Increased Inositol 1,4,5-Trisphosphate Accumulation Correlates with an Up-Regulation of Bradykinin Receptors in Alzheimer's Disease

Abstract: An alteration in signal transduction systems in Alzheimer's disease (AD) would likely be of pathophysiological significance, because these processes control normal brain functions. Previously, a diminished β-adrenergic-mediated cyclic AMP response was found in cultured fibroblasts from AD patients. Because cross-talk between the phosphoinositide and cyclic AMP pathways exists, the phosphoinositide cascade was studied under conditions that were similar to those for studying the cyclic AMP response. Cells from AD patients and age-matched controls responded to bradykinin (BK) and released inositol 1,4,5-trisphosphate Ins(1,4,5)P₃] in a time- and dose-dependent manner. The level of Ins(1,4,5)P₃ increased rapidly and transiently in response to BK, peaked at 5 s, but still remained 116–132% above the basal level by 30 s. Although the temporal patterns were similar in both groups, the Ins(1,4,5)P₃ concentrations in AD fibroblasts were 73 and 89% above levels in the age-matched controls at 5 and 10 s, respectively. Prostaglandin E₁ also increased Ins(1,4,5)P₃ formation, but this response was not different between the two groups. Although K _D (affinity) values for the BK receptor were similar in both control and AD cells, the number of BK receptors ( B _max) was significantly elevated in AD fibroblasts (186.8 ± 0.8 fmol/mg of protein) as compared with control fibroblasts (57.2 ± 15.3 fmol/mg of protein). These results indicate that the elevated Ins(1,4,5)P₃ production in response to BK in AD fibroblasts is positively correlated with an increase in the receptor numbers.