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Biomedical subjects

C H Cheung

Publications and source records attributed to C H Cheung.

15 recordsLinked to original sources
PubMed1997

Rabbit sex hormone binding globulin: primary structure, tissue expression, and structure/function analyses by expression in Escherichia coli.

Sex hormone binding globulin (SHBG) is a homodimeric plasma protein found in mammals that binds sex steroids with high affinity and regulates their bioavailability. The protein is identical in structure and properties to the androgen binding protein (ABP) found in the male reproductive tract. We have isolated a 1245-base pair rabbit SHBG cDNA encoding a reading frame for a signal peptide followed by a protein of 367 amino acids, which shares 79.0, 68.1 and 63.2% amino acid identity with the corresponding human, rat and mouse proteins respectively. Northern blot and hot-nested PCR analyses indicated that rabbit SHBG is produced from a 1.6 kilobase mRNA in the liver of both sexes and in the testis. The rabbit SHBG cDNA was inserted into pGEX-1 lambda T for expression of a glutathione S-transferase/SHBG fusion protein in Escherichia coli. The bacterial product bound 5 alpha-dihydrotestosterone (DHT) in the same manner as the corresponding protein in serum. The dissociation constants (Kd) for rabbit and human SHBGs produced in E. coli were 11.1 +/- 1.1 nM and 2.1 +/- 0.6 nM respectively, and rabbit SHBG formed a less stable protein-steroid complex (t1/2 = 5 min) than human SHBG (t1/2 > 60 min). Unlike human SHBG, rabbit SHBG does not bind estradiol with high affinity. To aid in the identification of differences in the sequences of rabbit and human SHBG, which determine species differences in steroid-binding affinity and specificity, chimeras containing the 5'-terminal half of SHBG from one species and 3'-terminal half of SHBG from the other species were constructed and expressed. It was found that the chimeric proteins assumed similar steroid-binding affinity and specificity as the wild-type proteins when the amino (N)-terminal half of SHBG was derived from the same species. Replacement of the carboxyl (C)-terminal half of rabbit SHBG by the corresponding region of the human molecule increased the integrity of its steroid-protein complex. This supports the concept that amino acids within the N-terminal half of SHBG constitute the steroid-binding domain while the C-terminal half of the molecule may provide structural stability to the protein and its steroid-binding site.

Amino Acid Sequence↗
PubMed1993

Structural requirements for the functional activity of a U1 snRNA gene enhancer.

The transcriptional enhancer of a chicken U1 small nuclear RNA (snRNA) gene contains a GC-box, an octamer motif, and an SPH motif that are recognized by the transcription factors Sp1, Oct-1, and SBF respectively. Previous work indicated that the octamer and the SPH motifs were both required for U1 gene enhancer activity in frog oocytes when the U1 gene was coinjected with a competing snRNA gene template. Here we show that neither two copies of the octamer motif, nor two copies of the SPH motif, can effectively substitute for the natural combination of octamer and SPH. Furthermore, neither the octamer nor the SPH motif (in the absence of the other) functioned efficiently in combination with a GC-box. Alteration of the spacing between the octamer and SPH motifs also reduced U1 template activity. Several potential cis-acting elements other than the SPH motif, with one possible exception among those tested, were unable to cooperate with the octamer motif to effectively enhance U1 gene expression. These results indicate that rather stringent structural requirements exist with respect to the essential cis-acting motifs present in the U1 enhancer, possibly reflecting the unique properties of the transcription complexes assembled on snRNA gene promoters.

Animals↗
PubMed1980

Molecular evidence of viral-like biochemical activities in human genitourinary malignancies.

We have demonstrated previously that core structures of urine samples from patients with genitourinary malignancies contain ribonucleic acid-directed deoxyribonucleic acid polymerase and a high molecular weight ribonucleic acid. If these particles originated from the existing genitourinary malignancies then the malignancies should contain similar characteristics. We examined 13 prostatic carcinomas, 4 bladder carcinomas, 1 urethral carcinoma and 1 hypernephroma. Positive reactions were noted in 10 of the 13 prostatic carcinomas (77 per cent), all 4 bladder carcinomas, the 1 urethral carcinoma and the hypernephroma with the simultaneous detection assay. The control samples consisted of 7 tissues of benign prostatic hypertrophy, and tissue from 2 normal bladders and 1 normal kidney. None of these tissues showed a positive response. Tritium labeled deoxyribonucleic acid probes synthesized from the malignant tissues hybridized to the polysomal ribonucleic acids but not to the corresponding normal tissues. Particles derived from the probes have a density of 1,1620 in sucrose gradient. No sequence homology could be demonstrated with various known oncogenic ribonucleic acid viruses nor with malignancies arising from other organs.

Adenocarcinoma↗
PubMed1978

Detection of viral-like cores from the urine of patients with genito-urinary malignancies.

Simultaneous detection assays on the core structures derived from the cerebrospinal fluid samples of patients with various types of central nervous system tumors have demonstrated the feasibility of this technique in detecting some of the diagnostic features of RNA tumor viruses. Similar assays done on urine samples from patients iwth various types of tumors in their genitourinary tracts have shown that of the 18 such samples from tumor patients, 15 or 83% were found to be positive. The control samples consisted of three from patients with benign prostatic hypertrophy and four from normal persons. None of these gave a positive reaction. [3H]DNA probes synthesized from the core structures from them hybridized readily to their corresponding polysomal RNAs but no to control tissues. The densities of particles from these samples have been found to be 1.168 g/ml for bladder carcinoma and 1.165 for prostatic carcinoma, the same densities as those found RNA tumor viruses.

Female↗
PubMed1977

Embryogeneic cell type, organ site sequence specificity in human cancers.

An extensive series of cross-hybridization studies were carried out with the DNA-RNA molecular hybridization technique. Molecular 70 S [3H]DNA probes synthesized from human central nervous system, gastrointestinal, pulmonary, and prostatic carcinomas were hybridized to cytoplasmic RNA's isolated from cancers of virtually all organ sites of the human body. Results indicated sequence homology between cancers of the same organ or cell type but not with cancers of different cell types. Thus cell types based on embryological origins determine the organ site specificity of the involved sequences. The designation of 70 S [3H]DNA denotes those [3H]DNA's that were copied off the template 70 S RNA, as distinguished from total [3H]DNA product, which includes all DNA's synthesized. It does not necessarily follow nor is it to be inferred that the 70 S [3H] DNA thus designated contains the full complement of the sequences found in the 70 S RNA template.

Base Sequence↗
PubMed1976

Biochemical evidences for ribonucleic acid viral-like characteristics in malignant diseases of gastrointestinal tract and lung in humans.

It has been demonstrated that malignant diseases of the gastrointestinal tract and lung in humans possess three characteristics invariably found in ribonucleic acid tumor viruses: the presence of a ribonucleic acid directed deoxyribonucleic acid polymerase, reverse transcriptase; a high molecular weight ribonucleic acid with a sedimentation coefficient of 70 Svedberg units, and particulate elements with densities of 1.16 to 1.18 grams per milliliter sucrose gradient. Twelve of 17 carcinomas of the colon, three of five carcinomas of the stomach, all three carcinomas of the rectum and seven of ten carcinomas of the lung displayed detectable evidence of these viral-like entities. None of the corresponding normal tissues had positive reactions.

Cell Transformation, Neoplastic↗