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Type 2 diabetes may be associated with elevated fracture risk, but the impact on bone loss is unknown. Analysis of 4-year change in hip BMD data from a cohort of white and black well-functioning men and women 70-79 years of age found that white women with diabetes had more rapid bone loss at the femoral neck than those with normal glucose metabolism.

INTRODUCTION

Type 2 diabetes may be associated with elevated fracture risk in older adults. Although type 2 diabetes is not associated with lower BMD, older diabetic adults have a higher prevalence of other risk factors for fracture, including more frequent falls, functional limitations, and diabetic complications. With this burden of risk factors, loss of BMD could place older adults with diabetes at higher risk of sustaining a fracture.

MATERIALS AND METHODS

To determine if bone loss is increased with type 2 diabetes, we analyzed data from the Health, Aging, and Body Composition Study of white and black well-functioning men and women 70-79 years of age. Hip BMD was measured at baseline and 4 years later in 480 (23%) participants with diabetes, 439 with impaired glucose metabolism, and 1172 with normal glucose homeostasis (NG).

RESULTS

Those with diabetes had higher baseline hip BMD and weight, but among white women, had more weight loss over 4 years. White women with diabetes lost more femoral neck and total hip BMD than those with NG in age-adjusted models. After multivariable adjustment, diabetes was associated with greater loss of femoral neck BMD (-0.32%/year; 95% CI: -0.61, -0.02) but not total hip BMD. In men and black women, change in hip BMD was similar for participants with diabetes and NG.

CONCLUSIONS

Despite having higher baseline BMD, diabetic white women, but not men or black women, had more rapid bone loss at the femoral neck than those with NG. This increased bone loss may contribute to the higher fracture risk observed in older diabetic women.

Although many skin disorders, including psoriasis and atopic dermatitis, are adversely affected by psychologic stress (PS), the pathophysiologic link between PS and disease expression remains unclear. Recent studies demonstrated PS-induced alterations in permeability barrier homeostasis, mediated by increased endogenous glucocorticoids. Here, we assessed the mechanisms by which PS alters stratum corneum (SC) function. Insomniac psychologic stress (IPS) altered both barrier homeostasis and SC integrity. IPS decreased epidermal cell proliferation, impaired epidermal differentiation, and decreased the density and size of corneodesmosomes (CD), which was linked to degradation of CD proteins (e.g., desmoglein1). Barrier compromise was linked to decreased production and secretion of lamellar bodies (LB), which in turn could be attributed to a decrease in de novo synthesis of epidermal lipids. Topical physiologic lipids (equimolar cholesterol, ceramides, and free fatty acids) normalized both barrier homeostasis and SC integrity in IPS mice, further evidence that lipid deficiency accounted for these functional abnormalities. Thus, PS inhibition of epidermal lipid synthesis results in decreased LB formation and secretion, as well as decreased CD, compromising both permeability barrier homeostasis and SC integrity. These studies suggest that topical treatment with epidermal physiologic lipids could be beneficial in stress-induced, barrier-associated dermatoses, such as psoriasis and atopic dermatitis.

Although stratum corneum (SC) hydration has been primarily of concern to the cosmetic industry, it serves an important biosensor function. In murine models, not only deiminated products of filaggrin-derived amino acids ("NMF") but also endogenous glycerol from circulation into the epidermis via aquaporin 3 channel and from triglyceride turnover in sebaceous glands (SG) are important determinants. We assessed here whether endogenous glycerol could also be linked to SC hydration in humans. SG-enriched sites are more hydrated than SG-impoverished sites, and SC hydration correlates with both sebum production and SC glycerol content, but the correlation is more significant for SC glycerol content than for sebum content. Moreover, gender-related differences in sebum content are not associated with altered SC hydration. SC hydration is also linked to SC glycerol content in SG-impoverished sites, suggesting a role for non-SG-derived (? from circulation) glycerol in SC hydration. Finally, short-term water immersion produces a parallel decline in SC hydration and SC glycerol content, with glycerol levels returning to normal over several hours. These results suggest that endogenous glycerol of both circulatory and SG origin comprises an H2O-extractable pool that influences SC hydration in humans. These results also provide a rationale for the development of glycerol-containing therapeutic moisturizers.

We showed recently that short-term increases in stratum corneum (SC) pH are accompanied by minor alterations in permeability barrier homeostasis and SC integrity/cohesion. Since prolonged SC neutralization more closely mirrors clinical situations (i.e., neonatal skin, occupational dermatitis conditions), we assessed here whether sustained elevations of SC pH by long-term application of 1,1,3,3-tetramethylguanidine superbase provoke profound alterations in SC function. Sustained SC neutralization altered not only barrier recovery kinetics but also basal permeability barrier function. These abnormalities were attributable to a decrease in beta-glucocerebrosidase (beta-GlcCer'ase) and acidic sphingomyelinase (aSMase) catalytic activity and enzyme degradation consequent to a pH-induced sustained serine protease (SP) activity. The role of SP in this process was shown by the normalization of enzyme activities/content by co-applied SP inhibitors (SPI). To address whether lipid-processing enzymes are potential substrates for the stratum corneum chymotryptic enzyme (SCCE), protein extracts from human SC were treated for 2 h at 37 degrees C with recombinant active SCCE at pH 7.2. Recombinant SCCE induced a significant decrease in the immunoblotting of both beta-GlcCer'ase or aSMase compared with control experiments performed in the absence of the active SCCE. Finally, with sustained SC neutralization, SC integrity/cohesion deteriorated, attributable to SP-mediated degradation of corneodesmosomes (CD) as well as CD constituent proteins, desmoglein 1. These abnormalities were again reversed by co-applied SPI. In conclusion, prolonged SC neutralization provokes profound abnormalities in SC function, due to pH-induced high SP activity that, in turn, degrades lipid processing enzymes and CD proteins.

Liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs) are potent regulators of keratinocyte proliferation, differentiation, and epidermal permeability barrier homeostasis. Cholesterol sulfotransferase type 2B isoform 1b (SULT2B1b) is a key enzyme in the synthesis of cholesterol sulfate (CS), a critical regulator of keratinocyte differentiation and desquamation, as well as a mediator of barrier homeostasis. In this study, we assessed the effect of activators of LXR, PPARalpha, PPARbeta/delta, and PPARgamma on SULT2B1b gene expression and enzyme activity in cultured human keratinocytes (CHKs). Our results demonstrate that PPAR and LXR activators increase SULT2B1b mRNA levels, with the most dramatic effect (a 26-fold increase) induced by the PPARgamma activator ciglitazone. Ciglitazone upregulates SULT2B1b mRNA in a dose- and time-dependent manner. Moreover, the stimulation of SULT2B1b gene expression by LXR and PPAR activators occurs in both undifferentiated and differentiated CHKs. The upregulation of SULT2B1b mRNA by ciglitazone appears to occur at a transcriptional level, because the degradation of SULT2B1b is not accelerated by ciglitazone. In addition, cycloheximide almost completely blocks the ciglitazone-induced increase in SULT2B1b mRNA, suggesting that the transcription of SULTB1b mRNA is dependent on new protein synthesis. Finally, LXR and PPAR activators also increased the activity of cholesterol sulfotransferase. Thus, LXR and PPAR activators regulate the expression of SULT2B1b, the key enzyme in the synthesis of CS, which is a potent regulator of epidermal differentiation and corneocyte desquamation.

Phospholipids are a major class of lipids in epidermis, where they serve as a source of free fatty acids that are important for the maintenance of epidermal permeability barrier function. The phospholipid biosynthetic enzyme, 1-acyl-sn-glycerol-3-phosphate acyltransferase (AGPAT), catalyzes the acylation of lysophosphatidic acid to form phosphatidic acid, the major precursor of all glycerolipids. We identified an expression pattern of AGPAT isoforms that is unique to epidermis, with relatively high constitutive expression of mouse AGPAT (mAGPAT) 3, 4, and 5 but low constitutive expression of mAGPAT 1 and 2. Localization studies indicate that all five isoforms of AGPAT were expressed in all nucleated layers of epidermis. Furthermore, rat AGPAT 2 and 5 mRNAs increased in parallel with both an increase in enzyme activity and permeability barrier formation late in rat epidermal development. Moreover, after two methods of acute permeability barrier disruption, mAGPAT 1, 2, and 3 mRNA levels increased rapidly and were sustained for at least 24 h. In parallel with the increase in mRNA levels, an increase in AGPAT activity also occurred. Because upregulation of mAGPAT mRNAs after tape-stripping could be partially reversed by artificial barrier restoration by occlusion, these studies suggest that an increase in the expression of AGPATs is linked to barrier requirements.

The etiology of atherosclerosis is complex and multifactorial but there is extensive evidence indicating that oxidized lipoproteins may play a key role. At present, the site and mechanism by which lipoproteins are oxidized are not resolved, and it is not clear if oxidized lipoproteins form locally in the artery wall and/or are sequestered in atherosclerotic lesions following the uptake of circulating oxidized lipoproteins. We have been focusing our studies on demonstrating that such potentially atherogenic oxidized lipoproteins in the circulation are at least partially derived from oxidized lipids in the diet. Thus, the purpose of our work has been to determine in humans whether oxidized dietary oxidized fats such as oxidized fatty acids and oxidized cholesterol are absorbed and contribute to the pool of oxidized lipids in circulating lipoproteins. When a meal containing oxidized linoleic acid was fed to normal subjects, oxidized fatty acids were found only in the postprandial chylomicron/chylomicron remnants (CM/RM) which were cleared from circulation within 8 h. No oxidized fatty acids were detected in low density lipoprotein (LDL) or high density lipoprotein (HDL) fractions at any time. However, when alpha-epoxy cholesterol was fed to human subjects, alpha-epoxy cholesterol in serum was found in CM/RM and also in endogenous very low density lipoprotein, LDL, and HDL and remained in the circulation for 72 h. In vitro incubation of the CM/RM fraction containing alpha-epoxy cholesterol with human LDL and HDL that did not contain alpha-epoxy cholesterol resulted in a rapid transfer of oxidized cholesterol from CM/RM to both LDL and HDL. We have suggested that cholesteryl ester transfer protein is mediating the transfer. Thus, alpha-epoxy cholesterol in the diet is incorporated into CM/RM fraction and then transferred to LDL and HDL contributing to lipoprotein oxidation. We hypothesize that diet-derived oxidized fatty acids in chylomicron remnants and oxidized cholesterol in remnants and LDL accelerate atherosclerosis by increasing oxidized lipid levels in circulating LDL and chylomicron remnants. This hypothesis is supported by our feeding experiments in animals. When rabbits were fed oxidized fatty acids or oxidized cholesterol, the fatty streak lesions in the aorta were increased by 100%. Moreover, dietary oxidized cholesterol significantly increased aortic lesions in apo-E and LDL receptor-deficient mice. A typical Western diet is rich in oxidized fats and therefore could contribute to the increased arterial atherosclerosis in our population.

In neonatal rat stratum corneum (SC), pH declines from pH 6.8 at birth to adult levels (pH 5.0-5.5) over 5-6 d. Liver X receptor (LXR) activators stimulate keratinocyte differentiation, improve permeability barrier homeostasis, and accelerate the in utero development of the SC. In this manuscript we determined the effect of LXR activators on SC acidification in the neonatal period and whether these activators correct the functional abnormalities in permeability barrier homeostasis and SC integrity/cohesion. Formation of the acid SC-buffer system was accelerated by topically applying the LXR activator, 22(R)-hydroxycholesterol, and non-oxysterol activators of LXR, TO-901317, and GW-3965. A sterol which does not activate LXR had no effect. LXR activation increased secretory phospholipase A(2) (sPLA(2)) activity and conversely, inhibition of sPLA(2) activity prevented the LXR induced increase in SC acidification, suggesting that increasing sPLA(2) accounts in part, for the LXR stimulation of acidification. LXR activation resulted in an improvement in permeability barrier homeostasis, associated with an increased maturation of lamellar membranes attributable to an increased beta-glucocerebrosidase activity. SC integrity cohesion also normalized in LXR-activator-treated animals and was associated with an increase in corneodesmosomes and in desmoglein 1 expression. These results demonstrate that LXR activators stimulate the formation of an acidic SC and improve both permeability barrier homeostasis and SC integrity/cohesion.

Epidermis and sebocyte-derived lipids are derived both from de novo synthesis and through uptake of fatty acids from the circulation. Plasma membrane proteins can significantly contribute to the latter process. In particular, fatty acid transport proteins (FATP/solute carrier family 27) are integral transmembrane proteins that enhance the uptake of long-chain fatty acids into cells. Using specific antisera against all six mammalian FATP, we found that both human and mouse skin express FATP1, -3, -4, and -6. In adult skin, FATP1 and -3 are expressed predominantly by keratinocytes, whereas FATP4 is strongly expressed by sebaceous glands and FATP6 by hair follicle epithelia. Sustained barrier disruption leads to increases in FATP1 and -6 levels as well as a robust increase in CD36 protein. Notably, expression of FATP1 by embryonic keratinocytes at day 18.5 was lower, and FATP4 increased in comparison with adult epidermis. Together, these findings indicate that FATP are not only expressed by different cell types within the skin, but also that their localization is dynamically regulated during development.