Kallikrein,
kininogen and
kinin receptors are present in human ocular tissues including the retinal pigment epithelium (RPE), suggesting a possible role of
bradykinin (BK) in physiological and/or pathological conditions. To test this hypothesis,
kinin receptors expression and function was investigated for the first time in human fetal RPE cells, a model close to native RPE, in both control conditions and
after treatment with proinflammatory
cytokines. Results showed that BK evoked intracellular Ca(2+) transients in human RPE cells by activating the
kinin B2 receptor. Pretreatment of the cells with TNF-α and/or IL-1β enhanced Ca(2+) response in a time- and concentration-dependent additive manner, whereas the potency of BK and that of the selective B2 receptor antagonist,
fasitibant chloride, both in the nanomolar range, remained unaffected.
Cytokines have no significant effect on cell number and viability and on the activity of other GPCRs such as the
kinin B1,
acetylcholine,
ATP and
thrombin receptors. Immunoblot analysis and immunofluorescence studies revealed that
cytokines treatment was associated with an increase in both
kinin B2 receptor and COX-2 expression and with the secretion of
prostaglandin E1 and E2 into the extracellular medium. BK, through activation of the
kinin B2 receptor, potentiated the COX-2 mediated
prostaglandin release in
cytokines-primed RPE cells while new
protein synthesis and
prostaglandin production contribute to the potentiating effect of
cytokines on BK-induced Ca(2+) response. In conclusion, overall data revealed a cross-talk between the
kinin B2 receptor and
cytokines in human RPE in promoting
inflammation, a key feature in
retinal pathologies including
diabetic retinopathy and
macular edema.