Measurement of protein metabolism in epidermis and dermis

Modeling Temperature-Dependent Dermal Absorption and Clearance for Transdermal and Topical Drug Applications

A computational model was developed to better understand the impact of elevated skin temperatures on transdermal drug delivery and dermal clearance. A simultaneous heat and mass transport model with emphasis on transdermal delivery system (TDS) applications was developed to address transient and steady-state temperature effects on dermal absorption. The model was tested using representative data from nicotine TDS applied to human skin either in vitro or in vivo. The approximately 2-fold increase of nicotine absorption with a 10°C increase in skin surface temperature was consistent with a 50-65 kJ/mol activation energy for diffusion in the stratum corneum, with this layer serving as the primary barrier for nicotine absorption. Incorporation of a dermal clearance component into the model revealed efficient removal of nicotine via the dermal capillaries at both normal and elevated temperatures. Two-compartment pharmacokinetic simulations yielded systemic drug concentrations consistent with the human pharmacokinetic data. Both in vitro skin permeation and in vivo pharmacokinetics of nicotine delivered from a marketed TDS under normal and elevated temperatures can be satisfactorily described by a simultaneous heat and mass transfer computational model incorporating realistic skin barrier properties and dermal clearance components.

Lipid functions in skin: Differential effects of n-3 polyunsaturated fatty acids on cutaneous ceramides, in a human skin organ culture model

Ceramides are important for skin health, with a multitude of species found in both dermis and epidermis. The epidermis contains linoleic acid-Ester-linked Omega-hydroxylated ceramides of 6-Hydroxy-sphingosine, Sphingosine and Phytosphingosine bases (CER[EOH], CER[EOS] and CER[EOP], respectively), that are crucial for the formation of the epidermal barrier, conferring protection from environmental factors and preventing trans-epidermal water loss. Furthermore, a large number of ceramides, derivatives of the same sphingoid bases and various fatty acids, are produced by dermal and epidermal cells and perform signalling roles in cell functions ranging from differentiation to apoptosis.

Supplementation with the n-3 polyunsaturated fatty acids (PUFA) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have shown promise as therapeutic agents in a number of inflammatory skin conditions, altering the lipid profile of the skin and production of bioactive lipids such as the eicosanoids, docosanoids and endocannabinoids. In this study we wished to investigate whether EPA and DHA could also affect the ceramide profile in epidermis and dermis, and, in this way, contribute to formation of a robust lipid barrier and ceramide-mediated regulation of skin functions.

Ex vivo skin explants were cultured for 6 days, and supplemented with EPA or DHA (50 μM). Liquid chromatography coupled to tandem mass spectrometry with electrospray ionisation was used to assess the prevalence of 321 individual ceramide species, and a number of sphingoid bases, phosphorylated sphingoid bases, and phosphorylated ceramides, within the dermis and epidermis.

EPA augmented dermal production of members of the ceramide families containing Non-hydroxy fatty acids and Sphingosine or Dihydrosphingosine bases (CER[NS] and CER[NDS], respectively), while epidermal CER[EOH], CER[EOS] and CER[EOP] ceramides were not affected. DHA did not significantly affect ceramide production. Ceramide-1-phosphate levels in the epidermis, but not the dermis, increased in response to EPA, but not DHA.

This ex vivo study shows that dietary supplementation with EPA has the potential to alter the ceramide profile of the skin, and this may contribute to its anti-inflammatory profile. This has implications for formation of the epidermal lipid barrier, and signalling pathways within the skin mediated by ceramides and other sphingolipid species. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

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Omega-3 fatty acid supplementation alters ex vivo skin ceramide profiles

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Eicosapentaenoic acid (EPA) increases dermal ceramides with non-hydroxy fatty acids (CER[NS] and CER[NDS])

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EPA increases ceramide-1-phosphate (C1P) in the epidermis but not dermis

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Long-chain linoleic-acid-containing ceramides were unaltered by EPA or docosahexaenoic acid (DHA)

Amino Acid Availability Regulates the Effect of Hyperinsulinemia on Skin Protein Metabolism in Pigs

The effects of amino acid supply and insulin infusion on skin protein kinetics (fractional synthesis rate (FSR), fractional breakdown rate (FBR), and net balance (NB)) in pigs were investigated. Four-month-old pigs were divided into four groups as follows: control, insulin (INS), amino acid (AA), and INS + AA groups based on the nutritional and hormonal conditions. l-[ring-(13)C6]Phenylalanine was infused. FBR was estimated from the enrichment ratio of arterial phenylalanine to intracellular free phenylalanine. Plasma INS was increased (p < 0.05) in the INS and INS + AA groups. Plasma glucose was maintained by infusion of glucose in the groups receiving INS. The interventions did not change the NB of skin protein. However, the interventions affected the FSR and FBR differently. An infusion of INS significantly increased both FSR and FBR, although AA infusion did not. When an AA infusion was added to the infusion of insulin (INS + AA group), FSR and FBR were both lower when compared with the INS group. Our data demonstrate that in anesthetized pigs INS infusion did not exert an anabolic effect, but rather it increased AA cycling into and out of skin protein. Because co-infusion of AAs with INS ameliorated this effect, it is likely that the increased AA cycling during INS infusion was related to AA supply. Although protein kinetics were affected by both INS and AAs, none of the interventions affected the skin protein deposition. Thus, skin protein content is closely regulated under normal circumstances and is not subject to transient changes in AAs or hormonal concentrations.

Distribution of bioactive lipid mediators in human skin

The skin produces bioactive lipids that participate in physiological and pathological states, including homeostasis, induction, propagation, and resolution of inflammation. However, comprehension of the cutaneous lipid complement, and contribution to differing roles of the epidermal and dermal compartments, remains incomplete. We assessed the profiles of eicosanoids, endocannabinoids, N-acyl ethanolamides, and sphingolipids, in human dermis, epidermis, and suction blister fluid. We identified 18 prostanoids, 12 hydroxy-fatty acids, 9 endocannabinoids and N-acyl ethanolamides, and 21 non-hydroxylated ceramides and sphingoid bases, several demonstrating significantly different expression in the tissues assayed. The array of dermal and epidermal fatty acids was reflected in the lipid mediators produced, whereas similarities between lipid profiles in blister fluid and epidermis indicated a primarily epidermal origin of suction blister fluid. Supplementation with omega-3 fatty acids ex vivo showed that their action is mediated through perturbation of existing species and formation of other anti-inflammatory lipids. These findings demonstrate the diversity of lipid mediators involved in maintaining tissue homeostasis in resting skin and hint at their contribution to signaling, cross-support, and functions of different skin compartments. Profiling lipid mediators in biopsies and suction blister fluid can support studies investigating cutaneous inflammatory responses, dietary manipulation, and skin diseases lacking biomarkers and therapeutic targets.

Natural isotopic signatures of variations in body nitrogen fluxes: a compartmental model analysis

Body tissues are generally ¹⁵N-enriched over the diet, with a discrimination factor (Δ¹⁵N) that varies among tissues and individuals as a function of their nutritional and physiopathological condition. However, both ¹⁵N bioaccumulation and intra- and inter-individual Δ¹⁵N variations are still poorly understood, so that theoretical models are required to understand their underlying mechanisms. Using experimental Δ¹⁵N measurements in rats, we developed a multi-compartmental model that provides the first detailed representation of the complex functioning of the body's Δ¹⁵N system, by explicitly linking the sizes and Δ¹⁵N values of 21 nitrogen pools to the rates and isotope effects of 49 nitrogen metabolic fluxes. We have shown that (i) besides urea production, several metabolic pathways (e.g., protein synthesis, amino acid intracellular metabolism, urea recycling and intestinal absorption or secretion) are most probably associated with isotope fractionation and together contribute to ¹⁵N accumulation in tissues, (ii) the Δ¹⁵N of a tissue at steady-state is not affected by variations of its P turnover rate, but can vary according to the relative orientation of tissue free amino acids towards oxidation vs. protein synthesis, (iii) at the whole-body level, Δ¹⁵N variations result from variations in the body partitioning of nitrogen fluxes (e.g., urea production, urea recycling and amino acid exchanges), with or without changes in nitrogen balance, (iv) any deviation from the optimal amino acid intake, in terms of both quality and quantity, causes a global rise in tissue Δ¹⁵N, and (v) Δ¹⁵N variations differ between tissues depending on the metabolic changes involved, which can therefore be identified using simultaneous multi-tissue Δ¹⁵N measurements. This work provides proof of concept that Δ¹⁵N measurements constitute a new promising tool to investigate how metabolic fluxes are nutritionally or physiopathologically reorganized or altered. The existence of such natural and interpretable isotopic biomarkers promises interesting applications in nutrition and health.

Body proteins ensure vital functions, and their constancy is maintained through the tight coordination of many nitrogen metabolic fluxes, but our understanding of how this flux system is regulated, and sometimes dysregulated, remains fragmentary and incomplete. Besides, body tissues are generally naturally enriched in the heavier stable nitrogen isotope (¹⁵N) over the diet: this ¹⁵N bioaccumulation (Δ¹⁵N) varies depending on tissues and metabolic orientations, likely as the result of isotope effects associated to some metabolic pathways. We used a novel approach, combining multi-tissue Δ¹⁵N measurements and their analysis using modeling, to understand how body Δ¹⁵N values relate to nitrogen fluxes. The multi-tissue model we have developed provides a clearer understanding of the metabolic processes that generate isotopic fractionation, and of how tissue Δ¹⁵N values are modulated in response to changes in the body distribution of specific nitrogen fluxes. We show that Δ¹⁵N values tend to rise when the amino acids intake does not optimally fit the metabolic demand, and that Δ¹⁵N values constitute natural and interpretable signatures of nutritionally-induced variations in nitrogen fluxes. This approach constitutes a new promising tool to investigate how nitrogen metabolism is nutritionally or physiopathologically reorganized or altered, and promises interesting applications in many areas (nutrition, pathology, ecology, paleontology, etc).

Isotopic and modeling investigation of long-term protein turnover in rat tissues

Fractional synthesis rates (FSR) of tissue proteins (P) are usually measured using labeled amino acid (AA) tracer methods over short periods of time under acute, particular conditions. By combining the long-term and non-steady-state (15)N labeling of AA and P tissue fractions with compartmental modeling, we have developed a new isotopic approach to investigate the degree of compartmentation of P turnover in tissues and to estimate long-term FSR values under sustained and averaged nutritional and physiological conditions. We measured the rise-to-plateau kinetics of nitrogen isotopic enrichments (δ(15)N) in the AA and P fractions of various tissues in rats for 2 mo following a slight increase in diet δ(15)N. Using these δ(15)N kinetics and a numerical method based on a two-compartment model, we determined reliable FSR estimates for tissues in which P turnover is adequately represented by such a simple precursor-product model. This was the case for kidney, liver, plasma, and muscle, where FSR estimates were 103, 101, 58, and 11%/day, respectively. Conversely, we identified tissues, namely, skin and small intestine, where P turnover proved to be too complex to be represented by a simple two-compartment model, evidencing the higher level of subcompartmentation of the P and/or AA metabolism in these tissues. The present results support the value of this new approach in gaining cognitive and practical insights into tissue P turnover and propose new and integrated FSR values over all individual precursor AA and all diurnal variations in P kinetics.

Intensive insulin treatment increases donor site wound protein synthesis in burn patients

BACKGROUND

In the treatment of burns, patients' own skin is the preferred material to cover burn wounds, resulting in the need to create a donor site wound. Enhancement of healing of the donor site wound would be beneficial in burn patients. Insulin, an anabolic agent, is used routinely to treat hyperglycemia after injury. We investigated whether intensive insulin treatment increases fractional synthesis rate (FSR) of the donor site wound protein and decreases the length of hospitalization normalized for total body surface area burned (LOS/TBSA).

METHODS

FSR of the donor site wound protein was measured in pediatric patients randomized to control (n = 13) and insulin (n = 10) treatments. Depending on the postoperative day when the tracer study was done, studies were divided into "early" (days < 5) and "late" (days ≥ 5) periods.

RESULTS

FSR of the donor site wound protein was greater in the insulin group at the "early" period of wound healing (control vs insulin, 8.2 ± 3.8 vs 13.1 ± 6.9% per day; P < .05); but not at the "late" (control vs insulin, 19.7 ± 4.6 vs 16.6 ± 4.0% per day; P > .05). Despite these differences, LOS/TBSA was not decreased in the insulin group. Correlation analyses demonstrated that, independent of the treatment regimen, FSR positively correlated (P < .05) with time after creation of the donor site and negatively correlated (P < .05) with LOS/TBSA.

CONCLUSION

Insulin treatment increased FSR of the donor site wound protein in the early period of wound healing; FSR correlated with LOS/TBSA independent of the treatment regimen.

Donor site wound protein synthesis correlates with length of acute hospitalization in severely burned children

Autografting of burn wounds results in generation of donor site wounds. Here we measured donor site wound protein fractional synthesis rate (FSR) in a burn pediatric population and showed that FSR increases over time postsurgery and correlates with the length of hospital stay (LOS) normalized for total body surface area (TBSA) burn size. 3.9 +/- 1.1 days after the grafting surgery patients participated in a metabolic study consisting of continuous infusion of l-[ring-(2)H(5)]-phenylalanine and donor site wound punch biopsies. Donor site wound protein FSR was 10.4 +/- 7.5%/day. Wound FSR demonstrated linear correlation with the time postsurgery (p<0.05). Multiple regression analysis showed that LOS/TBSA correlated with donor site wound protein FSR and time postsurgery (p<0.001) and the following equation describes the relationship: estimated LOS/TBSA=(FSR-12.95-1.414 x postsurgery day)/(-17.8). This equation predicted that FSR corrected for the postsurgery day when the metabolic study was conducted accounted for 67% of the variability (r(2)=0.673) in the LOS/TBSA. Donor site wound protein FSR correlated to LOS/TBSA of burn patients admitted to the intensive care unit. Measurement of protein deposition in regenerating donor site wound using stable isotope technique provides a quantitative measure of wound healing.

Determination of markers for collagen type I turnover in peritendinous human tissue by microdialysis: effect of catheter types and insertion trauma

OBJECTIVES

Previous results from our group have shown that loading of human tendon elevates tendinous type I collagen production measured by microdialysis. However, exclusion of the observed elevation as a response to trauma from inserting the microdialysis catheters or a possible influence from the collagen production in skin was not determined.

METHODS

Using the microdialysis method we measured the tissue levels of type I collagen metabolism markers [procollagen I COOH-terminal propeptide (PICP) and COOH-terminal telopeptide of type I collagen (ICTP)] in peritendinous tissue of the Achilles tendon in volunteers at two time points, 0 and 72 h. Using two different catheter types, an investigation of the contribution from the skin in the collagen results obtained was also examined.

RESULTS

The data showed no significant changes in the dialysate levels for PICP or ICTP (p>0.05) in either of the catheters.

CONCLUSION

Inserting microdialysis fibres around the Achilles tendon twice does not increase the collagen type I metabolism determined 3 days after the initial trauma, and when using microdialysis for measuring peritendinous collagen turnover the skin contribution can be regarded as negligible. These findings support microdialysis as a valid method for the determination of collagen metabolism in peritendinous tissue.

Growth of infants with IgE-mediated cow's milk allergy fed different formulas in the complementary feeding period

Observational studies have shown that allergic infants, irrespective of the type of diet, show various degrees of growth depression in the first year of life. We investigated whether the type of milk in the complementary feeding period (6-12 months of age) is associated with differences in the increase of standardized growth indices (weight-for-age, WA; length-for-age, LA; and weight-for-length, WL, z-scores) in infants with cow's milk allergy (CMA). Infants with immunoglobulin E-mediated CMA breastfed at least 4 months and progressively weaned in the 5- to 6-month period were randomly assigned to three special formulas, a soy formula (n = 32), a casein hydrolysate (n = 31), and a rice hydrolysate (n = 30). A fourth, non-randomized group was made up by allergic infants still breastfed up to 12 months (n = 32). Groups were compared for WA, LA, and WL z-scores at 6, 9 and 12 months of age. All groups showed low WA and LA z-scores at 6 months of age. Infants fed hydrolyzed products showed a trend toward higher WA z-score increments in the 6- to 12-month period. The use of casein- and rice-based hydrolyzed formulas resulted in higher changes in WA compared with soy formula. Further research should be aimed at optimizing the dietary needs and feeding regimens for infants with CMA.