Biomonitoring Equivalents for interpretation of urinary fluoride.

Exposure to fluoride is widespread due to its natural occurrence in the environment and addition to drinking water and dental products for the prevention of dental caries. The potential health risks of excess fluoride exposure include aesthetically unacceptable dental fluorosis (tooth mottling) and increased skeletal fragility. Numerous organizations have conducted risk assessments and set guidance values to represent maximum recommended exposure levels as well as recommended adequate intake levels based on potential public health benefits of fluoride exposure. Biomonitoring Equivalents (BEs) are estimates of the average biomarker concentrations corresponding to such exposure guidance values. The literature on daily urinary fluoride excretion rates as a function of daily fluoride exposure was reviewed and BE values corresponding to the available US and Canadian exposure guidance values were derived for fluoride in urine. The derived BE values range from 1.1 to 2.1mg/L (1.2-2.5µg/g creatinine). Concentrations of fluoride in single urinary spot samples from individuals, even under exposure conditions consistent with the exposure guidance values, may vary from the predicted average concentrations by several-fold due to within- and across-individual variation in urinary flow and creatinine excretion rates and due to the rapid elimination kinetics of fluoride. Thus, the BE values are most appropriately applied to screen population central tendency estimates for biomarker concentrations rather than interpretation of individual spot sample concentrations.

Differential sensitivity to apoptosis between the human small and large intestinal mucosae: linkage with segment-specific regulation of BCL-2 homologs and involvement of signaling pathways.

The small and large intestines differ in their expression profiles of Bcl-2 homologs. Intestinal segment-specific Bcl-2 homolog expression profiles are acquired as early as by mid-gestation (18-20 weeks) in man. In the present study, we examined the question whether such distinctions underlie segment-specific control mechanisms of intestinal cell survival. Using mid-gestation human jejunum and colon organotypic cultures, we analyzed the impact of growth factors (namely insulin; 10 microg/ml) and pharmacological compounds that inhibit signal transduction molecules/pathways (namely tyrosine kinases, Fak, P13-K/Akt, and MEK/Erk) on cell survival and Bcl-2 homolog expression (anti-apoptotic: Bcl-2, Bcl-X(L), Mcl-1; pro-apoptotic: Bax, Bak, Bad). The relative activation levels of p125Fak, p42Erk-2, and p57Akt were analyzed as well. Herein, we report that (1) the inhibition of signal transduction molecules/pathways revealed striking differences in their impact on cell survival in the jejunum and colon (e.g., the inhibition of p125Fak induced apoptosis with a significantly greater extent in the jejunum [approximately 43%] than in the colon [approximately 24%]); (2) sharp distinctions between the two segments were noted in the modulatory effects of the various treatments on Bcl-2 homolog steady-state levels (e.g., inhibition of tyrosine kinase activities in the jejunum down-regulated all anti-apoptotics analyzed while increasing Bax, whereas the same treatment in the colon down-regulated Bcl-X(L) only and increased all pro-apoptotics); and (3) in addition to their differential impact on cell survival and Bcl-2 homolog expression, the MEK/Erk and P13-K/Akt pathways were found to be distinctively regulated in the jejunum and colon mucosae (e.g., insulin in the jejunum increased p42Erk-2 activation without affecting that of p57Akt, whereas the same treatment in the colon decreased p42Erk-2 activation while increasing that of p57Akt). Altogether, these data show that intestinal cell survival is characterized by segment-specific susceptibilities to apoptosis, which are in turn linked with segmental distinctions in the involvement of signaling pathways and the regulation of Bcl-2 homolog steady-state levels. Therefore, these indicate that cell survival is subject to segment-specific control mechanisms along the proximal-distal axis of the intestine.

Early acquisition of bowel segment-specific Bcl-2 homolog expression profiles during development of the human ileum and colon.

The adult small and large intestines display distinct expression profiles of Bcl-2 homologs, known regulators of apoptosis. This is thought to indicate that control mechanisms of intestinal apoptosis are gut segment-specific. Little is known on the expression of Bcl-2 homologs during gut development. In man, intestinal features and functions are acquired largely by mid-gestation (18-20 wks); the question whether segment-specific controls of intestinal apoptosis are also acquired early during development remains open. In the present study, we approached this by investigating the expression of six Bcl-2 homologs (Bcl-2, Bcl-XL, Mcl-1, Bax, Bak, Bad), and one nonhomologous associated molecule (Bag-1), during development of the human ileum and colon (12-20 wks of gestation). Beginning at 18 wks, we found that the epithelial localization of Bcl-2 homologs displayed differential patterns (or gradients) in both the ileum and colon; however, the patterns of some of the homologs differed between the two segments. For instance, Bag-1 and Bcl-2 exhibited crypt-villus decreasing gradients of expression in the ileum but not in the colon, whereas Mcl-1 displayed differing compartimentalizations between the two segments. Further analyses indicated that the steady-state expression levels of Bcl-2 homologs underwent modulations between 12 and 20 wks; however, the observed developmental profiles contrasted significantly between the two segments. For example, Bcl-2, Bag-1 and Bak levels increased in the colon, but the levels of these same homologs decreased in the ileum. Furthermore, by 18-20 wks, we found that the expression levels of each Bcl-2 homolog analyzed differed greatly between the ileum and colon. Altogether, these data indicate that the expression of Bcl-2 homologs is modulated differentially during human gut development in order to establish, by mid-gestation, distinct expression profiles for the small and large intestines. This in turn suggests that gut segment-specific control mechanisms of human intestinal apoptosis are acquired early during fetal life.

Human intestinal epithelial cell survival: differentiation state-specific control mechanisms.

To investigate whether human intestinal epithelial cell survival involves distinct control mechanisms depending on the state of differentiation, we analyzed the in vitro effects of insulin, pharmacological inhibitors of Fak, MEK/Erk, and PI3-K/Akt, and integrin (beta1, beta4)-blocking antibodies on the survival of the well-established human Caco-2 enterocyte-like and HIEC-6 cryptlike cell models. In addition, relative expression levels of six Bcl-2 homologs (Bcl-2, Bcl-X(L), Mcl-1, Bax, Bak, and Bad) and activation levels of Fak, Erk-2, and Akt were analyzed. Herein, we report that 1) the enterocytic differentiation process results in the establishment of distinct profiles of Bcl-2 homolog expression levels, as well as p125(Fak), p42(Erk-2), and p57(Akt) activated levels; 2) the inhibition of Fak, of the MEK/Erk pathway, or of PI3-K, have distinct impacts on enterocytic cell survival in undifferentiated (subconfluent Caco-2, confluent HIEC-6) and differentiated (30 days postconfluent Caco-2) cells; 3) exposure to insulin and the inhibition of Fak, MEK, and PI3-K resulted in differentiation state-distinct modulations in the expression of each Bcl-2 homolog analyzed; and 4) Fak, beta1 and beta4 integrins, as well as the MEK/Erk and PI3-K/Akt pathways, are distinctively involved in cell survival depending on the state of cell differentiation. Taken together, these data indicate that human intestinal epithelial cell survival is regulated according to differentiation state-specific control mechanisms.

Role of p27(Kip1) in human intestinal cell differentiation.

BACKGROUND & AIMS: Growth arrest and differentiation are generally considered to be temporally and functionally linked phenomena in the intestinal epithelium. METHODS: To delineate the mechanism(s) responsible for the loss of proliferative potential as committed intestinal cells start to differentiate, we have analyzed the regulation of G(1)-phase regulatory proteins in relation to differentiation in the intact epithelium as well as in well-established intestinal cell models that allow the recapitulation of the crypt-villus axis in vitro. RESULTS: With intestinal cell differentiation, we have observed an induction of the cell cycle inhibitors p21(Cip), p27(Kip1), and p57(Kip2) expression with an increased association of p27(Kip1) and p57(Kip2) with cyclin-dependent kinase 2 (Cdk2). At the same time, there was an accumulation of the hypophosphorylated form of the pRb proteins and a strong decline in Cdk2 activity. Stable expression of a p27(Kip1) antisense complementary DNA in Caco-2/15 cells did not prevent growth arrest induced by confluence, but repressed villin, sucrase-isomaltase, and alkaline phosphatase expression. CONCLUSIONS: Our results indicate that the growth arrest that precedes differentiation involves the activation of Rb proteins and the inhibition of Cdk2. Furthermore, intestinal cell differentiation apparently requires a function of p27(Kip1) other than that which leads to inhibition of Cdks.

Early establishment of epithelial apoptosis in the developing human small intestine.

In the adult small intestine, the dynamic renewal of the epithelium is characterized by a sequence of cell production in the crypts, cell maturation and cell migration to the tip of villi, where apoptosis is undertaken. Little is known about enterocytic apoptosis during development. In man, intestinal architectural features and functions are acquired largely by mid-gestation (18-20 wks); the question whether the establishment of enterocytic apoptotic processes parallels or not the acquisition of other intestinal functional features remains open. In the present study, we approached this question by examining enterocytic apoptosis during development of the human jejunum (9-20 wks gestation), using the ISEL (in situ terminal uridine deoxynucleotidyl nick-end labelling) method. Between 9 and 17 wks, apoptotic enterocytes were not evidenced. However, beginning at the 18 wks stage, ISEL-positive enterocytes were regularly observed at the tip of villi. Since the Bcl-2 family of proteins constitutes a critical checkpoint in apoptosis, acting upstream of the apoptotic machinery, we investigated the expression of six Bcl-2 homologs (Bcl-2, Bcl-X(L), Mcl-1, Bax, Bak, Bad) and one non-homologous associated molecule (Bag-1). By immunofluorescence, we found that all homologs analyzed were expressed by enterocytes between 9 and 20 wks. However, Bcl-2 homologs underwent a gradual compartmentalization of epithelial expression along the maturing crypt-villus axis, to establish gradients of expression by 18-20 wks. Western blot analyses indicated that the expression levels of Bcl-2 homologs were modulated during morphogenesis of the crypt-villus axis, in parallel to their gradual compartmentalization of expression. Altogether, these data suggest that regulatory mechanisms of human enterocytic apoptosis become established by mid-gestation (18-20 wks) and coincide with the maturation of the crypt-villus axis of cell proliferation, differentiation and renewal.

Vertical Flux of Biogenic Carbon in the Ocean: Is There Food Web Control?

Models of biogenic carbon (BC) flux assume that short herbivorous food chains lead to high export, whereas complex microbial or omnivorous food webs lead to recycling and low export, and that export of BC from the euphotic zone equals new production (NP). In the Gulf of St. Lawrence, particulate organic carbon fluxes were similar during the spring phytoplankton bloom, when herbivory dominated, and during nonbloom conditions, when microbial and omnivorous food webs dominated. In contrast, NP was 1.2 to 161 times greater during the bloom than after it. Thus, neither food web structure nor NP can predict the magnitude or patterns of BC export, particularly on time scales over which the ocean is in nonequilibrium conditions.

Arctic air pollution and human health: what effects should be expected?

Persistent contaminants such as heavy metals and organochlorine compounds are transported from distant sources to the Arctic by oceanic and atmospheric currents. Natives inhabiting the Arctic can be exposed, because they exist at the highest trophic level of the arctic aquatic food chain, along which biomagnification of contaminants occurs. We reviewed the data available on heavy metal and organochlorine body burden in natives from different regions of Nunavik (northern Québec) and assessed the potential risk of health effects. In addition, we investigated the relationship between each contaminant plasma level and omega-3 fatty acid content of plasma phospholipid, a surrogate measure for aquatic food consumption. Cadmium exposure appears to be unrelated to the consumption of species from the aquatic food chain (r = 0.0004; P = 0.99), whereas PCBs and mercury were (r = 0.49 and 0.52, respectively; P < or = 0.0001). Mean blood mercury levels measured in northern Québec natives were below those associated with significant neurological disorders. Typical daily intakes of dioxin-like compounds, PCBs, DDE, and dieldrin were estimated from the mean concentration in milk fat and pharmacokinetic models. The calculated PCB intake (0.3 microgram/kg/day) exceeds the acceptable daily intake, with effects on reproduction and development being the most relevant to assess in future epidemiological studies.

On the presence of intermediate cells in the small intestine.

In the small intestine, the presence of transitional cells or cells intermediate between Paneth cells and goblet cells has been reported frequently for 100 years. Light microscopy and, more recently, fine structural studies have indicated that secretory granules observed in intermediate cells share some morphologic characteristics with those of granular goblet cells and of Paneth cells. In order to verify if intermediate cells in the jejunum and ileum of the adult mouse have functional similarities with either granular goblet or Paneth cells, we have studied the incorporation of sulfur-35 by radioautography and the localization of lysozyme by immunocytochemistry. After radioautography, goblet cells and, to a lesser extent, granular goblet cells had incorporated sulfur-35, whereas Paneth cells and intermediate cells were completely negative. Immunolocalization of lysozyme was done by using rabbit anti-rat lysozyme and protein A-peroxidase. After demonstration of peroxidase activity, only Paneth cells were stained and intermediate cells were negative. Therefore, intermediate cells do not contain sulfomucin or lysozyme, and they are functionally different from goblet and Paneth cells. Their function remains unknown.