Activation of angiotensin II type I receptor promotes protein kinase C translocation and cell proliferation in human cultured breast epithelial cells

The effect of angiotensin II (Ang II) on Ca²⁺ signalling in human primary cultured breast epithelial cells was investigated by using fura-2 as the Ca²⁺ probe. Ang II (0.1-1000 nM) induced an intracellular free calcium ([Ca²⁺]_i) transient peak which was unchanged by external Ca²⁺ removal. In Ca²⁺-free medium pretreatment with thapsigargin abolished Ang II-induced Ca²⁺ release. Suppression of 1,4,5-inositol trisphosphate formation by U73122, a phospholipase C inhibitor, blocked the Ang II-induced Ca²⁺ response. Losartan (DuP753), an inhibitor of Ang II type I receptor (AT1), decreased the [Ca²⁺]_i increase evoked by Ang II, while CGP4221A, an inhibitor of Ang II type II receptor (AT2) did not. AT1 desensitisation was demonstrated with respect to the Ca²⁺ response after subsequent exposure of cells to Ang II and also after pretreatment for 25 min with 1000 nM phorbol 12-myristate 13-acetate. Staurosporine, an inhibitor of protein kinases C (PKC), inhibited the AT1 desensitisation. Epithelial breast cells expressed PKC-α, -β1, -δ and -ζ isozymes, and Ang II provoked translocation from the cytosol to the membranes of PKC-α, -β1, and -δ (but not -ζ). Ang II was also able to stimulate cell proliferation in a dose-dependent manner; this effect was blocked by Gö 6976, a specific inhibitor of PKC-α and -β1, the Ca²⁺-dependent isozymes. The main conclusion of this study is that the the Ang II signalling mechanism in breast epithelial cells is based on the elevation of [Ca²⁺]_i released from intracellular stores through AT1 activation. In addition, Ang II stimulates cell proliferation by the activation of PKC isozymes.