A significant decrease of the transcriptional activity of p53 mutants deriving from human functional adrenal tumors

A novel p53 phosphorylation site within the MDM2 ubiquitination signal: II. a model in which phosphorylation at SER269 induces a mutant conformation to p53

The p53 DNA-binding domain harbors a conformationally flexible multiprotein binding site that regulates p53 ubiquitination. A novel phosphorylation site exists within this region at Ser(269), whose phosphomimetic mutation inactivates p53. The phosphomimetic p53 (S269D) exhibits characteristics of mutant p53: stable binding to Hsp70 in vivo, elevated ubiquitination in vivo, inactivity in DNA binding and transcription, increased thermoinstability using thermal shift assays, and λ(max) of intrinsic tryptophan fluorescence at 403 nm rather than 346 nm, characteristic of wild type p53. These data indicate that p53 conformational stability is regulated by a phosphoacceptor site within an exposed flexible surface loop and that this can be destabilized by phosphorylation. To test whether other motifs within p53 have similarly evolved, we analyzed the effect of Ser(215) mutation on p53 function because Ser(215) is another inactivating phosphorylation site in the conformationally flexible PAb240 epitope. The p53(S215D) protein is inactive like p53(S269D), whereas p53(S215A) is as active as p53(S269A). However, the double mutant p53(S215A/S269A) was transcriptionally inactive and more thermally unstable than either individual Ser-Ala loop mutant. Molecular dynamics simulations suggest that (i) solvation of phospho-Ser(215) and phospho-Ser(269) by positive charged residues or solvent water leads to local unfolding, which is accompanied by local destabilization of the N-terminal loop and global destabilization of p53, and (ii) the double alanine 215/269 mutation disrupts hydrogen bonding normally stabilized by both Ser(215) and Ser(269). These data indicate that p53 has evolved two serine phosphoacceptor residues within conformationally flexible epitopes that normally stabilize the p53 DNA-binding domain but whose phosphorylation induces a mutant conformation to wild type p53.

Mutation of p53 in recurrent hepatocellular carcinoma and its association with the expression of ZBP-89

p53 has recently been identified as a downstream target of ZBP-89, a zinc finger transcription factor. ZBP-89 promotes growth arrest through stabilization of the p53 protein. The aim of this study is to determine the status of the p53 gene in recurrent human hepatocellular carcinoma (HCC) and test the link between the expression of ZBP-89 and the p53 gene. The results showed that mutations in the p53 gene were frequently detected in recurrent HCC. The interval between surgical resection and the recurrence of HCC was significantly longer in patients with the wild-type p53 gene than those with mutations, strongly suggesting a pathological role for the mutant p53 gene in HCC recurrence. Among those positive for the p53 protein, nearly 85% (18 of 21) showed nuclear localization of the p53 protein while only about 14% (3 of 21) were positive for the p53 protein in the cytoplasm. ZBP-89 co-localized with p53 in the nucleus in about 67% (12 of 18) of all cases positive for the nuclear p53 protein, suggesting that ZBP-89 may play a role in the nuclear accumulation of the p53 protein in a subset of recurrent HCC. With accumulation of p53 protein in the nucleus, tumor cells undergo apoptosis and thus are more susceptible to radiotherapy and chemotherapy. Therefore, co-localization of p53 protein with ZBP-89 may define a subgroup of recurrent HCC that is more sensitive to treatment.

Mutant K-ras oncogene regulates steroidogenesis of normal human adrenocortical cells by the RAF-MEK-MAPK pathway

The result of our previous study has shown that the K-ras mutant (pK568MRSV) transfected human adrenocortical cells can significantly increase cortisol production and independently cause cell transformation. The aim of this study is to investigate the effect of the active K-ras oncogene on the cortisol production in normal human adrenocortical cells. First we used isopropyl thiogalactoside to induce the inducible mutant K-ras expression plasmid, pK568MRSV, in the stable transfected human adrenocortical cells. The result showed that the increase of RasGTP levels in transfected cells was time-dependent after isopropyl thiogalactoside induction. Additionally, results from Western blot analysis revealed significant elevation in phosphorylation of c-Raf-1 and Mitogen-activated protein kinase. We also detected the levels of mRNA encoding Cholesterol side-chain cleavage enzyme (P450(SCC)), 17alpha-Hydroxylase/17,20-lyase (P450(c17)) and 3beta-Hydroxysteroid dehydrogenase (3betaHSD) were increased in human adrenocortical cells transfected with mutant K-ras after IPTG treatment. The increase of mRNA amount in P450(scc) P450(c17) and 3betaHSD and the elevation of cortisol level were inhibited with a pretreatment of PD098059, a specific extracellular signal-regulated kinase inhibitor. In our previous report, we proved that lovastatin, a pharmacological inhibitor of p21(ras) function, also reversed the increase of cortisol level in mutant K-ras stably transfected human adrenocortical cells. Taken together, these findings proved that the active mutant Ras enhanced not only cell proliferation but also steroidogenesis in steroidogenic phenotype cells by activating Raf-MEK-MAPK related signal transduction pathway. Therefore, we believe that K-ras mutants influence regulation of steroidogenesis in adrenocortical cells through RAF-MEK-MAPK pathway.

Morphologic change and elevation of cortisol secretion in cultured human normal adrenocortical cells caused by mutant p21K-ras protein

In our previous study on the tumorigenesis of human functional adrenal tumors, we observed a high frequency of K-ras point mutations in clinical specimens. Furthermore, we cloned the mutated K-ras gene from the tumors and inserted it into vectors to transfect normal bovine adrenocortical cells to express the mutated K-ras gene. The mRNA level of steroidogenic enzymes such as cholesterol sidechain cleavage enzyme (P450SCC), 17alpha-hydroxylase/17,20-lyase (P450c17), and 3beta-hydroxysteroid dehydrogenase (3betaHSD) in the mutant K-ras stably transfected cells were elevated. Cultured normal adrenocortical cells from donors and patients with adrenocortical tumors were then transfected with mutant K-ras expression plasmids constructed from human adrenal tumors. Stable transfectants grew faster than normal cells. Additionally, morphologic change was observed in the transfected cells. Moreover, when the synthesis of hormones was analyzed, the mRNA of P450SCC, P450C17, and 3betaHSD was found to have increased, and the level of cortisol was 18 to 25 times that in control cells. The increased steroid hormone production in mutant K-ras-transfected cells was reversed by lovastatin, a pharmacologic inhibitor of p21ras function. These results, combined with previous reports of steroidogenic K-ras in bovine adrenocortical cells, suggest that the K-ras oncogene is involved in steroidogenesis in human adrenocortical cells.

The p53 codon 249 mutant--derived from human functional adrenal tumors--can modify the cell shape of normal adrenocortical transfected cells

Our previous study has indicated that p53 gene mutation occurred in 73% of human functional adrenal tumors, and the mutation hot spots were focused on codons 100, 102 (exon 4), and 249 (exon 7). Furthermore, a transcriptional activity study revealed that the mutant p53 protein derived from human functional adrenal tumors lost 90% transcriptional activity and the ability to bind with the p53 sequence. In order to investigate the influence of the mutant protein extracted from adrenocortical tumors on normal adrenal cells, we first cloned p53 cDNA from the human primary aldosteronism and constituted it with isopropyl thiogalactoside (IPTG) inducible expression vector as recombinant plasmid. The recombinant plasmid was then transfected to normal bovine adrenocortical cells through electroporation. The results showed that the p53 protein mutations at codons 100 and 102 could neither affect the cell morphology nor enable cell growth on the soft agar. In addition, no significant difference was found in cortisol level between the p53 transfected and the control cells. On the other hand, cell morphological changes and cell proliferation rate increase were observed when we used IPTG to induce the expression of the p53 protein, which mutated at codon 249, in adrenocortical cells. The cell morphology changes included less flattened and decreased elongation when compared to non-transfected cells. However, the cortisol level in transfected cells was not affected by the p53 mutants. Taken together, we concluded that the mutant p53 protein indeed participates in adrenal carcinogenesis; however, it has no influence on hormone production and secretion.

Mutations of K-ras oncogene in human adrenal tumours in Taiwan

Recently, we have found a high frequency of p53 gene mutations in human functional adrenal tumours. As the tumorigenesis is a multigene defect, we believe that other oncogenes may also be involved in the initiation or progression of adrenal tumours. Using the single-strand conformational polymorphism (SSCP) method, we chose the ras oncogenes as the target in this screening procedure because their high mutation rates were detected in thyroid tumours. For the ras oncogenes analysed, exon 1 to exon 2 of H-ras and K-ras genes in the tumour tissues of 13 Conn's syndrome, two adrenal Cushing's syndrome, two non-functional adrenal tumours, one adrenocortical hyperplasia and eight phaeochromocytomas and its paired adjacent normal adrenal tissues were amplified and sequenced. No mutations were detected in the H-ras gene. But mutations of the K-ras gene were detected in 46% (6 of 13) of Conn's syndrome; the hot spots were located at codon 15, 16, 18 and 31, which were different from those previously found in other tumours (codon 12, 13 and 61). Northern blot analysis with 1.1 kb K-ras cDNA revealed that K-ras mRNA was more than tenfold over-expressed in four of Conn's syndrome, one case of Cushing's syndrome and one case of adrenocortical hyperplasia. The mutation sites and mutation type were not found in other tissues, which conferred that this was highly related to adrenocortical tumours. Yet, the correlation between K-ras oncogene and adrenocortical tumours needs to be clarified by further studies.