Adult vascular progenitor cells and tissue regeneration in metabolic syndrome

Adult vascular progenitor cells (for example endothelial progenitor cells, EPC) have been studied for their contribution to vascular repair and angiogenesis. These cells can differentiate from bone marrow cells as well as circulating cells carrying hematopoetic stem cell markers. In vivo, they take part in vasculogenesis in different animal models of limb ischemia, myocardial infarction and wound healing. In metabolic disease, the outgrowth and function of EPC in vitro is defective and numbers of EPC correlate with classical risk factors of cardiovascular disease suggesting a role of EPC in the development of vascular complications. Pilot studies for the treatment of myocardial infarction and limb ischemia with autologous bone marrow showed a distinct therapeutic benefit that is presumably mediated by vasculogenesis in damaged tissues. However, little is known about the nature of EPC and their capability to differentiate into functional cells for tissue regeneration. In this article, we review and discuss the hitherto identified physiological function of EPC, the mechanisms leading to dysfunction of these cells and potential therapeutic applications in patients with metabolic syndrome or diabetes mellitus and vascular complications.

Serum resistin levels of obese and lean children and adolescents: biochemical analysis and clinical relevance

OBJECTIVE

It was hypothesized that resistin links obesity with diabetes, but this has not been studied in children and adolescents to date.

PATIENTS

We determined serum resistin levels of 135 obese (body mass index, 32.0 +/- 6.2 kg/m2; age, 12.6 +/- 3.4 yr) and 201 lean children (body mass index, 18.7 +/- 2.4 kg/m2; age, 12.5 +/- 2.5 yr) by a newly developed and extensively evaluated in-house immunoassay. These results were controlled for their association with markers of puberty, obesity, and insulin sensitivity.

RESULTS

The analytical evaluation of our assay revealed different resistin isoforms with major peaks of higher than 660 and 55 kDa in the size exclusion chromatography. Using this assay system we found no difference in the resistin levels of obese compared with lean subjects (P = 0.48). However, resistin was significantly higher in girls than in boys (6.74 +/- 2.42 vs. 5.79 +/- 2.45; P < 0.001). Interestingly, in both obese and lean children, resistin correlated with age (P < 0.01), Tanner stage, and testosterone and estradiol levels (P < 0.05). In contrast, no significant correlation was found with parameters of insulin resistance such as homeostasis model assessment, insulin sensitivity index, or insulin, proinsulin, and glucose concentrations in obese subjects.

CONCLUSIONS

Resistin appears to be not the main link between obesity and insulin resistance in children and adolescents but because of its association with Tanner stage, it may be related to the maturation of children during pubertal development. Additionally, we have demonstrated the presence of different molecular isoforms of resistin in human blood, and this may raise problems in comparing data from diverse assay systems.

Leptin and leptin receptor in human seminal plasma and in human spermatozoa

Leptin, a 167 amino acid peptide, is known to influence the gonads via direct and indirect effects. Recent studies provide contradictory proposition about the peripheral impact of leptin in the male gonads. Thus, we examined leptin and its receptors in human seminal plasma and in human ejaculated spermatozoa by Western blot technique and fluorescence microscopy. In seminal plasma we found a free leptin band (16 kDa) by an anti-leptin polyclonal antibody. Incubation of seminal plasma with recombinant leptin caused a statistically significant increase in the amount of free leptin (p < 0.01) and supports this finding. Furthermore, a soluble leptin receptor (145 kDa) was found in human seminal plasma in the same specimen. We also detected a 145-kDa leptin receptor isoform in ejaculated spermatozoa as a possible target of leptin action in the male genital tract, which was localized at the tail of spermatozoa by immunofluorescence microscopy only. This receptor was significantly associated with the intactness of sperm plasma membranes. Spermatozoa with deteriorated membranes contained 49.2 +/- 6.9% leptin receptor signal intensity compared with spermatozoa having intact membranes (p < 0.01). This finding is difficult to interpret and may be caused by a leakage of OB-R due to loss of membrane integrity. In conclusion, these data provide further hints for a peripheral role of leptin in the male genital tract, possibly, by an interaction between leptin and spermatozoa via sperm leptin receptors.

Different isoforms of the soluble leptin receptor in non-pregnant and pregnant mice

Leptin circulates in murine serum in a free and a bound form. As shown in humans, a soluble leptin receptor (sOB-R), which modulates the effects of its ligand, circulates in murine blood. The aim of our study was to determine abundance and biochemical nature of this protein. For the quantification of sOB-R we developed a ligand-immunofunctional assay (LIFA) which is based on both, leptin binding and immunological recognition. The use of this LIFA revealed that during late gestation sera of pregnant mice had a approximately 290-fold higher level of sOB-R than non-pregnant animals. As investigated by size exclusion chromatography these mice sera demonstrated a co-elution of their leptin binding activity with leptin immunoreactivity and levels of sOB-R measured by LIFA. Therefore, it has to be concluded that sOB-R represents the major leptin binding activity in murine circulation. The molecular analysis of sOB-R by Western blot and by cross-linking with 125I-leptin in sera of pregnant and non-pregnant mice demonstrated two different isoforms of sOB-R, which were capable of leptin binding. The sOB-R in serum migrated at a molecular weight of 150kDa in pregnant and only of 120kDa in non-pregnant animals. Deglycosylation of these isoforms led to sOB-R molecules which were found at the same molecular weight in SDS-PAGE. This finding indicates that both isoforms differ only in the degree of their glycosylation. In conclusion, the non-pregnant and the pregnant states are accompanied by differently glycosylated isoforms of sOB-R whose physiological relevance remains to be determined.

A rapid, quantitative immunofunctional assay for measuring human leptin

BACKGROUND

We have developed a rapid, sensitive and quantitative in vitro assay for leptin based upon its ability to bind to the soluble extracellular domain of the leptin receptor (sOB-R). Such an assay is theoretically capable of differentiating between physiologically active leptin molecules from those with modified, either enhanced or reduced, binding activity.

METHODS

A preparation of sOB-R was immobilized to capture leptin from serum samples or standards. Anti-leptin antibodies that had been raised in rabbits were added in a second incubation step to identify leptin molecules bound to sOB-R. Signal detection was performed in a third incubation step by anti-rabbit IgG labeled with peroxidase. The immunofunctional assay (IFA) was clinically validated by the comparison of leptin levels in adolescents (n = 41, age range 9-18 years, BMI range 13.4-33.8 kg/m(2) and adults (n = 80, age range 18-77 years, BMI range 16.4-54.7 kg/m(2) measured using the IFA with data of an in-house RIA performed with the same standards and leptin antibodies.

RESULTS

The functional sensitivity of the IFA was 0.4 ng/ml and comparable to the data of the RIA. Intra- and interassay coefficients of variation were below 12.5 % in both methods. Leptin levels correlated well with the BMI of the subjects studied (r = 0.70 for RIA, r = 0.72 for IFA; p < 0.0001) as well as between IFA (y) and RIA (x) (y = x -1.31 ng/ml; r = 0.97, p < 0.0001). The median of the quotient between IFA and RIA levels was 0.86 (quartile range 0.60-1.10) for all samples.

CONCLUSIONS

So far, only at the most minor differences between leptin measurements using the newly developed IFA and those using a conventional RIA have been detected. Additional studies using the IFA method are required to investigate whether or not discrepant results with the IFA will be seen in various states of relative leptin resistance and whether or not such differences are of biological relevance.

Circulating soluble leptin receptor and free leptin index during childhood, puberty, and adolescence

Leptin is bound in human blood by a high affinity binding protein, which appears to be identical with the soluble leptin receptor (sOB-R). Using a ligand-mediated immunofunctional assay for the determination of serum sOB-R, we investigated its course during childhood, puberty, and adolescence in a large cohort of 581 healthy children and adolescents and a small group of 13 patients with anorexia nervosa. In the first years of life, sOB-R is detectable in remarkably high concentrations. Thereafter, a continuous decline of sOB-R levels was found. Consequently, correlation analyses demonstrated significant inverse relationships (P < 0.001) of sOB-R with age, IGF-I levels, pubertal stage, auxological and body composition parameters, as well as with leptin concentrations. Multiple regression analysis revealed that height, IGF-I, and age (only in girls) were independent predictors of sOB-R levels; these variables account for approximately 65% and 48% of the variation of sOB-R levels in boys and girls, respectively. The courses of age-dependent median values for the free leptin index (FLI, ratio between leptin and sOB-R levels) and for leptin levels were parallel in both genders. Correlation analyses demonstrated that in particular parameters of growth and sexual maturation are more closely related to the FLI than to leptin alone; this closer relationship is more pronounced among boys. Weight gains of patients with anorexia nervosa resulted in a significant increase in leptin and IGF-I levels (P < 0.01), whereas the median of sOB-R values decreased (P < 0.01). sOB-R and IGF-I levels were again significantly correlated (r = -0.55, P < 0.01). These findings suggest that high levels of sOB-R in emaciation may reflect an up-regulation of the sOB-R to suppress leptin action during energy deficiency. Furthermore, determinations of sOB-R and FLI are additional valuable tools to investigate the leptin axis during growth and sexual maturation.

Leptin exists in tubuli seminiferi and in seminal plasma

Leptin is a 167-amino acid protein that stimulates gonadotrophin-releasing hormone secretion and exerts indirect effects on the gonads via neuropeptide Y, NPY. Recent research has suggested that leptin may also have an effect on testosterone secretion. To investigate the role of leptin in reproduction, leptin in testicular tissue and seminal plasma was examined in relation to leptin in serum, semen sample qualities and vasectomy. Seminal plasma and serum of 64 infertility patients, and 15 individuals after vasectomy, were assayed for leptin using a competitive 'in house' radioimmunoassay. The concentration of leptin in seminal plasma was significantly lower in the 'normal' semen sample group than in the 'pathological' group (Mean +/- SEM; 1.45 +/- 0.18 vs. 3.19 +/- 0.57 ng ml-1; P < 0.05), and showed a significantly negative correlation with percentage of motile spermatozoa (r = -0.46; P = 0.0005) and with the velocity straight line, VSL, (r = -0.30; P = 0.029). In contrast, leptin concentration in serum did not show any relationship with the spermiogram parameters. In testicular tissue, leptin was preferentially found within the tubuli seminiferi using anti-leptin polyclonal antibody, Ob A-20 Sc 842. The amount of leptin per ejaculate did not significantly change after vasectomy, and was not correlated to fructose, zinc or neutral alpha glucosidase in seminal plasma (P > 0.05). These results suggest that the amount of leptin in the genital tract, including the tubuli seminiferi, may influence the mechanisms involved in the motility development of spermatozoa.

Soluble leptin receptor represents the main leptin binding activity in human blood

In human blood leptin circulates both free and bound to high molecular weight proteins. Hypothesising that these proteins may modulate ligand bioavailability and bioactivity of leptin, we investigated their molecular nature. Therefore, leptin binding activity was partially purified from human plasma using a leptin affinity column. Subjecting this preparation to size exclusion chromatography (SEC) we observed a coelution of leptin binding activity with levels of the soluble leptin receptor (sOB-R) determined by a newly developed ligand immunofunctional assay. In Western blot analysis the partially purified leptin binding activity exhibited sOB-R immunoreactivity in two bands of 110 and 140 kD. Following N-deglycosylation these bands were replaced by two bands with the molecular weight of 90 and 60 kD, suggesting two isoforms which are capable of leptin binding, as determined by cross-linking. Furthermore, different ratios of these isoforms were detectable in fractions of the leptin binding activity after separation by SEC. These findings indicate the formation of heterodimers and homodimers complexed with and without leptin. As the two sOB-R bands from Western blot analysis correspond to only two specific bands in cross-linking experiments with 125l-leptin, the role of both isoforms as leptin binding proteins appears to be exclusive. Therefore, our results indicate that sOB-R is the major leptin binding protein in the circulating human blood.