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Biomedical subjects

J M Li

Publications and source records attributed to J M Li.

125 records · Page 7Linked to original sources
PubMed1986

Isolation and characterization of the TRM1 locus, a gene essential for the N2,N2-dimethylguanosine modification of both mitochondrial and cytoplasmic tRNA in Saccharomyces cerevisiae.

The trm1 mutation of Saccharomyces cerevisiae is a single nuclear mutation that affects a specific base modification of both cytoplasmic and mitochondrial tRNA. Transfer RNA isolated from trm1 cells lacks the modified base N2,N2-dimethylguanosine, and extracts from these cells do not have detectable N2,N2-dimethylguanosine-specific tRNA methyltransferase activity. As part of our efforts to determine how this mutation affects enzyme activities in two different cellular compartments we have isolated the TRM1 locus by genetic complementation. The TRM1 locus restores the N2,N2-dimethylguanosine modification to both cytoplasmic and mitochondrial tRNA in trm1 cells. An open reading frame in this TRM1 gene is essential for complementation of the trm1 phenotype. Expression of this open reading frame in Escherichia coli converts the organism from one that neither makes N2,N2-dimethylguanosine nor has N2,N2-dimethylguanosine-specific tRNA methyltransferase activity into one that does. This result suggests that the TRM1 locus is the structural gene for the tRNA modification enzyme and that both nuclear/cytoplasmic and mitochondrial forms of the methyltransferase are produced from the same gene.

Base Sequence↗
PubMed1986

Hormonal responses to 1,25-dihydroxyvitamin D3 in cultured mouse osteoblast-like cells--modulation by changes in receptor level.

The level of 1,25(OH)2D3 receptors in cultured mouse osteoblast-like (OB) cells is modulated by the rate of cell proliferation. We have studied two 1,25(OH)2D3-induced bioresponses to ascertain whether the changes in receptor levels during growth in culture alter cell responsiveness. Nuclear receptor levels were high (127 fmol/100 micrograms DNA) in rapidly dividing (log) cells and low (25 fmol/100 micrograms DNA) in quiescent (confluent) cells. The bioresponses we studied were induction of 25(OH)D3-24-hydroxylase activity (24-hydroxylase) and inhibition of collagen synthesis. The basal levels of 24-hydroxylase were low and similar in cells at log growth phase and confluence. At a maximal induction dose of 13 nM, 1,25(OH)2D3 induced a three-fold rise in enzyme activity at long growth phase, but only caused less than two-fold rise at confluence. The half-maximal dose (ED50) was slightly shifted from 0.6 nM to 0.8 nM. Daily measurement of 1,25(OH)2D3 receptor levels and maximal induction of 24-hydroxylase activity throughout the culture cycle showed a strong correlation between receptor abundance and enzyme induction. The basal level of collagen synthesized by cells in log growth phase was approximately 5% and increased to approximately 8% at confluence. Maximal inhibition of collagen synthesis by 1,25(OH)2D3 reached 80% of control levels in log cells, but was only 40% of control in confluent cells. The ED50 was approximately 0.1 nM in the log cells and increased to approximately 1 nM at confluence. Daily assay of 1,25(OH)2D3 receptor levels and 1,25(OH)2D3 responses during the culture cycle indicated a correlation between changes in receptor level and the extent of inhibition of collagen synthesis. These changes in bioresponse at various growth phases did not occur in rat OB cells where the 1,25(OH)2D3 receptor levels were independent of cell proliferation. The results indicate that cell proliferation rate, via change in receptor levels, determines the magnitude and sensitivity of the cellular responses to 1,25(OH)2D3.

Animals↗
PubMedDate not supplied

Transluminal catheter angioplasty of abdominal aorta in Takayasu's arteritis.

Nine patients with Takayasu's arteritis and a long stenotic segment of the abdominal aorta were treated by percutaneous transluminal angioplasty (PTA). Intermittent claudication disappeared in six of seven cases, the femoral pulse reappeared in all five; ankle/arm indices increased in seven cases; elevated blood pressure normalized in seven of eight cases. Seven patients were followed for 3 to 28 months. They were all free of symptoms from the lower extremities. In three patients with or without renal artery stenosis and with hypertension, the blood pressure decreased after PTA of the abdominal aorta only. PTA may be a valuable treatment in Takayasu's arteritis and stenosis of the abdominal aorta.

Adolescent↗