The asialoglycoprotein receptor is responsible for selective endocytosis of glycoproteins with galactosyl/N-acetylgalactosaminyl residues and is characteristic of fully differentiated hepatocytes. Enhanced expression of ASGR has been demonstrated during cGMP induction in a human derived hepatoblastoma cell line, HepG2. The long-term objectives of the studies proposed are to understand the underlying biochemical and molecular mechanisms which modulate hepatocellular differentiation at the translational level. The ASGR is a prototypical marker of this process.
The specific aims are to define the cis-element and trans-acting factors which govern the translation of ASGR in response to cGMP. The hypothesis is that cGMP regulates translational control via protein-RNA interactions localized to the 5' UTR of ASGR mRNA. The cis-acting elements will be defined by deletion analysis and expression of mutated reporter gene constructs in HepG2 cells. The postulated trans-acting factor responsive to cGMP will be purified by RNA affinity chromatography or will be cloned by a Northwestern ligand-expression technique. Findings will be extended to the regulation of ASGR expression during differentiation of HepG2 cells. The biosynthetic rate of ASGR will be compared to its mRNA steady-state level and polysome distribution to define the molecular level at which ASGR expression is regulated. The studies should ultimately lead to definition of the biochemical and molecular factors controlling ASGR expression. Characterization of these elements will determine whether common linkages exist between elements governing ASGR expression and are seminal to the understanding of molecular targets of cGMP.
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