Gene expression and protein array studies of folliculin-regulated pathways

The familial cancer syndrome Birt-Hogg-Dube syndrome is characterised by the development of skin (fibrofolliculomas) and renal tumours (and lung cysts) and is caused by mutations in the FLCN tumour suppressor gene. Though the FLCN gene product (folliculin) has been linked to the regulation of a variety of signalling pathways (e.g. the mTOR, AMPK, TGFbeta and hyoxia-responsive genes) the precise function of the folliculin protein is not well-defined. In order to identify potential novel pathways linked to folliculin function we analysed paired isogenic folliculin-deficient and folliculin-expressing cell lines by gene expression and protein (Kinexus) arrays. Gene expression microarray analysis in the folliculin +/- non-renal cancer line (FTC133), revealed 708 differentially expressed targets (fold change >2 and p<0.001) with enrichment of genes in the cadherin and Wnt signalling pathways. Comparison of the differentially expressed genes in the FTC133 datasets and previously reported gene expression data for a folliculin-deficient renal tumour and the UOK257 renal cell carcinoma cell line, revealed that RAB27B was dysregulated in all three datasets (increased expression in folliculin-deficient cells). The Kinexus protein array analysis suggested 73 candidate, differentially expressed, proteins and further investigation by western blot analysis of 5 candidates that were also differentially expressed in the FTC133 gene expression microarray data, revealed that EIF2AK2 (PKR) and CASP1 were reduced and PLCG2 was increased in folliculin-deficient FTC133 cells and in a BHD renal tumour. In view of the role of CASP1 in apoptosis we investigated whether other apoptosis-related proteins might be regulated by folliculin and found increased levels of SMAC/Diablo and HtrA2 in folliculin-expressing FTC133 cells. These findings identify novel pathways and targets linked to folliculin tumour suppressor activity.