The retinoblastoma gene is involved in multiple aspects of stem cell biology

The retinoblastoma tumor suppressor is a critical intrinsic regulator for hematopoietic stem and progenitor cells under stress

The retinoblastoma tumor suppressor protein (RB) plays important roles in the control of the cell division cycle. It is estimated that RB is dysfunctional/inactivated in up to 40% of human leukemias. The consequences of loss of RB on hematopoietic stem and progenitor cell (HSPC) function in vivo are incompletely understood. Here, we report that mice genetically deficient in Rb in all hematopoietic cells (Vav-Cre Rb knockout [KO] animals) showed altered contribution of distinct hematopoietic cell lineages to peripheral blood, bone marrow, and spleen; significantly increased extramedullary hematopoiesis in the spleen; and a 2-fold increase in the frequency of hematopoietic progenitor cells in peripheral blood. Upon competitive transplantation, HSPCs from Vav-Cre Rb KO mice contributed with an at least 4- to 6-fold less efficiency to hematopoiesis compared with control cells. HSPCs deficient in Rb presented with impaired cell-cycle exit upon stress-induced proliferation, which correlated with impaired function. In summary, Rb is critical for hematopoietic stem and progenitor cell function, localization, and differentiation.

Epigenetic modulation of estrogen receptor-alpha by pRb family proteins: a novel mechanism in breast cancer

Estrogen receptor-alpha (ER-alpha) plays a crucial role in normal breast development and has also been linked to mammary carcinogenesis and clinical outcome in breast cancer patients. However, ER-alpha gene expression can change during the course of disease and, consequently, therapy resistance can occur. The molecular mechanism governing ER-alpha transcriptional activity and/or silencing is still unclear. Here, we showed that the presence of a specific pRb2/p130 multimolecular complex on the ER-alpha promoter strongly correlates with the methylation status of this gene. Furthermore, we suggested that pRb2/p130 could cooperate with ICBP90 (inverted CCAAT box binding protein of 90 kDa) and DNA methyltransferases in maintaining a specific methylation pattern of ER-alpha gene. The sequence of epigenetic events for establishing and maintaining the silenced state of ER-alpha gene can be locus- or pathway- specific, and the local remodeling of ER-alpha chromatin structure by pRb2/p130 multimolecular complexes may influence its susceptibility to specific DNA methylation. Our novel hypothesis could provide a basis for understanding how the complex pattern of ER-alpha methylation and transcriptional silencing is generated and for understanding the relationship between this pattern and its function during the neoplastic process.

Differentiation is coupled to changes in the cell cycle regulatory apparatus of human embryonic stem cells

Mouse embryonic stem cells (mESC) exhibit cell cycle properties entirely distinct from those of somatic cells. Here we investigated the cell cycle characteristics of human embryonic stem cells (hESC). HESC could be sorted into populations based on the expression level of the cell surface stem cell marker GCTM-2. Compared to mESC, a significantly higher proportion of hESC (GCTM-2(+) Oct-4(+) cells) resided in G(1) and retained G(1)-phase-specific hypophosphorylated retinoblastoma protein (pRb). We showed that suppression of traverse through G(1) is sufficient to promote hESC differentiation. Like mESC, hESC expressed cyclin E constitutively, were negative for D-type cyclins, and did not respond to CDK-4 inhibition. By contrast, cyclin A expression was periodic in hESC and coincided with S and G(2)/M phase progression. FGF-2 acted solely to sustain hESC pluripotency rather than to promote cell cycle progression or inhibit apoptosis. Differentiation increased G(1)-phase content, reinstated cyclin D activity, and restored the proliferative response to FGF-2. Treatment with CDK-2 inhibitor delayed hESC in G(1) and S phase, resulting in accumulation of cells with hypophosphorylated pRb, GCTM-2, and Oct-4 and, interestingly, a second pRb(+) GCTM-2(+) subpopulation lacking Oct-4. We discuss evidence for a G(1)-specific, pRb-dependent restriction checkpoint in hESC closely associated with the regulation of pluripotency.

Lamins and lamin-associated proteins in aging and disease

Lamins, together with the lamin-associated proteins of the inner nuclear membrane, are structural proteins in the nucleus that mediate mechanical stress resistance. Novel findings show that lamin complexes also have scaffolding functions in the formation and regulation of higher order chromatin and in epigenetic regulatory pathways. Furthermore, lamins serve as scavenging complexes and regulators of signaling molecules in diverse pathways. Lamin complexes in the nuclear interior contribute to retinoblastoma-mediated cell cycle regulation. Because of their multiple and diverse roles, lamins are linked to an increasing number of human diseases. The molecular mechanisms of these diseases, which are just beginning to emerge, may involve cell cycle and differentiation defects in adult stem cells and genomic instability.

Regulatory networks: Linking microarray data to systems biology

Gene regulation and aging are intrinsically linked and these links often reach directly to transcription factors and their actions in gene regulation. However, it is very difficult to follow all the individual directions such factors can affect. Therefore, the opposite approach became more popular recently, i.e. observing the endpoints of all these actions. Microarrays are the preferred technology to monitor large-scale changes in transcripts across whole genomes. The trade-off for being able to survey whole genome transcriptomes is that the results are mere observations, which do not directly reveal the underlying mechanisms that represent the real link to transcription factors and their actions. Fortunately, a combination of knowledge mining (including but not restricted to literature mining) with genomics analyses can be harnessed to elucidate at least some of the regulatory networks orchestrating the transcriptional changes observed by microarray experiments. Thus, a considerable part of the functional system structure of cells and organisms can be revealed, which is a pivotal prerequisite for any meaningful systems biology approach towards aging related phenotypes.