Chromatin reprogramming and bone regeneration in vitro and in vivo via the microtopography-induced constriction of cell nuclei

Topographical cues on cells can, through contact guidance, alter cellular plasticity and accelerate the regeneration of cultured tissue. Here we show how changes in the nuclear and cellular morphologies of human mesenchymal stromal cells induced by micropillar patterns via contact guidance influence the conformation of the cells' chromatin and their osteogenic differentiation in vitro and in vivo. The micropillars impacted nuclear architecture, lamin A/C multimerization and 3D chromatin conformation, and the ensuing transcriptional reprogramming enhanced the cells' responsiveness to osteogenic differentiation factors and decreased their plasticity and off-target differentiation. In mice with critical-size cranial defects, implants with micropillar patterns inducing nuclear constriction altered the cells' chromatin conformation and enhanced bone regeneration without the need for exogenous signalling molecules. Our findings suggest that medical device topographies could be designed to facilitate bone regeneration via chromatin reprogramming.

[Regulatory effect of faciogenital dysplasia 6 gene on hepatic stem cell differentiation]

Objective: To investigate the regulatory effect of faciogenital dysplasia 6 (FGD6) gene on hepatic stem cell differentiation. Methods: FGD6 gene was selected for the co-intervention of target sequence, the AdEasy system was used for the construction of adenovirus vector and the packaging and multiplication of the recombinant adenovirus vector pSES-FGD6-siRNA, and the HP14.5 cells were infected. Immunofluorescence assay was used to measure the expression of FGD6 protein in HP14.5 cells, quantitative real-time PCR was used to measure the mRNA expression of FGD6, alpha-fetoprotein (AFP), and albumin (Alb), and Western blot was used to measure the protein expression of FGD6, AFP, and Alb. The empty pSES-Ad-RFP adenovirus vector was constructed as control in each group. All data were expressed as x±s, and a one-way analysis of variance was performed. Results: FGD6 protein was mainly expressed in the nucleus of HP14.5 cells. The pSES-FGD6-siRNA adenovirus vector was successfully constructed and it downregulated the expression of FGD6 gene and the mRNA and protein expression of AFP in HP14.5 cells and upregulated the mRNA and protein expression of Alb (P < 0.01). Conclusion: The inhibition of the expression of FGD6 gene in HP14.5 cells may differentiate HP14.5 cells into hepatocytes. Therefore, FGD6 gene plays an important role in the differentiation regulation of hepatic stem cells.

Induced maturation of hepatic progenitor cells in vitro

Hepatic progenitor cells (HPCs) are a potential cell source for liver cell transplantation but do not function like mature liver cells. We sought an effective and reliable method to induce HPC maturation. An immortalized HP14.5 albumin promoter-driven Gaussian luciferase (ALB-GLuc) cell line was established from HPCs isolated from fetal mouse liver of post coitus day 14.5 mice to investigate the effect of induction factors on ALB promoter. HP14.5 parental cells were cultured in DMEM with different combinations of 2% horse serum (HS), 0.1 µM dexamethasone (DEX), 10 ng/mL hepatic growth factor (HGF), and/or 20 ng/mL fibroblast growth factor 4 (FGF4). Trypan blue and crystal violet staining were used to assess cell proliferation with different induction conditions. Expression of hepatic markers was measured by semi-quantitative RT-PCR, Western blot, and immunofluorescence. Glycogen storage and metabolism were detected by periodic acid-Schiff and indocyanine green (ICG) staining. GLuc activity indicated ALB expression. The combination of 2% HS+0.1 µM Dex+10 ng/mL HGF+20 ng/mL FGF4 induced the highest ALB-GLuc activity. Cell proliferation decreased in 2% HS but increased by adding FGF4. Upon induction, and consistent with hepatocyte development, DLK, AFP, and CK19 expression decreased, while ALB, CK18, and UGT1A expression increased. The maturity markers tyrosine aminotransferase and apolipoprotein B were detected at days 3 and 6 post-induction, respectively. ICG uptake and glycogen synthesis were detectable at day 6 and increased over time. Therefore, we demonstrated that HPCs were induced to differentiate into functional mature hepatocytes in vitro, suggesting that factor-treated HPCs may be further explored as a means of liver cell transplantation.

Overexpression of Wnt-2 in colorectal cancers

The binding of the Wnt ligand to its receptor Frizzled, activates the Wnt canonical signaling pathway in carcinogenesis as well as many cellular processes, including cellular proliferation and differentiation. Wnt-2, one of 19 members of the <em>Wnt</em> gene family, is frequently overexpressed in malignant tissues. Here, in order to investigate the role of Wnt-2 in colorectal carcinogenesis, we examined the expression of the Wnt-2 protein in 120 colorectal cancers by immunohistochemistry. Wnt-2 protein was expressed in the cell membrane and cytoplasm and up-regulated in 74 (61.7%) of 120 colorectal cancers. Statistically, overexpression of Wnt-2 protein was not associated with the clinical and pathological parameters studied, including tumor location, tumor size, clinical stage, lymph node metastasis, and 5-year survival (<em>P > </em>0.05). These results indicate that up-regulation of the Wnt-2 protein might play a role in the development of colorectal cancers, as an early event of carcinogenesis.

KEYWORDS

Wnt-2 protein, expression, immunohistochemistry, tissue microarray, colon cancer.

The future of flexor tendon surgery

Clinical outcomes following flexor tendon repair have made significant improvements in the last 50 years. In that time standard treatment has evolved from secondary grafting to primary repair with postoperative rehabilitation protocols. Unfortunately, excellent results are not yet attained universally following treatment. Improving understanding of tendon healing at the cellular, molecular, and genetic levels will likely enable surgeons to modulate the normal repair process. We now look toward biologic augmentation of flexor tendon repairs to address the problems of increasing tensile strength while reducing adhesion formation following injury and operative repair.

Tetracycline-regulated gene expression mediated by a novel chimeric repressor that recruits histone deacetylases in mammalian cells

Regulated gene expression will provide important platforms from which gene functions can be investigated and safer means of gene therapy may be developed. Histone deacetylases have recently been shown to play an important role in regulating gene expression. Here we investigated whether a more tightly controlled expression could be achieved by using a novel chimeric repressor that recruits histone deacetylases to a tetracycline-responsive promoter. This chimeric repressor was engineered by fusing the tetracycline repressor (TetR) with an mSin3-interacting domain of human Mad1 and was shown to bind the tetO(2) element with high affinity, and its binding was efficiently abrogated by doxycycline. The chimeric repressor was shown to directly interact with mSin3 of the histone deacetylase complex. This inducible system was further simplified by using a single vector that contained both a chimeric repressor expression cassette and a tetracycline-responsive promoter. When transiently introduced into mammalian cells, the chimeric repressor system exhibited a significantly lower basal level of luciferase activity (up to 25-fold) than that of the TetR control. When stably transfected into HEK 293 cells, the chimeric repressor system was shown to exert a tight control of green fluorescent protein expression in a doxycycline dose- and time-dependent fashion. Therefore, this novel chimeric repressor provides an effective means for more tightly regulated gene expression, and the simplified inducible system may be used for a broad range of basic and clinical studies.

Adenoviral vector-mediated gene transfer for human gene therapy

Human gene therapy promises to change the practice of medicine by treating the causes of disease rather than the symptoms. Since the first clinical trial made its debut ten years ago, there are over 400 approved protocols in the United States alone, most of which have failed to show convincing data of clinical efficacy. This setback is largely due to the lack of efficient and adequate gene transfer vehicles. With the recent progress in elucidating the molecular mechanisms of human diseases and the imminent arrival of the post genomic era, there are increasing numbers of therapeutic genes or targets that are available for gene therapy. Therefore, the urgency and need for efficacious gene therapies are greater than ever. Clearly, the current fundamental obstacle is to develop delivery vectors that exhibit high efficacy and specificity of gene transfer. Recombinant adenoviruses have provided a versatile system for gene expression studies and therapeutic applications. Of late, there has been a remarkable increase in adenoviral vector-based clinical trials. Recent endeavors in the development of recombinant adenoviral vectors have focused on modification of virus tropism, accommodation of larger genes, increase in stability and control of transgene expression, and down-modulation of host immune responses. These modifications and continued improvements in adenoviral vectors will provide a great opportunity for human gene therapy to live up to its enormous potential in the second decade.

Peroxisome proliferator-activated receptors: roles in tumorigenesis and chemoprevention in human cancer

Peroxisome proliferator-activated receptors are nuclear receptors that were isolated for their ability to modulate lipid metabolism. Similar to other members of the nuclear receptor family, peroxisome proliferator-activated receptors bind ligand as heterodimers and exert their effects via transcriptional regulation through their DNA binding domains. During the past decade, it has become clear that peroxisome proliferator-activated receptors also contribute to a variety of different biologic processes, including atherosclerosis, insulin resistance, and more recently, cancer. In this review, we discuss the evidence for the different peroxisome proliferator-activated receptors' roles in tumorigenesis and also their potential application for the treatment and prevention of neoplastic diseases.

The beta-catenin binding domain of adenomatous polyposis coli is sufficient for tumor suppression

Inactivation of the adenomatous polyposis coli (APC) gene is a critical event in the development of human colorectal cancers. At the biochemical level, several functions have been assigned to the multidomain APC protein, but the cellular effects of APC expression and how they relate to its biochemical functions are less well defined. To address these issues, we generated a recombinant adenovirus (Ad-CBR) that constitutively expresses the central third of APC, which includes all of the known beta-catenin binding repeats. When expressed in colon cancer cells, Ad-CBR blocked the nuclear translocation of beta-catenin and inhibited beta-catenin/Tcf-4-mediated transactivation. Accordingly, expression of endogenous targets of the APC/beta-catenin/Tcf-4 pathway was down-regulated. Ad-CBR infection of colorectal cancer cell lines with mutant APC but wild-type beta-catenin resulted in substantial growth arrest followed by apoptosis. These effects were attenuated in lines with wild-type APC but with mutated beta-catenin. These findings suggest that the beta-catenin-binding domain in the central third of APC is sufficient for its tumor suppressor activity.

CDX2 is mutated in a colorectal cancer with normal APC/beta-catenin signaling

The majority of human colorectal cancers have elevated beta-catenin/TCF regulated transcription due to either inactivating mutations of the APC tumor suppressor gene or activating mutations of beta-catenin. Surprisingly, one commonly used colorectal cancer cell line was found to have intact APC and beta-catenin and no demonstrable beta-catenin/TCF regulated transcription. However, this line did possess a truncating mutation in one allele of CDX2, a gene whose inactivation has recently been shown to cause colon tumorigenesis in mice. Expression of CDX2 was found to be induced by restoring expression of wild type APC in a colorectal cancer cell line. These findings raise the intriguing possibility that CDX2 contributes to APC's tumor suppressive effects.

Identification of c-MYC as a target of the APC pathway

The adenomatous polyposis coli gene (APC) is a tumor suppressor gene that is inactivated in most colorectal cancers. Mutations of APC cause aberrant accumulation of beta-catenin, which then binds T cell factor-4 (Tcf-4), causing increased transcriptional activation of unknown genes. Here, the c-MYC oncogene is identified as a target gene in this signaling pathway. Expression of c-MYC was shown to be repressed by wild-type APC and activated by beta-catenin, and these effects were mediated through Tcf-4 binding sites in the c-MYC promoter. These results provide a molecular framework for understanding the previously enigmatic overexpression of c-MYC in colorectal cancers.

A simplified system for generating recombinant adenoviruses

Recombinant adenoviruses provide a versatile system for gene expression studies and therapeutic applications. We report herein a strategy that simplifies the generation and production of such viruses. A recombinant adenoviral plasmid is generated with a minimum of enzymatic manipulations, using homologous recombination in bacteria rather than in eukaryotic cells. After transfections of such plasmids into a mammalian packaging cell line, viral production is conveniently followed with the aid of green fluorescent protein, encoded by a gene incorporated into the viral backbone. Homogeneous viruses can be obtained from this procedure without plaque purification. This system should expedite the process of generating and testing recombinant adenoviruses for a variety of purposes.

14-3-3sigma is a p53-regulated inhibitor of G2/M progression

Exposure of colorectal cancer (CRC) cells to ionizing radiation results in a cell-cycle arrest in G1 and G2. The G1 arrest is due to p53-mediated induction of the cyclin-dependent kinase inhibitor p21WAF1/CIP1/SDI1, but the basis for the G2 arrest is unknown. Through a quantitative analysis of gene expression patterns in CRC cell lines, we have discovered that 14-3-3sigma is strongly induced by gamma irradiation and other DNA-damaging agents. The induction of 14-3-3sigma is mediated by a p53-responsive element located 1.8 kb upstream of its transcription start site. Exogenous introduction of 14-3-3sigma into cycling cells results in a G2 arrest. As the fission yeast 14-3-3 homologs rad24 and rad25 mediate similar checkpoint effects, these results document a molecular mechanism for G2/M control that is conserved throughout eukaryotic evolution and regulated in human cells by p53.

Homeosis and polyposis: a tale from the mouse

Homeobox genes play essential roles in specifying the fates of different cell types during embryogenesis. In Drosophila, the homeotic gene caudal is important for the generation of posterior structures. In the mouse, the caudal homologue Cdx2 has been implicated in directing early processes in intestinal morphogenesis and in the maintenance of the differentiated phenotype. A recent study showed that Cdx2 null mutation was embryonically lethal, whereas Cdx2+/- mice developed multiple intestinal polyps in the proximal colon in addition to developmental defects. There are striking phenotypic similarities and differences between Cdx2+/- and other mice predisposed to intestinal neoplasia. The possible role of Cdx2 in human colorectal tumorigenesis is discussed.

Genetic determinants of p53-induced apoptosis and growth arrest

Previous studies have suggested that expression of p53 in cancer cells can result in either growth arrest or apoptosis. Accordingly, expression of p53 in a series of colorectal cancer cell lines yielded growth arrest in some lines (A-lines) and apoptosis in others (D-lines). To investigate the basis of this difference, we evaluated the role of p21WAF1/Cip1, a known mediator of p53-induced growth arrest. Inactivation of p21 by homologous recombination converted an A-line to a D-line, suggesting that p21 could protect cells from apoptosis. However, examination of p53-induced p21 expression in naturally occurring D-lines and A-lines demonstrated that the induction of p21 could not account for the differential response to p53. Moreover, when a D-line was fused to an A-line, the resulting hybrid cells underwent apoptosis in response to p53, indicating that the apoptosis pathway was dominant over the growth arrest pathway. Therefore, the apoptotic response to p53 in colorectal cancer cells is modulated by at least two factors: p21-mediated growth arrest that can protect cells from apoptosis in A-cells, and trans-acting factors in D-cells that can overcome this protection, resulting in cell death.

The box1 domain of the erythropoietin receptor specifies Janus kinase 2 activation and functions mitogenically within an interleukin 2 beta-receptor chimera

Several distinct classes of cytokine receptors engage Jak kinases as primary effectors. Among type 1 receptors, Janus-activated kinase (Jak) recruitment is mediated by membrane-proximal cytoplasmic domains, which typically contain conserved box motifs. In the erythropoietin receptor (Epo-R), two such motifs (box1 and box2) have been suggested to be essential for the activation of Jak2 and mitogenesis. Presently, an Epo-R chimera containing the extracellular and box1 domains of the Epo-R (Jak2-associated receptor) and the box2 and carboxyl-terminal domains of the interleukin 2 beta-receptor (IL2beta-R; a Jak1-associated subunit) is shown to activate Jak2. Interestingly, Jak2 also was activated in FDC-P1 cells by a control Epo-R chimera containing the complete IL2beta-R cytoplasmic domain, and mitogenesis was supported by each of these above chimeras. By comparison, in BaF3 cells expressing IL2 receptor alpha and gamma subunits, an ectopically expressed IL2beta-R chimera containing the box1 domain of the Epo-R, activated Jak2 and Jak3 and was as mitogenically active as the wild-type IL2beta-R (Jak1 and Jak3 activation). Thus, the box1 domain of the Epo-R specifies Jak2 activation and functions efficiently within a heterologous IL2 receptor complex that normally activates Jak1 and Jak3.

Converting cancer genes into killer genes

Over the past decade, it has become clear that tumorigenesis is driven by alterations in genes that control cell growth or cell death. Theoretically, the proteins encoded by these genes provide excellent targets for new therapeutic agents. Here, we describe a gene therapy approach to specifically kill tumor cells expressing such oncoproteins. In outline, the target oncoprotein binds to exogenously introduced gene products, resulting in transcriptional activation of a toxic gene. As an example, we show that this approach can be used to specifically kill cells overexpressing a mutant p53 gene in cell culture. The strategy may be generally applicable to neoplastic diseases in which the underlying patterns of genetic alterations or abnormal gene expression are known.

Erythropoietin-dependent inhibition of apoptosis is supported by carboxyl-truncated receptor forms and blocked by dominant-negative forms of Jak2

Apoptosis, or programmed cell death (PCD), recently has emerged as an important homeostatic mechanism within several hematopoietic lineages. This process is subject to both positive and negative modulation by cytokines and within the erythroid lineage is inhibited by interleukin-3, stem cell factor, and erythropoietin (Epo). Through the expression of carboxyl-truncated Epo receptor mutants in FDC-P1 cells, a receptor form possessing 80 membrane-proximal cytoplasmic residues is shown to efficiently mediate Epo-dependent inhibition of PCD. This is in contrast to previous studies that attributed this activity to a distal carboxyl-terminal receptor subdomain (and/or heterodimerization of wild type Epo receptors with a truncated non-functional receptor form). Epo-dependent inhibition of PCD also is shown to be blocked by ectopic expression of kinase-deficient dominant-negative forms of Jak2 (Jak2 delta VIII and Jak2-829), further underlining a role of this membrane-proximal subdomain of the Epo receptor in the inhibition of PCD. To our knowledge, this comprises the first direct evidence for an essential role for a Jak tyrosine kinase (Jak2) in this apoptotic response pathway.

Erythropoietin-induced recruitment of Shc via a receptor phosphotyrosine-independent, Jak2-associated pathway

Based on the recently implicated role of Shc as a signaling effector for type I cytokine receptors, factors which mediate the recruitment and phosphorylation of Shc in the erythropoietin receptor (EPOR) system have been studied. FDC-P1 cells stably expressing the wild type murine EPOR supported the EPO-induced association of Shc with Jak2 and its rapid tyrosine phosphorylation. However, this did not depend upon the presence of phosphotyrosine sites within the EPOR and was mediated by a mitogenically deficient receptor form (EPOR329) lacking cytoplasmic tyrosine residues. This was shown both by Western blotting of Shc and Jak2 co-immunoprecipitates and through the development of an in vitro assay for cytokine-induced Shc phosphorylation. The direct association of Shc with Jak2 also was observed and was shown to depend upon EPO-exposure and the SH2 subdomain of Shc. Together, these studies indicate that Jak2, in part, may mediate the EPO-induced phosphorylation of Shc.

Dominant negative effects of a carboxy-truncated Jak2 mutant on Epo-induced proliferation and Jak2 activation

Members of the Janus family of protein tyrosine kinases are emerging as primary, receptor-associated transducing factors among numerous cytokine systems. However, little is understood regarding mechanisms of recruitment of these kinases to receptor complexes and their ligand-dependent activation. To initially address these questions, we have assessed effects of ectopically expressing a carboxy-truncated form of Jak2 (Jak2-829) in Epo-responsive DAER cells. Expression of this truncation mutant at low levels efficiently inhibited both Epo-dependent activation of endogenous Jak2 and Epo-induced mitogenesis (10% to 39% of parental DAER cells). These results suggest that amino-terminal domains of Jak2 may mediate the assembly of Jak2/Epo receptor complexes and that integration of Jak2-829 into receptor complexes may effectively inhibit the activity of oligomeric Jak2/receptor assemblages.

Inhibition of erythropoietin-induced mitogenesis by a kinase-deficient form of Jak2

Receptors for a variety of hematopoietins, interferons alpha/beta and gamma, and growth hormone have recently been shown to mediate rapid, ligand-dependent activation of the Janus-type cytosolic protein-tyrosine kinases Jak1, Jak2, and/or tyk-2. This finding extends relatedness among class I and II cytokine receptors to a functional context and provides an initially satisfying mechanistic analogy to protein-tyrosine kinase-encoding receptors of the epidermal growth factor/platelet-derived growth factor/insulin family. Through the construction and expression of a kinase-deficient form of Jak2 (JK2 delta VIII) in interleukin-3 (IL-3)/erythropoietin (Epo)-dependent DAER cells, we have tested whether activation of Jak2 is required for induced mitogenesis via these class I cytokine receptors. Ectopic expression of JK2 delta VIII inhibited Epo- and IL-3-induced activation of endogenous wild-type Jak2, transiently attenuated IL-3-dependent growth, and essentially abrogated Epo-induced proliferation in this model system. These dominant-negative effects provide the first direct experimental evidence for an essential role for Janus kinase activation in mitogenesis and suggest that distinct effectors may mediate IL-3-induced versus Epo-induced pathways.

The extended box 2 subdomain of erythropoietin receptor is nonessential for Jak2 activation yet critical for efficient mitogenesis in FDC-ER cells

The development of erythroid progenitor cells depends upon exposure to the glycoprotein hormone, erythropoietin (EPO). Binding of EPO to its transmembrane receptor leads to the rapid tyrosine phosphorylation of several cellular targets including Shc, Raf-1, Gap120, the cloned EPO receptor (EPOR), pp100/97, and a M(r) 130,000 EPO-activated receptor-associated Janus protein tyrosine kinase, Jak2. A membrane-proximal cytosolic region of the EPOR recently has been shown to be essential for the activation of Jak2 and sufficient for EPO-induced mitogenesis. This cytosolic region includes 8-12 amino acid box 1 and box 2 subdomains, which are conserved in certain class I receptors as well as a more distal 10-40 amino acid subdomain (extended box 2 subdomain, ExBx2), which likewise is implicated in mitogenic signaling. Through the expression of EPOR carboxyl-terminal truncation mutants in FDC-P1 cells, we presently show that an EPOR form truncated within the ExBx2 domain efficiently activates Jak2, yet is deficient in mitogenesis. Efficient expression of this mutant receptor at the cell surface and its ability to activate Jak2 indicate that poor mitogenic activity does not result from aberrant transport or folding. Rather, failure of this mutant to support proliferation above nominal rates underlines an apparent role for the EPOR ExBx2 subdomain in the activation of a distinct primary mitogenic effector.

Association of the p85 regulatory subunit of phosphatidylinositol 3-kinase with an essential erythropoietin receptor subdomain

Using an active, HAI epitope-tagged form of the murine erythropoietin (EPO) receptor and via direct coimmunoprecipitation, the p85 regulatory subunit of phosphatidyl inositol-3 kinase (p85/PI3-K) is shown to associate with the EPO receptor in transfected FDC-P1 cell lines. Coimmunoprecipitation of p85 with epitope-tagged EPO receptors was observed initially in FDC-HER cells labeled metabolically with [32P]orthophosphate, and association of these factors was confirmed by Western analyses of receptor immunoprecipitates using p85 antiserum. Interestingly, this association occurred in the absence of ligand, and exposure of FDC-HER cells to EPO did not detectably affect levels of receptor-associated p85 or overall levels of p85 phosphorylation. However, EPO was observed to stimulated the rapid formation of phosphatidylinositol 32P-phosphate in FDC-HER and FDC-ER cells. Through baculovirus-mediated expression of epitope-tagged EPO receptor forms in SF9 cells, domains for p85 association were mapped. Analyses of receptor forms with cytosolic truncations and deletions delineated a candidate subdomain for p85 binding to an essential extended box-2 region (P329-E374; including a putative motif for SH2 binding, Y343LVL). These findings extend a mechanistic alignment between the EPO receptor and protein tyrosine kinase-encoding receptors that likewise activate PI3-K, and expand the importance of further defining pathways to PI3-K activation.

Signal transduction in the erythropoietin receptor system

Developing erythroid cells require the glycoprotein hormone, erythropoietin (EPO) as an activator of the rapid proliferation of early proerythroblasts (colony forming units-erythroid [CFU-e]), and subsequently as an activator of late erythroid gene expression. Activation of these growth and differentiation events proceeds from the binding of EPO at its transmembrane receptor (Class I cytokine receptor), to the engagement of a complex set of signaling pathways. Studies of reconstituted activities of the cloned EPO receptor in transfected hematopoietic cell lines have served well in identifying receptor domains and downstream mediators involved in proliferative signaling. Extracellular domains have been defined which contribute to ligand binding, receptor processing and transport, and possible dimerization. Cytosolic regions have been delineated which mediate induced mitogenesis, early gene transcription, activated protein tyrosine phosphorylation, down modulation of EPO- and granulocyte-macrophage colony-stimulating factor (GM-CSF)-induced proliferation, and direct association with PI3- and JAK-2 kinases. These newly defined properties begin to align the EPO receptor mechanistically with growth factor receptors (GFR) which encode, or likewise associate with, regulated protein tyrosine kinases including the Class II cytokine receptors for interferons alpha/beta and gamma. An improved understanding of factors which mediate EPO-induced late erythroid gene activation also is emerging. These factors and pathways may be distinct from those associated with EPO-induced proliferation and may involve induced increases in cellular Ca++, cAMP and arachidonic acid, as well as the modulation of GATA-1, and/or SCL. Attributes of model systems used in studies of the role of EPO in late erythroid differentiation also are considered.