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S100P is a member of the S100 family of calcium-binding proteins and there have been several recent reports of its overexpression in pancreatic ductal adenocarcinoma (PDAC). We have used Far Western screening and in vitro interaction assays to identify and confirm a novel target protein for S100P. We have named this protein S100PBPR, and shown that its interaction with S100P is dependent on Ca(2+) or Mg(2+). S100PBPR was found to localize to cell nuclei where S100P is also present, and the two proteins co-immunoprecipitate. By in situ hybridization, S100PBPR transcript was found in islet cells but not duct cells of the healthy pancreas. Both S100P and S100PBPR were detected by quantitative real-time polymerase chain reaction in pancreatic intraepithelial neoplasia (PanIN) and PDAC samples, and in situ hybridization revealed the presence of S100PBPR transcript in malignant PDAC cells. These data suggest that an interaction between S100P and S100PBPR may be involved in early pancreatic cancer. S100P was further investigated in PanIN lesions and immunohistochemical analysis showed its expression to correlate significantly with increasing grade of PanINs, being found as early as PanIN-1 with more prevalent expression in PanIN-2 and -3. These data suggest that S100P can be added to the genetic progression model for PDAC.

A fertilization-induced increase in intracellular Ca2+ is responsible for initiating all of the events of egg activation. In mammals, the Ca2+ increase takes the form of a series of Ca2+ oscillations showing complex temporal and spatial properties. To understand the nature of these changes, we have investigated the expression patterns of the three isoforms of the inositol trisphosphate receptor (InsP3R) during oocyte maturation and preimplantation development. We find that mouse oocytes express mRNAs for all three InsP3R subtypes. Semiquantitative ratio reverse-transcriptase polymerase chain reaction shows that the type II isoform is the predominant message in mature oocytes, representing 67% of the InsP3R mRNA. In contrast, protein analysis reveals that the type I isoform accounts for all of the detectable InsP3R protein, despite representing only 20% of the InsP3R mRNA. The levels of InsP3R protein were examined to determine whether they correlated with the Ca2+ signaling events surrounding the fertilization process. Type I InsP3R protein increased during oocyte maturation and, in addition, within 8 h of fertilization underwent a dramatic decrease. During development to the blastocyst the level of type I InsP3R protein did not return to prefertilization levels and types II and III remained below our detection limit. The decrease in InsP3R protein after fertilization was found to correlate with a decrease in the sensitivity of InsP3-induced Ca2+ release. These studies show that the expression of InsP3R mRNA is developmentally regulated, that Ca2+ signaling at fertilization is mediated exclusively through the type I InsP3R, and that the InsP3R is downregulated after fertilization., (Copyright 1998 Academic Press.)