Gq-Dependent Signaling Upregulates COX2 in Glomerular Podocytes

Abstract

Accumulating evidence suggests that upregulation of cyclooxygenase 2 (COX2) in glomerular podocytes promotes podocyte injury. Because Gq signaling activates calcineurin and calcineurin-dependent mechanisms are known to mediate COX2 expression, this study investigated the role of Gq␣ in promoting COX2 expression in podocytes. A constitutively active Gq ␣ subunit tagged with the TAT HIV protein sequence was introduced into an immortalized podocyte cell line by protein transduction. This stimulated inositol trisphosphate production, activated an nuclear factor of activated T cells-responsive reporter construct, and enhanced levels of both COX2 mRNA and protein compared with cells treated with a Gq protein lacking the TAT sequence. Induction of COX2 was associated with increased prostaglandin E 2 production and podocyte death, both of which were attenuated by selective COX2 inhibition. In vivo, levels of COX2 mRNA and protein were significantly enhanced in podocytes from transgenic mice that expressed podocyte-targeted constitutively active Gq␣ compared with nontransgenic littermates. These data suggest that Gq-dependent signaling cascades stimulate calcineurin and, in turn, upregulate COX2 mRNA and protein, increase eicosanoid production, and cause podocyte injury. 19: 210819: -211819: , 200819: . doi: 10.1681 Glomerular podocytes are highly differentiated cells that play a key role in maintaining glomerular permselectivity. [1][2][3] In glomerular diseases, podocyte damage causes proteinuria and progressive loss of kidney function. 2 The underlying mechanisms that predispose podocytes to injury, however, are incompletely understood. Recent studies suggested that upregulation of cyclooxygenase 2 (COX2) in glomerular podocytes promotes podocyte damage. In this regard, COX2 expression in podocytes is increased in diabetic kidney disease, 4 subtotal nephrectomy, 5-9 Thy-1 glomerulonephritis, 10 and human renal allograft rejection, 11 and selective COX2 inhibition reduces renal injury and proteinuria in animal models of diabetes, 12 renal ablation, 5,6,8,9 and salt-sensitive hypertension. 13 More recently, Kennedy et al. 14 demonstrated that mechanical stress induces COX2 expression in cultured podocytes. Moreover, overexpression of COX2 specifically in glomerular podocytes enhances albuminuria and foot process effacement and reduces glomerular nephrin expression in adriamycin nephropathy. 15 These data suggest that COX2 may be an important mediator of renal injury in glomerular disease processes. J Am Soc NephrolCOX2 expression is stimulated, in part, by activation of the serine/threonine phosphatase calcineurin, through activation of nuclear factor of activated T cells (NFAT)-responsive elements in the COX2 promoter. 16 NFAT transcription factors were originally thought to be expressed only in cells of the lymphoid lineage, but abundant evidence now indicates that

A Maladaptive Role for EP4 Receptors in Podocytes

“…Similarly, Harris and coworkers 31 showed that puromycin aminonucleoside-induced apoptosis was attenuated in response to antagonism of the Gq-coupled TP receptor but not antagonism of the Gs-coupled EP4 receptor. The work of Spurney and colleagues 36 is consistent with these findings; they demonstrated that overexpression of a constitutively active Gq ␣ subunit stimulated calcineurin activity and was associated with podocyte apoptosis in vitro. The notion that Gq-but not Gs-coupled signaling promotes podocyte apoptosis is supported by a number of studies examining effects of the angiotensin II type 1 (AT1) receptor.…”

The EP4 Receptor for Prostaglandin E2 in Glomerular Disease

“…In the context of previous studies, 13 these findings suggest that a mechanistic pathway of increased expression of COX-2 in the injured glomerulus triggers local generation of PGE 2 with activation of the EP4 receptor in podocytes, causing exaggerated proteinuria and glomerular damage. This pathway may work in parallel with enhanced thromboxane production and TP receptor activation.…”

“…These data are also consistent with a more recent report from a different group. 13 Thus, it is probably safe to conclude that either deficient or excess podocyte-derived VEGF is bad for the filtration barrier. Both perturbations lead to proteinuria but by dichotomous pathobiological routes.…”

Gq signaling causes glomerular injury by activating TRPC6

“…As previously reported, induction of GqQ>L does not cause glomerular injury (28), suggesting that a "second hit" may be required to induce podocyte injury, as has been suggested for Familial forms of focal segmental glomerulosclerosis (FSGS) have been linked to gain-of-function mutations in the gene encoding the transient receptor potential channel C6 (TRPC6). GPCRs coupled to Gq signaling activate TRPC6, suggesting that Gq-dependent TRPC6 activation underlies glomerular diseases.…”

“…To investigate the role of Gq signaling in the pathogenesis of glomerular diseases, we expressed a constitutively active Gq α subunit (Gq Q209L , herein referred to as GqQ>L) specifically in podocytes in a doxycycline-inducible (DOX-inducible) fashion (28). As previously reported, induction of GqQ>L does not cause glomerular injury (28), suggesting that a "second hit" may be required to induce podocyte injury, as has been suggested for Familial forms of focal segmental glomerulosclerosis (FSGS) have been linked to gain-of-function mutations in the gene encoding the transient receptor potential channel C6 (TRPC6).…”