Activators of peroxisome proliferator-activated receptor-alpha partially inhibit mouse skin tumor promotion.

Several recent reports have suggested that peroxisome proliferator-activated receptors (PPARs) may be involved in the development of neoplasias in different tissue types. The present study was undertaken to determine whether PPARs play a role in skin physiology and tumorigenesis. In an initiation-promotion study, SENCAR mice treated topically with the PPARalpha ligands conjugated linoleic acid and 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (Wy-14643) exhibited an approximately 30% lower skin tumor yield compared with mice treated with vehicle. The PPARgamma and PPARdelta activators troglitazone and bezafibrate, respectively, exerted little, if any, inhibitory activity. PPARalpha was detected in normal and hyperplastic skin and in papillomas and carcinomas by immunohistochemistry. In addition, PPARalpha, PPARdelta/PPARbeta, and PPARgamma protein levels were analyzed by immunoblotting in normal epidermis and papillomas. Surprisingly, the levels of all three isoforms were increased significantly in tumors as opposed to normal epidermis. In primary keratinocyte cultures, protein levels of PPARalpha and, to a lesser extent, PPARgamma were markedly increased when the cells were induced to differentiate with high-calcium (0.12 mM) conditions. In addition, we observed that Wy-14643 enhanced transcriptional activity of a peroxisome proliferator-response element-driven promoter in a mouse keratinocyte cell line. These results demonstrate that keratinocytes express functional PPARalpha, that PPARalpha may play a role in differentiation, and that ligands for PPARalpha are moderately protective against skin tumor promotion. We conclude that selective PPARalpha ligands may exert their protective role against skin tumor promotion by ligand activation of PPARalpha.

Characterization of 25-hydroxyvitamin D binding protein from intestinal cells.

We have previously purified a cytosolic vitamin D metabolite binding protein (cDBP) from rat enterocytes, which has characteristics distinct from other vitamin D binding proteins. In these studies, we demonstrate that cDBP in a semi-purified fraction from human intestinal cells (Caco-2 cells) binds 25-hydroxyvitamin D (25OHD) with at least a 1000-fold greater affinity than 1, 25-dihydroxyvitamin D (1,25(OH)(2)D) or 24,25-dihydroxyvitamin D. Treatment of cells with 1,25(OH)(2)D reduced 25OHD binding to approximately one third that of the untreated cells (0.42 CPM/mg total protein vs 1.34 CPM/mg total protein, respectively). Finally, the cDBP is not immunoreactive to antibodies prepared against the C-terminus of the nuclear vitamin D receptor (VDR). In summary, cDBP bound 25OHD with greater affinity than either 1,25(OH)(2)D or 24,25 dihydroxyvitamin D, the cytosolic binding activity was down-regulated by 1,25(OH)(2)D and cBDP is distinct from the nuclear VDR.

Inositol hexaphosphate reduces 12-O-tetradecanoylphorbol-13-acetate-induced ornithine decarboxylase independent of protein kinase C isoform expression in keratinocytes.

High-fiber diets have been shown to have beneficial effects on preventing tumorigenesis. Inositol hexaphosphate (InsP6 or phytic acid) which is a fiber-associated component of cereals and legumes has been demonstrated to inhibit cell proliferation and enhance cell differentiation, indicating its potential for chemopreventive roles. In this study, we investigated the effect of InsP6 on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced ornithine decarboxylase (ODC) activity, an essential event in tumor promotion in HEL-30 cells, a murine keratinocyte cell line and SENCAR mouse skin. ODC activity was significantly reduced by 0.5 mM InsP6 in keratinocytes (P < 0.01). Furthermore, when mouse skin was treated with 10 mM InsP6, ODC induction was significantly inhibited (P < 0.05). In addition, the expression of TPA-induced c-myc mRNA was significantly inhibited by the same InsP6 treatments in HEL-30 cells and CD-1 mouse skin (P < 0.01). No changes in protein kinase C (PKC) isoform expression and phorbol dibutyrate binding due to InsP6 treatment were found in HEL-30 cells. These results indicate that InsP6 reduces TPA-induced ODC activity independent of PKC isoform expression.

Dietary conjugated linoleic acid normalizes impaired glucose tolerance in the Zucker diabetic fatty fa/fa rat.

Conjugated linoleic acid (CLA) is a naturally occurring fatty acid which has anti-carcinogenic and anti-atherogenic properties. CLA activates PPAR alpha in liver, and shares functional similarities to ligands of PPAR gamma, the thiazolidinediones, which are potent insulin sensitizers. We provide the first evidence that CLA is able to normalize impaired glucose tolerance and improve hyperinsulinemia in the pre-diabetic ZDF rat. Additionally, dietary CLA increased steady state levels of aP2 mRNA in adipose tissue of fatty ZDF rats compared to controls, consistent with activation of PPAR gamma. The insulin sensitizing effects of CLA are due, at least in part, to activation of PPAR gamma since increasing levels of CLA induced a dose-dependent transactivation of PPAR gamma in CV-1 cells cotransfected with PPAR gamma and PPRE X 3-luciferase reporter construct. CLA effects on glucose tolerance and glucose homeostasis indicate that dietary CLA may prove to be an important therapy for the prevention and treatment of NIDDM.

Calcium bioavailability of vegetarian diets in rats: potential application in a bioregenerative life-support system.

Calcium bioavailability of vegetarian diets containing various proportions of candidate crops for a controlled ecological life-support system (CELSS) was determined by femur 45Ca uptake. Three vegetarian diets and a control diet were labeled extrinsically with 45Ca and fed to 5-wk old male rats. A fifth group of rats fed an unlabeled control diet received an intraperitoneal (IP) injection of 45Ca. There was no significant difference in mean calcium absorption of vegetarian diets (90.80 +/- 5.23%) and control diet (87.85 +/- 5.25%) when calculated as the percent of an IP dose. The amounts of phytate, oxalate, and dietary fiber in the diets did not affect calcium absorption.

Calcium bioavailability from bovine milk and dairy products in premenopausal women using intrinsic and extrinsic labeling techniques.

Stable isotopes were used to compare calcium fractional absorption from intrinsically and extrinsically labeled bovine milk as well as intrinsically labeled dairy product and cheese analogue. Healthy Caucasian women were fed a controlled diet for 4 d during the follicular phase of their menstrual cycle. With breakfast on the third day, participants ingested milk containing 44Ca (intrinsic) and 42CaCl2 (extrinsic) or dairy products containing 44Ca. Total feces were collected for 2 d prior to and 10 d after isotope ingestion. Polyethylene glycol was administered to monitor completeness of fecal collections. Total calcium was determined by atomic absorption spectrophotometry, and isotopic abundance was determined by high resolution fast atom bombardment mass spectrometry. Fractional absorption was determined as the difference between the administered isotopic dose and the quantity of 44Ca or 42Ca excreted in feces. The fractional absorption of calcium from milk was not affected by the method of labeling, lactose content, fermentation or the chemical form of calcium in dairy products or cheese analogue.

Dietary conjugated linoleic acid modulation of phorbol ester skin tumor promotion.

The fatty acid derivative conjugated dienoic linoleate (CLA) has been shown to inhibit initiation and postinitiation stages of carcinogenesis in several experimental animal models. The goal of the present study was to determine the role of increasing levels of dietary CLA in mouse skin tumor promotion elicited by 12-O-tetradecanoylphorbol-13-acetate (TPA). Mice were fed control (no CLA) diet during initiation, then switched to diets containing 0.0%, 0.5%, 1.0%, or 1.5% (wt/wt) CLA during skin tumor promotion by TPA. Body weights of mice fed 0.5%, 1.0%, or 1.5% CLA were similar to each other but were significantly lower (p < 0.05) than weights of mice fed no CLA (0.0%) throughout promotion. A reduction in papilloma incidence was observed in mice fed 1.5% CLA from Weeks 8 to 24 compared with mice fed diets containing 0.0-1.0% CLA (p < 0.05). Twenty-four weeks after tumor promotion was begun, diets containing 1.0% and 1.5% CLA inhibited tumor yield (4.94 and 4.35 tumors/mouse, respectively) compared with diets without CLA (0.0% CLA, 6.65 tumors/mouse, p < 0.05) or 0.5% CLA (5.92 tumors/mouse, p < 0.05). These data indicate that CLA inhibits tumor promotion in a manner that is independent of its anti-initiator activity. Further studies are warranted in identifying cellular mechanisms that are likely to be involved with the antipromoter effects of CLA.