In maturing T lineage cells, the helix-loop-helix protein E47 has been shown to enforce a critical proliferation and developmental checkpoint commonly referred to as ␤ selection. To examine how E47 regulates cellular expansion and developmental progression, we have used an E2A-deficient lymphoma cell line and DNA microarray analysis to identify immediate E47 target genes. Hierarchical cluster analysis of gene expression patterns revealed that E47 coordinately regulates the expression of genes involved in cell survival, cell cycle progression, lipid metabolism, stress response, and lymphoid maturation. These include Plc␥2, Cdk6, CD25, Tox, Gadd45a, Gadd45b, Gfi1, Gfi1b, Socs1, Socs3, Id2, Eto2, and Xbp1. We propose a regulatory network linking Janus kinase (JAK)͞signal transducer and activator of transcription (STAT)-mediated signaling, E47, and suppressor of cytokine signaling (SOCS) proteins in a common pathway. Finally, we suggest that the aberrant activation of Cdk6 in E47-deficient T lineage cells contributes to the development of lymphoid malignancy.L ymphocyte development is regulated, in part, by a distinct class of helix-loop-helix (HLH) proteins, named E proteins (1). Four E proteins, E12, E47, E2-2, and HEB, are expressed in developing lymphocytes. E12 and E47 are encoded by one gene, designated as E2A, and arise through differential splicing in an exon encoding the HLH domain. E2A-deficient mice exhibit a complete block in B cell development at the onset of lineage commitment (2-4). T cell development is partially blocked in E2A-deficient mice before the onset of T cell receptor (TCR) ␤ V(D)J gene rearrangement (5-7).E proteins, and E2A proteins in particular, play critical roles in regulating developmental progression at the pre-TCR and TCR checkpoints (8, 9). Signals emanating from the pre-TCR and TCR complex act to suppress E protein DNA-binding activity, induce the expression of the E protein inhibitor Id3, and lower the abundance of E47 protein levels (10). Lowering the dose of E47 has been demonstrated to release the block in differentiation and proliferation observed in thymocytes with defects in pre-TCR expression and pre-TCR-mediated signaling (8,9,11). The E proteins also function at the TCR checkpoint, because an E2A deficiency has been shown to accentuate positive selection (12). In contrast, positive selection in Id3-ablated mice is blocked, albeit partially (13). Taken together, these data suggest that the E2A proteins act as gatekeepers at the pre-TCR and TCR checkpoints, and passage through these checkpoints requires pre-TCR or TCR-mediated modulation of E2A activity.E proteins also function as tumor suppressors. E2A-deficient mice rapidly develop thymic lymphoma (5, 14). Reintroduction of E2A activity into cell lines adapted from lymphomas that developed in E2A-ablated mice caused rapid apoptosis (15). In contrast, enforced expression of E47 in conjunction with Bcl-2 induced cell cycle arrest, suggesting a direct role for E2A in the suppression of cell growth (11). As a first approach...

The response of thymocytes to pre-T cell receptor (pre-TCR) signaling includes proliferation and gene rearrangement, two cellular processes that are incompatible. The control of proliferation by pre-TCR signals depends on the activities of the transcription factors RORgammat, Egr3, E12, and E47. Here, we describe a regulatory network in which interplay between these factors ensures transient proliferation that is temporally distinct from gene rearrangement. RORgammat expression was elevated after pre-TCR signaling, and RORgammat promoted gene rearrangement in CD4+, CD8+ cells by inhibiting cell division, promoting survival via Bcl-X(L), and inducing Rag2. Egr3 was transiently induced by pre-TCR signals and promoted a distinct proliferative phase by reducing E protein-dependent RORgammat expression and interacting with RORgammat to prevent induction of target genes. After Egr3 subsided, the expression and function of RORgammat increased. Thus, transient induction of Egr3 delays the effects of RORgammat and enables pre-TCR signaling to induce both proliferation and gene rearrangement.

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This investigation was undertaken to seek quantitative information concerning the relative activity of certain enzymes in hepatic cells in various parts of the lobule. The enzymes selected for study are chiefly those with important nwtabohc roles.

Collards, turnip greens, leaf lettuce, and spinach, grown in nutrient solution so that their Mg content was 80 to 90% 26Mg, were tested in ambulant male volunteers stabilized on a constant metabolic diet. The freeze-dried vegetables were incorporated in bran muffins in which the vegetables replaced part of the bran. Bran muffins without vegetables were consumed for breakfast each day. They were also used as a standard test meal to which the vegetable muffins were compared. All subjects participated in three consecutive isotope absorption tests: one of the standard test meal and two of the vegetables. The standard test was carried out after at least 30 days on the controlled diet. Subsequent tests of vegetables followed at 4-wk intervals. Each test meal contained 30 microCi 28MgCl2 and 50 mg stable 26Mg, the latter either as the intrinsic label of a test vegetable or as 26MgCl2 in solution taken with the standard bran muffins. Net absorption of both isotopes was measured to establish exchangeability and to determine relative Mg absorption from the vegetables. Exchangeability was 90% or higher from all meals tested. Relative Mg absorption was highest from collards and least from the standard test meal. Net absorption values ranged from 40 to 60%.

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Calcium homeostasis and bone pathology were studied in weanling rats fed a low (70 ppm) magnesium diet for 2-21 days. The rats developed significant, progressive hypercalcemia after 6 days on the diet. The increase in blood calcium was accompanied by progressive hypoactivity of the parathyroid gland (PTG), as determined by histologic and morphometric analyses. Thus hyperactivity of the PTG could not have been responsible for the hypercalcemia observed. Histologic examination of femora and humeri from magnesium-deficient rats showed progressive subperiosteal hyperplasia, consisting of undifferentiated osteoprogenitor cells and fibrous tissue, after 7 days of deficiency. The presence of unmineralized osteoid tissue in the metaphyses indicated that mineralization was not proceeding normally. The alterations in differentiation of osteoprogenitor cells, together with the failure of mineralization, resulted in significantly lower rates of bone formation (as measured by fluorochrome labeling) in the magnesium-deficient rats. Basophilic cementing lines and inactive osteocytes in the cortices of bones from magnesium-deficient rats indicated that bone resorption was also severely reduced in magnesium deficiency. We postulate that bone magnesium depletion (66% by day 21) has a direct negative effect on osteoblastic and osteocytic activity, and may explain, in part, the decreased responsiveness of bone to parathyroid hormone (PTH) that has been observed in magnesium-deficient animals.

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