Quercetin Dihydrate is the dihydrate preparation of Quercetin, a ubiquitous natural flavonoid with antiproliferative properties. Quercetin is described to inhibit PLA2 and PI 3-kinase (IC50 = 3.8 microM), blocking the ATP binding site of this kinase and interrupting the role of this enzyme in propagating cellular responses to growth factors. Quercetin is further described to inhibit nucleotide-dependent kinase enzymes through competition with ATP and GTP, demonstrating inhibition of pp60src phosphotransferase, tyrosine protein kinase, calcium- and phospholipid-dependent protein kinase, and cyclic GMP phosphodiesterases. Inhibition of PI 4-kinase (1-phosphatidylinositol 4-kinase) and PI-4-phosphate 5-kinase (1-phosphatidylinositol 4-phosphate 5-kinase) by Quercetin blocks the participation of these enzymes in signal transduction pathways and reduces cellular levels of the second messenger IP3 (inositol 1,4,5-trisphosphate), decreasing the release of intracellular Ca2+ stores. The antiproliferative and cytotoxic effects of Quercetin on cultured breast cancer cells are attributed to inhibition of PI 4-kinase and PI-4-phosphate 5-kinase, reduction of IP3 levels, and blockade of oncogenic growth signaling. Quercetin was shown to induce apoptosis in tumor cell lines at the G1 and S phases. Potential fatty acid synthase, PKC, and ATP5 (mitochondrial ATPase) inhibitor as well as an SIRT1 activator. SIRT1 deacetylates histones. Quercetin Dihydrate is an inhibitor of Pim-1 and an activator of GPR30.