Response of rat connective tissues to streptozotocin-diabetes. Tissue-specific effects on collagen metabolism

Different Stages of Alveolar Bone Repair Process Are Compromised in the Type 2 Diabetes Condition: An Experimental Study in Rats

Simple Summary

Type 2 diabetes (T2D) affects more than 90% of all patients diagnosed with diabetes, and among its risk factors, unhealthy eating habits are worth mentioning. With the notorious increase in the incidence of diabetic patients, there has also been an increase in surgical complications in dentistry, so this work presents a study model that mimics the T2D condition in rats, where animals receive a diet composed of foods rich in sugar and fat equivalent to the poor diet of the current population. The animals were submitted to dental extraction to perform analyzes at different stages of the alveolar bone. It is important to highlight that with the development of this experimental model it will be possible to simulate different conditions that are observed in clinics and in consequence and improve the characterization of the cellular responses involved in this complex condition of T2D. The scientific evidence presented in this study shows that T2D prolongs the local inflammatory process, which impairs the organization and maturation of collagen fibers, delaying bone formation and bone turnover. This fact implies in a series of disorders in dental practice, that would need to compensate in other ways, either with systemic medications or local therapies.

Abstract

The aim of this study was to analyze the stages of the alveolar bone repair in type 2 diabetic rats evaluating the mechanism of mineralization and bone remodeling processes after dental extraction. Forty-eight rats were divided into normoglycemic (NG) and type 2 diabetes (T2D) groups. The upper right incisor was extracted and after 3, 7, 14 and 42 days the animals were euthanized. The following analyses were performed: immunolabeling against antibodies TNFα, TGFβ, IL6, WNT, OCN and TRAP, collagen fibers maturation, microtomography and confocal microscopy. Data were submitted to statistical analysis. The immunolabeling analysis showed that the T2D presented a more pronounced alveolar inflammation than NG. Labeling of proteins responsible for bone formation and mineralization was higher in NG than T2D, which presented greater resorptive activity characterized by TRAP labeling. Also, T2D group showed a decrease in the amount of collagen fibers. Micro-CT analysis showed that T2D causes a decrease in bone volume percentage due to deficient trabecular parameters and higher porosity. The T2D bone dynamics show a loss in bone remodeling process. T2D prolongs the local inflammatory process, which impairs the organization and maturation of collagen fibers, delaying bone formation that generates impact on mineralization and bone turnover.

The role of extracellular matrix in the pathophysiology of diabetic wounds

Impaired healing leading to the formation of ulcerated wounds is a critical concern in patients with diabetes. Abnormalities in extracellular matrix (ECM) production and remodeling contribute to tissue dysfunction and delayed healing. Specifically, diabetes-induced changes in the expression and/or activity of structural proteins, ECM-modifying enzymes, proteoglycans, and matricellular proteins have been reported. In this review, we provide a summary of the key ECM molecules and associated changes in skin and diabetic wounds. Such information should allow for new insights in the understanding of impaired wound healing and lead to the development of ECM-based therapeutic strategies.

A slow release formulation of insulin as a treatment for osteoarthritis

OBJECTIVE

To examine the potential of insulin, in a sustained delivery system, as a treatment for arthritis.

DESIGN

The effect of insulin on matrix synthesis, matrix breakdown, and nitric oxide production in primary cartilage explants was examined. The activity of insulin on diseased cartilage from Dunkin Hartley guinea pigs, diabetic mice, and osteoarthritic patients was measured. The specificity of insulin stimulation was compared to that of IGF-I using osteoblasts and fibroblasts. Finally, the stability of insulin in a biologically relevant system was tested, and a slow-release formulation of insulin was developed and characterized.

RESULTS

In articular cartilage explants, insulin stimulated proteoglycan (PG) synthesis, inhibited PG release and nitric oxide production, and overcame the detrimental effects of interleukin 1 (IL-1). The mechanism whereby insulin decreased matrix breakdown was through inhibition of aggrecanase activity. Insulin was active on cartilage at concentrations at which insulin does not cross-react with insulin-like growth factor I (IGF-I) receptors nor stimulate proliferation of other cells types. The response of cartilage to insulin did not diminish with age or disease. Insulin stimulated matrix synthesis in osteoarthritic cartilage and local treatment with insulin overcame endogenous suppression of matrix synthesis in diabetic cartilage. Poly-lactic-coglycolic acid (PLGA) was found to be an effective carrier for delivery of insulin, and PLGA-Insulin was active on articular cartilage in vitro and in vivo.

CONCLUSIONS

As the incidence of arthritis increases with the aging population, an effective therapy to induce repair of cartilage is needed. Based on its biological activities, insulin appears to be an attractive protein therapeutic candidate. Maximum insulin effectiveness may require a sustained delivery system.

Strength of colonic anastomoses and skin incisional wounds in old rats - influence by diabetes and growth hormone

The influence of advanced age on the mechanical strength of colonic anastomoses and skin incisional wounds in diabetic rats was investigated after 0 (suture binding capacity) and after 7 days of healing. Furthermore, the effects of growth hormone (GH) injections to old diabetic rats were investigated. Diabetes in old rats did not influence the strength of colonic anastomoses after 0 and 7 days. However, in these diabetic animals, the strength of skin incisional wounds was reduced by 27% after 7 days of healing (P< 0.01). GH injections administered to old diabetic rats doubled the mortality compared with that of saline-injected old diabetic rats (P< 0.01). GH injections did not influence the strength formation of either colonic anastomoses or skin incisional wounds in old normal rats. In conclusion, the healing of colonic anastomoses in diabetic rats was not compromised by old age, while the strength of skin wounds was decreased.

The effect of diabetes mellitus on histopathological changes in the denture-supporting tissues under continuous mechanical pressure in rat

The histopathological changes in denture-supporting tissues of streptozotocin-induced diabetes mellitus rat were studied in relation to continuous mechanical pressure exerted through an experimental denture base. The experimental dentures were designed to load continuous mechanical pressures (3.4, 1.5 or 0.5 kPa) to the hard palate of the molar region of a rat. From the results of this study, the following conclusions were drawn: the streptozotocin-induced diabetic condition (1) tended to prolong the appearance period of a shortened epithelial ridge, and weakened the phenomenon, (2) reduced the appearance period and severity of the compression of epithelium, (3) delayed the time of manifestation of the proliferative change in the recovery process of the shortened epithelial ridge, and weakened the phenomenon, (4) prolonged the appearance period of the compressed lamina propria mucosae, and enhanced the phenomenon, (5) reduced the number of osteoclasts, (6) lowered the threshold for induction of the osteoclastic bone resorption to between 1.5 and 0.5 kPa, (7) inhibited the appearance of osteoblast which follows the disappearance of osteoclasts, (8) did not induce any inflammatory change, and (9) tended to enhance the longitudinal change of the continuous mechanical pressure. The histopathological changes in the denture-supporting tissues of the streptozotocin-induced diabetes mellitus rat related highly to the longitudinal change of the continuous mechanical pressure exerted through the denture base. From the results of this study, it was suggested that the streptozotocin-induced diabetic condition lowers the tolerance of the denture-supporting tissues to continuous mechanical pressure.

The effect of diabetes mellitus on histopathological changes in the tissues under denture base and without mechanical pressure

Histopathological changes in the tissues under a denture base were studied with respect to the difference between diabetic and normal rats. Diabetes mellitus was induced in Wistar rats by streptozotocin. The experimental denture base was designed to make contact with the palatal mucosa without any continuous nor masticatory pressure. Histopathological observation periods were 1, 2, 4, 8, 12 and 20 weeks after the insertion of the denture base. Histopathological examination revealed that only the transient slight proliferative reaction observed in mucosal epithelium under a denture base in normal non-diabetic rats, was lessened by induced diabetes mellitus. Moreover, covering the palatal mucosa with a denture base without any continuous nor masticatory pressure and with the maintenance of mucosal cleanliness under it and of the denture itself, did not cause any inflammatory change or bone resorption in the tissues under the base.

A chemically modified tetracycline inhibits streptozotocin-induced diabetic depression of skin collagen synthesis and steady-state type I procollagen mRNA

Wasting of connective tissues including skin, bone, and cartilage have been closely associated with elevated matrix metalloproteinase (MMP) activity and depressed collagen content in the streptozotocin (STZ)-induced diabetic rat, while tetracyclines have been reported to normalize total body weight, skin hydroxyproline and collagen content in this model, in part through inhibition of MMPs. In the present study, we report the effect of CMT-1, a chemically modified tetracycline that lacks antimicrobial properties but retains divalent cation binding and MMP inhibitory activity, on diabetic skin collagen synthesis and steady-state levels of procollagen alpha 1(I) mRNA. Male, 4-month old Sprague-Dawley rats received a single injection of 75 mg/kg STZ or citrate vehicle alone and diabetic status was confirmed by positive glucosuria. Some diabetic animals received 10 mg/day of CMT-1 by oral gavage and, 28 days after STZ treatment, body weight, blood glucose values and the in vivo rates of skin collagen production were measured using the pool-expansion technique. Steady-state levels of procollagen alpha 1(I) mRNA were analyzed 21 days after STZ treatment by hybridization of total RNA with a 32P labelled cDNA to rat type I procollagen alpha 1(I) mRNA in a dot-blot assay. STZ treatment was found to significantly depress body weight, skin collagen hydroxyproline content, the in vivo rate of collagen production, and hybridizable levels of type I procollagen alpha 1(I) mRNA. CMT-1 administered daily to STZ-treated rats inhibited the diabetic depression of these parameters but had little or no effect on non-diabetic controls or on STZ-induced hyperglycemia. Thus, in addition to the inhibition of MMP mediated extracellular collagen degradation, these results suggest CMT-1 also acts to inhibit diabetic connective tissue breakdown in STZ-induced diabetes by increasing both steady-state levels of type I procollagen mRNA and collagen synthesis through mechanism(s) that are independent of the antibacterial properties of tetracyclines.

Tetracycline prevents cancellous bone loss and maintains near-normal rates of bone formation in streptozotocin diabetic rats

The skeletal consequences of streptozotocin-induced (STZ) diabetes in the rat are characterized by decreased bone formation and, consequently, reductions in bone mass. Given the ability of tetracyclines to inhibit the breakdown of connective tissue collagen in experimental diabetes (and in other diseases), we examined the potential of this drug to prevent the osteopenia associated with STZ diabetes. To evaluate drug efficacy, the cortical and trabecular bone histomorphometry were analyzed and compared between vehicle-treated control and diabetic rats and control and diabetic rats treated orally with 20 mg/day of minocycline, a semisynthetic tetracycline. In addition, blood and urine glucose, body weight change, tibia lengths, cortical bone densities, and bone ash content were compared. At the end of the 26 day experimental period, diabetic (D) and minocycline-treated diabetic (MTD) rats were polyuric with reduced body weights and significantly elevated blood and urinary glucose levels (p < 0.01). Compared to control (C) and minocycline-treated control (MTC) animals, the periosteal and cancellous bone formation in the D rats had virtually ceased (p < 0.001), and the cancellous bone mass in the tibial metaphysis was reduced 47% (p < 0.01). In contrast, bone formation rates in the MTD animals were increased compared to the D rats (p < 0.001), while cancellous bone areas in the MTD animals were essentially equivalent to those observed in the C and MTC groups. Moreover, growth plate thickness, reduced 43% in the D rats, was preserved in the diabetic animals treated with minocycline. These results demonstrate that minocycline treatment of the streptozotocin diabetic rat maintains normal bone formation, normalizes growth plate thickness, and prevents cancellous bone loss.

Plasminogen activator activity is decreased in rat gingiva during diabetes

Diabetes produces extensive alterations of collagen metabolism including enhanced gingival collagenase activity. However, the mechanism for this enhanced enzyme activity is unclear. Collagenase is secreted from cells in a latent form and plasmin has been proposed as an important in vivo activator of procollagenase. Plasmin is converted from its precursor, plasminogen, by the proteolytic action of a serine proteinase, plasminogen activator (PA). The current study was therefore undertaken to determine the effect of diabetes on gingival PA activity in the rat. Since doxycycline is a potent collagenase inhibitor, the effect of doxycycline on gingival PA activity was also investigated. Eighteen male, Sprague-Dawley rats were made diabetic by streptozotocin injection (7 mg/100 g). Control rats (N = 8) were sham-treated. Doxycycline (5 mg/day/rat) was administered to 9 of the 18 diabetic rats by gavage on a daily basis. The other 9 diabetic rats were administered with saline. After 3 weeks, blood and gingival tissue were collected from each rat for the determination of glucose level and gingival PA activity. The tissues were then minced and extracted with 5 mM sodium phosphate containing 1% Triton X-100. PA assay was performed using chromatogenic substrate to determine PA activity in the extracts. Gingival PA activity in the diabetic rats was significantly reduced compared to the control (13.5 +/- 1.6 vs. 36.0 +/- 3.3 microunits/100 micrograms protein, P < 0.01). Doxycycline administration to diabetic rats had no effect on the already reduced gingival PA activity (10.4 +/- 3.5 in doxycycline-treated rats vs. 13.5 +/- 1.6 mu units/100 micrograms protein in untreated diabetic rats). PA activities in gingival tissues from the diabetic, nondiabetic control and doxycycline-treated diabetic groups were also demonstrated on zymographs as lytic bands. Regarding the well-known fact that gingival collagenase activity is enhanced during diabetes, our results did not support the notion that this biochemical alteration is attributed to increased activation of procollagenase by PA.

Response of immature diabetic rat bone-ligament junctions to insulin and exercise

The mechanical and morphological characteristics of femur-medial collateral ligament-tibia units and the histomorphometry of medial collateral ligament-tibial insertion were examined in female Sprague-Dawley rats with diabetes mellitus (type I, insulin-dependent diabetes). Diabetes was induced with the streptozotocin, a drug with toxic effects on insulin-producing islet cells in the pancreas. The groups studied included rats with untreated streptozotocin-induced diabetes mellitus (Diabetes), insulin-treated streptozotocin-induced diabetes (Diabetes-Insulin), exercise with streptozotocin-induced diabetes (Diabetes-Exercise), exercise with insulin-treated streptozotocin-induced diabetes (Diabetes-Insulin-Exercise), and age-matched sedentary-control rats (Control). Diabetes and Diabetes-Exercise groups had significantly lower body mass and higher blood glucose than Diabetes-Insulin, Diabetes-Insulin-Exercise, and Control groups, indicating that exercise alone did not prevent growth retardation or improve blood glucose control in the streptozotocin-induced diabetes. The strength of Diabetes femur-medial collateral ligament-tibia units was significantly less than Control, but exercise (with or without insulin treatment) maintained the bone-ligament-bone unit strength at a normal level. The load (per unit body mass) of Diabetes-Exercise femur-medial collateral ligament-tibia unit was significantly greater than Control, Diabetes, and Diabetes-Insulin groups. The tensile stiffness (per unit body mass) of the femur-medial collateral ligament-tibia unit for the Diabetes-Exercise group was also significantly greater than Control and Diabetes-Insulin groups. The fibroblast-like cell density in medial collateral ligament at its tibial-insertion site and medial collateral ligament insertion area was significantly less in Diabetes rats. RELEVANCE--:Diabetes mellitus can have deleterious effects on connective tissues, such as bone and ligament. Thus the bone-ligament junction becomes vulnerable. The results of this experiment suggest, however, that exercise, with or without insulin treatment, can have a positive effect on the bone-ligament interface in the rat with type I diabetes mellitus. Furthermore exercise (with insulin treatments) can mitigate growth retardation and many of the adverse effects of streptozotocin-induced diabetes on the structure and mechanical properties of immature bone-ligament junctions.

Diabetes impairs the development of early strength, but not the accumulation of collagen, during intestinal anastomotic healing in the rat

The evidence that diabetes mellitus adversely affects healing is derived from data on skin, where diminished wound strength is accompanied by a reduced collagen content. As no such data are available on intestinal anastomotic repair, and it has been reported that collagen metabolism in skin and intestine is affected differently by the diabetic state, anastomotic healing in the rat intestine during uncontrolled diabetes was examined. Wistar rats underwent resection and anastomosis of both ileum and colon 1 week after a single injection of streptozotocin (diabetic group) or citrate (control group). After 3 days the mean bursting pressure of ileal anastomoses was 4.1 kPa in controls and 1.0 kPa in diabetic animals (P < 0.05). For colon, these values were 12.0 and 4.9 kPa respectively (P < 0.05). This negative effect of diabetes persisted after 7 days in ileum but not in colon. Diabetic animals had more anastomotic abscesses, especially in the ileum: nine of 16 animals at 3 days after operation versus one of 15 in the control group (P < 0.01). Anastomotic hydroxyproline concentration and content in diabetic animals were not reduced. The postoperative collagen synthetic capacity, as measured in tissue explants, was strongly stimulated in the wound area and appeared essentially unchanged in the diabetic animals. Uncontrolled streptozotocin-induced diabetes severely impairs the development of anastomotic strength in the rat intestine but, unlike impaired wound healing in skin, this phenomenon is not caused by deficient accumulation of collagen.

Kinetic properties of osseous plate alkaline phosphatase from diabetic rats

1. Increased levels of bone alkaline phosphatase activity were observed in diabetic rats. These animals exhibited impaired bone development without concomitant alterations of the sequence of cellular transformations. 2. Alkaline phosphatase activity was delayed in diabetic rats but the kinetic parameters for the hydrolysis of p-Nitrophenylphosphate (PNPP) were virtually the same observed for controls (N = 1.2 and K0.5 = 43 microM). 3. Alkaline phosphatase from diabetic rats had a better affinity (K0.5 = 38 microM) for magnesium ions than controls (K0.5 = 91 microM). 4. Zinc ions affected alkaline phosphatase activity from control and diabetic rats in the same way (K0.5 = 10 microM).

Glucose and glucose analogs modulate collagen metabolism

Patients with diabetes often develop complications involving collagen-containing connective tissues. Previous in vitro studies have demonstrated that glucose inhibits collagen fibril formation and subsequent cross-linking. Collagen with diminished cross-linking is more susceptible to collagenolytic degradation. This may underlie the decreased collagen levels. To test this hypothesis, D-glucose and its two analogs, L-glucose and 2-deoxy-D-glucose, were used in chick calvaria organ cultures to examine parameters of collagen metabolism. L-Glucose is not used by the cell and functions as an extracellular glucose-like molecule, while 2-deoxy-D-glucose inhibits normal D-glucose uptake by blockading the glucose transport mechanism. Each of these three sugars had the ability to inhibit collagen fibril formation. D-Glucose stimulated collagen synthesis; L-glucose had no effect; and deoxyglucose inhibited collagen synthesis. D-Glucose was able to reverse the inhibitory effect of deoxyglucose. D-Glucose did not change levels of degradation of newly synthesized collagen while both L-glucose and deoxyglucose stimulated collagen degradation. When glucose transport was inhibited by deoxyglucose, collagen degradation was further enhanced. We suggest that decreased collagen levels in the connective tissues of diabetics may result from a combination of inhibition of collagen fibril formation and subsequent cross-linking, as well as increased collagen degradation.

STZ-induced diabetes in SHR and renovascular hypertensive rats: dissociation between changes in arterial pressure and vascular collagen synthesis

Streptozotocin (STZ)-induced diabetes depresses the rate of vascular collagen synthesis in the spontaneously hypertensive rat (SHR), but it also reduces arterial pressure (SAP) in this strain. We investigated this phenomenon further by comparing the SHR with the renovascular hypertensive (RVH) rat, because diabetes does not affect SAP in the latter model of hypertension. Renovascular hypertension was induced by clipping the left renal artery of Wistar-Kyoto (WKY) rats; sham-operated WKY were included as normotensive controls. Collagen synthesis of arterial tissue in vitro was quantified as prolyl hydroxylase activity and the rate of radioactive proline incorporation into collagen. Arterial collagen synthesis of nondiabetic SHR and RVH animals was elevated compared to that of the nonhypertensive WKY controls. STZ-induced diabetes (8 weeks) reduced SAP of SHR, but had no effect on SAP of either RVH or normotensive WKY rats. However, diabetes significantly depressed vascular collagen synthesis of both SHR and RVH rats, and, less consistently, of the WKY. The results strongly suggest that STZ-induced diabetes in SHR impairs arterial collagen synthesis independent of associated changes in arterial pressure.

Collagen synthesis and degradation in vivo. Evidence for rapid rates of collagen turnover with extensive degradation of newly synthesized collagen in tissues of the adult rat

Collagen turnover is now known to occur more rapidly in body tissues than traditionally believed, but the kinetics and mechanisms for degradation are still poorly understood. Here we measure collagen synthesis rates and the proportion of newly synthesized collagen (probably procollagen) which is rapidly degraded, in tissues of the adult rat after injection of [14C]-proline with a large "flooding" dose of unlabelled proline. Incorporation of [14C]-proline into lung, heart, skeletal muscle and skin collagen and its appearance as hydroxy [14C]-proline, free or in small molecular weight moieties, at various times up to one hour, suggested extremely rapid synthesis and degradation for some tissues of the adult rat. Values in heart, lung, skeletal muscle and skin (with the proportion of degradation of newly synthesized collagen shown in parentheses) were 5.2 +/- 0.7%/day (53 +/- 5%), 9.0 +/- 0.7%/day (37 +/- 2%), 2.2 +/- 0.3%/day (38 +/- 7%) and 4.4 +/- 1.3%/day (8.8 +/- 0.5%). These data provide in vivo evidence, which are consistent with the observation in isolated cells, that a proportion of newly synthesized collagen is degraded rapidly, and probably intracellularly, after its synthesis. They also indicate that collagen may be synthesized and degraded rapidly in normal rat tissues, but the mean turnover rates and the proportions of collagen degraded intracellularly vary widely between tissues.

In vivo collagen metabolism in spontaneously diabetic (db/db) mice

To further define the pathogenesis of diabetic connective tissue lesions, collagen synthesis and degradation were measured in vivo in spontaneously diabetic db/db mice. A double isotopic labeling technique, in which 14C-labeled and 3H-labeled proline were injected into the same mouse 7 days apart, was applied. Collagen synthesis and degradation were assessed in skins, intestines, hearts, and kidneys. There were no changes in collagen metabolism in the intestines of the diabetic mice. In all other tissues, collagen degradation was accelerated. Collagen synthesis was decreased in skins, but increased in the hearts and kidneys of the diabetic mice. These tissue-specific changes in collagen metabolism resulted in a net loss of collagen in all tissues examined except intestines. The results of this study provide insight into the mechanisms leading to connective tissue defects occurring in diabetes mellitus.

Diabetes stimulates procollagen degradation in rat tendon in vitro

To identify the mechanisms responsible for the paucity of recently synthesized collagen in connective tissues during diabetes, in vitro procollagen metabolism was studied in non-diabetic (control) and diabetic rats. Achilles tendons from the two groups were incubated for 1-8 h (35 degrees C) in medium containing [14C]proline and the radiolabeled collagen in the tissue, and that released into the media, were examined by SDS-polyacrylamide gel electrophoresis and fluorography. The bulk of the radiolabeled collagen in tendon from the diabetics was recovered as degradation products; these, but also procollagen and collagen components, were prominent in the control tissues. Moreover, the collagenous components synthesized by the diabetic rat tendons were more readily digested in vitro by trypsin than those produced by control tissues. We conclude that diabetes reduces collagen accretion in connective tissues in part due to increased intracellular degradation of procollagen.

Depressor effect of diabetes in spontaneously hypertensive rat: role of vascular reactivity and prolyl hydroxylase and lysyl oxidase activities

Streptozotocin (STZ)-induced diabetes (8 weeks) produced a marked depressor effect in the spontaneously hypertensive rat (SHR), confirming earlier studies, but had no effect on arterial pressure of normotensive controls (WKY). We investigated the phenomenon further by examining the effects of diabetes on the activities of aortic prolyl hydroxylase (PH) and lysyl oxidase (LO), marker enzymes for collagen biosynthesis, and on the reactivity of isolated mesenteric arteries to vasoactive agents. PH and LO activities of nondiabetic SHR were greater than those of the WKY controls. Diabetes markedly reduced PH and LO activities of SHR aortae, but had no significant effect on PH and LO activities of the WKY strain. The effects of diabetes on vascular collagen biosynthetic enzymes of SHR were not associated with reductions in mesenteric arterial responsiveness or sensitivity to norepinephrine, methoxamine, serotonin or KC1. These results suggest that the depressor effect of diabetes in SHR is associated with a reduction in vascular collagen biosynthesis but not a reduction in vascular reactivity.

Dietary ascorbic acid normalizes diabetes-induced underhydroxylation of nascent type I collagen molecules

Nascent collagen alpha chains (types I and III) isolated from diabetic rat skin were shown to be underhydroxylated, an underhydroxylation normalized (type I) or partially corrected (Type III) by dietary ascorbic acid. Increased hydroxylation occurred concomitantly with reduced intracellular procollagen degradation and increased production of nascent collagen molecules, both contributing to an increased total skin collagen mass. Overall, by correcting a defect (underhydroxylation) in a posttranslational event and by increasing collagen production, dietary ascorbic acid improved the collagen status of a diabetes-perturbed connective tissue.

Evidence for glucose-mediated covalent cross-linking of collagen after glycosylation in vitro

Rabbit forelimb tendons incubated for 15 or 21 days at 35 degrees C in the presence of 8 or 24 mg of glucose/ml were shown to change their chemical, biochemical and mechanical characteristics. The tendons treated with glucose contained up to three times as much hexosyl-lysine and hexosylhydroxylysine as did control tendons as judged by assay of NaB3H4-reduced samples. Measurement of the force generated on thermal contraction showed significant increases in glycosylated tendons compared with controls, indicating the formation of new covalent stabilizing bonds. This conclusion was supported by the decreased solubility of intact tendons and re-formed fibres glycosylated in vitro, and by the evidence from peptide maps of CNBr-digested glucose-incubated tendons. The latter, when compared with peptide maps of control tendons, revealed the presence of additional high-Mr peptide material. These peptides appear to be cross-linked by a new type of covalent bond stable to mild thermal and chemical treatment. This system in vitro provides a readily controlled model for the study of the chemistry of changes brought about in collagen by non-enzymic glycosylation in diabetes.

The effect of alloxan diabetes on prolyl and lysyl hydroxylase activity in uninflamed and inflamed rat gingiva

The influence of diabetes on gingival inflammation was studied through its effect on prolyl and lysyl-hydroxylase activities and on tissue-collagen content. Inflammation induced for 7 days with either endotoxin or antigen-antibody complexes reduced the activity of both enzymes by about 45 per cent, and decreased the concentration of soluble and insoluble collagens. Diabetes alone decreased the enzyme activity by more than 50 per cent but prevented the loss of the soluble and insoluble collagens which occurs during inflammation. The complex interaction of diabetes and inflammation on collagen metabolism in gingival tissue may be explained in part by abnormalities of collagen synthesis; alterations in collagen maturation and degradation, and in leukocyte function, may also occur.

Inhibition of collagen fibril formation in vitro and subsequent cross-linking by glucose

Glucose inhibits collagen fibril formation in vitro. A linear dose response was observed, with half-maximum inhibition of fibril formation occurring at 50 mM glucose. Nonfibrillar collagen cannot be cross-linked by lysyl oxidase, an enzyme that catalyzes the initial cross-linking reaction. The degree of decreased fibril formation correlated with the loss of ability of the collagen to serve as a substrate for lysyl oxidase. Collagen that is not cross-linked is unstable and more susceptible to collagenolytic attack. Interference with collagen cross-linking and more rapid degradation may explain the decreased amounts of interstitial collagen and the poor healing of wounds associated with diabetes mellitus.

Skin collagen metabolism in the streptozotocin-induced diabetic rat: free hydroxyproline, the principal in vivo degradation product of newly synthesized collagen--probably procollagen

We characterized the degradation products of recently synthesized collagen present in skins of control and diabetic rats. Specifically, the TCA-soluble fractions of homogenized skins from control and diabetic rats (killed 1 and 4 hours after [3H]-proline injection) were fractionated by molecular sieve chromatography, and eluted fractions were analyzed for hydroxyproline and [3H]-hydroxyproline. Free [3H]-hydroxyproline was the principal (greater than 95%), low molecular weight (greater than 2000 daltons), [3H]-hydroxyproline-containing material eluted from the molecular sieve column, this amount representing approximately 80% (controls) and approximately 87% (diabetics) of [3H]-hydroxyproline-containing material in TCA-soluble fractions of skin homogenates. These observations are similar to those from the intracellular degradation of cellular and secretory proteins in that the principal--almost exclusive--degradation product was the free amino acid. The free hydroxyproline had a greater specific radioactivity than that in any other [3H]-hydroxyproline-containing fraction (soluble and insoluble, see below); furthermore, the total radioactivity of free [3H]-hydroxyproline was greater at 1 hour than 3 hours later. These two properties (identity with free amino acid; time-dependent decrease in amounts) are consistent with [3H]-hydroxyproline arising from the intracellular degradation of procollagen. The [3H]-hydroxyproline-containing material eluting before free hydroxyproline (designated peptidyl [3H]-hydroxyproline) was similar to free [3H]-hydroxyproline in terms of specific radioactivity and the time-dependent decreases of specific and total radioactivities, these similarities indicating that the peptidyl [3H]-hydroxyproline are intermediates in the degradative pathway of procollagen to free amino acids. Results for control and diabetic rats were qualitatively similar, with regard to the inter-fraction ratios of specific radioactivities and their time-dependent changes. However, the degradative process, as assessed by the release of free and peptidyl [3H]-hydroxyproline, was dramatically enhanced by the diabetic state, extending our previous results based on analyses of uncharacterized degradation products.

Extensive degradation of recently synthesized collagen in gingiva of normal and streptozotocin-induced diabetic rats

The degradation of recently synthesized collagen (probably procollagen) in rat incisor gingiva was three times greater than that in skin. Concomitantly, the formation of undegraded (intact) collagen molecules in gingiva was slower than that in skin. This high basal rate of degradation in gingiva was just slightly increased in streptozotocin-induced diabetic rats, whereas the low basal rate in skin was dramatically increased by the diabetic state. The degradation of recently synthesized collagen was measured by the relative amounts(%) of [3H]hydroxyproline-containing material in the TCA-soluble fraction of a tissue, compared with the total amount (TCA-soluble + TCA-insoluble) of [3H]hydroxyproline-containing material. Separation of the TCA fractions allowed the formation of collagen degradation products (TCA-soluble) to be viewed separately from the formation of undegraded collagen molecules (TCA-insoluble). The [3H]hydroxyproline-containing material in the TCA-soluble fraction was greatest in amount and in specific activity, 30 min after [3H] proline injection, supporting the origin of this material as being procollagen. At this time period, the relative amounts of TCA-soluble [3H]hydroxyproline-containing material were 40.3% (gingiva) and 12.7% (skin). For diabetic rats, the values were 55% and 48.8%, respectively. For the [3H]hydroxyproline-containing material in the TCA-insoluble fraction, at 30 min, the specific activity of [3H]hydroxyproline was 4.3 for gingiva and 7.4 for skin. At all other time periods, the values were also greater for skin than for gingiva, making it unlikely that the formation of intact collagen molecules occurred faster in gingiva than in skin.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of streptozotocin-induced diabetes on prolyl hydroxylase activity in rat gingiva

The diabetes resulted in marked decrease in prolyl hydroxylase (EC 1.14.11.2) activity in rat gingiva. Insulin treatment of diabetic rats returned the decreased enzyme activity to the level of normal controls. The results suggest that diabetes decreases the rate of collagen biosynthesis in gingival tissue by inhibiting the hydroxylation of peptidyl proline in collagen precursors.

Intracellular degradation of newly synthesized collagen

The intracellular degradation of newly synthesized collagen is a cellular pathway that accounts for the destruction of 10-60% of collagen synthesized by a variety of cell types prior to secretion. This pathway can serve in a regulatory role to limit the secretion of defective molecules, and, in response to some extracellular mediators, regulates the amount and type of collagens secreted. In addition, this pathway may contribute to the pathogenesis of a variety of conditions affecting the extracellular matrix including fibrosis, diabetes mellitus, and scurvy.

Thermal stability, mechanical properties and reducible cross-links of rat tail tendon in experimental diabetes

Thermal stability (measured as isometric contraction force), biomechanical properties and reducible cross-links were measured in tail tendons from streptozotocin diabetic rats, with and without insulin treatment. After 10 days of diabetes the maximum thermal contraction force was unchanged, but the relaxation following the maximal contraction was retarded. After 30 days the maximum contraction force was increased and the relaxation rate was decreased. The maximum strength and stiffness of the tendons were increased after 10 days of diabetes and even more after 30 days. There was no change in the density of reducible cross-links. However, diabetes increased the amount of glucose attached to the lysine and hydroxylysine residues of collagen. Insulin treatment prevented all changes in thermal stability and mechanical properties. The results indicate that stabilization of collagen fibres in diabetes does not follow the same pattern as that seen in normal ageing.

Influence of streptozotocin and alloxan induced diabetes on the metabolism of dermal collagen in albino rats

The metabolism of collagen in male rats made diabetic by treatment with either streptozotocin or alloxan was studied after the injection of 3H-proline by estimating specific and total 3H-hydroxyproline activity in skin collagen fractions and urine. Experimentally induced diabetes was found to decrease the neutral salt-soluble and acid-soluble collagen with no change in insoluble collagen as compared to a control group. The specific and total radioactivity of 3H-hydroxyproline in soluble and insoluble collagen fractions were also decreased. Studies of total 3H-hydroxyproline activities in soluble collagens and insoluble collagen showed that the conversion of soluble to insoluble collagen was influenced by diabetes. Both streptozotocin and alloxan were found to increase urinary excretion of total hydroxyproline and 3H-hydroxyproline during the first 12 h after the administration of 3H-proline. Weekly analyses of urinary hydroxyproline also indicated a similar pattern. The results of the present investigation clearly indicate decreased synthesis and increased catabolism of collagen accompanied by accelerated conversion of soluble to insoluble collagen in experimentally induced diabetes.