Proinsulin C-peptide regulates ribosomal RNA expression

The role of C-peptide in diabetes and its complications: an updated review

Worldwide, diabetes and its complications have seriously affected people's quality of life and become a serious public health problem. C-peptide is not only an indicator of pancreatic β-cell function, but also a biologically active peptide that can bind to cell membrane surface signaling molecules and activate downstream signaling pathways to play antioxidant, anti-apoptotic and inflammatory roles, or regulate cellular transcription through internalization. It is complex how C-peptide is related to diabetic complications. Both deficiencies and overproduction can lead to complications, but their mechanisms of action may be different. C-peptide replacement therapy has shown beneficial effects on diabetic complications in animal models when C-peptide is deficient, but results from clinical trials have been unsatisfactory. The complex pattern of the relationship between C-peptide and diabetic chronic complications has not yet been fully understood. Future basic and clinical studies of C-peptide replacement therapies will need to focus on baseline levels of C-peptide in addition to more attention also needs to be paid to post-treatment C-peptide levels to explore the optimal range of fasting C-peptide and postprandial C-peptide maintenance.

FoxA2 and RNA Pol II mediate human islet amyloid polypeptide turnover in ER-stressed pancreatic β-cells

Here, we investigated transcriptional and trafficking mechanisms of human islet amyloid polypeptide (hIAPP) in normal and stressed β-cells. In high glucose-challenged human islets and rat insulinoma cells overexpressing hIAPP, cell fractionation studies revealed increased accumulation of hIAPP. Unexpectedly, a significant fraction (up to 22%) of hIAPP was found in the nuclear soluble and chromatin-enriched fractions of cultured human islet and rat insulinoma cells. The nucleolar accumulation of monomeric forms of hIAPP did not have any adverse effect on the proliferation of β-cells nor did it affect nucleolar organization or function. However, intact nucleolar organization and function were essential for hIAPP expression under normal and ER-stress conditions as RNA polymerase II inhibitor, α-amanitin, reduced hIAPP protein expression evoked by high glucose and thapsigargin. Promoter activity studies revealed the essential role of transcription factor FoxA2 in hIAPP promoter activation in ER-stressed β-cells. Transcriptome and secretory studies demonstrate that the biosynthetic and secretory capacity of islet β-cells was preserved during ER stress. Thus, the main reason for increased intracellular hIAPP accumulation is its enhanced biosynthesis under these adverse conditions.

Nuclear localization and actions of the insulin-like growth factor 1 (IGF-1) system components: Transcriptional regulation and DNA damage response

Insulin-like growth factor (IGF) system stimulates growth, proliferation, and regulates differentiation of cells in a tissue-specific manner. It is composed of two insulin-like growth factors (IGF-1 and IGF-2), six insulin-like growth factor-binding proteins (IGFBPs), and two insulin-like growth factor receptors (IGF-1R and IGF-2R). IGF actions take place mostly through the activation of the plasma membrane-bound IGF-Rs by the circulating ligands (IGFs) released from the IGFBPs that stabilize their levels in the serum. This review focuses on the IGF-1 part of the system. The IGF-1 gene, which is expressed mainly in the liver as well as in other tissues, comprises six alternatively spliced exons that code for three protein isoforms (pro-IGF-1A, pro-IGF-1B, and pro-IGF-1C), which are processed to mature IGF-1 and E-peptides. The IGF-1R undergoes autophosphorylation, resulting in a signaling cascade involving numerous cytoplasmic proteins such as AKT and MAPKs, which regulate the expression of target genes. However, a more complex picture of the axis has recently emerged with all its components being translocated to the nuclear compartment. IGF-1R takes part in the regulation of gene expression by forming transcription complexes, modifying the activity of chromatin remodeling proteins, and participating in DNA damage tolerance mechanisms. Four IGFBPs contain a nuclear localization signal (NLS), which targets them to the nucleus, where they regulate gene expression (IGFBP-2, IGFBP-3, IGFBP-5, IGFBP-6) and DNA damage repair (IGFBP-3 and IGFBP-6). Last but not least, the IGF-1B isoform has been reported to be localized in the nuclear compartment. However, no specific molecular actions have been assigned to the nuclear pro-IGF-1B or its derivative EB peptide. Therefore, further studies are needed to shed light on their nuclear activity. These recently uncovered nuclear actions of different components of the IGF-1 axis are relevant in cancer cell biology and are discussed in this review.

Analysis of Metal Effects on C-Peptide Structure and Internalization

The connecting peptide (C-peptide) has received increased attention for its potential therapeutic effects in ameliorating illnesses such as kidney disease and diabetes. Although the mechanism of C-peptide signaling remains elusive, evidence supports its internalization and intracellular function. Emerging research is uncovering the diverse biological roles metals play in controlling and affecting the function of bioactive peptides. The work presented herein investigates interactions between C-peptide and first-row d-block transition metals, as well as their effects on C-peptide internalization into cells. Through spectroscopic techniques, it is demonstrated that Cr^III , Cu^II , and Zn^II bind to C-peptide with differing stoichiometries and biologically relevant affinities. In addition, metal binding elicits both subtle changes in secondary structure and inhibits adoption of an α-helical character in environments where the dielectric constants are reduced. This study shows how metal ions can modulate peptide hormone activity through subtle structural changes to disrupt cellular uptake.

Transcriptome profiling of four candidate milk genes in milk and tissue samples of temperate and tropical cattle

The expression of four genes involved in milk regulation and production in bovine milk and tissue samples profiled using quantitative PCR to identify differential gene expression. Our goal focussed on the differential mRNA expression of milk genes (KCN, PRL, BLG and PIT-1) in milk samples and different tissues from four different breeds of ecologically adapted and geographically separated cattle species. The mRNA expression identified the four milk genes under studied most upregulated in mammary gland and milk samples as compared with other tissues. The expression of PIT-1 gene in the brain identified to have influenced the expression of PRL and K-CN in the mammary and milk samples. Among the four genes, PRL had the highest mRNA expression (144.19-fold change) in Holstein followed by K-CN with 100.89-fold change, while the smallest relative expression for most genes in this study are in the range from 0.79 to 7.35-fold difference. White Fulani cattle was identified to have a higher expression for K-CN, PRL and BLG compared with Angus and Ndama cattle, while Holstein cattle is on top of the list on the basis of the gene expression and gene regulation for all the four genes in this study. Also, White Fulani and Holstein are in the same cluster based on their mRNA expression for milk genes. Our data showed the first evidence of the molecular identification of indigenous White Fulani cattle ofhaving potential for higher milk production.

C-peptide prevents SMAD3 binding to alpha promoters to inhibit collagen type IV synthesis

Activation of transforming growth factor β1 (TGFB1)/SMAD3 signaling may lead to additional synthesis of collagen type IV (COL4), which is a major contributor to extracellular matrix (ECM) accumulation in diabetic nephropathy (DN). C-peptide can attenuate fibrosis to have unique beneficial effects in DN. However, whether and how C-peptide affects TGFB1/SMAD3-activated COL4 synthesis is unclear. In this study, pathological changes, expression of COL4 a1-a5 chains (Col4a1-a5), COL4 distribution and protein and TGFB1 and SMAD3 protein were first assessed in a rat model of diabetes. Then, rat mesangial cells were treated with high glucose (HG) and/or C-peptide to investigate the underlying mechanism. Col4a1-a5 expression, COL4 protein and secretion, TGFB1 protein, SMAD3 nuclear translocation and binding of SMAD3 to its cognate sites in the promoters of Col4a1a2, Col4a3a4 and Col4a5 were measured. It was found that C-peptide attenuated glomerular pathological changes and suppressed renal Col4a1-a5 mRNA expression, COL4 protein content and TGFB1 protein content. C-peptide had a dose-dependent effect to inhibit Col4a1-a5 mRNA expression, COL4 protein content and secretion, in HG-stimulated mesangial cells. In addition, the HG-induced increase in TGFB1 protein content was significantly reduced by C-peptide. Although not apparently affecting SMAD3 nuclear translocation, C-peptide prevented SMAD3 from binding to its sites in the Col4a1a2, Col4a3a4 and Col4a5 promoters in HG-stimulated mesangial cells. In conclusion, C-peptide could prevent SMAD3 from binding to its sites in the Col4a1a2, Col4a3a4 and Col4a5 promoters, to inhibit COL4 generation. These results may provide a mechanism for the alleviation of fibrosis in DN by C-peptide.

C-peptide prevents NF-κB from recruiting p300 and binding to the inos promoter in diabetic nephropathy

C-peptide (CP) has demonstrated unique beneficial effects in diabetic nephropathy (DN), but whether and how CP regulates NF-κB and its coactivator, p300, to suppress inducible iNOS and antagonize DN are unknown. iNOS expression, NF-κB nuclear translocation, colocalization and binding of NF-κB to p300, binding of NF-κB to the inos promoter, and the bound NF-κB, p300, and histone 3 lysine 9 acetylation (H3K9ac) at binding sites were measured in high glucose-stimulated mesangial cells. We evaluated pathologic changes, iNOS expression, NF-κB, and p300 contents in diabetic rats. We found that CP inhibited iNOS expression and notably prevented colocalization and binding of NF-κB and p300. CP prevented NF-κB from binding to the inos promoter, especially at the distal site, and reduced bound NF-κB, p300, and H3K9ac. N-terminal plus middle fragment could mostly mimic the antagonizing effects of CP against the pathologic changes of DN and equally suppresses renal iNOS expression as CP. In conclusion, CP prevented NF-κB from recruiting p300 and binding to the inos promoter, and decreased H3K9ac at the binding sites to suppress iNOS expression and antagonize DN, with the effect region identified as N-terminal plus middle fragment.-Li, Y., Li, X., He, K., Li, B., Liu, K., Qi, J., Wang, H., Wang, Y., Luo, W. C-peptide prevents NF-κB from recruiting p300 and binding to the inos promoter in diabetic nephropathy.

C-peptide exhibits a late induction effect on matrix metallopeptidase-9 in high glucose-stimulated rat mesangial cells

Insufficient matrix metalloproteinase (MMP)-9 and MMP-2 is considered to be a contributor of extracellular matrix (ECM) accumulation in diabetic nephropathy (DN). C-peptide can reverse fibrosis, thus exerting a beneficial effect on DN. Whether C-peptide induces MMP-9 and MMP-2 to reverse ECM accumulation is not clear. In the present study, in order to determine ECM metabolism, rat mesangial cells were treated with high glucose (HG) and C-peptide intervention, then the early and late effects of C-peptide on HG-affected MMP-9 and MMP-2 were evaluated. Firstly, it was confirmed that HG mainly suppressed MMP-9 expression levels. Furthermore, C-peptide treatment induced MMP-9 expression at 6 h and suppressed it at 24 h, revealing the early dual effects of C-peptide on MMP-9 expression. Subsequently, significant increase in MMP-9 expression at 72, 96 and 120 h C-peptide treatment was observed. These changes in MMP-9 protein content confirmed its expression changes following late C-peptide treatment. Furthermore, at 96 and 120 h C-peptide treatment reversed the HG-inhibited MMP-9 secretion, further indicating the late induction effect of C-peptide on MMP-9. The present results demonstrated that C-peptide exerted a late induction effect on MMP-9 in HG-stimulated rat mesangial cells, which may be associated with the underlying mechanism of C-peptide's reversal effects on DN.

C-peptide evolution: generation from few structural restrictions of bioactivities not necessarily functional

The proinsulin C-peptide has molecular, cellular and organismal activities but lacks disease-associated mutations or short-term loss-of-function effects. This dilemma between activity and function may be explained from its evolutionary setting with insulin as an ancestral partner. The charge, approximate length and flexibility of C-peptide are all that is required for the insulin interactions, while remaining aspects are free to evolve, where new bioactivities can emerge. They can initially be transient, weak, and non-functional, but may gradually be consolidated. In this manner, C-peptide may have acquired multiple bioactivities, explaining why some yet have limited functions but could represent early-stage hormonal-like activities.

Evidence for inhibitory autocrine effects of proinsulin C-peptide on pancreatic β-cell function and insulin secretion

AIMS

Autocrine and paracrine regulatory mechanisms ensure integrated secretion of islet hormones that respond efficiently to changes in metabolic need. As proinsulin C-peptide exerts various biological effects and binds to cell membranes including insulin-secreting β cells, its physiological role in insulin release was examined.

METHODS

Insulin releasing activity of human and rat C-peptides were studied in the clonal pancreatic cell line, BRIN-BD11, with findings substantiated using isolated islets and in vivo studies employing SWISS TO mice.

RESULTS

Acute exposure of clonal β cells to human C-peptide resulted in concentration-dependent inhibitory effects on insulin secretion at 5.6 mM (p < 0.05-p < 0.001) and 16.7 mM (p < 0.01-p < 0.001) glucose. At physiologically relevant intra-islet concentrations (10(-9) -10(-6) M), C-peptide suppressed the insulin-secretory responses to a range of secretagogues acting at different points in the β cell stimulus-secretion coupling pathway including alanine (p < 0.05), Ca(2+) (p < 0.001), arginine (p < 0.05), tolbutamide (p < 0.001), glucagon-like peptide 1 (GLP-1) (p < 0.001), isobutylmethylxanthine (IBMX) (p < 0.01) and KCl (p < 0.05). Similar results were obtained using isolated mouse pancreatic islets. Human C-peptide (3 × 10(-7) M, p < 0.001), somatostatin-14 (3 × 10(-7) M, p < 0.01) and diazoxide (300 µM, p < 0.001) reduced both alanine and glucose-stimulated insulin release by 43, 25 and 48%, respectively. The effects of human C-peptide were reproduced using rat C-peptide I and II. C-peptide also inhibited in vivo glucose-stimulated insulin release and impaired glucose tolerance in mice.

CONCLUSIONS

C-peptide is a biologically active endogenous peptide hormone that exerts inhibitory autocrine effects on pancreatic β-cell function. Mechanisms involving the activation of K(+) channels and a distal effect downstream of increased cytoplasmic Ca(2+) appear to be implicated in the inhibition of insulin secretion.

C-peptide replacement therapy as an emerging strategy for preventing diabetic vasculopathy

Lack of C-peptide, along with insulin, is the main feature of Type 1 diabetes mellitus (DM) and is also observed in progressive β-cell loss in later stage of Type 2 DM. Therapeutic approaches to hyperglycaemic control have been ineffective in preventing diabetic vasculopathy, and alternative therapeutic strategies are necessary to target both hyperglycaemia and diabetic complications. End-stage organ failure in DM seems to develop primarily due to vascular dysfunction and damage, leading to two types of organ-specific diseases, such as micro- and macrovascular complications. Numerous studies in diabetic patients and animals demonstrate that C-peptide treatment alone or in combination with insulin has physiological functions and might be beneficial in preventing diabetic complications. Current evidence suggests that C-peptide replacement therapy might prevent and ameliorate diabetic vasculopathy and organ-specific complications through conservation of vascular function, as well as prevention of endothelial cell death, microvascular permeability, vascular inflammation, and neointima formation. In this review, we describe recent advances on the beneficial role of C-peptide replacement therapy for preventing diabetic complications, such as retinopathy, nephropathy, neuropathy, impaired wound healing, and inflammation, and further discuss potential beneficial effects of combined C-peptide and insulin supplement therapy to control hyperglycaemia and to prevent organ-specific complications.

Proinsulin C-peptide potentiates the inhibitory action of insulin on glucose synthesis in primary cultured rabbit kidney-cortex tubules: Metabolic studies

Effects of equimolar concentrations of proinsulin C-peptide and insulin on glucose synthesis were studied in primary cultures of rabbit kidney-cortex tubules grown in the presence of alanine, glycerol, and octanoate. The rhodamine-labeled C-peptide entered renal tubular cells and localized in nuclei, both in the presence and absence of insulin; preincubations with the unlabeled compound inhibited internalization. C-peptide did not affect glucose formation when added alone but potentiated the inhibitory action of insulin by about 20% due to a decrease in flux through glucose-6-phosphate isomerase (GPI) and (or) glucose-6-phosphatase (G6Pase). GPI inhibition was caused by: (i) increased intracellular contents of fructose-1,6-bisphosphate and fructose-1-phosphate, inhibitors of the enzyme and (ii) reduced level of the phosphorylated GPI, which exhibits higher enzymatic activity in the presence of casein kinase 2. A decrease in flux through G6Pase, due to diminished import of G6P by G6P-transporter from the cytoplasm into endoplasmic reticulum lumen, is also suggested. The data show for the first time that in the presence of insulin and C-peptide, both GPI and G6P-ase may act as regulatory enzymes of renal gluconeogenic pathway.

Dynamic localization and functional implications of C-peptide might for suppression of iNOS in high glucose-stimulated rat mesangial cells

Although C-peptide has unique beneficial effects on diabetic nephropathy (DN), its functional localization and molecular mechanism have not been fully clarified. Whether C-peptide exhibits its protective role through the regulation of inducible nitric oxide synthase (iNOS), a key enzyme in oxidative stress, is not clear. In this study, it was revealed that C-peptide could enter the nucleus of high glucose-stimulated mesangial cells, especially in a time-dependent manner by high glucose pretreatment, while no C-peptide was detected in low glucose-cultured mesangial cells. The dynamic functional localization of C-peptide might be the intrinsic cause of its unique beneficial effects for DN, which may provide a foundation for further clarification of its underlying mechanism. Our preliminarily data also shown C-peptide suppressed the iNOS expression. Taking together, these results revealed the dynamic functional localization of C-peptide by high glucose stimulation in rat mesangial cells, which might suppress expression of iNOS to exhibit its protective effects.

Human Eb peptide: not just a by-product of pre-pro-IGF1b processing?

Several physiological activities have been assigned to E-peptides derived from pre-pro-insulin-like growth factor (IGF1) processing; however, the whole range of the E-peptides' functions is still unknown. The objective of this study was to investigate human Eb peptide (hEb) in terms of its bioactivity, cellular localization, and intracellular trafficking using human cancer cells. Human Eb fused with red fluorescence protein (RFP) or green fluorescence protein (GFP) localizes strongly to nucleoli and to a lesser extent to nuclei of HeLa and U2-OS cells. Mutagenesis of hEb nucleolus localization sequence (NoLS) leads to its partial delocalization from nuclei and nucleoli to cytoplasm of transfected cells. Thus, NoLS is not sufficient for the hEb to be localized in nucleoli of the cells and a different mechanism may be involved in hEb targeting. A BrdU ELISA showed that the proliferation index of cells expressing hEb hybrid proteins increased up to 28%. For comparison, the same assay was performed using HeLa cells treated extracellularly with synthetic hEb. A significant increase in the proliferation index was observed (41-58% for concentrations ranging from 10-100 nM, respectively). Additionally, a cell migration assay was performed using stable U2-OS cell lines expressing hEb fused with RFP or RFP alone as a negative control. The migration index of hEb expressing cells was 38.3% greater. The increase in cell proliferation index and in motile properties of hEb expressing cells demonstrate that hEb is more than a pre-pro-IGF1b processing product, and has intrinsic activity of biological significance.

Insulin solubility transitions by pH-dependent interactions with proinsulin C-peptide

Proinsulin processing into insulin and C-peptide in the secretory granules of the pancreatic β-cells occurs under mildly acidic conditions and at high peptide concentrations (> 10 mm). Mature insulin has reduced solubility and a propensity to adopt an amyloid-like structure, but is physiologically released as a mixture of a zinc-containing core and a zinc-free, C-peptide-rich fluid phase. C-peptide is known to function in the insulin secretion, but its exact mode of interaction is not established. We now demonstrate that C-peptide in sub-stoichiometric amount versus insulin coprecipitates with insulin at the pH found in secretory vesicles. Precipitation is reversible and the precipitate is dissolved by elevation of the pH. This effect was found to be dependent on relatively conserved glutamate residues in the otherwise poorly conserved C-peptide. Together, the data show that C-peptide has the ability to influence insulin solubility. The physiological pH changes between insulin processing and release sites may therefore affect the quaternary structure of insulin, as well as the phase transitions during insulin sorting and secretion.

STRUCTURED DIGITAL ABSTRACT

Insulin and C-peptide bind by molecular sieving (View Interaction: 1, 2) C-peptide and Insulin bind by dynamic light scattering (View Interaction: 1, 2) C-peptide and Insulin bind by fluorescence technology (View Interaction: 1, 2, 3, 4).

NOL11, implicated in the pathogenesis of North American Indian childhood cirrhosis, is required for pre-rRNA transcription and processing

The fundamental process of ribosome biogenesis requires hundreds of factors and takes place in the nucleolus. This process has been most thoroughly characterized in baker's yeast and is generally well conserved from yeast to humans. However, some of the required proteins in yeast are not found in humans, raising the possibility that they have been replaced by functional analogs. Our objective was to identify non-conserved interaction partners for the human ribosome biogenesis factor, hUTP4/Cirhin, since the R565W mutation in the C-terminus of hUTP4/Cirhin was reported to cause North American Indian childhood cirrhosis (NAIC). By screening a yeast two-hybrid cDNA library derived from human liver, and through affinity purification followed by mass spectrometry, we identified an uncharacterized nucleolar protein, NOL11, as an interaction partner for hUTP4/Cirhin. Bioinformatic analysis revealed that NOL11 is conserved throughout metazoans and their immediate ancestors but is not found in any other phylogenetic groups. Co-immunoprecipitation experiments show that NOL11 is a component of the human ribosomal small subunit (SSU) processome. siRNA knockdown of NOL11 revealed that it is involved in the cleavage steps required to generate the mature 18S rRNA and is required for optimal rDNA transcription. Furthermore, abnormal nucleolar morphology results from the absence of NOL11. Finally, yeast two-hybrid analysis shows that NOL11 interacts with the C-terminus of hUTP4/Cirhin and that the R565W mutation partially disrupts this interaction. We have therefore identified NOL11 as a novel protein required for the early stages of ribosome biogenesis in humans. Our results further implicate a role for NOL11 in the pathogenesis of NAIC.

Ribosomes are the cellular factories that produce proteins. Making a ribosome is a complex and energy intensive process that requires hundreds of different factors. Ribosome biogenesis is an essential process, and therefore mutations that partially disrupt this process lead to disease. One such disease is North American Indian childhood cirrhosis (NAIC), which is caused by a mutation in a ribosome biogenesis protein called hUTP4/Cirhin. We looked for proteins that interact with hUTP4/Cirhin, since we hypothesized that disruption of this interaction could play a role in the development of NAIC. We identified a novel protein called NOL11, which is only found in animals and not in any other organisms. We showed that NOL11 is required for ribosome biogenesis and acts at one of the earliest steps in this process. We then showed that NOL11 interacts with the region of hUTP4/Cirhin that contains the NAIC mutation and that the NAIC mutation interferes with the interaction between hUTP4/Cirhin and NOL11. Further study of the interaction between hUTP4/Cirhin and NOL11 will give insight into the development of NAIC, as well as elucidate some of the differences in ribosome biogenesis between animals and other organisms.

Proinflammatory effects of C-Peptide in different tissues

Atherosclerosis is well known as an inflammatory disease that can lead to clinical complications such as heart attack or stroke. C-peptide as a cleavage product of proinsulin is in the last few decades known as an active peptide with a number of different effects on microvascular and macrovascular complications in type 2 diabetic patients. Patients with insulin resistance and early type 2 diabetes show elevated levels of C-peptide in blood. Several last findings demonstrated deposition of C-peptide in the vessel wall in ApoE-deficient mice and induction of local inflammation. Besides that, C-peptide has proliferative effects on human mesangial cells. This review discusses recently published proinflammatory effects of C-peptide in different tissues.

C-peptide: a new mediator of atherosclerosis in diabetes

Diabetes type 2 and insulin resistance are the risk factors for cardiovascular disease. It is already known that atherosclerosis is an inflammatory disease, and a lot of different factors are involved in its onset. C-peptide is a cleavage product of proinsulin, an active substance with a number of effects within different complications of diabetes. In this paper we discuss the role of C-peptide and its effects in the development of atherosclerosis in type 2 diabetic patients.

Evaluation of bone mineral density in type 2 diabetes mellitus patients before and after treatment

BACKGROUND

The relationship between bone mineral density (BMD) and type 2 diabetes mellitus (T2DM) has been controversial. Recent studies have revealed adverse impact of antidiabetic drugs on BMD in type 2 diabetic patients. However, the influence of various antihyperglycaemic agents on BMD has not been well studied.

METHOD

A total of 200 patients with T2DM were screened initially for the study. Finally 67 patients (M:34, F:33) who satisfied the requirement of having been on one year of prescribed therapy were included for analysis.

RESULTS

Bone mineral density was lower in diabetic patients as compared to controls (hip 0.962 ± 0.167 g/cm(2) vs 1.013 ± 0.184 g/cm(2), P = 0.05; spine 0.929 ± 0.214 g/cm(2) vs 1.113 ± 0.186 g/cm(2), P < 0.00001). In males BMD was significantly lower at spine (P < 0.00001) and in females BMD was significantly lower in both at the spine (P < 0.00001) and hip (P < 0.032). On multivariate analysis significant positive correlation was found between spine BMD and body mass index (BMI) (r = 0.372, P = 0.002), total cholesterol (r = 0.272, P = 0.026), low-density lipoprotein (r = 0.242, P = 0.047), and triglycerides (r = 0.282, P = 0.021). There was no correlation between BMD and glycosylated haemoglobin (r = 0.158, P = 0.265). A significant decrease in BMD at spine and hip was seen with the use of glitazones and metformin while increase was noted with sulphonylurea and its combination.

CONCLUSION

Men and women with T2DM have lower BMD. Bone mineral density did not have correlation to glycaemic control. Glitazones, metformin, and insulin are associated with decrease in BMD at spine, and hip, while sulphonylureas are associated with increase in BMD.

Early transcriptional regulation by C-peptide in freshly isolated rat proximal tubular cells

BACKGROUND

Clinical studies have shown that proinsulin C-peptide exerts renoprotective effects in type 1 diabetes, although the underlying mechanisms are poorly understood. As C-peptide has been shown to induce several intracellular events and to localize to nuclei, we aimed to determine whether gene transcription is affected in proximal tubular kidney cells, and if so, whether the genes with altered transcription include those related to protective mechanisms.

METHODS

The effect of C-peptide incubation (2 h) on gene expression was investigated in freshly isolated proximal tubular cells from streptozotocin-diabetic Sprague-Dawley rats using global gene expression profiling and real-time quantitative polymerase chain reaction. Protein expression was assayed using western blotting. Different bioinformatic strategies were employed.

RESULTS

Gene transcription profiling demonstrated differential transcription of 492 genes (p < 0.01) after 2 h of C-peptide exposure, with the majority of these genes repressed (83%). Real-time quantitative polymerase chain reaction validation supported a trend of several G protein-coupled receptors being activated, and certain transcription factors being repressed. Also, C-peptide repressed the transcription of genes associated with the pathways of circulatory and inflammatory diseases.

CONCLUSION

This study shows that C-peptide exerts early effects on gene transcription in proximal tubular cells. The findings also bring further knowledge to the renoprotective mechanisms of C-peptide in type 1 diabetes, and support a transcriptional activity for C-peptide. It is suggested that C-peptide may play a regulatory role in the gene expression of proximal tubular cells.

N-terminal segment of proinsulin C-peptide active in insulin interaction/desaggregation

Evidence has emerged that proinsulin C-peptide has at least three types of functional interactions in addition to its role during synthesis and secretion of insulin. Thus, C-peptide has been shown (i) to bind to cell membranes triggering G-protein-mediated intracellular signaling; (ii) to be internalized into cells and nuclei promoting transcription of rRNA and expression of particular genes; and (iii) to interact with peptides, including insulin, causing desaggregation of insulin oligomers like a chaperone, and with itself, causing homo-oligomers potentially capable of forming aggregates and deposits. In this work, we studied the insulin-C-peptide interactions by monitoring desaggregation and binding effects of C-peptide fragments on insulin. We find that the N-terminal segment of C-peptide harbors an interaction with insulin and that Glu11 appears to play a role in this action. We conclude that C-peptide fragments with this residue can mimic C-peptide in biophysical interactions with insulin, and that the insulin-interacting and membrane-interacting effects of C-peptide are distinct, ascribable to separate C-peptide segments, N- and C-terminally, respectively. The findings may have relevance to peptide effects in diabetic and healthy states.

Structural features of proinsulin C-peptide oligomeric and amyloid states

The formation and structure of proinsulin C-peptide oligomers has been investigated by PAGE, NMR spectroscopy and dynamic light scattering. The results obtained show that C-peptide forms oligomers of different sizes, and that their formation and size distribution is altered by salt and divalent metal ions, which indicates that the aggregation process is mediated by electrostatic interactions. It is further demonstrated that the size distribution of the C-peptide oligomers, in agreement with previous studies, is altered by insulin, which supports a physiologically relevant interaction between these two peptides. A small fraction of oligomers has previously been suggested to be in equilibrium with a dominant fraction of soluble monomers, and this pattern also is observed in the present study. The addition of modest amounts of sodium dodecyl sulphate at low pH increases the relative amount of oligomers, and this effect was used to investigate the details of both oligomer formation and structure by a combination of biophysical techniques. The structural properties of the SDS-induced oligomers, as obtained by thioflavin T fluorescence, CD spectroscopy and IR spectroscopy, demonstrate that soluble aggregates are predominantly in β-sheet conformation, and that the oligomerization process shows characteristic features of amyloid formation. The formation of large, insoluble, β-sheet amyloid-like structures will alter the equilibrium between monomeric C-peptide and oligomers. This leads to the conclusion that the oligomerization of C-peptide may be relevant also at low concentrations.