Phospholipase C β3, and its role in endocrine tumorigenesis

Sammanfattning: The Phospholipase C β3 (PLCB3) gene is located on chromosome 11q13, in the vicinity of theMultiple Endocrine Neoplasia type 1 (MEN1) suppressor gene. PLCB3 has been implicated inendocrine tumorigenesis based on findings of low expression in endocrine tumors, and the probableexistence of other suppressor genes located at 11q13. In order to study the extent of PLCB3 loss inendocrine tumors, we evaluated the expression at the mRNA level using in situ-RNA-RNAhybridization on a total of 82 samples, including 34 from MEN1 patients. The expression was lost in31% of MEN1 related tumors and in a subset of sporadic exocrine pancreatic cancers, bronchialcarcinoids and an adrenocortical carcinoma. Further, we isolated three endocrine tumor cell lineswith low PLCB3 expression, and transfected them with a PLCB3 construct. The growth rate andtumorigenicity in nude mice were assessed. Restoration of PLCB3 expression inhibited the growthrate and suppressed the malignant phenotype both in vitro and in vivo. We hypothesized that genes differentially expressed between transfected and non-transfected tumor cells might be involved in mediating these effects? By using RT Differential cDNA Display we isolated 8 genes dffferentially expressed in the PLCB3 expressing cells. Chromogranin A and S100A3 were downregulated, and human mismatch repair protein 3, cytochrome oxidase subunit1, and a transcript homologous tomouse topoisomerase inhibitor suppressed mRNA were up-regulated. In addition, we found threeup-regulated unknown transcripts. To study the biological function of PLCB3 in vivo we performed a targeted disruption of PLCB3 in mice. Homozygous inactivation of the gene was lethal at embryonic day 2.5. Although expression of the protein in the heterozygous mice was reduced by approximately 50% compared to wild type littermates, they showed no specific phenotype and remained healthy for at least two generations.In conclusion, these results implicate that PLCB3 has tumor suppressor characteristics in endocrine tumor cells, and that it possibly regulates genes of high interest in tumorigenesis. In addition, PLCB3 seems to be involved in early embryonic development.