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Biomedical subjects

K C Chow

Publications and source records attributed to K C Chow.

At least 73 records · Page 4Linked to original sources
PubMed1987

Selection of AUG initiation codons differs in plants and animals.

The influence of the nucleotide at position -3 relative to the AUG initiation codon on the initiation of protein synthesis was studied in two different in vitro translation systems using synthetic mRNAs. The four mRNAs, transcribed from cDNAs directed by an SP6 promoter, were identical except for mutations at nucleotide -3. In each case, translation of mRNAs produced a single protein of Mr = 12,600. Relative translational efficiencies showed a hierarchy in the reticulocyte lysate system (100, 85, 61 and 38% for A, G, U and C in position -3, respectively) but no differences in the wheat germ system. Differential mRNA degradation or polypeptide chain elongation were excluded as causes of the differences observed in translation in the reticulocyte lysate. mRNA competition increased the differences observed in translational efficiencies in reticulocyte lysate but showed no effect in wheat germ. Analysis of 61 plant and 209 animal mRNA sequences revealed qualitative and quantitative differences between the consensus sequences surrounding AUG initiation codons. Whereas the consensus sequence for animals was CACCAUG that for plants was AACAAUGGC. Both the structural and functional findings suggest that the factors which select AUG initiation codons in plants and animals differ significantly.

Animals↗
PubMed1987

Topoisomerase-specific drug sensitivity in relation to cell cycle progression.

The nuclear enzyme DNA topoisomerase II catalyzes the breakage and resealing of duplex DNA and plays an important role in several genetic processes. It also mediates the DNA cleavage activity and cytotoxicity of clinically important anticancer agents such as etoposide. We have examined the activity of topoisomerase II during the first cell cycle of quiescent BALB/c 3T3 cells following serum stimulation. Etoposide-mediated DNA break frequency in vivo was used as a parameter of topoisomerase II activity, and enzyme content was assayed by immunoblotting. Density-arrested A31 cells exhibited a much lower sensitivity to the effects of etoposide than did actively proliferating cells. Upon serum stimulation of the quiescent cells, however, there was a marked increase in drug sensitivity which began during S phase and reached its peak just before mitosis. Maximal drug sensitivity during this period was 2.5 times greater than that of log-phase cells. This increase in drug sensitivity was associated with an increase in intracellular topoisomerase II content as determined by immunoblotting. The induction of topoisomerase II-mediated drug sensitivity was aborted within 1 h of exposure of cells to the protein synthesis inhibitor cycloheximide, but the DNA synthesis inhibitor aphidicolin had no effect. In contrast to the sensitivity of cells to drug-induced DNA cleavage, maximal cytotoxicity occurred during S phase. A 3-h exposure to cycloheximide before etoposide treatment resulted in nearly complete loss of cytotoxicity. Our findings indicate that topoisomerase II activity fluctuates with cell cycle progression, with peak activity occurring during the G2 phase. This increase in topoisomerase II is protein synthesis dependent and may reflect a high rate of enzyme turnover. The dissociation between maximal drug-induced DNA cleavage and cytotoxicity indicates that the topoisomerase-mediated DNA breaks may be necessary but are not sufficient for cytotoxicity and that the other factors which are particularly expressed during S phase may be important as well.

Animals↗
PubMed1987

Role of proliferation in determining sensitivity to topoisomerase II-active chemotherapy agents.

We have examined the relationship between topoisomerase II content and the DNA cleavage activity and cytotoxicity of etoposide during proliferative and quiescent culture conditions. In proliferating cultures of Chinese hamster ovary (CHO) cells, human lymphoblastic CCRF cells, and mouse leukemia L1210 cells, there was easily detectable topoisomerase II by immunoblotting. In contrast, quiescent CHO cells contained virtually no detectable topoisomerase II, while the content of L1210 cells was unchanged. Enzyme content of quiescent CCRF cells was diminished but detectable. DNA cleavage activity induced by etoposide correlated well with enzyme content in proliferating and quiescent cells. Quiescent CHO and CCRF cultures were highly resistant to the cytotoxic effects of etoposide as expected. However, despite unchanged enzyme content and DNA cleavage activity, there was also significant resistance observed in plateau L1210 cells. We have also investigated topoisomerase content and drug activity as a function of cell cycle progression. Following serum stimulation of confluent BalbC/3T3 cells, maximal etoposide-induced DNA cleavage activity is observed in G2/M and is associated with an increase in topoisomerase II content. Maximum cytotoxicity, however, occurs during mid to late S phase. Our data suggest that topoisomerase II content may be an important determinant of chemotherapeutic sensitivity during alterations in the proliferative status of the cell. However, it is clear that other factors must be involved in cell sensitivity, and elucidation of these may contribute to our understanding of the mechanism of action of these drugs.

Animals↗
PubMed1985

Adenovirus infection elevates levels of cellular topoisomerase I.

We have developed a specific, sensitive, and quantitative assay for topoisomerase I, which is based on the formation of a covalent enzyme-DNA intermediate. Our assay measures the quantitative transfer of 32P radioactivity from 32P-labeled DNA to topoisomerase I. Since 32P-labeled topoisomerase molecules are resolved by NaDodSO4/PAGE, HeLa topoisomerase I (100 kDa) and calf thymus topoisomerase I (82 kDa) can be quantitatively assayed in the same reaction mixture. The assay can detect at least 0.3 ng (3 fmol) of topoisomerase I. We have used our assay to measure the levels of topoisomerase I activity in crude extracts of nuclei prepared from uninfected, adenovirus-infected, and adenovirus-transformed human cells. The evidence suggests that an adenovirus early gene product, presumably a protein encoded in early region 1A (E1A), increases cellular topoisomerase I activity at least 10-fold. Immunoblotting analysis with antiserum against calf thymus topoisomerase I shows that the increase in activity is due to an increase in the amount of enzyme.

Adenoviridae↗
PubMed1984

Site-specific nicking within the adenovirus inverted terminal repetition.

Site-specific nicking occurs on the l-strand, but not on the r-strand, of the adenovirus left inverted terminal repeat. Nicks are presumably introduced into double- or single-stranded DNA by a cellular endonuclease in an ATP-independent reaction. The consensus nick site has the sequence: (sequence in text).

Adenoviridae↗
PubMed1983

In vitro replication directed by a cloned adenovirus origin.

A 5.7-kb recombinant plasmid, called XD-7, contains the terminal XbaI-E fragment from the left end of type 2 adenovirus cloned into the EcoRI site of pBR322. An average of 9% +/- 1% of input supercoiled, protein-free XD-7 DNA replicated as rolling circles with single-stranded tails ranging up to unit length and longer in reaction mixtures containing nuclear and cytoplasmic extracts from adenovirus-infected, but not uninfected, HeLa cells. The adenovirus origin was mapped on XD-7 by electron microscopy at the left boundary of the cloned adenovirus segment. Since replication proceeded rightwards, we conclude that the adenovirus l strand was displaced during replication. No origin was located at or near the EcoRI site on pBR322. Reversing the orientation of the adenovirus origin reversed the direction of replication, and deletion of the adenovirus origin abolished replication.

Adenoviridae↗
PubMed1983

Mutational mapping of a cloned adenovirus origin.

We have developed a standardized, quantitative assay to study the function of a cloned adenovirus origin. We have shown that the adenovirus origin is located within the first 20 bp of the adenovirus inverted terminal repetition (ITR), a region containing a sequence conserved among human, simian, murine, and avian adenoviruses. Deletions removing or penetrating from either direction into the conserved sequence inactivated the cloned adenovirus origin. A point mutation within the conserved sequence impaired the adenovirus origin, but point mutations outside the conserved sequence had no effect. These results strongly suggest that the conserved sequence within the first 20 bp of the ITR alone constitutes the adenovirus origin (ori) signal.

Adenoviridae↗
PubMed1982

A calorimetric study of the thermal transitions of Halobacterium cutirubrum.

The thermal transitions of Halobacterium cutirubrum have been examined by differential scanning calorimetry. Two distinct peaks corresponding to the denaturation of two major protein components were observed in the heating curves. One of the peaks has been assigned to the denaturation of the envelope glycoprotein. The variations of the denaturation temperatures with the addition of glucose, glycerol, NaNO3, and NaSCN are consistent with the previous proposal that hydrophobic interactions are essential in stabilizing the glycoprotein.

Calorimetry, Differential Scanning↗
PubMed1982

Gastrointestinal bleeding and mycotic superior mesenteric aneurysm.

This is a case report of management of a delayed mycotic superior mesenteric artery aneurysm occurring in a patient 2 years after aortic valve replacement for endocarditis. A chronic ulcer history, anticoagulation therapy, episodic gastrointestinal bleeding associated with negative gastrointestinal series, and gastritis seen at endoscopy delayed the ultimate diagnosis. An episode of massive hemorrhage precipitated angiography with subsequent surgical confirmation of the diagnosis. Management included debridement and extirpation of the major part of the aneurysm, Doppler assessment of the inadequacy of collateral mesenteric arterial blood flow, and restoration of flow with a bypassing saphenous vein graft segment. Although this technique of reconstruction has been suggested, we can find no other report of such a similar case among the few reported surgical successes with superior mesenteric artery mycotic aneurysm.

Aneurysm, Infected↗
PubMed1981

Stress fractures of the femoral neck: a scintigraphic sign for early diagnosis.

Two patients with hip pain are presented; both patients were on steroid therapy. The diagnosis of a stress fracture was initially missed because the patients were not exceedingly active and the plain roentgenograms were essentially normal. The bone scan demonstrated a discretely localized area of increased radionuclide uptake in the femoral neck, which alerted us to the correct diagnosis. This scintigraphic sign is helpful in arriving at an early diagnosis and preventing prolonged disability.

Adult↗
PubMed1981

The aortic isthmus.

In chest radiographs of seven infants, the aortic knob, isthmus, and descending aorta rendered a figure 3 image identical to that seen in coarctation of the aorta. Awareness of this anatomic variation may aid in diagnosis and may help to prevent unnecessary cardiac catheterization.

Aorta, Thoracic↗
PubMed1980

Severe abdominal vasculitis with hepatitis B antigenemia.

Two cases of necrotizing abdominal vasculitis associated with hepatitis B are reported. The diagnosis of mesenteric vasculitis is often delayed, and intestinal perforation is common. Early recognition of the disease and early treatment with corticosteroid and immunosuppressive drugs may alter the course of the disease. Preoperative diagnosis is best made by mesenteric arteriography and rectal biopsy.

Abdomen↗