The rate of protein synthesis per cell in cultured hamster embryo fibroblasts in the stationary growth phase falls to about one third of the rate in the exponential growth phase. This reduction can be entirely accounted for by the following observations: (1) the average cell in stationary phase contains about one-half the number of ribosomes per cell compared to the average cell in exponential phase; (2) only two thirds of the ribosomes are bound to polysomes in stationary phase, while nearly all of the ribosomes are polysome-bound in exponential phase.In stationary phase, ribosomes which are polysome-bound function with the same efficiency and produce proteins of approximately the same average length as in exponential phase. Experimental findings are presented which suggest that the generation of a higher proportion of free ribosomes in stationary phase is not due to a limitation in messenger RNA, but to a decreased attachment probability of ribosomes to messenger RNA.

No abstract

Regions of the normal arterial intima predisposed to atherosclerosis are sites of ongoing monocyte trafficking and also contain resident myeloid cells with features of dendritic cells. However, the pathophysiological roles of these cells are poorly understood. Here we found that intimal myeloid cells underwent reverse transendothelial migration (RTM) into the arterial circulation after systemic stimulation of pattern-recognition receptors (PRRs). This process was dependent on expression of the chemokine receptor CCR7 and its ligand CCL19 by intimal myeloid cells. In mice infected with the intracellular pathogen Chlamydia muridarum, blood monocytes disseminated infection to the intima. Subsequent CCL19-CCR7-dependent RTM was critical for the clearance of intimal C. muridarum. This process was inhibited by hypercholesterolemia. Thus, RTM protects the normal arterial intima, and compromised RTM during atherogenesis might contribute to the intracellular retention of pathogens in atherosclerotic lesions.

Rapid signaling and structural adaptations to the actin cytoskeleton enable leukocytes to stabilize α4 integrin–mediated adhesion and resist detachment from inflamed endothelium.

Integrins are adhesion molecules critical for the recruitment of leukocytes from blood into peripheral tissues. However, whether integrins are also involved in leukocyte exit from peripheral tissues via afferent lymphatics to the draining lymph node remains poorly understood. In this article, we show that adhesion by the collagen IV–binding integrin α1β1 unexpectedly inhibited macrophage exit from inflamed skin. We monitored macrophages exiting mouse footpads using a newly developed in situ pulse labeling technique. Blockade of α1β1 integrin or genetic deletion (Itga1−/−) increased macrophage exit efficiency. Chemotaxis assays through collagen IV showed more efficient migration of Itga1−/− macrophages relative to wild type. Given that macrophages are key orchestrators of inflammation, α1β1 integrin adhesion may represent a mechanism for regulating inflammatory responses by controlling macrophage exit or persistence in inflamed tissues.

Using a new, sensitive and quantitative technique for determining the ribosomal-RNA content of a measured number of cells, the cellular ribosome complement was compared for cultured hamster embryo cells in the stationary growth phase and in the early GI phase of the cell cycle. Cells from stationary phase cultures were found to contain less than 70% of the ribosome complement of the early GI phase cells, though the volumes of the two cell types were similar. This would imply that the stationary phase cell is physiologically different from a cell merely arrested at some point in the cell cycle.We have previously observed (Stanners and Becker, '71) that as cultures of hamster embryo cells progress from the exponential to the stationary growth phase, there is a three-fold decrease in the rate of protein synthesis per cell. This depression of protein synthesis could be entirely accounted for by a decrease in the number of ribosomes per cell together with a reduced attachment probability of the remaining ribosomes to messenger-RNA.The average cell in a perfectly asynchronous, exponentially growing culture (Stanners and Till, '60) should have about 1.40 times the number of ribosomes of an early GI cell.3 Our finding that the stationary phase hamster cell has about one-half of the ribosome complement of the average exponential phase cell (Stanners and Becker, '71 ) indicates, therefore, that the stationary phase cell should have only about 0.7 times the ribosome complement of the early GI phase cell, that is, a ribosome complement below that expected of any growing cell in our exponential cultures. This would imply that the stationary phase cell is not merely arrested at some position in the cell cycle, but rather has undergone some physiological change from a proliferating cell. The importance of this J. CELL, PHYSIOL., 77: 43-50. prediction required direct experimental verification. The results, using cells from exponential and stationary phase cultures and synchronized cells in the early G1 phase of the cell cycle are described in this report. 3 This calculation assumes the measured cycle parameters shown in figures l a and b: If one assumes that the cellular rate of ribosomal RNA synthesis is constant from early GI to the middle of S phase, after which time it increases by a factor of two and remains constant from the middle of S to the end of Gz. such that a doubling in the tptal amount of I-RNA occurs (Pfeiffer and Tolmach, 68). then the average cell in a perfectly asynchronous, exponentially growing culture should have 1.36 times the number of ribosomes of an early GI cell. On the other hand, if one assumes that the cellular rate of RNA synthesis ,is constant around the ce!l cycle (Enger and Tobey, 69; Scharff and Robbins, 65) the figure becomes 1.44. Thus, depending on the kinetic model for the doubling of the RNA content around the cell cycle, the average cell in a perfectly asynchronous exponentially growing culture should have about 1.40 times the ribosome complement of the early GI cell. fig. la...

No abstract

The ultraviolet (u.v.) irradiation of the 3' isomer of cytidylic acid (Cp) produces the hydrate (Cp*) with water added across the 5-6 double bond. The yield of this photoproduct has been measured by, (a) separating the photoproduct by electrophoresis and (b) by observing the loss in absorbance. When corrections are made for reversal of the hydrate during the experiment, both methods gave the same result. Cross sections and quantum yields for the production of the hydrate were measured over the wavelength range 220 to 290 nm and over a pH range from 1 to LO. The quantum yield is markedly dependent on pH being higher by a factor of 6 to 10 for the neutral form. We have also demonstrated the existence of a very short lived photoproduct (half life 8-9 min) in both Cp3' and Cp5': The nature of this short lived product is not known.