"Up-regulation of Parathyroid Hormone-Related Protein (PTHrP) and its receptor in experimental diabetic nephropathy"

Pedro Garrido, Adriana Izquierdo, Arantxa Ortega**, Pedro Esbrit**, Julie Matthews, Ignacio Arribas*, Ma Antonia Gutiérrez-Tarrés, Pilar López-Luna, Jordi Bover*** and Ricardo J. Bosch.

Laboratory of Renal Physiology and Experimental Nephrology, Department of Physiology, University of Alcalá and Hospital Príncipe de Asturias*, Alcalá de Henares; Bone and Mineral Metabolism Laboratory, Fundación Jiménez Díaz**, Madrid; Department of Nephrology, Fundació Puigvert***,
Barcelona, Spain.

This work was supported in part by grants from Spanish MEC (PB98-0700-C02-01 y 02) and CAM (08./0038./2000).

Parathyroid hormone-related protein (PTHrP) is present in the kidney, where it modulates glomerular function, and acts as a renal mitogenic factor. PTHrP expression increases in several experimental nephropathies, and its upregulation appears to be a common event associated with renal injury and/or repair. We analyzed herein whether PTHrP might be involved in diabetic nephropathy.

The streptozotocin (STZ) (65 mg/kg i.p.) model of diabetes mellitus was used in CD1 mice. Some animals were also injected with a daily dose (15 IU/kg s.c.) of insulin (Lente MC, Novo) for two months. PTHrP and PTH/PTHrP receptor expression: At 1-2 months after STZ, kidneys were removed. Total RNA and proteins were isolated, and used for RT-PCR and Western blot, respectively. Immunohistochemistry for PTHrP was performed using antiserum anti-PTHRP in paraffin-embedded kidney samples.

Diabetes Induction: STZ-induced diabetes mellitus in mice [glycemia (mean±SEM; mg/dl): 310±2 vs 90±3 (control) (p<0.05)].
Urinary albumin excretion in these animals was increased at month 1, and remained so up to month 2 [10.6±2.4 vs 2.6±0.6 (control) mg/24 h; p<0.01], but it sharply increased at month 3 (23.7±3.1 mg/24 h; p<0.01), following STZ injection.
PTHrP expression: Through RT-PCR and Western-blot a significant increase of PTHrP (3-fold over control) was observed in the kidney at month one after STZ diabetes induction, decreasing to control values at month two.
PTH/PTHrP receptor (PTH1R) expression: The same occured in western blot analysis of the receptor (PTH1R) in diabetic mice at month 1 after STZ diabetes induction, which increased 3-fold over control. At month 2, this value return to control values.
Renal PTHRP immunohistochemistry: showed dramatic PTHrP staining in the glomerulus, mainly in epithelial cells, at this time period in these mice. Renal PTHrP overexpression was abrogated by insulin, and it returned to control values at month 1 after STZ injection.

Our results indicate that the changes in PTHrP and PTH1R expression in the kidney are associated with the initial stages of diabetic nephropathy. These findings suggest that PTHrP might have a role in the pathophysiology of this condition. Further studies are required to assess the interaction between insulin and PTHrP in renal tissue.