No abstract

The uterine endometrium is exquisitely sensitive to steroid hormones that act through well-described nuclear receptors. Estrogen drives epithelial proliferation, and progesterone inhibits growth and causes cell differentiation. The importance of progesterone as a key inhibitor of carcinogenesis is reflected by the observation that women who ovulate and produce progesterone almost never get endometrial cancer. In this review we describe seminal research findings that define progesterone as the major endometrial tumor suppressor. We discuss the genes and diverse signaling pathways that are controlled by progesterone through progesterone receptors (PRs) and also the multiple factors that regulate progesterone/PR activity. By defining these progesterone-regulated factors and pathways we identify the principal therapeutic opportunities to control the growth of endometrial cancer.

Distributions of ErbB receptors on membranes of SKBR3 breast cancer cells were mapped by immunoelectron microscopy. The most abundant receptor, ErbB2, is phosphorylated, clustered and active. Kinase inhibitors ablate ErbB2 phosphorylation without dispersing clusters. Modest co-clustering of ErbB2 and EGFR, even after EGF treatment, suggests that both are predominantly involved in homointeractions. Heregulin leads to dramatic clusters of ErbB3 that contain some ErbB2 and EGFR and abundant PI 3-kinase. Other docking proteins, such as Shc and STAT5, respond differently to receptor activation. Levels of Shc at the membrane increase two- to five-fold with EGF, whereas pre-associated STAT5 becomes strongly phosphorylated. These data suggest that the distinct topography of receptors and their docking partners modulates signaling activities.

Often considered to be a "dead" kinase, erbB3 is implicated in escape from erbB-targeted cancer therapies. Here, heregulin stimulation is shown to markedly upregulate kinase activity in erbB3 immunoprecipitates. Intact, activated erbB3 phosphorylates tyrosine sites in an exogenous peptide substrate, and this activity is abolished by mutagenesis of lysine 723 in the catalytic domain. Enhanced erbB3 kinase activity is linked to heterointeractions with catalytically active erbB2, since it is largely blocked in cells pretreated with lapatinib or pertuzumab. erbB2 activation of erbB3 is not dependent on equal surface levels of these receptors, since it occurs even in erbB3-transfected CHO cells with disproportionally small amounts of erbB2. We tested a model in which transient erbB3/erbB2 heterointeractions set the stage for erbB3 homodimers to be signaling competent. erbB3 homoand heterodimerization events were captured in real time on live cells using single-particle tracking of quantum dot probes bound to ligand or hemagglutinin tags on recombinant receptors. ErbB3 is a member of the epidermal growth factor receptor (EGFR)/erbB family of receptor tyrosine kinases that has been implicated in escape from targeted therapies (1, 2). In the classic view of erbB3 signaling, ligand binding to this "kinase-dead" receptor leads to dimerization with erbB2 or another kinase-competent family member (3, 4). The dimerizing partner is then solely responsible for phosphorylating erbB3 and any binding partners recruited to the erbB3 tail (5). When expressed alone, the erbB3 extracellular domain or full-length receptor is refractory to activation by ligand (6-9). These data, along with experiments using chimeric forms of erbB3 (10), led to the assumption that erbB3 does not homodimerize and that erbB2 is obligatory for erbB3 signaling. Interpretation of these findings must now be revisited in the context of the evolving understanding of both extracellular and intracellular erbB dimerization interfaces (11), as well as new evidence that the erbB3 cytoplasmic tail is capable of measurable kinase activity (12, 13). We reexplored erbB3's catalytic activity, using an in vitro peptide substrate and immunoisolated wild-type or mutant forms of the receptor. One goal of this work was to determine whether native, full-length erbB3 can engage in productive homointeractions (dimers or possibly higher-order oligomers). By conducting our work in live cells, these results strengthen the case that erbB3 signals in part through its modest catalytic activity, as proposed recently by the molecular dynamic simulations of Telesco et al. (14).Several recent advances now make it possible to directly observe dimerization dynamics for receptors in live cell membranes. When conjugated to ligands or antibodies, quantum dots (QDs) serve as improved probes for single-particle tracking (SPT) (15-17). Our group recently used two-color QD-based probes to track the diffusion-limited interactions of EGFR/erbB1, including observations of homodimerization in real tim...