Regulation of chimeric phosphoenolpyruvate carboxykinase genes by the trans-dominant locus TSE1

Normal human melanocytes that express a bFGF transgene still require exogenous bFGF for growth in vitro

The expression of basic fibroblast growth factor (bFGF) has been implicated as an important factor in the development of malignant melanoma. The timing of this expression suggests that bFGF plays a role early in melanoma tumor progression. Benign nevi produce bFGF, and cells cultured from these lesions show a loss of dependence on exogenous bFGF for growth. We have examined the effects of constitutive bFGF expression on the in vitro growth requirements of normal human melanocytes. bFGF was overexpressed in normal human epidermal melanocytes through genomic insertion of a human bFGF cDNA in a retroviral vector. These melanocytes produced the 18 kDa bFGF isoform as well as the higher molecular weight isoforms. The bFGF was not released into the culture medium, but it was present in the cell nucleus. The bFGF produced by these cells was mitogenic for 3T3 fibroblasts and therefore possessed functional activity; however, melanocytes producing bFGF had the same appearance and growth patterns as those infected with control virus or uninfected melanocytes. Expression of bFGF did not confer independence from the exogenous mitogen, nor would these cells form colonies in a soft-agar medium. These results indicate that expression of bFGF alone is not enough to cause aberrant growth of normal human melanocytes.

Expression and subcellular distribution of phosphoenolpyruvate carboxykinase in primary cultures of rabbit kidney proximal tubule cells: comparative study with renal and hepatic PEPCK in vivo

The behaviour of the phosphoenolpyruvate carboxykinase (PEPCK) in rabbit proximal tubule cells in primary culture was investigated and compared with renal and hepatic PEPCK in vivo. The enzyme activity decreased rapidly in rabbit proximal tubule cells developed in hormonally defined medium supplemented with glucose and insulin. In this condition, the cytosolic form disappears with time. Without glucose and insulin, the subcellular location of PEPCK is similar to the location observed in proximal tubule freshly isolated and in renal cortex, with approx. 50% of mitochondrial form and approx. 50% of cytosolic form. However, the levels of mRNA that encode the cytosolic PEPCK are not detectable in cell cultures, whatever the medium composition. Treatment with dibutyryl cAMP caused a 14-fold induction of PEPCK mRNA in 6 h. This result indicates that the transcription of cytosolic PEPCK can be induced in cell cultures. Lactate or pyruvate additions did not modify the levels of PEPCK mRNA whereas specific activity increased rapidly, suggesting an activation of an inactive form in cell cultures. Moreover, lactate induced increased specific activity of the sole mitochondrial form while pyruvate induced increased specific activities of both mitochondrial and cytosolic form. Thus, subcellular location of PEPCK in rabbit proximal tubule cells appears to be modulated by the available substrate in culture medium. This observation parallels the changes observed in vivo since a modification of subcellular location of this enzyme was seen between fed and fasted rabbit, when subcellular distribution remains similar between fed and starved rats. Moreover, in the fasted liver of rabbit, a decrease of the mitochondrial PEPCK specific activity is seen concomitant with an increase in cytosolic PEPCK activity. These results point out the relative contributions of the cytosolic and mitochondrial PEPCK to rabbit gluconeogenesis.

Multiple elements regulate phosphoenolpyruvate carboxykinase gene expression in hepatoma hybrid cells

The phosphoenolpyruvate carboxykinase (PEPCK) gene is highly expressed in cultured rat hepatoma cells, but extinguished in hepatoma x fibroblast hybrids. Extinction of PEPCK gene expression in hybrids is a polygenic process that involves several fibroblast loci, only one of which (tissue-specific extinguisher-1, TSE1) has been characterized to date. To identify sequence elements of the PEPCK gene that are involved both in TSE1-mediated extinction and in TSE1-independent processes, we assayed expression of chimeric PEPCK transgenes in transiently and stably transfected hybrid cells. We report that TSE1 responsiveness mapped to the PEPCK CRE (cAMP response element), as shown previously for the tyrosine aminotransferase gene. This was expected from the recent identification of the TSE1 gene product as a regulatory subunit of protein kinase A. However, none of the transgenes we assayed were responsive to TSE1-independent extinction mechanisms, suggesting that these controls require DNA sequences and/or chromatin structures that were not present in the transfected reporters. The implications of these findings are discussed.

The tissue-specific extinguisher locus TSE1 encodes a regulatory subunit of cAMP-dependent protein kinase

The tissue-specific extinguisher locus TSE1, a dominant negative regulator of transcription in somatic cell hybrids, acts via a cAMP response element (CRE) to repress activity of a hepatocyte-specific enhancer. Guided by the antagonism between TSE1 and cAMP-mediated signal transduction, we identified the regulatory subunit RI alpha of protein kinase A (PKA) as the product of the TSE1 locus. The evidence derives from concordant expression of RI alpha mRNA and TSE1 genetic activity, high resolution mapping of the RI alpha gene and TSE1 on human chromosome 17, and the ability of a transfected RI alpha cDNA to generate a phenocopy of TSE1-mediated extinction. The mechanism of TSE1/RI alpha-mediated extinction involves repression of basal PKA activity, reduced phosphorylation of CREB at Ser-133, and a corresponding reduction of in vivo protein binding at the target CRE.