Elevated serum preptin concentrations in patients with diabetic nephropathy

Preptin: A New Bone Metabolic Parameter?

Preptin is a 34-aminoacid peptide derived from the E-peptide of pro-insulin-like growth factor 2 (pro-IGF2) that is co-secreted with insulin and upregulates glucose-mediated insulin secretion. High serum preptin levels were described in conditions associated with insulin resistance, such as polycystic ovary syndrome and type 2 diabetes mellitus (T2M). Insulin and also IGF2 are known to be anabolic bone hormones. The "sweet bone" in T2M usually associates increased density, but altered microarchitecture. Therefore, preptin was proposed to be one of the energy regulatory hormones that positively impacts bone health. Experimental data demonstrate a beneficial impact of preptin upon the osteoblasts. Preptin also appears to regulate osteocalcin secretion, which in turn regulates insulin sensitivity. Preptin is greatly influenced by the glucose tolerance status and the level of physical exercise, both influencing the bone mass. Clinical studies describe low serum preptin concentrations in osteoporosis in both men and women, therefore opening the way towards considering preptin a potential bone anabolic therapy. The current review addresses the relationship between preptin and bone mass and metabolism in the experimental and clinical setting, also considering the effects of preptin on carbohydrate metabolism and the pancreatic-bone loop.

Is Preptin a New Bone Metabolism Parameter in Hemodialysis Patients?

BACKGROUND

Preptin is a bone-anabolic pancreatic peptide hormone. Its role in bone metabolism has been studied in rats and in patients with diabetes, but its levels and significance in bone metabolism in hemodialyzed (HD) patients is unknown.

METHODS

The relationships between preptin and anthropometric and biochemical parameters related to bone metabolism were studied in 73 patients on chronic hemodialysis (48 males, 25 females; mean age of 57 years; HD vintage of 69.7 months). Of these subjects, 36 patients had diabetes or impaired glucose tolerance (DM/IGT), and 37 patients had normal glucose tolerance (NGT). Dual-energy X-ray absorptiometry of the femoral neck and lumbar spine were also performed.

RESULTS

No differences were observed in preptin levels between DM/IGT and NGT HD patients. Preptin was positively correlated with HD vintage (r = 0.312, p = 0.007). Negative correlations between preptin and bone mineral density (BMD), T-score, and Z-score in the lumbar spine (L2-L4) were observed (r = -0.319, p = 0.009; r = -0.341, p = 0.005; r = -0.375, p = 0.002). Preptin was positively correlated with parathormone (PTH) levels (r = 0.379, p < 0.001) and osteocalcin levels (r = 0.262, p = 0.027).

CONCLUSIONS

The results indicate that preptin may reflect on bone and mineral metabolism disturbances seen in HD patients. The significant correlation of preptin with PTH and osteocalcin suggests that preptin may be important in indirect measurement of bone turnover in HD patients.

Insulin-like growth factor-II and bioactive proteins containing a part of the E-domain of pro-insulin-like growth factor-II

Insulin-like growth factor (IGF)-II is considered to function as an important fetal growth factor, which is structurally and functionally related to IGF-I and proinsulin. At least in vitro, IGF-II actions are mediated through the IGF-I receptor and to a lesser extent the insulin receptor. After birth, the function of IGF-II is less clear although in adults the serum level of IGF-II exceeds that of IGF-I several fold. The IGF-II gene is maternally imprinted, with exception of the liver and several parts of the brain, where it is expressed from both alleles. The regulation, organization, and translation of the IGF-II gene is complex, with five different putative promotors leading to a range of noncoding and coding mRNAs. The 180-amino acid pre-pro-IGF-II translation product can be divided into five domains and include a N-terminal signal peptide of 24 amino acid residues, the 67 amino acid long mature protein, and an 89 residues extension at the COOH terminus, designated as the E-domain. After removal of the signal peptide, the processing of pro-IGF-II into mature IGF-II requires various steps including glycosylation of the E-domain followed by the action of endo-proteases. Several of these processing intermediates can be found in the human circulation. There is increasing evidence that, besides IGF-II, several incompletely processed precursor forms of the protein, and even a 34-amino acid peptide (preptin) derived from the E-domain of pro-IGF-II, exhibit distinct biological activities. This review will focus on the current insights regarding the specific roles of the latter proteins in cancer, glucose homeostasis, and bone physiology. To address this topic clearly in the right context, a concise overview of the biological and biochemical properties of IGF-II and several relevant aspects of the IGF system will be provided.