Non-genomic effects of tamoxifen on the activation of membrane-bound guanylate cyclase GC-A

Abstract

Abstract Oestrogen is known to exert both genomic and non-genomic effects on target tissues. Unlike the genomic effects, the identity of receptors mediating the non-genomic effects of oestrogen remains controversial. 17β-Estradiol has been shown to activate membrane-bound guanylate cyclase GC-A in PC12 cells in a non-genomic manner. To examine whether 17β-estradiol exerts a similar effect in other cell types, we measured the effect of 17β-estradiol and tamoxifen, an anti-oestrogen, on guanylate cyclase activity in porcine kidney proximal tubular LLC-PK1 cells. 17β-Estradiol increased cGMP levels in LLC-PK1 cells. Interestingly, addition of tamoxifen also increased cGMP levels in a concentration-dependent manner in LLC-PK1 cells. The effects of both 17β-estradiol and tamoxifen on guanylate cyclase activity were not additive, suggesting that oestrogen and tamoxifen activate the same enzyme. Similar phenomena were also observed in LLC-PK1 cell membrane preparation. LLC-PK1 cells do not express membrane-bound guanylate cyclase GC-B and express low levels of membrane-bound guanylate cyclase GC-C. Tamoxifen inhibited the activation of GC-A by atrial natriuretic factor (ANF). However, it did not affect membrane-bound guanylate cyclase GC-C stimulated by guanylin or Escherichia coli heat-stable toxin STa. These results indicate that 17β-estradiol and tamoxifen activate GC-A in LLC-PK1 cells. Thus, tamoxifen functions as an agonist rather than an antagonist for the membrane oestrogen receptor coupled to the activation of GC-A.