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Myofibroblast phenotypes expression in experimental renal scarring.

BACKGROUND
Myofibroblasts have been implicated in the pathogenesis of wound healing and tissue fibrosis. A role has also been put forward for these cells in the development of experimental and clinical renal scarring.


SUBJECTS AND METHODS
We examined the expression of myofibroblast phenotypes by immunohistochemistry, relying on an avidin-biotin-peroxidase method, during the course of renal scarring in rats submitted to subtotal (5/6) nephrectomy (SNx). We also attempted to identify changes in immunoreactive transforming growth factor-beta (TGF-beta) and collagen (III and IV) within remnant kidneys in order to determine their association with the expression of the myofibroblasts.


RESULTS
In normal sham-operated rats, alpha-smooth muscle actin (alpha-SMA) was confined to the media of renal arteries and arterioles. In contrast, in rats with renal ablation we observed the early (day 7) appearance of myofibroblasts expressing alpha-SMA (A) in the interstitium of remnant kidneys particularly around vessels. Interstitial cells expressing alpha-SMA increased with time as tubulointerstitial fibrosis progressed. By day 30 some interstitial cells also expressed vimentin (V). Various interstitial myofibroblast phenotypes (A, V, VA) were expressed during the course of experimental renal scarring. Interstitial myofibroblasts appeared to be associated with TGF-beta as these cells' cytoplasm stained for both this growth factor and alpha-SMA. Interstitial fibrosis was also associated with increased interstitial expression of both collagen III and IV. Some atrophic tubular cells showed positive immunostaining for vimentin during the late stages of renal scarring (days 90-150). In the glomeruli, a segmental expression of alpha-SMA was noted from day 21 after SNx onward. Normal glomerular endothelial cells appeared to express vimentin while epithelial cells expressed both vimentin and desmin (D). The glomerular immunostain for vimentin increased with time but decreased as glomerulosclerosis progressed. In contrast, glomerular desmin and alpha-SMA immunostain continued to rise with progressive glomerulosclerosis. This was associated with the appearance of type III collagen within scarred glomeruli. Both vimentin and desmin appeared within the walls of the renal arterioles and increased with time from day 7 and 15, respectively. Vimentin was also expressed in the peritubular capillaries of remnant kidneys. By contrast, alpha-SMA, normally present in the media of arterioles, decreased as arteriolar sclerosis progressed. These changes cannot be exclusively attributed to systemic hypertension as they were absent in a group of age-matched, sham-operated, spontaneously hypertensive rats.


DISCUSSION
Myofibroblasts may play a role in the pathogenesis of glomerulosclerosis, tubulointerstitial fibrosis and vascular sclerosis. Further, the acquisition of new myofibroblastic phenotypes by glomerular and tubular cells may contribute to renal fibrosis.