Insulin-like growth factor I-dependent regulation of prolidase activity in cultured human skin fibroblasts

KLF6 activates Sp1-mediated prolidase transcription during TGF-β1 signaling

Prolidase (PEPD) is the only hydrolase that cleaves the dipeptides containing C-terminal proline or hydroxyproline-the rate-limiting step in collagen biosynthesis. However, the molecular regulation of prolidase expression remains largely unknown. In this study, we have identified overlapping binding sites for the transcription factors Krüppel-like factor 6 (KLF6) and Specificity protein 1 (Sp1) in the PEPD promoter and demonstrate that KLF6/Sp1 transcriptionally regulate prolidase expression. By cloning the PEPD promoter into a luciferase reporter and through site-directed deletion, we pinpointed the minimal sequences required for KLF6 and Sp1-mediated PEPD promoter-driven transcription. Interestingly, Sp1 inhibition abrogated KLF6-mediated PEPD promoter activity, suggesting that Sp1 is required for the basal expression of prolidase. We further studied the regulation of PEPD by KLF6 and Sp1 during transforming growth factor β1 (TGF-β1) signaling, since both KLF6 and Sp1 are key players in TGF-β1 mediated collagen biosynthesis. Mouse and human fibroblasts exposed to TGF-β1 resulted in the induction of PEPD transcription and prolidase expression. Inhibition of TGF-β1 signaling abrogated PEPD promoter-driven transcriptional activity of KLF6 and Sp1. Knock-down of KLF6 as well as Sp1 inhibition also reduced prolidase expression. Chromatin immunoprecipitation assay supported direct binding of KLF6 and Sp1 to the PEPD promoter and this binding was enriched by TGF-β1 treatment. Finally, immunofluorescence studies showed that KLF6 co-operates with Sp1 in the nucleus to activate prolidase expression and enhance collagen biosynthesis. Collectively, our results identify functional elements of the PEPD promoter for KLF6 and Sp1-mediated transcriptional activation and describe the molecular mechanism of prolidase expression.

Collagen metabolism as a regulator of proline dehydrogenase/proline oxidase-dependent apoptosis/autophagy

Recent studies on the regulatory role of amino acids in cell metabolism have focused on the functional significance of proline degradation. The process is catalysed by proline dehydrogenase/proline oxidase (PRODH/POX), a mitochondrial flavin-dependent enzyme converting proline into ∆1-pyrroline-5-carboxylate (P5C). During this process, electrons are transferred to electron transport chain producing ATP for survival or they directly reduce oxygen, producing reactive oxygen species (ROS) inducing apoptosis/autophagy. However, the mechanism for switching survival/apoptosis mode is unknown. Although PRODH/POX activity and energetic metabolism were suggested as an underlying mechanism for the survival/apoptosis switch, proline availability for this enzyme is also important. Proline availability is regulated by prolidase (proline supporting enzyme), collagen biosynthesis (proline utilizing process) and proline synthesis from glutamine, glutamate, α-ketoglutarate (α-KG) and ornithine. Proline availability is dependent on the rate of glycolysis, TCA and urea cycles, proline metabolism, collagen biosynthesis and its degradation. It is well established that proline synthesis enzymes, P5C synthetase and P5C reductase as well as collagen prolyl hydroxylases are up-regulated in most of cancer types and control rates of collagen biosynthesis. Up-regulation of collagen prolyl hydroxylase and its exhaustion of ascorbate and α-KG may compete with DNA and histone demethylases (that require the same cofactors) to influence metabolic epigenetics. This knowledge led us to hypothesize that up-regulation of prolidase and PRODH/POX with inhibition of collagen biosynthesis may represent potential pharmacotherapeutic approach to induce apoptosis or autophagic death in cancer cells. These aspects of proline metabolism are discussed in the review as an approach to understand complex regulatory mechanisms driving PRODH/POX-dependent apoptosis/survival.

PROLIDASE: A Review from Discovery to its Role in Health and Disease

Prolidase (peptidase D), encoded by the PEPD gene, is a ubiquitously expressed cytosolic metalloproteinase, the only enzyme capable of cleaving imidodipeptides containing C-terminal proline or hydroxyproline. Prolidase catalyzes the rate-limiting step during collagen recycling and is essential in protein metabolism, collagen turnover, and matrix remodeling. Prolidase, therefore plays a crucial role in several physiological processes such as wound healing, inflammation, angiogenesis, cell proliferation, and carcinogenesis. Accordingly, mutations leading to loss of prolidase catalytic activity result in prolidase deficiency a rare autosomal recessive metabolic disorder characterized by defective wound healing. In addition, alterations in prolidase enzyme activity have been documented in numerous pathological conditions, making prolidase a useful biochemical marker to measure disease severity. Furthermore, recent studies underscore the importance of a non-enzymatic role of prolidase in cell regulation and infectious disease. This review aims to provide comprehensive information on prolidase, from its discovery to its role in health and disease, while addressing the current knowledge gaps.

Current Understanding of the Emerging Role of Prolidase in Cellular Metabolism

Prolidase [EC 3.4.13.9], known as PEPD, cleaves di- and tripeptides containing carboxyl-terminal proline or hydroxyproline. For decades, prolidase has been thoroughly investigated, and several mechanisms regulating its activity are known, including the activation of the β₁-integrin receptor, insulin-like growth factor 1 receptor (IGF-1) receptor, and transforming growth factor (TGF)-β₁ receptor. This process may result in increased availability of proline in the mitochondrial proline cycle, thus making proline serve as a substrate for the resynthesis of collagen, an intracellular signaling molecule. However, as a ligand, PEPD can bind directly to the epidermal growth factor receptor (EGFR, epidermal growth factor receptor 2 (HER2)) and regulate cellular metabolism. Recent reports have indicated that PEPD protects p53 from uncontrolled p53 subcellular activation and its translocation between cellular compartments. PEPD also participates in the maturation of the interferon α/β receptor by regulating its expression. In addition to the biological effects, prolidase demonstrates clinical significance reflected in the disease known as prolidase deficiency. It is also known that prolidase activity is affected in collagen metabolism disorders, metabolic, and oncological conditions. In this article, we review the latest knowledge about prolidase and highlight its biological function, and thus provide an in-depth understanding of prolidase as a dipeptidase and protein regulating the function of key biomolecules in cellular metabolism.

Clinical Genetics of Prolidase Deficiency: An Updated Review

Prolidase is a ubiquitous enzyme that plays a major role in the metabolism of proline-rich proteins. Prolidase deficiency is a rare autosomal recessive inborn metabolic and multisystemic disease, characterized by a protean association of symptoms, namely intellectual disability, recurrent infections, splenomegaly, skin lesions, auto-immune disorders and cytopenia. To our knowledge, no published review has assembled the different clinical data and research studies over prolidase deficiency. The aim of this study is to summarize the actual state of the art from the descriptions of all the patients with a molecular diagnosis of prolidase deficiency reported to date regarding the clinical, biological, histopathological features, therapeutic options and functional studies.

Proline-dependent regulation of collagen metabolism

This review is focused on recent data on the role of proline (Pro) in collagen biosynthesis and cellular metabolism. It seems obvious that one of the main substrates for collagen biosynthesis Pro is required to form collagen molecule. The question raised in this review is whether the Pro for collagen biosynthesis is synthesized "de novo", comes directly from degraded proteins or it is converted from other amino acids. Recent data provided evidence that extracellular Pro (added to culture medium) had significant, but relatively little impact on collagen biosynthesis in fibroblasts (the main collagen synthesized cells) cultured in the presence of glutamine (Gln). However, extracellular Pro drastically increased collagen biosynthesis in the cells cultured in Gln-free medium. It suggests that Pro availability determines the rate of collagen biosynthesis and demand for Pro in fibroblasts is predominantly met by conversion from Gln. The potential mechanism of this process as well as possible implication of this knowledge in pharmacotherapy of connective tissue diseases is discussed in this review.

Acetylenic derivative of betulin induces apoptosis in endometrial adenocarcinoma cell line

Since betulin (Bet) and its acetylenic derivative, 28-O-propynoylbetulin (proBet) were shown to induce apoptosis in several cancer cell lines, we studied the mechanism of this process in human endometrial adenocarcinoma cells (EA). Previous studies suggested that this group of compounds affect prolidase activity (proline releasing enzyme from imidodipeptides) and collagen biosynthesis (proline utilizing process) providing substrate (proline) for proline oxidase (POX) dependent apoptosis. Here we provide evidence that Bet and proBet exhibit prolidase-inducing activity in EA cell line. However, in contrast to Bet, proBet inhibited collagen biosynthesis, increased intracellular proline concentration and induced apoptosis in EA cells, as detected by caspase-3, and -9 expressions and annexin V staining. Although POX expression was not affected by both compounds, the process of apoptosis was accompanied by increase in cytoplasmic level of proline. The mechanism for proBet-induced prolidase activity was found at the level of β1 integrin signaling. The inhibition of collagen biosynthesis was due to up-regulation of NF-κB p65, an inhibitor of collagen type I gene transcription. Although Bet and proBet induced expression of pro-apoptotic p53 in EA cells, the effect of proBet on the processes was much stronger. In contrast to proBet, Bet strongly induced expression of pro-survival factors, HIF-1α and VEGF. The data suggest that massive production of proline by proBet-dependent activation of prolidase and inhibition of proline utilization for collagen biosynthesis may represent mechanism for POX-dependent apoptosis in EA cells.

Exogenous proline stimulates type I collagen and HIF-1α expression and the process is attenuated by glutamine in human skin fibroblasts

Abundance of proline (Pro) in collagen molecule led us to investigate whether Pro supply affects collagen biosynthesis in human skin fibroblasts. Treatment of the cells with milimolar concentrations (5 and 10 mM) of Pro for 24 and 48 h contributed to increase in α₁ subunit of collagen type I (COL1A1) expression in both cells and culture medium. However, the effect was more pronounced in glutamine-free medium. In such condition, Pro induced collagen expression by about twofold in the cells, while in the medium only by about 30% during 24 h incubation, compared to control. In the presence of glutamine (Gln), exogenous Pro stimulated intracellular collagen expression only by about 30% during 24 h of fibroblasts incubation, and it was not accompanied by adequate increase of collagen secretion into medium. Gln alone stimulated the processes by about 2–3 fold during the course of the experiment. Pro-dependent increase in collagen expression in Gln-free medium was accompanied by increase in prolidase activity and expression of pAkt. In both Gln-free medium and Gln-supplemented medium, Pro induced expression of p53 and HIF-1α. The data suggest that availability of Gln, as a substrate for Pro biosynthesis, determine the utilization of exogenous Pro for the collagen biosynthesis.

Prolidase-proline dehydrogenase/proline oxidase-collagen biosynthesis axis as a potential interface of apoptosis/autophagy

Prolidase is a cytosolic imidodipeptidase that specifically splits imidodipeptides with C-terminal proline or hydroxyproline. The enzyme plays an important role in the recycling of proline from imidodipeptides for resynthesis of collagen and other proline-containing proteins. The mechanism of prolidase-dependent regulation of collagen biosynthesis was found at both transcriptional and post-transcriptional level. The increase in the enzyme activity is due to its phosphorylation on serine/threonine residues. Prolidase-dependent transcriptional regulation of collagen biosynthesis was found at the level of NF-κB, known inhibitor of type I collagen gene expression. Proline dehydrogenase/proline oxidase (PRODH/POX) is flavin-dependent enzyme associated with the inner mitochondrial membrane. The enzyme catalyzes conversion of proline into Δ(1) -pyrroline-5-carboxylate (P5C), during which reactive oxygen species (ROS) are produced, inducing intrinsic and extrinsic apoptotic pathways. Alternatively, under low glucose stress, PRODH/POX activation produces ATP for energy supply and survival. Of special interest is that PRODH/POX gene is induced by P53 and peroxisome proliferator-activated gamma receptor (PPARγ). Among down-regulators of PRODH/POX is an oncogenic transcription factor c-MYC and miR-23b*. On the other hand, PRODH/POX suppresses HIF-1α transcriptional activity, the MAPK pathway, cyclooxygenase-2, epidermal growth factor receptor and Wnt/b-catenin signaling. PRODH/POX expression is often down-regulated in various tumors, limiting mitochondrial proline utilization to P5C. It is accompanied by increased cytoplasmic level of proline. Proline availability for PRODH/POX-dependent ATP or ROS generation depends on activity of prolidase and utilization of proline in process of collagen biosynthesis. Therefore, Prolidase-PRODH/POX-Collagen Biosynthesis axis may represent potential interface that regulate apoptosis and survival. © 2016 BioFactors, 42(4):341-348, 2016.

Prolidase-dependent mechanism of (Z)-8,9-epoxyheptadeca-1,11,14-triene-induced inhibition of collagen biosynthesis in cultured human skin fibroblasts

The effects of polyolefinic compound from roots of Cirsium palustre, (Z)-8,9-epoxyheptadeca-1,11,14-triene (EHT) on collagen biosynthesis, prolidase activity, expression of insulin-like growth factor receptor (IGF-IR), β1 integrin, MAP kinases (pERK1/2), the transcription factors such as nuclear factor kappa B (NF-κB) and hypoxia-inducible factor-1α (HIF-1α) were evaluated in human dermal fibroblasts treated with micromolar concentrations (40-200 μM) for 24 h. It was found that EHT-dependent inhibition of collagen biosynthesis was accompanied by parallel inhibition in prolidase activity. Since IGF-I is the most potent regulator of both processes and prolidase is regulated by β1 integrin signalling, the effect of EHT on IGF-IR and β1 integrin receptor expressions were evaluated. Exposure of the cells to EHT contributed to distinct increase in IGF-IR and slight increase in β1 integrin receptor expressions. It was accompanied by decrease in expression of pERK1/2, HIF-1α and NF-κB. EHT-dependent inhibition of collagen biosynthesis results from inhibition of prolidase activity, the enzyme involved in collagen biosynthesis.

Enalapril stimulates collagen biosynthesis through prolidase-dependent mechanism in cultured fibroblasts

The mechanism of a lower incidence of dermatological manifestations in patients treated with enalapril compared to patients treated with other ACE-inhibitors, e.g., captopril, is not known. The finding that prolidase plays an important role in collagen biosynthesis and that some angiotensin-converting enzyme inhibitors affect prolidase activity led us to evaluate its effect on collagen biosynthesis in cultured human skin fibroblasts. Since insulin-like growth factor (IGF-I) and transforming growth factor beta 1 (TGF-β1) are the most potent stimulators of both collagen biosynthesis and prolidase activity, and prolidase is regulated by β₁ integrin signaling, the effect of enalapril and enalaprilat on IGF-IR, TGF-β1, and β₁ integrin receptor expressions was evaluated. Cells were treated with milimolar concentrations (0.3 and 0.5 mM) of enalapril and enalaprilat for 24 h. The activity of prolidase was determined by colorimetic assay. Collagen biosynthesis was evaluated by radiometric assay. Expression of signaling proteins was evaluated using Western blot. It was found that enalapril- and enalaprilat-dependent increase in prolidase activity and expression was accompanied by parallel increase in collagen biosynthesis. The exposure of the cells to 0.5 mM enalapril and enalaprilat contributed to increase in IGF-IR and α₂β₁ integrin receptor as well as TGF-β1 and NF-κB p65 expressions. Enalapril- and enalaprilat-dependent increase of collagen biosynthesis in fibroblasts results from increase of prolidase activity and expression, which may undergo through activation of α₂β₁ integrin and IGF-IR signaling as well as upregulation of TGF-β1 and NF-κB p65, the inhibitor of collagen gene expression.

The mechanism of oxythiamine-induced collagen biosynthesis in cultured fibroblasts

The oxythiamine (OXY) is antivitamin of thiamine. The finding that OXY increases the cytoplasmic concentration of pyruvate, known to enhance collagen biosynthesis, led us to investigate the mechanism of this antivitamin action on collagen biosynthesis in cultured human skin fibroblasts. Confluent fibroblasts were treated with micromolar concentrations (30–1,000 µM) of OXY for 24 and 48 h. It was found that OXY-dependent increase in collagen biosynthesis was accompanied by parallel increase in prolidase activity and level, compared to untreated cells. Since phosphoenolpyruvate (PEP) is known as an inhibitor of prolidase—the enzyme that plays important role in collagen biosynthesis, the mechanism of pyruvate interconversion was considered as a regulatory switch in collagen biosynthesis. In fact, 3-MPA, specific inhibitor of phosphoenolpyruvate carboxykinase (PEPCK), contributed to up-regulation of prolidase activity, suggesting that down-regulation of PEP formation is an underlying mechanism. Since collagen biosynthesis and prolidase activity are regulated by signal induced by activated α₂β₁ integrin receptor as well as insulin-like growth factor-I receptor (IGF-IR), the expression of these receptors was measured by Western immunoblot analysis. The exposure of the cells to OXY contributed to decrease in IGF-IR, α₂β₁ integrin receptor, pERK1/2, and NF-κB p65 expressions. It was accompanied by increase in total ERK1/2 expression and induction of phosphorylation of Akt protein. The data suggest that OXY-dependent increase of collagen biosynthesis in cultured human skin fibroblasts results from activation of prolidase activity and level, induction in pAkt expression and down-regulation of pERK1/2 and NF-κB p65, the known inhibitor of collagen gene expression.

Cross-talk between integrin receptor and insulin-like growth factor receptor in regulation of collagen biosynthesis in cultured fibroblasts

PURPOSE

Cellular processes are regulated by signals generated by adhesion receptors and growth factor receptors. IGFbinding protein 1 (IGFBP-1) is a molecule which may affect the both signaling pathways through inactivation of IGF-I (ligand for IGF-IR) and binding to RGD region of integrin receptors. Whether this phenomenon is important in communication between insulin-like growth factor receptor (IGF-IR) and β1-integrin receptor in regulation of prolidase activity and collagen biosynthesis is the aim of this study.

MATERIAL AND METHOD

We studied the effects of IGFBP-1, IGF-I, thrombin (integrin activator), echistatin (disintegrin), phosphatidylinositol 3-kinase inhibitor (LY-294002) and ERK 1/2 inhibitors (PD98059 and UO126) on prolidase activity, collagen biosynthesis and expression of proteins participating in pathways generated by these receptors.

RESULTS

Stimulation of β1-integrin and IGF-I receptors by standard ligands was proved to up-regulate collagen synthesis in cultured fibroblasts. IGFBP-1, similarly as echistatin and studied inhibitors, contributed to down-regulation of ERK1/2, Akt, mTOR expression and up-regulation of NFκB. It was accompanied by parallel decrease in prolidase activity and collagen biosynthesis.

CONCLUSION

The data suggest that "cross talk" between IGF-I receptor and integrin receptor may play important role in regulation of prolidase activity and collagen biosynthesis.

UVC inhibits collagen biosynthesis through up-regulation of NF-κB p65 signaling in cultured fibroblasts

The effects of UVC on collagen biosynthesis, prolidase activity, expression of α₂β₁ integrin, IGF-I receptor, FAK, MAP-kinases (ERK1 and ERK2) and the transcription factor NF-κB p65 were evaluated in human dermal fibroblasts. Confluent fibroblasts were treated with UVC light at a rates of 30 and 60 J/m(2). It was found that UVC-dependent decrease in collagen biosynthesis was not accompanied by parallel decrease in prolidase activity and expression. Since insulin-like growth factor receptor (IGF-IR) and α₂β₁ integrin signaling are the most potent regulators of collagen biosynthesis, the effect of UVC on IGF-IR and α₂β₁ integrin receptor expressions were evaluated. It was found that the exposure of the cells to UVC contributed to decrease in α₂β₁ integrin receptor and FAK expression and to an increase in IGF-IR and pERK1, pERK2 expressions. It was accompanied by an increase in the expression of NF-κB p65, the known inhibitor of collagen gene expression. The data suggest that UVC-dependent decrease of collagen biosynthesis in cultured human skin fibroblasts results from decrease in α₂β₁ integrin receptor signaling and activation of NF-κB p65, that is responsible for down-regulation of collagen gene expression.

Serum prolidase activity in diabetic foot ulcers

Impaired wound healing is an important problem in diabetes mellitus; however, its pathogenesis remains unclear. We aimed to evaluate serum prolidase activity (SPA), an important marker of collagen turnover, in subjects with and without diabetic foot ulcers as compared with healthy controls. Twenty-seven patients with diabetic foot ulcers (foot ulcer group), 27 patients without diabetic foot ulcers (diabetic control group) and 27 healthy controls were enrolled. The study groups had similar age, sex distribution and body mass index. Metabolic and inflammatory parameters as well as SPA were determined. The diabetic foot ulcer group had significantly higher SPA (both p < 0.001) when compared with the diabetic and the healthy control groups. SPA showed a positive correlation with high-sensitive C-reactive protein and a negative correlation with high-density lipoprotein cholesterol levels (r = 0.313, p = 0.021 and r = -0.233, p = 0.036, respectively). No correlation was detected between SPA and glycaemic parameters. SPA appears to be higher in patients with diabetic foot ulcers when compared with patients without diabetic foot ulcers and healthy controls. The underlying mechanisms of elevated SPA and its clinical significance in predicting the natural course of wound healing in diabetic foot ulcers needs to be further evaluation.

The mechanism of hydralazine-induced collagen biosynthesis in cultured fibroblasts

The finding that hydralazine (HYD) affects collagen metabolism led us to investigate the mechanism of its action on collagen biosynthesis, prolidase expression and activity, expression of α₂β₁ integrin, insulin-like growth factor-I receptor (IGF-IR), focal adhesion kinase (FAK), mitogen-activated protein (MAP) kinases (ERK₁, ERK₂), and transcription factors hypoxia-inducible factor-1α (HIF-1α) and nuclear factor-κB p65 (NF-κB p65) in human dermal fibroblasts. Confluent fibroblasts were treated with micromolar concentrations (50–500 μM) of HYD for 24 h. HYD had no influence on cell viability. It was found that HYD-dependent increase in collagen biosynthesis was accompanied by a parallel increase in prolidase activity and expression, HIF-1α expression, and decrease in DNA biosynthesis, compared to untreated cells. Since collagen biosynthesis and prolidase activity are regulated by a signal induced by activated α₂β₁ integrin receptor as well as IGF-IR, the expression of these receptors was measured by Western immunoblot analysis. The exposure of the cells to HYD contributed to the increase in IGF-IR expression without any effect on α₂β₁ integrin receptor and FAK expressions. It was accompanied by a decrease in expression of MAP kinases and NF-κB p65, the known inhibitor of collagen gene expression. The data suggest that the HYD-dependent increase of collagen biosynthesis in cultured human skin fibroblasts results from activation of IGF-IR expression and prolidase activity and downregulation of NF-κB p65.

The retinoic acid-induced up-regulation of insulin-like growth factor 1 and 2 is associated with prolidase-dependent collagen synthesis in UVA-irradiated human dermal equivalents

BACKGROUND

Ultraviolet (UV) A irradiation causes the degeneration of extracellular matrix in the skin dermis, mainly due to disrupted collagen homeostasis, resulting in the photo-aging of human skin. All-trans retinoic acid (ATRA) improves photo-aged human skin in vivo.

OBJECTIVES

Although the effects of ATRA on collagen synthesis and MMP regulation are well known, the effects of ATRA on other collagen homeostasis-associated genes have not been elucidated. This study was aimed to study the factors that are pharmacologically associated with the effect of ATRA on collagen homeostasis.

METHODS

The gene transcription profile of collagen homeostasis-associated genes was systematically evaluated in three-dimensional human dermal equivalents (HDEs) following UVA-irradiation and/or ATRA treatment.

RESULTS

In addition to the expected changes in MMPs and collagen synthesis in HDEs in response to ATRA, prolidase, an important enzyme in the recycling of proline and hydroxyproline from degraded collagen molecules, was significantly decreased by UVA irradiation, and its down-regulation was antagonized by ATRA. Transfection with a prolidase-specific siRNA led to a significant decrease in procollagen synthesis in human fibroblasts. ATRA inhibited the UVA irradiation-induced decrease in prolidase activity through an insulin-like growth factor (IGF) receptor signaling pathway in HDEs. ARTA increased IGF1 and IGF2 production in HDEs, and neutralizing IGFs with anti-IGF antibodies abolished the effect of ATRA on proliase activity.

CONCLUSIONS

These data demonstrate that ATRA regulates prolidase activity in HDEs via IGF receptor signaling, suggesting one of the pharmacological mechanisms by which improves photo-aged human skin.

Scutellarin-dependent inhibition of collagen biosynthesis in cultured fibroblasts

The effects of the flavonoid compound scutellarin (SCUT) on collagen biosynthesis, prolidase activity, expression of β₁ integrin, insulin-like growth factor-I (IGF-I) receptor and the transcription factor NF-κB were evaluated in human dermal fibroblasts. Confluent fibroblasts were treated with micromolar concentrations (10-30 µM) of SCUT for 24 h. It was found that a SCUT-dependent decrease in collagen biosynthesis was accompanied by an increase in prolidase activity. Since the IGF-I receptor (IGF-IR) is the most potent regulator of both collagen biosynthesis and prolidase activity, and prolidase is regulated by β₁ integrin signalling, the effect of SCUT on IGF-IR and β₁ integrin receptor expressions were evaluated. It was found that the exposure of the cells to SCUT contributed to an increase in IGF-IR and β₁ integrin receptor expressions. This was accompanied by an increase in expression of NF-κB, the known inhibitor of collagen gene expression. These data suggest that the SCUT-dependent decrease of collagen biosynthesis in cultured human skin fibroblasts results from activation of NF-κB, which is responsible for the down-regulation of collagen gene expression.

Proline metabolism and microenvironmental stress

Proline, the only proteinogenic secondary amino acid, is metabolized by its own family of enzymes responding to metabolic stress and participating in metabolic signaling. Collagen in extracellular matrix, connective tissue, and bone is an abundant reservoir for proline. Matrix metalloproteinases degrading collagen are activated during stress to make proline available, and proline oxidase, the first enzyme in proline degradation, is induced by p53, peroxisome proliferator-activated receptor gamma (PPARgamma) and its ligands, and by AMP-activated protein kinase downregulating mTOR. Metabolism of proline generates electrons to produce ROS and initiates a variety of downstream effects, including blockade of the cell cycle, autophagy, and apoptosis. The electrons can also enter the electron transport chain to produce adenosine triphosphate for survival under nutrient stress. Pyrroline-5-carboxylate, the product of proline oxidation, is recycled back to proline with redox transfers or is sequentially converted to glutamate and alpha-ketoglutarate. The latter augments the prolyl hydroxylation of hypoxia-inducible factor-1alpha and its proteasomal degradation. These effects of proline oxidase, as well as its decreased levels in tumors, support its role as a tumor suppressor. The mechanism for its decrease is mediated by a specific microRNA. The metabolic signaling by proline oxidase between oxidized low-density lipoproteins and autophagy provides a functional link between obesity and increased cancer risk.

Serum prolidase and IGF-1 as non-invasive markers of hepatic fibrosis during four different periods after bile-duct ligation in rats

AIM

Our aim was to study the correlation of serum prolidase and insulin like growth factor-1 to liver collagen and assess their utility as markers of fibrosis during four different periods of hepatic injury and fibrosis after bile-duct ligation in rats.

METHODS

Forty-eight Wistar albino rats were included in the study and divided into six groups. Seven rats served as the control group (Control), while seven rats had a sham operation (Sham group). Thirty-four rats underwent bile-duct ligation. Bile-duct ligated (BDL) animals were sacrificed at the end of the first week (Group 1; n = 8), second week (Group 2; n = 8), third week (Group 3; n = 9), or fourth week (Group 4; n = 9) after BDL. Liver collagen, liver prolidase, and serum prolidase and IGF-I, were determined.

RESULTS

There was a positive correlation between liver collagen and serum prolidase (r(s): 0.843, P < 0.001) levels and a negative correlation among liver collagen and serum IGF-1 levels (r(s): -0.667, P < 0.001). The peak levels of liver collagen and serum prolidase were reached in the third week while the lowest levels of IGF-1 were found at the end of the third week.

CONCLUSION

Serum prolidase and IGF-1 either independently or in combination correlate with liver collagen content in hepatic fibrosis.

Prolidase-dependent regulation of collagen biosynthesis

Prolidase [EC.3.4.13.9] is a cytosolic imidodipeptidase, which specifically splits imidodipeptides with C-terminal proline or hydroxyproline. The enzyme plays an important role in the recycling of proline from imidodipeptides (mostly derived from degradation products of collagen) for resynthesis of collagen and other proline-containing proteins. The enzyme activity is up-regulated by beta(1)-integrin receptor stimulation. The increase in the enzyme activity is due to its phosphorylation on serine/threonine residues. Collagen is not only structural component of extracellular matrix. It has been recognized as a ligand for integrin receptors, which play an important role in signaling that regulate ion transport, lipid metabolism, kinase activation and gene expression. Therefore, changes in the quantity, structure and distribution of collagens in tissues may affect cell signaling, metabolism and function. Several line of evidence suggests that prolidase activity may be a step-limiting factor in the regulation of collagen biosynthesis. It has been shown in different physiologic and pathologic conditions. It is of great importance during wound healing, inflammation, aging, tissue fibrosis and possibly skeletal abnormalities seen in Osteogenesis Imperfecta. The mechanism of prolidase-dependent regulation of collagen biosynthesis was found at both transcriptional and post-transcriptional levels. In this study, we provide evidence for prolidase-dependent transcriptional regulation of collagen biosynthesis. The mechanism was found at the level of NF-kB, known inhibitor of type I collagen gene expression. Modulation of integrin-dependent signaling by stimulatory (i.e. thrombin) or inhibitory (i.e. echistatin) beta(1)-integrin ligands or by nitric oxide donors (i.e. DETA/NO) affect prolidase at post-transcriptional level. All those factors may represent novel approach to pharmacotherapy of connective tissue disorders.

Protective effect of hyaluronic acid on interleukin-1-induced deregulation of beta1-integrin and insulin-like growth factor-I receptor signaling and collagen biosynthesis in cultured human chondrocytes

The mechanism of protective action of hyaluronic acid (HA) on collagen metabolism disturbances in tissues during inflammation is not known. Insulin-like growth factor-I (IGF-I) receptor and beta1-integrin receptor signaling plays an important role in the regulation of collagen biosynthesis at both transcriptional and post-transcriptional levels. The present study was undertaken to evaluate the effect of IL-1beta (inductor of experimental inflammation) on the signaling pathways as well as on collagen biosynthesis, gelatinases and prolidase activity in cultured human chondrocytes and the effect of HA on these processes. It was found that IL-1beta-dependent inhibition of collagen biosynthesis was accompanied by increase in beta1-integrin receptor, NF-kB expressions, and increase in phosphorylation of FAK, that resulted in stimulation of metalloproteinase MMP-2 and MMP-9 activities, but not prolidase activity and expression. Simultaneously, decrease in expression of IGF-I receptor and phosphorylation of Akt and p38 were found. All those processes were counteracted by HA. This suggests that cross talk between beta1-integrin and IGF-I receptors is disturbed by IL-1beta, and HA recovers their proper signaling in cultured chondrocytes. We propose that IGF-I receptor and beta1-integrin signaling may play an important role in protective effect of hyaluronic acid on interleukin-1-induced inhibition of collagen biosynthesis in cultured human chondrocytes.

Alpha-ketoglutarate stimulates procollagen production in cultured human dermal fibroblasts, and decreases UVB-induced wrinkle formation following topical application on the dorsal skin of hairless mice

Alpha-ketoglutarate is a key intermediate in the Krebs cycle, and a rate-limiting cofactor of prolyl-4-hydroxylase. It also has a potent effect on increasing the proline pool during collagen production, but the details underlying the boosting effect on collagen production by alpha-ketoglutarate remain as yet unreported. To investigate the effects of alpha-ketoglutarate on procollagen production and wrinkle formation, we conducted experiments in cultured human dermal fibroblasts and UVB-irradiated hairless mice. Based on ELISA measurements, alpha-ketoglutarate (10 microM) stimulated procollagen production in fibroblasts by 25.6+/-4.6% compared to vehicle (dH(2)O)-treated control cells. Also, we demonstrated that alpha-ketoglutarate increased activities of prolidase, which is known to play an important role in collagen metabolism, in fibroblasts and N-benzyloxycarbonyl-L-proline (Cbz-Pro), prolidase inhibitor, inhibited procollagen synthesis by alpha-ketoglutarate in fibroblasts. To determine the effect of topically applied alpha-ketoglutarate on wrinkle formation, alpha-ketoglutarate (1%) and vehicle (70% propylene glycol, 30% ethanol) were applied on the dorsal skin of UVB-induced hairless mice for twelve weeks. We found that alpha-ketoglutarate decreased wrinkle formation upon long-term topical application. These results suggest that alpha-ketoglutarate diminishes UVB-induced wrinkle formation by increasing collagen production, through a pathway that involves prolidase activation. Therefore, application of alpha-ketoglutarate may represent an effective anti-wrinkle agent for the cosmetic field.

Enhanced prolidase activity and decreased collagen content in breast cancer tissue

Adherent interactions between integrins and extracellular matrix (ECM) proteins play an important role in tumorigenicity and invasiveness. The major component of ECM is collagen that plays a central role in the interaction with integrins. The expression of certain collagenases (gelatinases) by tumour cells is one of the characteristic features of the so-called metastatic phenotype, presumably by breaking down ECM barriers as well by altering the ECM-cell interaction. Although extracellular collagenases initiate the breakdown of collagen, the final step of collagen degradation is catalysed by intracellular prolidase. Collagen deposition, gelatinolytic and prolidase activities, expression of beta(1)-integrin receptor and their possible relationships were studied in seven operable breast cancer cases. In breast cancer tissue, we have found significant decrease in the amount of collagen. The decrease in collagen deposition in breast cancer tissue was accompanied by increase in the tissue gelatinolytic and prolidase activities. Simultaneously, a slight decrease in the expression of beta(1)-integrin receptor in breast cancer tissue was observed. These results suggest that alteration in collagen metabolism in breast cancer tissue may reflect tissue remodelling, characteristic for invasive phenotype of cancer cells. Increased gelatinolytic and prolidase activities in breast cancer tissue may enhance stromal matrix degradation and thus may promote metastatic dissemination. On the basis of the data, it seems that compounds endowed with gelatinolytic and prolidase inhibitory activities may be considered as a potential drug candidates for breast cancer therapy.

Hyaluronic acid counteracts interleukin-1-induced inhibition of collagen biosynthesis in cultured human chondrocytes

Although, hyaluronic acid (HA) is used in the treatment of osteoarthritis for 30 years, the mechanism of its protective action on collagen metabolism disturbances in tissues during inflammation is not known. Therefore, the present study was undertaken to evaluate the mechanism of IL-1beta action (inductor of experimental inflammation) on deregulation of collagen biosynthesis in cultured human chondrocytes and the effect of HA on the process. It has been found that IL-1beta strongly induced inhibition of collagen biosynthesis, while HA counteracted the process. The mechanism of this phenomenon was found at both transcriptional and post-transcriptional level. IL-1 was found to down regulate the expression of mRNA for type II collagen and to inhibit prolidase activity, an enzyme that plays an important role in collagen biosynthesis at post-translational level. HA was shown to counteract the IL-1beta-dependent inhibition of both processes. During experimental inflammation of chondrocytes cultured in 0.1% FBS there was no differences in the expression of beta(1)-integrin independently of cell number and the presence of HA in growth medium. In chondrocytes cultured in 5% FBS, IL-1beta up-regulated the expression of beta(1)-integrin receptor while HA abolished the effect. The data suggest that HA-dependent up-regulation of collagen biosynthesis in IL-1beta-treated chondrocytes may involve stimulation of prolidase activity in serum "starved" cells and may also originate at the transcriptional level in the cells cultured in standard conditions.

Nitric oxide regulates prolidase activity by serine/threonine phosphorylation

Prolidase [E.C. 3.4.13.9], a member of the matrix metalloproteinase (MMP) family, is a manganese-dependent cytosolic exopeptidase that cleaves imidodipeptides containing C-terminal proline or hydroxyproline. It plays an important role in collagen metabolism, matrix remodeling and cell growth. Nitric oxide (NO), a versatile signaling molecule, regulates many processes including collagen synthesis and matrix remodeling and, thereby, may modulate angiogenesis, tumor invasiveness, and metastasis. Thus, we considered that prolidase may be an important target of NO regulation. In our study, SIN I and DETA/NO were used as NO donors. Both donors increased prolidase activity in a time-dependent and dose-dependent manner. Prolidase activity increased not only with NO donors but also with endogenous NO in cells transfected with iNOS. The effect of iNOS was abolished by treatment with S-methylisothiourea (SMT), a selective inhibitor of iNOS. However, with either exogenous or endogenous sources of NO, the increase in prolidase activity was not accompanied by increased prolidase expression. Therefore, we suspected phosphorylation of prolidase as a potential mechanism regulating enzyme activation. We observed increased serine/threonine phosphorylation on prolidase protein in cells treated with NO donors and in cells transfected with iNOS. To determinate the pathways that may mediate prolidase induction by NO, we first used 8-Br-cGMP, a cGMP agonist, and found that 8-Br-cGMP strongly and rapidly stimulated prolidase activity accompanied by increased phosphorylation. Rp-8-Br-pCPT-cGMP, an inhibitor of cGMP, reduced NO donor-stimulated prolidase activity to control levels. To test whether the MAPK pathway is involved in this NO-dependent activation, we used an ERK1/2 inhibitor and found that it had no effect on prolidase activity increased by NO donors. These results demonstrate that NO stimulates prolidase activity by increasing serine/threonine phosphorylation through PKG-cGMP pathway, but independent of MAPK and suggest an interaction between inflammatory signaling pathways and regulation of the terminal step of matrix degradation.

Butyrate-induced collagen biosynthesis in cultured fibroblasts is independent on alpha2beta1 integrin signalling and undergoes through IGF-I receptor cascade

The potential role of butyrate to modulate cellular metabolism through integrin receptor led to evaluation of its effect on collagen biosynthesis in cultured fibroblasts. Confluent human dermal fibroblasts were treated with 2 mM and 4 mM of sodium butyrate (NaB) for 48 h. It was found that butyrate induced collagen biosynthesis and prolidase activity independently of alpha2beta1 integrin signaling. The expressions of both alpha2 and beta1 integrin subunits as well as integrin-induced activation of focal adhesion kinase (FAK) were not affected in the cells treated with NaB. Since insulin-like growth factor-I (IGF-I) is the most potent stimulator of collagen biosynthesis in fibroblasts, the effect of butyrate on IGF-I receptor (IGF-IR) expression was evaluated. It was found that the exposure of the cells to 4 mM butyrate contributed to a distinct increase in IGF-IR. It was accompanied by a parallel increase in the expression of Sos protein and MAP-kinases (ERK1, ERK2). The data suggests that butyrate-dependent stimulation of collagen biosynthesis in cultured human skin fibroblasts undergoes through IGF-IR signaling.

Gly511 to Ser substitution in the COL1A1 gene in osteogenesis imperfecta type III patient with increased turnover of collagen

Osteogenesis imperfecta (OI) is a result of heterozygous mutations in the COL1A1 or COL1A2 genes, encoding type I procollagen chains. Here we described the molecular and biochemical defects detected in a case of severe type III OI. Cultured skin fibroblasts from the proband produced both normal and mutant type I collagen which was secreted into the medium. The mutation site was localized in alpha 1(I)-CB3 by CNBr cleavage of collagen chains. Subsequent reverse transcription-PCR amplification and direct sequencing of single-stranded PCR product led to identification of G to A transition in the COL1A1 gene, resulting in Gly511Ser substitution in the a1 chain of type I collagen. The new mutation conforms to the chain-specific non-lethal microdomain of Gly to Ser substitutions in the genotype-phenotype map. We have found that biosynthesis of collagen was increased in OI cells to about 160% of the control value. However, the amount of collagen deposed to the insoluble matrix was decreased as compared to the control. This suggests increased degradation of collagen, since the collagenolytic activity of OI cells was increased. Furthermore, the activity of prolidase, which is a marker of collagen turnover, was increased in OI cells. In regulation of activity of the enzyme are involved beta1 integrin and insulin-like growth factor (IGF) receptors. Western immunoblot analysis showed that the expressions of both receptors were markedly increased in OI cells. These results suggest that increase in activity of prolidase can be associated with increase in intensity of collagen metabolism in type III OI patient with identified new G511S mutation.

Defects of type I procollagen metabolism correlated with decrease of prolidase activity in a case of lethal osteogenesis imperfecta

We have studied the structure and metabolism of type I procollagen in a case of perinatal lethal osteogenesis imperfecta (OI) type II. Cultured skin fibroblasts from the proband synthesized both normal and abnormal forms of type I procollagen. Some abnormal, overmodified molecules were secreted by OI cells, although less efficiently than normal molecules from control cells. The OI fibroblasts accumulated large amounts of abnormal proalpha1(I) and proalpha2(I) chains intracellularly. The extracellular collagenolytic activity was decreased compared to control cells. Furthermore, OI cells produced less type I procollagen and demonstrated lower capacity to synthesize DNA than control cells. We have found that in contrast to prolinase activity, the activity of prolidase (an enzyme essential for collagen synthesis and cell growth) is also significantly reduced in OI cells. No differences were found in the amount of the enzyme protein recovered from both the OI and control cells. However, we found that expressions of beta1 integrin and insulin-like growth factor-I receptor (receptors known to play an important role in up regulation of prolidase activity) were decreased in OI cells compared to control cells. The decrease in prolidase activity may provide an important mechanism of altered cell growth and collagen metabolism involved in producing the perinatal lethal form of the OI phenotype.

Phosphorylation of prolidase increases the enzyme activity

Prolidase [EC 3.4.13.9] is a ubiquitously distributed imidodipeptidase that catalyzes the hydrolysis of C-terminal proline-containing dipeptides. The enzyme plays an important role in the recycling of proline for collagen synthesis and cell growth. Although, the increase in the enzyme activity is correlated with increased rate of collagen turnover, the mechanism by which prolidase is regulated remain largely unknown. In the present study we found that phosphorylation of fibroblast's prolidase may be an underlying mechanism for up regulation of the enzyme activity. Supporting evidence comes from the following observations: (1) immunoprecipitated prolidase was detected as a phosphotyrosine protein as shown by western immunoblot analysis, (2) tyrosine kinase inhibitor-erbstatin induced (in a dose dependent manner) a decrease in prolidase activity in cultured human skin fibroblasts, (3) anti-phosphotyrosine antibody reduced and phosphotyrosine phosphatase 1B antibody (anti-PTP 1B) increased (in a dose dependent manner) the prolidase activity in extract of fibroblast's homogenate, (4) decrease in prolidase activity from collagenase treated or serum starved fibroblasts can be partially prevented by incubating fibroblast's homogenate extract with anti-PTP 1B antibody. These results provide evidence that prolidase is phosphotyrosine enzyme and suggest that the activity of prolidase may be up regulated by the enzyme phosphorylation.

Inhibition of collagen biosynthesis and increases in low molecular weight IGF-I binding proteins in the skin of fasted rats

Insulin-like growth factor-I (IGF-I) is an important stimulator of collagen biosynthesis and prolidase activity in connective tissue cells. The disturbances in skin collagen metabolism (reflected by significant decrease in skin collagen content, collagen biosynthesis and prolidase activity) in fasted rats were accompanied by decrease in serum IGF-I level. Fasted rat serum was found to contain about 58% of IGF-I (101.6 +/- 15.4 ng/ml) as compared to control rat serum (175.7 +/- 19.8 ng/ml), while the skin of control and fasted rats contained similar concentrations of IGF-I (about 77 ng/g tissue). The insulin-like growth factor binding proteins (IGFBPs) of sera and tissue extracts (known to regulate IGF-I activity) were analysed by ligand blotting. In the serum of control rats one IGFBP band of about 46 kDa (corresponding to the acid-dissociated IGFBP-3) was detected. In the serum of fasted rats the 46 kDa IGFBP was not observed, however, an other IGFBP of about 30 kDa (corresponding to low molecular weight IGFBPs, e.g. IGFBP-1 or IGFBP-2) was found. The intensity of IGF-I binding to the 30 kDa IGFBP was much higher than that of IGFBP-3, found in control rat serum. Control and fasted rat skin contained similar IGFBPs, however their IGF-I binding abilities were much lower, compared to their serum counterparts. It was found that 46 kDa and 30 kDa proteins, observed in ligand blotting represent IGFBP-3 and IGFBP-1 or IGFBP-2. respectively as demonstrated by western immunoblot analysis. An increase in IGF-binding to 30 kDa IGFBP-1 and/or IGFBP-2 (known as an inhibitors of IGF-dependent functions) in the skin of fasted rats may explain the mechanism of reduced collagen biosynthesis and deposition in tissues during fasting.