Parathyroid hormone secretion in diabetes mellitus

Impaired response of FGF-23 to oral phosphate in patients with type 2 diabetes: a possible mechanism of atherosclerosis

BACKGROUND

Fibroblast growth factor (FGF)-23, secreted from osteocytes/osteoblasts, plays major roles in phosphate (Pi)-mediated stimulation of PTH secretion and consequently in regulation of serum Pi. Osteocyte/osteoblast dysfunction develops in patients with type 2 diabetes mellitus (DM).

OBJECTIVE

Our objective was to examine whether increases in serum FGF-23 and PTH after oral Pi stimulation are impaired in type 2 DM.

DESIGN AND METHODS

The subjects were 10 DM and 10 non-DM patients without chronic kidney disease stage 3-5. Serum FGF-23, intact PTH (iPTH), and Pi were measured serially after oral Pi administration at a daily dose of 2.0 g.

RESULTS

Pi administration caused significant increases of FGF-23 by 2 h and iPTH by 4 h in non-DM patients. These increases were attenuated in DM patients. After 2 d of Pi stimulation, serum FGF-23 and iPTH remained elevated in non-DM patients but not in DM. In all subjects, initial changes of serum FGF-23 (0-2 h) and iPTH (0-4 h) were positively correlated (r = 0.528) and showed significant negative correlations with later changes in serum Pi (2-4 h) (r = -0.457 and r = -0.673, respectively). Serum Pi (2-4 h) significantly increased in DM patients, consistent with the lack of change in serum FGF-23 and iPTH, whereas serum Pi did not change significantly in non-DM patients.

CONCLUSION

These results show that increases of serum FGF-23 and PTH in response to Pi stimulation are impaired in type 2 DM and that serum Pi is significantly increased thereafter. This may be a mechanism underlying advanced atherosclerosis in type 2 DM.

Serum calcium is positively correlated with fasting plasma glucose and insulin resistance, independent of parathyroid hormone, in male patients with type 2 diabetes mellitus

Patients with primary hyperparathyroidism have impaired glucose tolerance more often than do controls, and parathyroid resection sometimes improves this derangement. However, it is unclear whether serum calcium (Ca) or parathyroid hormone (PTH) is more strongly related to impaired glucose metabolism in subjects without primary hyperparathyroidism. In this cross-sectional study, we examined patients with type 2 diabetes mellitus (DM) (271 men and 209 women) and analyzed the relationships between serum concentrations of Ca or intact PTH and DM-related variables. Simple regression analyses showed that the level of serum Ca was significantly and positively correlated with the levels of fasting plasma glucose, immunoreactive insulin, and homeostasis model assessment insulin resistance in men (P < .05), but not in women. In contrast, intact PTH was not significantly correlated with DM-related parameters in either sex. Multiple regression analyses showed that the significant and positive correlations between serum Ca vs fasting plasma glucose and homeostasis model assessment insulin resistance in men still remained after adjustment for intact PTH as well as age, body weight, height, creatinine, albumin, phosphate, bone metabolic markers, and estradiol (P < .05). Serum Ca level is positively associated with impaired glucose metabolism, independent of PTH or bone metabolism, in men with type 2 DM.

Serum osteocalcin/bone-specific alkaline phosphatase ratio is a predictor for the presence of vertebral fractures in men with type 2 diabetes

We examined whether or not BMD or bone markers were useful for assessing the risk of vertebral fractures in 248 Japanese men with type 2 diabetes. We analyzed the relationships between bone markers (osteocalcin [OC], bone-specific alkaline phosphatase [BAP], urinary N-terminal cross-linked telopeptide of type-I collagen) or BMD and HbA(1c), urinary C-peptide, insulin-like growth factor-I (IGF-I), parathyroid hormone, 1,25(OH)(2) vitamin D, and the presence of prevalent vertebral fractures. Multiple regression analysis adjusted for age, body height, weight, duration of diabetes, and serum creatinine showed that serum OC and OC/BAP ratio were correlated negatively with HbA(1c) (P < 0.01) and positively with IGF-I (P < 0.01). Multivariate logistic regression analysis adjusted for the above parameters showed that serum OC/BAP ratio was inversely associated with the presence of vertebral fractures (odds ratio = 0.695, P < 0.05). This association was still significant after additional adjustment for lumbar or femoral neck BMD. Our results suggest that poor diabetic control and lower IGF-I level are linked to impaired bone formation and resultant reduction in OC/BAP ratio in men with type 2 diabetes. The OC/BAP ratio could be clinically useful for assessing the risk of vertebral fractures independent of BMD in diabetic men.

Combination of obesity with hyperglycemia is a risk factor for the presence of vertebral fractures in type 2 diabetic men

Although patients with type 2 diabetes show no bone mineral density (BMD) reduction, fracture risks are known to increase. It is unclear why the patients have an increased risk of fracture despite sufficient BMD. We investigated the relationships of body mass index (BMI), HbA(1c), and urinary C-peptide (uC-peptide) versus BMD, bone metabolic markers, serum adiponectin, and prevalent vertebral fracture (VF). A total of 163 Japanese type 2 diabetic men were consecutively recruited, and radiographic and biochemical data were collected. BMI was positively correlated with BMD at the whole body, lumbar spine, and femoral neck (P < 0.05) and negatively correlated with osteocalcin and urinary N-terminal cross-linked telopeptide of type-I collagen (uNTX) (P < 0.01). HbA(1c) was negatively correlated with osteocalcin (P < 0.01) but not BMD at any site. Subjects were classified into four groups based on BMI and HbA(1c) (group LL BMI < 24 and HbA(1c) < 9, group LH BMI < 24 and HbA(1c) > or = 9, group HL BMI > or = 24 and HbA(1c) < 9, group HH BMI > or = 24 and HbA(1c) > or = 9). Serum adiponectin, osteocalcin, and uNTX were lower and the incidence of VF was higher despite sufficient BMD in the HH group. Multivariate logistic regression analysis adjusted for age, duration of diabetes, uC-peptide, and estimated glomerular filtration rate showed that the HH group was associated with the presence of a VF and multiple VFs (odds ratio [OR] = 3.056, 95% confidence interval [CI] 1.031-9.056, P = 0.0439, and OR = 5.415, 95% CI 1.126-26.040, P = 0.0350, respectively). Combination of obesity with hyperglycemia was a risk factor for VF despite sufficient BMD in diabetic men.

Bone formation in spontaneously diabetic Torii-newly established model of non-obese type 2 diabetes rats

It is well known that patients with type 1 diabetes mellitus exhibit bone abnormalities as one of the complications of the disease. Whether this occurs in type 2 diabetes is controversial. This uncertainty could be because type 2 diabetes includes several pathological types such as obese and non-obese. To examine the bone abnormalities in non-obese type 2 diabetes, we used Spontaneously Diabetic Torii (SDT) rats, which is a newly established model of non-obese type 2 diabetes. Sprague-Dawley (SD) rats were used as a control group (n=17). SDT rats were divided into two groups: the diabetic (DM) group (n=18) and the DM+insulin (INS) group (n=18) at 20 weeks of age. The DM+INS group received subcutaneously implanted insulin pellets every 2 weeks. At 36 weeks of age, the rats were killed, and we evaluated bone formation and the effect of insulin on bone formation, blood and urine analyses, bone mineral density (BMD), histomorphometry, and mRNA expression of alkaline phosphatase (ALP) and osteocalcin (OCN). Despite renal function not being impaired, BMD and bone strength were significantly lower in the DM group than in the control group. Osteoid volume per bone volume, osteoblast surface per bone surface, eroded surface per bone surface, osteoclast surface per bone surface, the mineral apposition rate, and the bone formation rate per bone surface were significantly lower in the DM group than in the control and DM+INS groups. The mRNA expression of ALP and OCN was significantly lower in the DM group than in the control group. Furthermore, 8-hydroxydeoxyguanosine, which is an oxidative stress marker, was remarkably elevated in the DM group. These abnormalities were recovered by insulin therapy. Our data support the notion that non-obese type 2 diabetes is associated with a low turnover of bone and that the abnormalities are ameliorated by insulin. The SDT rat may be a useful animal model for examining the mechanisms of bone abnormalities in non-obese type 2 diabetes.

Increased incidence of vertebral fracture in older female hemodialyzed patients with type 2 diabetes mellitus

Bone disease in hemodialysis (HD) patients with type 2 diabetes mellitus (DM) is characterized by low bone turnover (Inaba M, et al. Am J Kidney Dis 2002; 39:1261-1269), although their bone quality is yet to be determined. The present study was designed to examine whether the prevalence of vertebral fracture in female HD patients with type 2 DM, age 65 years and older, might be increased, and the relation of this fracture to bone mineral density (BMD) determined by dual X-ray absorptiometry (DXA), since few data are available on the effect of DM on bone strength at lumbar spine. The prevalence of vertebral fracture in type 2 DM HD patients was 32.3%, which was greater than that of non-DM HD patients (13.3%) when adjusted for age and HD duration. Logistic regression analysis elucidated the presence of DM and age as independent risk factors for an increased prevalence of vertebral fracture in HD patients. In non-DM HD patients, those with vertebral fracture showed age significantly higher and BMD in either lumbar spine or distal one third of radius significantly lower than the respective value in those without fracture. However, in DM HD patients, neither BMD in lumbar spine nor distal one third of radius was significantly lower in those with vertebral fracture than in those without. Furthermore, age did not differ significantly between DM HD patients with and without fracture. In conclusion, female type 2 DM HD patients, age 65 years and older, showed significantly higher incidence of vertebral fracture than non-DM HD patients. Although age and low BMD emerged as independent risk factors for vertebral fracture in non-DM HD patients, those factors failed to be a risk factor in DM HD patients, suggesting that BMD determined by DXA might not be reliable in assessing bone strength in DM HD patients.

Impaired secretion of parathyroid hormone, but not refractoriness of osteoblast, is a major mechanism of low bone turnover in hemodialyzed patients with diabetes mellitus

Diabetic bone disease is characterized by low bone turnover resulting from either impaired secretion of parathyroid hormone (PTH) or refractoriness of osteoblasts to PTH. The present study was performed to elucidate which factor contributes more to the reduction in bone turnover by comparison between 64 hemodialyzed patients with diabetes mellitus and 106 hemodialyzed patients without diabetes mellitus. Only men were enrolled to avoid the influence of the menstrual cycle on bone metabolism. Serum intact PTH (iPTH) levels were significantly lower in hemodialyzed patients with diabetes than those without diabetes, although no significant difference existed in age, duration of hemodialysis therapy, or serum calcium or phosphate levels. Of the biochemical markers measured, serum intact osteocalcin (iOC) and deoxypyridinoline levels were significantly lower in patients with diabetes, although serum bone-specific alkaline phosphatase (BAP) and pyridinoline levels did not differ significantly between the two groups of patients. When patients were restricted to those with serum iPTH levels greater than 180 pg/mL, this parameter correlated significantly in a positive manner with both serum iOC and BAP levels and negatively with bone mineral density at distal radius 1/3. Regression slopes between iPTH levels and these parameters were not significantly different between the two groups of patients, indicating the absence of refractoriness of bone to PTH in patients with diabetes. In conclusion, our findings suggest that impaired PTH secretion, but not refractoriness of osteoblasts to PTH, may be responsible for the low bone turnover in hemodialyzed patients with diabetes.

Impaired secretion of parathyroid hormone is coherent to diabetic hemodialyzed patients

Diabetic bone disease is characterized by low bone turnover resulting from impaired secretion of parathyroid hormone (PTH). However, it was suggested that the difference in duration of hemodialysis (HD) therapy and age of patients between HD patients with and without diabetes mellitus (DM) may be responsible for a significant reduction in serum intact PTH (iPTH) level in HD patients with DM. The present study showed that although such major factors affecting PTH secretion as age, sex, HD duration, and serum calcium, phosphate, and magnesium levels did not differ significantly between HD patients with and without DM, serum iPTH levels were still significantly lower in HD patients with than without DM. Among biochemical markers for bone metabolism, serum levels of intact osteocalcin (iOC) and deoxypyridinoline (DPD) were significantly lower in HD patients with than without DM, whereas serum bone-specific alkaline phosphatase, pyridinoline, and beta-crosslaps did not differ significantly between the two groups of patients. In summary, our findings indicate that PTH secretion may be significantly impaired in HD patients with DM compared with those without DM, and serum iOC and DPD are bone markers sensitive enough to detect low bone turnover in HD patients with DM.