bilirubin metabolism contribute to observed bilirubin concentrations. Genotype frequencies for UGT1A6*2 were 0.478, 0.392, 0.029, 0.090, 0.012 for wild-type (wt)/wt, wt/T181A + R184S, wt/R184S, T181A + R184S/T181A + R184S and T181A + R184S/R184S, respectively. The co-occurrence of polymorphisms in UGT1A1 and UGT1A6 differed from that expected (P < 0.0001): individuals homozygous wild-type for UGT1A1 and UGT1A6 were observed at twice the expected frequency; individuals homozygous variant for both genes were ten-fold more frequent and individuals homozygous wild-type for one gene and homozygous variant for the other were ten-fold less frequent than expected. Overall, 8% were homozygous variant for both UGT1 polymorphisms and 43% had at least one variant allele for both UGT1A1*28 and UGT1A6*2. These highly prevalent polymorphisms, which result in modified expression and activity of UGTs, may influence susceptibility to cancers associated with altered metabolism of endogenous and xenobiotic compounds.

Methylenetetrahydrofolate reductase, diet, and risk of colon cancer

Individuals with different forms of the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene, carriers of the C677T mutation versus wild type, show differences in enzyme levels; these differences have been hypothesized to be related to DNA methylation and, perhaps, to the nucleotide pool size. Using data from an incident case-control study, we evaluated the combined effect of dietary intake of folate, methionine, vitamin B6, vitamin B12, and alcohol and various forms of the MTHFR gene on risk of colon cancer. Individuals homozygous for the variant form of the MTHFR gene (TT) had a slightly lower risk of colon cancer than did individuals who were wild type [CC, odds ratio (OR) = 0.8, 95% confidence interval (CI) = 0.6-1.1 for men; and OR = 0.9, 95% CI = 0.6-1.2 for women]. High levels of intake of folate, vitamin B6, and vitamin B12 were associated with a 30-40% reduction in risk of colon cancer among those with the TT relative to those with low levels of intake who were CC genotype. Associations were stronger for proximal tumors, in which high levels of intake of these nutrients were associated with a halving of risk among those with the TT genotype. The inverse association with high levels of these nutrients in those with the TT genotype was stronger among those diagnosed at an older age. Although imprecise, the inverse association with the low-risk diet that was high in folate and methionine and without alcohol was observed for both the TT genotype (OR = 0.4 95% CI = 0.1-0.9) and the CC/CT genotype (OR = 0.6, 95% CI = 0.4-1.0), but this association was not seen with the high-risk diet for either the TT or CC/CT genotype. Although associations were generally weak, these findings suggest that those with differing MTHFR genotypes may have different susceptibilities to colon cancer, based on dietary consumption of folate, vitamin B6, and vitamin B12.

Intake of fluids and methylxanthine-containing beverages: association with colon cancer

Although diet and its constituents have been studied extensively as risk factors for colon cancer, much less is known about how specific types of fluid influence colon cancer risk. In this study, we explored associations between colon cancer and total fluids, water and methylxanthine-containing beverages such as coffee, tea and cola; data were obtained from 1,993 incident cases of colon cancer and 2,410 population-based controls living in California, Utah and Minnesota. Our primary objective was to determine the influence on associations of amount consumed, confounding and effect modification. We observed few important differences between colon cancer and fluid consumption for all subjects combined. Among men, low levels of coffee intake were associated with an increased risk of colon cancer relative to non-consumers of coffee (OR 1.32, 95% CI 1.02-1.67), while at high levels of consumption, an inverse association was observed (OR 0.81, 95% CI 0.58-1.12). The observed associations were only slightly influenced by consumption of water or other potential confounding factors, but changing the referent group to those consuming one cup of coffee per day or less resulted in a stronger association and a more significant inverse linear trend (OR 0.71, 95% CI 0.53-0.96). The associations with coffee and caffeine- and methylxanthine-containing beverages were strongest for proximal tumors in men. High levels of water intake, however, were protective for distal tumors (OR for men 0.68, 95% CI 0.49-0.96). Assessment of the impact of smoking on associations between colon cancer and coffee showed a significant interaction between smoking and coffee for both men and women.

Intake of fluids and methylxanthine-containing beverages: association with colon cancer

Although diet and its constituents have been studied extensively as risk factors for colon cancer, much less is known about how specific types of fluid influence colon cancer risk. In this study, we explored associations between colon cancer and total fluids, water and methylxanthine-containing beverages such as coffee, tea and cola; data were obtained from 1,993 incident cases of colon cancer and 2,410 population-based controls living in California, Utah and Minnesota. Our primary objective was to determine the influence on associations of amount consumed, confounding and effect modification. We observed few important differences between colon cancer and fluid consumption for all subjects combined. Among men, low levels of coffee intake were associated with an increased risk of colon cancer relative to non-consumers of coffee (OR 1.32, 95% CI 1.02-1.67), while at high levels of consumption, an inverse association was observed (OR 0.81, 95% CI 0.58-1.12). The observed associations were only slightly influenced by consumption of water or other potential confounding factors, but changing the referent group to those consuming one cup of coffee per day or less resulted in a stronger association and a more significant inverse linear trend (OR 0.71, 95% CI 0.53-0.96). The associations with coffee and caffeine- and methylxanthine-containing beverages were strongest for proximal tumors in men. High levels of water intake, however, were protective for distal tumors (OR for men 0.68, 95% CI 0.49-0.96). Assessment of the impact of smoking on associations between colon cancer and coffee showed a significant interaction between smoking and coffee for both men and women.

A novel method of extraction of TnC from skeletal muscle myofibrils

Incubation of mechanically skinned barnacle myofibrillar bundles in 10 mM orthovanadate (pH 6.6) results in the loss of Ca2+-dependent force generation, which reduces to 0.98+/-0.006% (mean +/-SEM, n=25) of control levels. Analysis of myofibrillar bundles by gel electrophoresis showed that tension loss is primarily due to the extraction of troponin C (TnC) (65.4+/-5.04% mean +/-SEM, n=5). This is a novel finding, since treating cardiac fibres with orthovanadate results in the removal of both TnC and troponin I (TnI) (28). Ca2+ dependence was restored to the myofibrillar bundles following reconstitution with either native isoform of barnacle TnC (BTnC1: 78. 72+/-12.8%, n=9, BTnC2: 82.73+/-20.3%, n=3). The reversible loss of Ca2+-dependent tension generation following the removal and replacement of TnC indicates that the regulation of contraction in the barnacle is controlled by thin-filament regulatory proteins.

Parental cigarette smoking and the risk of acute leukemia in children

BACKGROUND

Studies of the relation between parental smoking and childhood leukemia have produced inconsistent results. In the largest case-control studies of childhood acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) conducted to date, the authors evaluated leukemia risk relative to parental self-report of cigarette smoking.

METHODS

In telephone interviews in which a structured questionnaire was used, parents of 1842 ALL patients, 517 AML patients, and their matched controls were asked about their cigarette smoking habits before, during, and after the pregnancy with the index child. Risk of leukemia was examined by histologic type, age of the child at diagnosis, immunophenotype (for ALL), and French-American-British morphology group (for AML).

RESULTS

The risk of ALL was not associated with the father's ever having smoked (odds ratio [OR] = 1.04, 95% confidence interval [CI] 0.90-1.20) or the mother's ever having smoked (OR = 1.04, 95% CI 0.91-1.19). Similarly, no significant risk of AML was observed for paternal (OR = 0.88, 95% CI 0.67-1.16) or maternal smoking (OR = 0.95, 95% CI 0.74-1.22). The relative risk of leukemia was not significantly different from the null for parental smoking in any time period during or around the index pregnancy, nor was it related to the number of cigarettes, the number of years of smoking, or the number of pack-years. A small number of sporadic, statistically significant associations were found, but overall there appeared to be no association between parental cigarette smoking and ALL or AML, or any subgroup of leukemia.

CONCLUSIONS

Parental smoking while pregnant or exposure to cigarette smoke shortly after birth is unlikely to contribute substantially to the risk of childhood leukemia in North America.

A diazo-2 study of relaxation mechanisms in frog and barnacle muscle fibres: effects of pH, MgADP, and inorganic phosphate

The aim of this study was to compare the effects of increased concentrations of MgADP, inorganic phosphate (Pi) and H+ ([MgADP], [Pi] and [H+], respectively) on the rate of relaxation in two different muscle types: skinned muscle fibres from the frog Rana temporaria and myofibrillar bundles from the giant Pacific acorn barnacle Balanus nubilus. Relaxation transients are produced by the photolysis of diazo-2 and are well fitted with a double exponential curve, giving two rate constants: k1 [5.6+/-0.1 s-1 for barnacle, n=30; 26.3+/-0.7 s-1 for frog, n=14 (mean+/-SEM)] and k2 [0.6+/-0.1 s-1 in barnacle, n=30; 10.4+/-1.0 s-1 in frog, n=14 (mean+/-SEM)], at 10 degrees C. Decreasing the pH by 0.5 pH units did not significantly affect k1 for barnacle relaxation [5.6+/-0.1 s-1 (mean+/-SEM), n=15] compared to the decrease in k1 of 40% seen in frog. Use of the Ca2+-sensitive fluorescent label acrylodan on barnacle wild-type troponin C demonstrated that decreasing the pH from 7.0 to 6.6 only alters the pCa50 value by 0.23 in the cuvette, while stopped-flow experiments with acrylodan revealed no significant change in koff from the labelled protein [322+/-32 s-1 at pH 7.0 and 381+/-24 s-1 (mean+/-SEM) at pH 6.6]. Increasing [MgADP] by 20 microM (50 microM added ADP) from control values of 50 microM in frog decreased k1 to 12.3+/-0.4 s-1 (mean+/-SEM, n=8), and at 400 microM MgADP, k1=9.6+/-0.1 s-1 (mean+/-SEM, n=12). In barnacle, 500 microM MgADP had a much smaller effect on k1 (4.0+/-0. 9 s-1, mean+/-SEM, n=8). Increasing the free [Pi] from the contaminant level of 0.36 mM to 1.9 mM slowed k1 by approximately 15% in barnacle [4.8+/-0.8 s-1, mean+/-SEM, n=7], compared to a approximately 30% reduction seen in frog. We conclude that the differences between barnacle and frog seen here are most probably due to different isomers of the contractile proteins, and that events underlying the crossbridge cycle are the same or similar. We interpret our results according to a model of crossbridge transitions during relaxation.

Meat consumption, genetic susceptibility, and colon cancer risk: a United States multicenter case-control study

Meat consumption may especially increase risk of colon cancer when the meat is prepared at high temperatures and consumed by subjects with an inherited susceptibility to well-done meat. In this United States case-control study, the association between meat consumption, genetic susceptibility, and colon cancer risk was studied. Meat consumption data were available from a detailed diet history questionnaire and from questions about methods of preparation. Molecular variants in the carcinogen-metabolizing genes NAT2 and GSTM1 were determined in DNA extracted from WBCs. A total of 1542 cases and 1860 population-based controls were included in these analyses. The amount of red and white meat consumed was not associated with overall colon cancer risk. Processed meat consumption was weakly positively associated with colon cancer risk in men only (odds ratio for highest versus lowest quintile of intake = 1.4, 95% confidence interval = 1.0-1.9). The frequency of fried, broiled, baked, or barbecued meat, use of drippings, and doneness of meat were not significantly associated with risk. The Mutagen Index, as an estimate for exposure to mutagenic or carcinogenic substances, was slightly positively associated with colon cancer risk in men (odds ratio = 1.3, 95% confidence interval = 1.0-1.7). No significant associations with colon cancer risk were observed for different NAT2 and GSTM1 gene variants. The observed associations with processed meat and the Mutagen Index were strongest for those with the intermediate or rapid NAT2 acetylator phenotype. Associations were not markedly influenced by lack of the GSTM1 gene. This study provides little support for an association between meat consumption and colon cancer risk but does provide some, albeit not strong, evidence for a modifying effect of molecular variants of the NAT2 gene.

Cholecystectomy and the risk of colon cancer

OBJECTIVE

The relationship between cholecystectomy and the occurrence of subsequent colon cancer has been controversial. Using data collected as part of an incident case-control study of colon cancer conducted in northern California, Minnesota, and Utah, we evaluated this association.

METHODS

Participants were between 30 and 79 yr of age and had a first primary colon cancer diagnosed between October 1, 1991 and September 30, 1994. Analyses were adjusted for age, gender, family history of colorectal cancer, body mass index, dietary energy and fiber intake, use of aspirin or nonsteroidal antiinflammatory drugs, and long-term leisure-time vigorous physical activity.

RESULTS

A weak positive association between cholecystectomy and proximal colon cancer (odds ratio [OR] and 95% confidence interval [CI] 1.3 [1.0-1.6]) was observed. This was counterbalanced by a weak, nonsignificant negative association (OR 0.8, 95% CI 0.6-1.1) with distal colon cancer leading to no overall association (OR 1.0, 95% CI 0.9-1.2). The association between colon cancer and cholecystectomy did not differ by gender or race, but it did differ by study area, with most of the increased association being attributed to the Minnesota population. The elevated risk of proximal colon cancer increased after cholecystectomy but disappeared after 14 years.

CONCLUSIONS

Our results suggest that cholecystectomy or the underlying gallstone disease that prompts it may be related weakly to the risk of subsequent proximal colon cancer. However, the association may differ by geographic area of the country, and may be artifactual at least in part.

Colorectal adenomatous and hyperplastic polyps: smoking and N-acetyltransferase 2 polymorphisms

Arylamine N-acetyltransferase 2 (NAT2) is involved in both the detoxification and bioactivation of carcinogenic arylamines and other mutagens. This enzyme is polymorphic, and the fast and slow phenotypes are thought to be risk factors for colon and bladder cancer, respectively. Here, we report on a case-control study of adenomatous and hyperplastic polyps, with particular attention to tobacco smoking, a known risk factor for adenomas, and polymorphisms of NAT2. All participants underwent complete colonoscopy and were subsequently divided into case and control groups on the basis of pathology. Cases were diagnosed with confirmed adenomas (n = 527) or hyperplastic polyps (n = 200); controls (n = 633) had no history of colonic neoplasia and no polyps at colonoscopy. NAT2 genotype was determined using an oligonucleotide ligation assay and fast, intermediate, or slow phenotype imputed. Multivariate-adjusted odds ratios (ORs) and 95% confidence intervals were computed using logistic regression adjusting for age, sex, nonsteroidal anti-inflammatory drug use, and hormone replacement therapy use. Smoking was associated with an increased risk of adenomas [current versus never smoking OR = 2.0 (95% confidence interval, 1.4-2.9)] and hyperplastic polyps [current versus never smoking OR = 4.1 (2.6-6.5)]. NAT2 status among adenomatous polyp patients and hyperplastic polyp patients, respectively, showed ORs of 1.1 (0.8-1.4) and 1.2 (0.8-1.6; intermediate versus slow) and 1.1 (0.6-1.9) and 0.9 (0.4-1.9; fast versus slow). There were no differences in risk when adenoma patients were stratified on multiplicity, size, or histopathological subtype of polyps. Never-smokers showed no variation in risk across acetylator status for either species of polyp, whereas current smokers showed ORs of 2.0 (1.2-3.2) and 2.3 (1.4-3.9) for adenomas and 3.9 (2.1-7.1) and 4.9 (2.6-9.4) for hyperplastic polyps for slow and intermediate/fast NAT2, respectively, compared with slow-NAT2 never-smokers. Risks of both multiple [OR = 4.3 (2.1-8.8)] and large [OR = 3.8 (1.9-7.5)] adenomas were somewhat elevated in current smokers with an intermediate/fast phenotype compared with smokers with a slow NAT2 phenotype, but the interaction was not statistically significant. Risk of hyperplastic polyps and adenomatous polyps is strongly related to smoking. There is little suggestion of interaction between NAT2 status and smoking and no relationship with NAT2 genotype alone.

Effects of isoproterenol on twitch contraction of wild type and phospholamban-deficient murine ventricular myocardium

Ablation of the gene for phospholamban (PLB), a transmembrane peptide regulator for the cardiac sarcoplasmic reticulum Ca2+ pump, in mice brings about a complete loss of the myocardial responses to beta-adrenergic agonists (e.g., Luo et al., Circ. Res. 1994; 75: 401). We have evaluated the functional significance of PLB-independent mechanisms in the myocardial responses to beta-adrenergic stimulation in isolated intact ventricular myocardium. We compared the effects of (-)-isoproterenol (ISO) on isometric twitch contraction of paced right ventricular muscle strips of wild type (WT) and PLB-deficient (PLBKO) mice. At 37 degrees C, frequent spontaneous contractions in both types of muscles required the inclusion of lidocaine, an antiarrhythmic, in the bathing medium. Thus the experiments were also performed at two lower temperatures, 30 degrees C and 25 degrees C, at which lidocaine was not needed. Under three conditions, in the absence of ISO, PLBKO ventricular muscles exhibited substantially shortened time to peak tension (TPT) and half relaxation time (TR1/2), compared with the WT muscles. In both WT and PLBKO muscles ISO increased the peak developed tension and decreased TPT and TR1/2 in a dose-dependent manner although the effects were generally smaller in PLBKO than in WT muscles. One micromolar ISO caused TPT and TR1/2 to decrease by 7.3+/-1.2% (mean+/-SEM) and 7.5+/-1.2% in PLBKO vs. 22.8+/-0.7% and 29.1+/-1.7% in WT at 37 degrees C; by 13.5+/-0.4% and 14.1+/-1.2% in PLBKO vs. 31.3+/-0.8%, and 44.8+/-1.3% in WT at 30 degrees C; by 15.0+/-2.3% and 21.1+/-4.9% in PLBKO vs. 25.8+/-1.9% and 54.0+/-1.9% in WT at 25 degrees C. These findings strongly suggest that PLB-independent mechanisms play a significant role in mediating the positive inotropic and lusitropic effects of beta-adrenergic agonists on ventricular myocardium.