Synthesis and degradation of collagens in skin of healthy and protein-malnourished rats in vivo, studied by 18O2 labelling

Tracer-based estimates of protein flux in cases of incomplete product renewal: evidence and implications of heterogeneity in collagen turnover

The synthesis of various molecules can be estimated by measuring the incorporation of a labeled precursor into a product of interest. Unfortunately, a central problem in many studies has been an inability to estimate the intracellular dilution of the precursor and therein correctly calculate the synthesis of the product; it is generally assumed that measuring the true product labeling is straightforward. We initiated a study to examine liver collagen synthesis and identified an apparent problem with assumptions regarding measurements of the product labeling. Since it is well known that collagen production is relatively slow, we relied on the use of [(2)H]H2O labeling (analogous to a primed infusion) and sampled animals over the course of 16 days. Although the water labeling (the precursor) remained stable and we observed the incorporation of labeled amino acids into collagen, the asymptotic protein labeling was considerably lower than what would be expected based on the precursor labeling. Although this observation is not necessarily surprising (i.e., one might expect that a substantial fraction of the collagen pool would appear "inert" or turn over at a very slow rate), its implications are of interest in certain areas. Herein, we discuss a novel situation in which tracers are used to quantify rates of flux under conditions where a product may not undergo complete replacement. We demonstrate how heterogeneity in the product pool can lead one to the wrong conclusions regarding estimates of flux, and we outline an approach that may help to minimize errors surrounding data interpretation.

Combination of BCAAs and glutamine enhances dermal collagen protein synthesis in protein-malnourished rats

Skin collagen decreases in protein-malnourished states. Amino acids regulate protein metabolism, glutamine stimulates collagen synthesis through the conversion process to proline and provides 75 % of the intracellular free proline in fibroblasts. However, the impact of these amino acids on collagen synthesis under malnutrition has not been examined. We investigated the effect of amino acids on dermal tropocollagen synthesis in protein-malnourished rats. The fractional synthesis rate (FSR, %/h) of dermal tropocollagen was evaluated by the incorporation of L-[ring-(2)H(5)]-phenylalanine after 4 h infusion of each amino acid and the stable isotope. None of the infused 12 single amino acids (glutamine, proline, alanine, arginine, glutamate, glycine, aspartate, serine, histidine, lysine, phenylalanine and threonine) significantly increased the FSR (P = 0.343, one-way ANOVA). In contrast, amino acid mixtures of essential amino acids + glutamine + arginine (EAARQ) and branched-chain amino acids + glutamine (BCAAQ) significantly increased the FSR compared to saline, but the branched-chain amino acids (BCAAs) and amino acid mixture of collagen protein (AAC) did not alter the FSR (saline, 0.96 ± 0.24 %/h; EAARQ, 1.76 ± 0.89 %/h; BCAAQ 1.71 ± 0.36 %/h; BCAAs, 1.08 ± 0.20 %/h and AAC 1.39 ± 0.35 %/h, P < 0.05, Tukey's test). Proline conversion from glutamine represented only 3.9 % of the free proline in skin, as evaluated by the primed-constant infusion of L-d7-proline and L-α-15N-glutamine in rats. These results suggested that the combination of BCAAQ is a key factor for the enhancement of skin collagen synthesis in protein-malnourished rats. The contribution of extracellular free glutamine on de novo proline synthesis and collagen synthesis is very low in vivo compared to the contribution in vitro.

New approaches and recent results concerning human-tissue collagen synthesis

PURPOSE OF REVIEW

Knowledge of the physiological regulation of human-tissue collagen metabolism in vivo is poor, due to the lack of appropriately robust methods. Recent application of stable isotope tracer techniques to measure human collagen synthesis has provided some insights into the role of nutrition and exercise on collagen turnover in the extracellular matrix of the musculoskeletal system.

RECENT FINDINGS

Collagen turnover in the musculoskeletal system is faster than previously thought. Bone collagen synthesis is increased by feeding, whereas both muscle collagen and tendon are unresponsive. Exercise stimulates collagen synthesis in both muscle and tendon in an apparently coordinated manner. There are also sex differences and normal aging is associated with increased muscle collagen synthesis and reductions in bone collagen synthesis, particularly in mature bone collagen.

SUMMARY

Collagen turnover appears to be faster than previously thought and is regulated by feeding and exercise, in a tissue-specific manner. Further application of these approaches, coupled with measures of gene and protein expression, to measure the acute regulation of collagen, will lead to a better understanding of the physiology and pathophysiology of human collagen turnover. This is particularly important for developing new therapies to improve bone health and minimize tissue fibrosis.

The measurement of tissue protein turnover

Tissue protein turnover can be assessed by a number of semi-, quantitative and qualitative methods. There are a number of static indices of the state of turnover of protein, for example amount of RNA per DNA or protein, the state of aggregation of ribosomes (i.e. the polyribosome index), the abundance of mRNA for particular proteins, and the enzymatic activity of proteins such as proteases, ribonuclease, etc. In addition, the concentration of particular amino acids such as glutamine or non-re-utilizable amino acids, formed post-translationally, such as 3-methylhistidine or hydroxyproline, are able to provide snapshot indices. However, since turnover is a dynamic process it should, ideally, be probed using methods such as the incorporation of tracer amino acids into protein or the dilution of tracer amino acids in the free pool by protein breakdown. The combination of tracer and tissue or limb balance methods is especially powerful since all the dynamic processes can potentially be quantified. The use of stable isotopes to label metabolic tracers has dramatically increased the feasibility of carrying out measurements of protein synthesis and breakdown and there has been a substantial growth in the application of the methods to a wide variety of tissues sampled by biopsy or at operation. Summaries of a number of currently feasible methods are provided, together with commentary on the relative efficacy of the methods and of the instrumental techniques required. There is also a discussion of suitable tracer labels and amino acids, plus a summary of the most reliable current values for protein turnover in a variety of tissues. The review also contains descriptions of potential methods which have not yet been applied in human beings but which are feasible, given the current recent increases in the accuracy and sensitivity of instrumentation for measurement of stable isotope labelling.

Absence of change in hepatic lactate metabolism after burn injury

In response to severe injury, extraordinary quantities of lactate that are released from the peripheral tissues serve as substrate for hepatic gluconeogenesis. It is possible that as a result of burn injury, reaction kinetics involving lactate could be directly changed within the liver. The metabolic fate of [U-14C]lactate was examined in vitro in fresh liver slices after a 20 per cent total body surface area injury. Glucose and CO2 were produced in vitro by the liver tissues after the injury and no differences were seen in the metabolism of these substrates between the injured and control animals. These findings suggest that the intrinsic enzymatic processes within the liver are not directly altered by injury itself or by any of the associated inflammatory mediators which appear early after burns.

Increased collagen-linked pentosidine levels and advanced glycosylation end products in early diabetic nephropathy

RATIONALE

Advanced glycosylation end products (AGEs) may play an important role in the development of diabetic vascular sequelae. An AGE cross-link, pentosidine, is a sensitive and specific marker for tissue levels of AGEs.

OBJECTIVES

To evaluate the role of AGEs in the development of diabetic nephropathy and retinopathy, we studied pentosidine levels and the clinical characteristics of 48 subjects with insulin-dependent diabetes mellitus. Diabetic nephropathy was classified as normal, microalbuminuria, or gross proteinuria, and retinopathy was graded as none, background, or proliferative. NEWLY OBSERVED FINDINGS: Significant elevation of pentosidine (P = 0.025) was found in subjects with microalbuminuria or gross proteinuria (73.03 +/- 9.47 vs 76.46 +/- 6.37 pmol/mg col) when compared with normal (56.96 +/- 3.26 pmol/mg col). Multivariate analysis to correct for age, duration of diabetes, and gender did not modify the results. Elevated pentosidine levels were also found in those with proliferative when compared with those with background retinopathy (75.86 +/- 5.66 vs 60.42 +/- 5.98 pmol/mg col) (P < 0.05).

CONCLUSIONS

Microalbuminuria is associated with elevated levels of pentosidine similar to those found in overt diabetic nephropathy suggesting that elevated AGE levels are already present during the earliest detectable phase of diabetic nephropathy.

Gas chromatography-mass spectrometry determination of 18O2 in 18O-labelled 4-hydroxyproline for measurement of collagen synthesis and intracellular degradation

The use of gas chromatography-mass spectrometry (GC-MS) and 18O2, a stable isotope which is incorporated into collagen during the post-translational conversion of proline to hydroxyproline, offers the potential advantages of high levels of sensitivity and specificity as compared to other techniques for measuring rates of collagen synthesis and degradation in vitro and vivo. Trifluoracetylation and methanol esterification of hydroxyproline yields two derivatives of hydroxyproline: N,O-trifluoroacetyl methyl 4-hydroxy-L-proline (N,O-TFA-Hyp) and N-trifluoroacetyl methyl 4-hydroxy-L-proline (N-TFA-Hyp). In the past, N-TFA-Hyp, which yields the 16O/18O-containing m/z 182/184 ion pair [M-COOH3]+ when analyzed by electron impact ionization GC-MS, has been proposed for analysis of 18O-enriched collagen. Although N,O-TFA-Hyp can be converted to N-TFA-Hyp by solvolysis, we find that this leads to degradation of the chromatography in GC-MS and demonstrate here that this extra chemical step is unnecessary if the m/z 278/280 ion pair (representing the [M-COOCH3]+. fragment) is measured by selected ion monitoring. By labelling fibroblasts in culture with 18O2, a sample of isotope-enriched collagen was obtained which was used to calibrate the GC-MS over the range 0.5-49% atom percent enrichment (APE). The greater sensitivity of 18O2 versus [15N]proline for labelling newly synthesized collagen was demonstrated by the finding of a ten-fold higher enrichment in the former isotope when administered to cell cultures at the same precursor APE. Thus, the approach described herein permits the determination of total hydroxyproline and APE on the same sample avoiding additional processing steps while maintaining the quality of chromatography and the sensitivity of detection. Measurement of absolute rates of both collagen synthesis and intracellular degradation of newly synthesized collagen in cell cultures is thus possible. Preliminary results comparing collagen metabolism in pairs of fibroblasts from hypertrophic scars and normal skin in post-burn patients are presented.

Decrease in skin collagen glycation with improved glycemic control in patients with insulin-dependent diabetes mellitus

Glycation, oxidation, and nonenzymatic browning of protein have all been implicated in the development of diabetic complications. The initial product of glycation of protein, fructoselysine (FL), undergoes further reactions, yielding a complex mixture of browning products, including the fluorescent lysine-arginine cross-link, pentosidine. Alternatively, FL may be cleaved oxidatively to form N(epsilon)-(carboxymethyl)lysine (CML), while glycated hydroxylysine, an amino-acid unique to collagen, may yield N(epsilon)-(carboxymethyl)hydroxylysine (CMhL). We have measured FL, pentosidine, fluorescence (excitation = 328 nm, emission = 378 nm), CML, and CMhL in insoluble skin collagen from 14 insulin-dependent diabetic patients before and after a 4-mo period of intensive therapy to improve glycemic control. Mean home blood glucose fell from 8.7 +/- 2.5 (mean +/- 1 SD) to 6.8 +/- 1.4 mM (P less than 0.005), and mean glycated hemoglobin (HbA1) from 11.6 +/- 2.3% to 8.3 +/- 1.1% (P less than 0.001). These changes were accompanied by a significant decrease in glycation of skin collagen, from 13.2 +/- 4.3 to 10.6 +/- 2.3 mmol FL/mol lysine (P less than 0.002). However, levels of browning and oxidation products (pentosidine, CML, and CMhL) and fluorescence were unchanged. These results show that the glycation of long-lived proteins can be decreased by improved glycemic control, but suggest that once cumulative damage to collagen by browning and oxidation reactions has occurred, it may not be readily reversed. Thus, in diabetic patients, institution and maintenance of good glycemic control at any time could potentially limit the extent of subsequent long-term damage to proteins by glycation and oxidation reactions.

Age-dependent accumulation of N epsilon-(carboxymethyl)lysine and N epsilon-(carboxymethyl)hydroxylysine in human skin collagen

N epsilon-(Carboxymethyl)lysine (CML) is formed on oxidative cleavage of carbohydrate adducts to lysine residues in glycated proteins in vitro [Ahmed et al. (1988) J. Biol. Chem. 263, 8816-8821; Dunn et al. (1990) Biochemistry 29, 10964-10970]. We have shown that, in human lens proteins in vivo, the concentration of fructose-lysine (FL), the Amadori adduct of glucose to lysine, is constant with age, while the concentration of the oxidation product, CML, increases significantly with age [Dunn et al. (1989) Biochemistry 28, 9464-9468]. In this work we extend our studies to the analysis of human skin collagen. The extent of glycation of insoluble skin collagen was greater than that of lens proteins (4-6 mmol of FL/mol of lysine in collagen versus 1-2 mmol of FL/mol of lysine in lens proteins), consistent with the lower concentration of glucose in lens, compared to plasma. In contrast to lens, there was a slight but significant age-dependent increase in glycation of skin collagen, 33% between ages 20 and 80. As in lens protein, CML, present at only trace levels in neonatal collagen, increased significantly with age, although the amount of CML in collagen at 80 years of age, approximately 1.5 mmol of CML/mol of lysine, was less than that found in lens protein, approximately 7 mmol of CML/mol of lysine. The concentration of N epsilon-(carboxymethyl)hydroxylysine (CMhL), the product of oxidation of glycated hydroxylysine, also increased with age in collagen, in parallel with the increase in CML, from trace levels at infancy to approximately 5 mmol of CMhL/mol of hydroxylysine at age 80.(ABSTRACT TRUNCATED AT 250 WORDS)

Microderivatization of 4-[18O]hydroxyproline and quantitation with a benchtop mass spectrometer

Accurate estimation of in vivo turnover rates of collagen is complicated by amino acid reutilization. It was previously shown that the ideal, non-recycling tracer was [18O]hydroxyproline synthesized in vivo. The analytical method for measuring turnover rates with [18O]hydroxyproline must include analyte quantitation for pool size determination and isotope ratio measurement for determining levels of label incorporation. For ease of use and widest availability, a benchtop gas chromatograph-mass spectrometer in the electron-impact ionization mode was chosen. Here we present a versatile procedure for hydroxyproline derivatization that is well suited for routine, large-scale determination of analyte concentrations and relative levels of 18O incorporation.

Determination of 4-hydroxyproline in collagen by gas chromatography/mass spectrometry

Derivatization of 4-hydroxyproline (Hyp) in collagen using trifluoroacetylation and methanol esterification produces two derivatives when analyzed by gas chromatography/mass spectrometry (GC/MS). The diacyl derivative N,O-bis(trifluoroacetyl)-4-hydroxy-L-proline methyl ester (N,O-TFA-Hyp) formed in this manner has a shorter retention time and different fragmentation pattern by GC/MS as compared to the slower eluting monoacetylated species N-trifluoroacetyl-4-hydroxy-L-proline methyl ester (N-TFA-Hyp). By selected ion monitoring of the appropriate ions of either N,O-TFA-Hyp (m/z 164, 278) or N-TFA-Hyp (m/z 164, 182) efficient quantitation of Hyp in collagen is possible within the broad range of 5-1000 ng with a lower limit of detection of 0.5 ng per injection. Measurement of 18O2 incorporation into collagen is possible by selected ion monitoring of the m/z 182 ion formed only from the monoacetylated derivative, N-TFA-Hyp, produced by methanol solvolysis of the N,O-TFA-Hyp derivative, as proposed herein.