Fas-dependent fratricidal apoptosis is a mechanism of tubular epithelial cell deletion in chronic renal failure.

Renal tubular atrophy predicts a poor prognosis in chronic renal failure, but the molecular mechanisms regulating this process remain unknown. Because the Fas apoptosis pathway has recently been implicated in disease pathogenesis and Fas is expressed in the kidney, we hypothesized that Fas-mediated apoptosis of renal tubule epithelial cells (RTC) contributes to tubular atrophy in chronic renal failure. In vivo, immunohistochemical analyses of renal sections from two murine models of progressive renal disease revealed coordinate increases in RTC Fas expression and apoptosis compared with tissue sections from age-matched control kidneys. Increased RTC apoptosis was not accompanied by compensatory hyperplasia, suggesting that RTC targeted for Fas-dependent apoptotic deletion contribute to tubular atrophy. These data are supported by in vitro studies showing that interleukin-1alpha and tumor necrosis factor-alpha, cytokines secreted in chronic renal failure, stimulated increases in Fas expression in cultured RTC. Both murine kidney cortex and RTC in culture demonstrated constitutive expression of transmembrane and soluble forms of RTC Fas ligand, features that are primarily restricted to lymphocytes and immune-privileged tissues and that have been previously unrecognized in RTC. Functional studies revealed that interleukin-1alpha-stimulated RTC Fas expression was accompanied by increased apoptosis, which was inhibited by blocking anti-Fas ligand antibodies. In contrast to the conventional paradigm, which holds that Fas-dependent apoptosis is initiated by the binding of lymphocyte Fas ligand to target cell Fas, our data suggest that up-regulated RTC Fas binds to Fas ligand on adjacent RTC, which then leads to RTC death by fratricide. We propose this pathway as an initiating mechanism of tubular atrophy.

Angiotensin II activates the beta 1 isoform of phospholipase C in vascular smooth muscle cells.

Vascular smooth muscle cells (VSMC) contribute to the pathophysiology of hypertension through cell growth and contraction, and phospholipase C (PLC) is a critical effector enzyme in growth factor and vasoconstrictor signaling. There is indirect evidence that angiotensin II (ANG II) receptors are linked to the PLC-beta isoform signaling pathways. However, recent studies suggest that PLC-beta isoforms may not be expressed in VSMC. Our data demonstrate that in human aortic VSMC, PLC-beta 1 and PLC-gamma 1 proteins were detected by immunoblot analysis, and PLC-beta 1 mRNA was identified by reverse transcriptase-polymerase chain reaction in rat aortic VSMC. Incubation of permeabilized VSMC with anti-PLC-beta 1 or anti-Gq alpha antibodies inhibited ANG II-dependent inositol polyphosphate (IP) formation, while anti-PLC-gamma 1 antibodies did not inhibit ANG II-regulated IP formation. Conversely, anti-PLC-gamma 1 antibodies completely abolished platelet-derived growth factor (PDGF)-dependent IP generation, whereas anti-PLC-beta 1 antibodies had no effect on PDGF-induced PLC activation. Inhibition of tyrosine phosphorylation with genistein or herbimycin A did not diminish ANG II-stimulated IP formation or cytosolic free Ca2+ concentration transients, thereby confirming that ANG II signals via a PLC-gamma 1-independent mechanism. In summary, PLC-beta 1 and PLC-gamma 1 are expressed in human aortic VSMC, and PLC-beta 1 is the isoform that is critical for ANG II-regulated PLC signaling in these cells.