Activation of a local tissue angiotensin system in podocytes by mechanical strain.
BACKGROUND
Glomerular capillary hypertension, a common denominator in various forms of progressive glomerular disease, results in mechanical distention of the capillary tuft, and subsequent injury of the overlying podocyte layer. The mechanisms by which elevated intraglomerular pressure is translated into a maladaptive podocyte response remain poorly understood. Angiotensin II plays a central role in the pathogenesis of chronic renal injury, largely through its actions on the subtype 1 receptor. Accordingly, we have tested the hypothesis that mechanical strain up-regulates local angiotensin II in podocytes, thereby resulting in a progressive reduction in podocyte number.


METHODS
Conditionally immortalized mouse podocytes were subjected to cyclical stretch of 10% amplitude. Nonstretched podocytes served as controls. Angiotensin II levels were measured in whole cell lysate by competitive enzyme-linked immunosorbent assay (ELISA). Expression of angiotensin II receptors (AT1R, AT2R) was measured by quantitative polymerase chain reaction (PCR) and Western blot analysis. Apoptosis was measured by Hoechst staining. Immunostaining for AT1R was performed in tissue sections from rats with 5/6 remnant kidney disease, a model of glomerular hypertension.


RESULTS
Mechanical strain increased angiotensin II production in podocytes at 24, 48, and 72 hours (P < 0.05 vs. nonstretched controls). Stretching podocytes resulted in a fivefold increase in AT1R mRNA expression at 24 hours and a twofold increase in protein levels vs. controls (P < 0.05), and also an increase in transforming growth hormone-beta (TGF-beta) mRNA expression. AT1R staining was increased in a podocyte distribution in the 5/6 remnant kidney, consistent with our in vitro findings. Mechanical strain resulted in a 2.5-fold increase in apoptosis (P < 0.001 vs. nonstretched controls) in an angiotensin II-dependent fashion.


CONCLUSIONS
Mechanical strain leads to up-regulation of the AT1R and increased angiotensin II production in conditionally immortalized podocytes. The resulting activation of a local tissue angiotensin system leads to an increase in podocyte apoptosis, mainly in an AT1R-mediated fashion.
Durvasula
Raghu V
RV
Department of Medicine, Division of Nephrology, University of Washington School of Medicine, Seattle, Washington 98195, USA.
Petermann
Arndt T
AT
Hiromura
Keiju
K
Blonski
Mary
M
Pippin
Jeffrey
J
Mundel
Peter
P
Pichler
Raimund
R
Griffin
Sian
S
Couser
William G
WG
Shankland
Stuart J
SJ
D000804:Angiotensin II / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D017209:Apoptosis / Q000502:physiology; D002196:Capillaries / Q000502:physiology; D002461:Cell Line, Transformed; D015870:Gene Expression / Q000502:physiology; D006977:Hypertension, Renal / Q000503:physiopathology; D007678:Kidney Glomerulus / Q000098:blood supply / Q000187:drug effects / Q000502:physiology*; D040542:Mechanotransduction, Cellular / Q000502:physiology*; D051379:Mice; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D044140:Receptor, Angiotensin, Type 1 / Q000235:genetics / Q000378:metabolism; D044139:Receptor, Angiotensin, Type 2 / Q000235:genetics / Q000378:metabolism; D012079:Renal Circulation / Q000187:drug effects / Q000502:physiology; D013314:Stress, Mechanical; D016212:Transforming Growth Factor beta / Q000235:genetics; D015854:Up-Regulation; D014662:Vasoconstrictor Agents / Q000378:metabolism* / Q000494:pharmacology
D012333:RNA, Messenger; D044140:Receptor, Angiotensin, Type 1; D044139:Receptor, Angiotensin, Type 2; D016212:Transforming Growth Factor beta; D014662:Vasoconstrictor Agents; D000804:Angiotensin II