High specific activity estradiol labeled with iodine-125 was used to detect approximately 200 saturable, high-affinity (dissociation constant approximately equal to 1.0 nM) nuclear binding sites in rat (ROS 17/2.8) and human (HOS TE85) clonal osteoblast-like osteosarcoma cells. Of the steroids tested, only testosterone exhibited significant cross-reactivity with estrogen binding. RNA blot analysis with a complementary DNA probe to the human estrogen receptor revealed putative receptor transcripts of 6 to 6.2 kilobases in both rat and human osteosarcoma cells. Type I procollagen and transforming growth factor-beta messenger RNA levels were enhanced in cultured human osteoblast-like cells treated with 1 nM estradiol. Thus, estrogen can act directly on osteoblasts by a receptor-mediated mechanism and thereby modulate the extracellular matrix and other proteins involved in the maintenance of skeletal mineralization and remodeling.

Phosphatidylinositol 3-kinase (PI3-K) phosphorylates the 3-position of phosphatidylinositol to give rise to three signaling phospholipids. Binding of the pleckstrin homology (PH) domain of Akt to membrane PI(3)P's causes the translocation of Akt to the plasma membrane bringing it into contact with membrane-bound Akt kinase (PDK1 and 2), which phosphorylates and activates Akt. Akt inhibits apoptosis by phosphorylating Bad, thus promoting its binding to and blockade of the activity of the cell survival factor Bcl-x. Herein we present the synthesis and biological activity of several novel phosphatidylinositol analogues and demonstrate the ability of the carbonate group to function as a surrogate for the phosphate moiety. Due to a combination of their PI3-K and Akt inhibitory activities, the PI analogues 2, 3, and 5 proved to be good inhibitors of the growth of various cancer cell lines with IC(50) values in the 1-10 microM range. The enhanced Akt inhibitory activity of the axial hydroxymethyl-bearing analogue 5 compared to its equatorial counterpart 6 is rationalized based upon postulated differences in the H-bonding patterns of these compounds in complex with a homology modeling generated structure of the PH domain of Akt. This work represents the first attempt to examine the effects of 3-modified PI analogues on these two crucial cell signaling proteins, PI3-K and Akt, in an effort to better understand their cell growth inhibitory properties.

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