Nuclear export and mitochondrial and endoplasmic reticulum localization of IGF-binding protein 3 regulate its apoptotic properties

Tumor suppression by IGF-binding protein 3 (IGFBP3) may occur in an IGF-independent manner, in addition to its role as a regulator of IGF bioavailability. After secretion, IGFBP3 is internalized, rapidly localized to the nucleus, and is later detected in the cytoplasm. We identified a putative nuclear export sequence (NES) in IGFBP3 between amino acids 217 and 228, analogous to the leucine-rich NES sequence of p53 and HIV Rev. Mutation of the NES prevents nucleocytoplasmic shuttling of IGFBP3 and blocks its ability to induce apoptosis. Targeting of IGFBP3 to the mitochondria and endoplasmic reticulum (ER) was confirmed by co-localization with organelle markers using fluorescence confocal microscopy and subcellular fractionation. Mitochondrial targeting was also demonstrated in vivo in IGFBP3-treated prostate cancer xenografts. These results show that IGFBP3 shuttles from the nucleus to the mitochondria and ER, and that nuclear export is essential for its effects on prostate cancer apoptosis.

Estradiol acts directly on bone marrow myeloid progenitors to differentially regulate GM-CSF or Flt3 ligand-mediated dendritic cell differentiation

Estrogen receptor (ER) ligands modulate hemopoiesis and immunity in the normal state, during autoimmunity, and after infection or trauma. Dendritic cells (DC) are critical for initiation of innate and adaptive immune responses. We demonstrate, using cytokine-driven culture models of DC differentiation, that 17-beta-estradiol exerts opposing effects on differentiation mediated by GM-CSF and Flt3 ligand, the two cytokines that regulate DC differentiation in vivo. We also show that estradiol acts on the same highly purified Flt3+ myeloid progenitors (MP) to differentially regulate the DC differentiation in each model. In GM-CSF-supplemented cultures initiated from MP, physiological amounts of estradiol promoted differentiation of Langerhans-like DC. Conversely, in Flt3 ligand-supplemented cultures initiated from the same MP, estradiol inhibited cell survival in a dose-dependent manner, thereby decreasing the yield of plasmacytoid and conventional myeloid and lymphoid DC. Experiments with bone marrow cells from ER-deficient mice and the ER antagonist ICI182,780 showed that estradiol acted primarily via ERalpha to regulate DC differentiation. Thus, depending on the cytokine environment, pathways of ER signaling and cytokine receptor signaling can differentially interact in the same Flt3+ MP to regulate DC development. Because the Flt3 ligand-mediated differentiation pathway is important during homeostasis, and GM-CSF-mediated pathways are increased by inflammation, our data suggest that endogenous or pharmacological ER ligands may differentially affect DC development during homeostasis and disease, with consequent effects on DC-mediated immunity.

Contribution of the orphan nuclear receptor Nur77 to the apoptotic action of IGFBP-3

Tumor suppression by insulin-like growth factor-binding protein-3 (IGFBP-3) has been demonstrated to occur via insulin-like growth factor-dependent and -independent mechanisms in vitro and in vivo. We have recently described IGFBP-3-induced mitochondrial translocation of the nuclear receptors RXRalpha/Nur77 in the induction of prostate cancer (CaP) cell apoptosis. Herein, we demonstrate that IGFBP-3 and Nur77 associate in the cytoplasmic compartment in 22RV1 CaP cells. Nur77 is a major component of IGFBP-3-induced apoptosis as shown by utilizing mouse embryonic fibroblasts (MEFs) derived from Nur77 wild-type and knockout (KO) mice. However, dose-response experiments revealed that a small component of IGFBP-3-induced apoptosis is Nur77 independent. Reintroduction of Nur77 into Nur77 KO MEFs restores full responsiveness to IGFBP-3. IGFBP-3 induces phosphorylation of Jun N-terminal kinase and inhibition of Akt phosphorylation and activity, which have been associated with Nur77 translocation. Finally, IGFBP-3 administration to CaP xenografts on SCID mice induced apoptosis and translocated Nur77 out of the nucleus. Taken together, our results verify an important role for the orphan nuclear receptor Nur77 in the apoptotic actions of IGFBP-3.

Estrogen selectively promotes the differentiation of dendritic cells with characteristics of Langerhans cells

The steroid hormone estrogen regulates the differentiation, survival, or function of diverse immune cells. Previously, we found that physiological amounts of 17beta-estradiol act via estrogen receptors (ER) to promote the GM-CSF-mediated differentiation of dendritic cells (DC) from murine bone marrow progenitors in ex vivo cultures. Of the two major subsets of CD11c(+) DC that develop in these cultures, estrogen is preferentially required for the differentiation of a CD11b(int)Ly6C(-) population, although it also promotes increased numbers of a CD11b(high)Ly6C(+) population. Although both DC subsets express ERalpha, only the CD11b(high)Ly6C(+) DC express ERbeta, perhaps providing a foundation for the differential regulation of these two DC types by estrogen. The two DC populations exhibit distinct phenotypes in terms of capacity for costimulatory molecule and MHC expression, and Ag internalization, which predict functional differences. The CD11b(int)Ly6C(-) population shows the greatest increase in MHC and CD86 expression after LPS activation. Most notably, the estrogen-dependent CD11b(int)Ly6C(-) DC express langerin (CD207) and contain Birbeck granules characteristic of Langerhans cells. These data show that estrogen promotes a DC population with the unique features of epidermal Langerhans cells and suggest that differentiation of Langerhans cells in vivo will be dependent upon local estrogen levels and ER-mediated signaling events in skin.

The Selective Estrogen Receptor Modulators, Tamoxifen and Raloxifene, Impair Dendritic Cell Differentiation and Activation

Most immune cells, including myeloid progenitors and terminally differentiated dendritic cells (DC), express estrogen receptors (ER) making these cells sensitive to estrogens. Our laboratory recently demonstrated that 17-beta-estradiol (E2) promotes the GM-CSF-mediated development of CD11c+ CD11b(int) DC from murine bone marrow precursors. We tested whether the therapeutic selective estrogen receptor modulators (SERM), raloxifene and tamoxifen, can perturb DC development and activation. SERM, used in treatment of breast cancer and osteoporosis, bind to ER and mediate tissue-specific agonistic or antagonistic effects. Raloxifene and tamoxifen inhibited the differentiation of estrogen-dependent DC from bone marrow precursors ex vivo in competition experiments with physiological levels of E2. DC differentiated in the presence of SERM were assessed for their capacity to internalize fluoresceinated Ags as well as respond to inflammatory stimuli by increasing surface expression of molecules important for APC function. Although SERM-exposed DC exhibited increased ability to internalize Ags, they were hyporesponsive to bacterial LPS: relative to control DC, they less efficiently up-regulated the expression of MHC class II, CD86, and to a lesser extent, CD80 and CD40. This phenotype indicates that these SERM act to maintain DC in an immature state by inhibiting DC responsiveness to inflammatory stimuli. Thus, raloxifene and tamoxifen impair E2-promoted DC differentiation and reduce the immunostimulatory capacity of DC. These observations suggest that SERM may depress immunity when given to healthy individuals for the prevention of osteoporosis and breast cancer and may interfere with immunotherapeutic strategies to improve antitumor immunity in breast cancer patients.

Estrogen preferentially promotes the differentiation of CD11c+ CD11b(intermediate) dendritic cells from bone marrow precursors

Sex biases in autoimmunity and infection suggest that steroid sex hormones directly modulate immune cells. We show in this study that 17-beta-estradiol (E2) promotes the differentiation of functional dendritic cells (DC) from murine bone marrow precursor cells. Remarkably, ex vivo DC differentiation was inhibited in steroid hormone-deficient medium, and was restored by addition of physiological amounts of E2, but not dihydrotestosterone. DC differentiation was inhibited by the estrogen receptor (ER) antagonists ICI 182,780 and tamoxifen, and from ERalpha(-/-) bone marrow cells, indicating that E2 acted via ERs. E2 addition was most effective in promoting DC differentiation immediately ex vivo, but did not increase DC proliferation. E2 treatment specifically promoted differentiation of a CD11c(+) CD11b(int) DC population that displayed high levels of cell surface MHC class II and CD86, suggesting that E2 could augment numbers of potent APC. DC that differentiated in E2-supplemented medium were fully functional in their capability to mediate presentation of self and foreign Ags and stimulate the proliferation of naive CD4(+) T cells. The requirement for estrogen during DC differentiation suggests a mechanism by which E2 levels in peripheral tissues might modulate both the number and functional capabilities of DC in vivo, thereby influencing immune responses.

In vivo MHC class II presentation of cytosolic proteins revealed by rapid automated tandem mass spectrometry and functional analyses

We report a strategy for high through-put sequence analyses of large MHC class II-bound peptide repertoires which combines automated electrospray ionization tandem mass-spectrometry with computer-assisted interpretation of the tandem mass spectra using the algorithm SEQUEST. This powerful approach discerned 128 peptide sequences displayed by the murine MHC class II molecule I-Ab in activated B cells and macrophages, including a surprisingly large number of peptides derived from self cytosolic proteins. Mice lacking the chaperone molecule H-2M were used to generate T cells specific for selected self peptides. Functional T cell analyses of ex vivo antigen-presenting cells indicated that peptides originating from cytosolic proteins are efficiently presented by splenic and thymic dendritic cells, but less so by resting B cells or thymic cortical epithelial cells. These results suggest that central tolerance to at least some MHC class II-bound self peptides derived from cytosolic proteins exists in vivo.