In cells of higher eukaryotes, cyclin D-dependent kinases Cdk4 and Cdk6 and, possibly, cyclin E-dependentCdk2 positively regulate the G 1-to S-phase transition, by phosphorylating the retinoblastoma protein (pRb), thereby releasing E2F transcription factors that control S-phase genes. Here we performed microinjection and transfection experiments using rat R12 fibroblasts, their derivatives conditionally overexpressing cyclins D1 or E, and human U-2-OS cells, to explore the action of G~ cyclins and the relationship of E2F and cyclin E in S-phase induction. We demonstrate that ectopic expression of cyclin E, but not cyclin D1, can override G~ arrest imposed by either the p16 INK4a Cdk inhibitor specific for Cdk4 and Cdk6 or a novel phosphorylation-deficient mutant pRb. Several complementary approaches to assess E2F activation, including quantitative reporter assays in live cells, showed that the cyclin E-induced S phase and completion of the cell division cycle can occur in the absence of E2F-mediated transactivation. Together with the ability of cyclin E to overcome a G~ block induced by expression of dominant-negative mutant DP-1, a heterodimeric partner of E2Fs, these results provide evidence for a cyclin E-controlled S phase-promoting event in somatic cells downstream of or parallel to phosphorylation of pRb and independent of E2F activation. They furthermore indicate that a lack of E2F-mediated transactivation can be compensated by hyperactivation of this cyclin E-controlled event.[Key Words: Cyclin E; cyclin D1; p16; pRb phosphorylation; E2F; S phase] Received November 11, 1996; revised version accepted April 9, 1997.Progression from G~ to S phase of the mammalian cell cycle is regulated by cyclin D-dependent kinases Cdk4 and Cdk6 complexed to D-type cyclins, and by Cdk2 bound to cyclins E or A
Hematopoiesis occurs in the liver and the bone marrow (BM) during murine development. Newborn mice with a ubiquitous deletion of I kappa B alpha develop a severe hematological disorder characterized by an increase of granulocyte/erythroid/monocyte/macrophage colony-forming units (CFU-GEMM) and hypergranulopoiesis. Here, we report that this particular myeloproliferative disturbance is mediated by continuously deregulated perinatal expression of Jagged1 in I kappa B alpha-deficient hepatocytes. The result is a permanent activation of Notch1 in neutrophils. In contrast, in mice with a conditional deletion of I kappa B alpha only in the myeloid lineage (ikba(flox/flox) x LysM-Cre) and in fetal liver cell chimeras (ikba(FL delta/FL delta)), a cell-autonomous induction of the myeloproliferative disease was not observed. Coculture of I kappa B alpha-deficient hepatocytes with wild-type (wt) BM cells induced a Jagged1-dependent increase in CFUs. In summary, we show that cell-fate decisions leading to a premalignant hematopoietic disorder can be initiated by nonhematopoietic cells with inactive I kappa B alpha.
DNA damage caused by ultraviolet (UV) irradiation is considered the main etiologic factor contributing to the development of skin cancer. Systemic or topical application of antioxidants has been suggested as a protective measure against UV-induced skin damage. We investigated the effect of long-term oral administration of a combination of the antioxidants ascorbic acid (vitamin C) and D-alpha-tocopherol (vitamin E) in human volunteers on UVB-induced epidermal damage. The intake of vitamins C and E for a period of 3 mo significantly reduced the sunburn reaction to UVB irradiation. Detection of thymine dimers in the skin using a specific antibody revealed a significant increase of this type of DNA damage following UVB exposure. After 3 mo of antioxidant administration, significantly less thymine dimers were induced by the UVB challenge, suggesting that antioxidant treatment protected against DNA damage.
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