Hypoxia-induced irreversible S-phase arrest involves down-regulation of cyclin A.

We have studied hypoxia-induced cell cycle arrest in human cells where the retinoblastoma tumour suppressor protein (pRB) is either functional (T-47D cells) or abrogated by expression of the HPV18 E7 oncoprotein (NHIK 3025 cells). All cells in S phase are immediately arrested upon exposure to extreme hypoxia. During an 18-h extreme hypoxia regime, the cyclin A protein level is down-regulated in cells of both types when in S-phase, and, as we have previously shown, pRB re-binds in the nuclei of all T-47D cells (Amellem et al. 1996). Hence, pRB is not necessary for the down-regulation of cyclin A during hypoxia. However, our findings indicate that re-oxygenation cannot release pRB from its nuclear binding following this prolonged exposure. The result is permanent S-phase arrest even after re-oxygenation, and this is correlated with a complete and permanent down-regulation of cyclin A in the pRB functional T-47D cells. In contrast, both cell cycle arrest and cyclin A down-regulation in S phase are reversed upon re-oxygenation in non-pRB-functional NHIK 3025 cells after prolonged exposure to extreme hypoxia. Our results indicate that pRB is involved in permanent S-phase arrest and down-regulation of cyclin A after extreme hypoxia.

Counteraction of pRb-dependent protection after extreme hypoxia by elevated ribonucleotide reductase.

We have studied hypoxia-induced cell cycle arrest in human cells where the retinoblastoma tumour suppressor protein (pRb) is either functional (T-47D and T-47DHU-res cells) or abrogated by expression of the HPV18 E7 oncoprotein (NHIK 3025 cells). We have previously found that pRb is dephosphorylated and rebound in the nucleus in T-47D cells arrested in S-phase during hypoxia and that this binding is protracted even following re-oxygenation. In the present study, however, we show that the long-lasting arrest following re-oxygenation induced by pRb-binding in the cell nuclei may be overruled by an elevated level of ribonucleotide reductase (RNR). This seems to create a forced DNA-synthesis, uncoordinated with cell division, which induces endoreduplication of the DNA. The data indicate that the cells initiating endoreduplication continue DNA-synthesis until all DNA is replicated once and then may start cycling and cell division with a doubled DNA-content. Corresponding data on the pRb-incompetent NHIK 3025-cells show similar endoreduplication in these. Thus, the data indicate that endoreduplication of DNA following re-oxygenation may come, either as a result of hypoxic arrest of DNA-synthesis when pRb-function is absent in the cells, or if it is overruled by increased RNR. The present study further shows that pRb not only protects the culture by arresting most of the cells that are exposed to extreme hypoxia in S-phase, but also increases cell survival by means of increased clonogenic ability of these cells. Interestingly, however, cells having an elevated level of RNR have equally high survival as wild-type cells following 20 h extreme hypoxia. If RNR-overruling of pRb-mediated arrest following re-oxygenation results in an unstable genome, this may therefore represent a danger of oncogenic selection as the protective effect of pRb on cell survival seems to be maintained.

Propranolol and amitriptyline in prophylaxis of migraine. Pharmacokinetic and therapeutic effects.

OBJECTIVES: To determine if the effectiveness of propranolol hydrochloride and amitriptyline hydrochloride are correlated with blood levels and/or with standardized test of pharmacologic effect and to determine which clinical variables are predictors of response to one or the other medication. DESIGN: Three-month modules of treatment with each drug and placebo in a randomized crossover design. Headache scores from daily diaries were calculated at monthly intervals, as were simultaneous blood levels of drug, supine and standing blood pressure, pulse rise with exercise, and salivary flow. SETTING: Outpatient headache clinic at the University of Kansas Medical Center, Kansas City. PATIENTS: Thirty consecutive patients with a history of frequent migraine. MAIN OUTCOME MEASUREMENTS: From headache scores, patients were classified as either propranolol responders, amitriptyline responders, or nonspecific responders. Clinical variables as predictors of response to medications were studied, as were effects on frequency, duration, and/or severity of headache. RESULTS AND CONCLUSIONS: No significant correlations were found between changes in headache score and blood level of drug or change in any of the physiologic measurements. Amitriptyline significantly reduced the severity, frequency, and duration of headache attacks; propranolol reduced the severity of attacks only. Amitriptyline response was correlated with female gender and baseline headaches of shortest duration and of highest frequency. Propranolol response was associated with attacks of greatest duration at baseline and with low pulse rise with exercise at baseline. Nonspecific response was associated with male gender and most frequent headaches by history.

Major improvements of the photorefractive and photovoltaic properties in potassium niobate.

Optical and electrical measurements have been made on a new codoped potassium niobate crystal (KNbO3:Fe,Ag) that yields a significant enhancement of the photorefractive and photovoltaic effects when compared with the published results for singly doped potassium niobate crystals. The codoped Ag impurity enters the K site, rather than the typical Nb site, thus changing the local field in the lattice. It is believed that Fe perturbed by the Ag in the K site is responsible for an enhancement of the linear absorption and photocurrent, as well as a probable increase in the effective trap density. An enhanced trap density is likely the cause of the increased photorefractive counterpropagating two-beam coupling efficiency.

[Effects of sunscreening agents and reactions with ultraviolet radiation]. / Solbeskyttende midlers virkemåte og reaksjoner ved ultrafiolett bestråling.

The use of sunscreens is extensive. During the last few years there have been indications that UV radiation causes breakdown of the sunlight absorbing filters in the sunscreens, i.e. the sunscreens are not photostable. We describe briefly UV propagation in skin, the chemical and physical properties of sunscreens, and how these may react during UV irradiation. We have studied the stability of several sunscreens in vitro. The stability tests were performed by applying a thin film of the sunscreen preparation to the wall of a quartz window, irradiating it with a sun simulator, and measuring the absorbance with spectrophotometry before and during irradiation. The sunscreen agent studied most thoroughly was the UVB filter octyl methoxy cinnamate, but other UVA and UVB filters and some commercial products were also tested. Considerable breakdown of most filters was observed after doses of irradiation equivalent to moderate sun exposure. It can be questioned whether the breakdown products of sunscreens also possess other physical or biological properties. General practitioners should be able to advise their patients on sun protection and the proper use of sunscreens, considering the extensive use of sunscreens and the fact that sunbathing may be a health hazard.

Abnormal complement-mediated immune clearance in MRL-lpr/+ heterozygote mice: dose-dependent effect of the Fas antigen mutation.

We have previously demonstrated decreased complement-mediated clearance of IgG-opsonized erythrocytes in mice homozygous for the lpr mutation of the fas gene (BALB/c-lpr/lpr, C57BL/6-lpr/lpr, and MRL-lpr/lpr). To further test the hypothesis that the lpr mutation leads to a series of events resulting in selectively decreased complement-mediated immune clearance, in vivo clearance rate data were obtained from MRL-lpr/+F1, F2, and reciprocal backcross mice. Southern analysis of genomic DNA extracted from F2 and backcross mice was used to correlate the presence of the normal fas gene or the lpr mutation with normal or decreased complement-mediated clearance, respectively. Mean clearance rate constants for complement-dependent sequestration and phagocytosis were significantly decreased in the group of F1, F2, and backcross mice compared to control BALB/c mice (P < 0.0001). Data correlating clearance rate parameters from F2 and backcross mice with their respective genotype demonstrated a dose effect of the lpr mutation on abnormal complement-dependent sequestration and phagocytosis (r > 0.98), with heterozygote mice expressing mean values approximately half of those observed in +/+ homozygotes. These data demonstrate that the presence of the lpr mutation of the fas gene is strongly correlated in a dose-dependent manner with abnormal complement-mediated immune clearance. This clearance defect may be one mechanism through which the lpr mutation acts as an enhancer of autoimmune disease. Reduced clearance of immune complexes would increase the likelihood of tissue deposition and immune-mediated damage.