The nuclear Ki-67 protein (pKi-67) has previously been shown to be exclusively expressed in proliferating cells. As a result, antibodies against this protein are widely used as prognostic tools in cancer diagnostics. Here we show, that despite the strong downregulation of pKi-67 expression in non-proliferating cells, the protein can nevertheless be detected at sites linked to ribosomal RNA (rRNA) synthesis. Although this finding does not argue against the use of pKi-67 as a proliferation marker, it has wide ranging implications for the elucidation of pKi-67 function. Employing the novel antibody TuBB-9, we could further demonstrate that also in proliferating cells, a fraction of pKi-67 is found at sites linked to the rRNA transcription machinery during interphase and mitosis. Moreover, chromatin immunoprecipitation (ChIP) assays provided evidence for a physical association of pKi-67 with chromatin of the promoter and transcribed region of the rRNA gene cluster. These data strongly suggest a role for pKi-67 in the early steps of rRNA synthesis.

In ␤-thalassemia, the mechanism driving ineffective erythropoiesis (IE) is insufficiently understood. We analyzed mice affected by ␤-thalassemia and observed, unexpectedly, a relatively small increase in apoptosis of their erythroid cells compared with healthy mice. Therefore, we sought to determine whether IE could also be characterized by limited erythroid cell differentiation. In thalassemic mice, we observed that a greater than normal percentage of erythroid cells was in Sphase, exhibiting an erythroblast-like morphology. Thalassemic cells were associated with expression of cell cycle-promoting genes such as EpoR, Jak2, Cyclin-A, Cdk2, and Ki-67 and the antiapoptotic protein Bcl-X L . The cells also differentiated less than normal erythroid ones in vitro. To investigate whether Jak2 could be responsible for the limited cell differentiation, we administered a Jak2 inhibitor, TG101209, to healthy and thalassemic mice. Exposure to TG101209 dramatically decreased the spleen size but also affected anemia. Although our data do not exclude a role for apoptosis in IE, we propose that expansion of the erythroid pool followed by limited cell differentiation exacerbates IE in thalassemia. In addition, these results suggest that use of Jak2 inhibitors has the potential to profoundly change the management of this disorder. (Blood. 2008;112:875-885) Introduction ␤-Thalassemia, one of the most common congenital anemias, arises from partial or complete lack of ␤-globin synthesis. ␤-Thalassemia major, also known as Cooley anemia, 1 is the most severe form of this disease, and is characterized by ineffective erythropoiesis (IE) and extramedullary hematopoiesis (EMH), requiring regular blood transfusions to sustain life. [1][2][3][4][5] In ␤-thalassemia intermedia, where a larger amount of ␤-globin is synthesized, the clinical picture is milder and the patients do not require frequent transfusions. The ineffective production of red blood cells in both forms of the disease has been attributed to erythroid cell death during the maturation process mediated by apoptosis or hemolysis. It was proposed that accumulation of alpha-globin chains leads to the formation of aggregates, which impair erythroid maturation triggering apoptosis. [6][7][8][9][10][11][12][13] Ferrokinetic studies done in 1970 suggested that 60% to 80% of the erythroid precursors in ␤-thalassemia major die in the marrow or extramedullary sites. 14 However, several observations call into question the view that cell death is the only cause of IE in ␤-thalassemia.First, the number of apoptotic erythroid cells in thalassemic patients is low compared with that anticipated by ferrokinetic studies. 14,15 In fact, only 15% to 20% of bone marrow (BM) erythroid precursors (CD45 Ϫ /CD71 ϩ ) present apoptotic features in aspirates from affected patients. 6,8,16 Second, hemolytic markers in young ␤-thalassemic patients are normal or only slightly increased, unless the patients suffer from splenomegaly or the liver has been damaged by iron overload or viral infections. 17 Third...

We have compared the distribution of endogenous heterochromatin protein 1 (HP1) proteins (α, β and γ) in different epithelial lines, pluripotent stem cells and embryonic fibroblasts. In parallel, we have interrogated assembly and dynamics of newly expressed HP1-GFP proteins in cells lacking both HP1α and HP1β alleles, blocked at the G1-S boundary, or cultured in the presence of HDAC and HAT inhibitors. The results reveal a range of cell type and differentiation state-specific patterns that do not correlate with `fast' or `slow' subunit exchange in heterochromatin. Furthermore, our observations show that targeting of HP1γ to heterochromatic sites depends on HP1α and H1β and that, on an architectural level, HP1α is the most polymorphic variant of the HP1 family. These data provide evidence for HP1 plasticity under shifting microenvironmental conditions and offer a new conceptual framework for understanding chromatin dynamics at the molecular level.

Targeting molecular markers and pathways implicated in cancer cell growth is a promising avenue for developing effective therapies. Although the Ki-67 protein (pKi-67) is a key marker associated with aggressively proliferating cancer cells and poor prognosis, its full potential as a therapeutic target has never before been successfully shown. In this regard, its nuclear localization presents a major hurdle because of the need for intracellular and intranuclear delivery of targeting and therapeutic moieties. Using a liposomally encapsulated construct, we show for the first time the specific delivery of a Ki-67-directed antibody and subsequent lighttriggered death in the human ovarian cancer cell line OVCAR-5. Photoimmunoconjugate-encapsulating liposomes (PICEL) were constructed from anti-pKi-67 antibodies conjugated to fluorescein 5(6)-isothiocyanate, as a photoactivatable agent, followed by encapsulation in noncationic liposomes. Nucleolar localization of the PICELs was confirmed by confocal imaging. Photodynamic activation with PICELs specifically killed pKi-67-positive cancer cells both in monolayer and in three-dimensional (3D) cultures of OVCAR-5 cells, with the antibody TuBB-9 targeting a physiologically active form of pKi-67 but not with MIB-1, directed to a different epitope. This is the first demonstration of (a) the exploitation of Ki-67 as a molecular target for therapy and (b) specific delivery of an antibody to the nucleolus in monolayer cancer cells and in an in vitro 3D model system. In view of the ubiquity of pKi-67 in proliferating cells in cancer and the specificity of targeting in 3D multicellular acini, these findings are promising and the approach merits further investigation.