Cellular imbalances of cholesterol and fatty acid metabolism result in pathological processes, including atherosclerosis and metabolic syndrome. Recent work from our group and others has shown that the intronic microRNAs hsa-miR-33a and hsa-miR-33b are located within the sterol regulatory element-binding protein-2 and -1 genes, respectively, and regulate cholesterol homeostasis in concert with their host genes. Here, we show that miR-33a and -b also regulate genes involved in fatty acid metabolism and insulin signaling. miR-33a and -b target key enzymes involved in the regulation of fatty acid oxidation, including carnitine O-octaniltransferase, carnitine palmitoyltransferase 1A, hydroxyacyl-CoAdehydrogenase, Sirtuin 6 (SIRT6), and AMP kinase subunit-α. Moreover, miR-33a and -b also target the insulin receptor substrate 2, an essential component of the insulin-signaling pathway in the liver. Overexpression of miR-33a and -b reduces both fatty acid oxidation and insulin signaling in hepatic cell lines, whereas inhibition of endogenous miR-33a and -b increases these two metabolic pathways. Together, these data establish that miR-33a and -b regulate pathways controlling three of the risk factors of metabolic syndrome, namely levels of HDL, triglycerides, and insulin signaling, and suggest that inhibitors of miR-33a and -b may be useful in the treatment of this growing health concern.lipid homeostasis | posttranscriptional regulation | cardiovascular disease

Human tumours exhibit a large degree of cellular and genetic heterogeneity 1. Complex cell interactions in the tumour and its microenvironment are thought to play a significant role in tumourigenesis and cancer progression 2. It is also known that cooperation between oncogenic genetic lesions is required for tumour development 3. However, it is not known how cell interactions contribute to oncogenic cooperation. The genetic techniques available in the fruit fly Drosophila melanogaster allow analysis of the behavior of cells with distinct mutations 4, giving this model organism a privileged position to study cell interactions and oncogenic cooperation. In Drosophila eye-antennal discs, cooperation between the oncogenic protein RasV12 5 and loss-of-function mutations in the conserved tumour suppressor scribble (scrib) 6,7 gives rise to metastatic tumours that display many characteristics observed in human cancers 8-11. Here we show that clones of cells bearing different mutations can cooperate to promote tumour growth and invasion in Drosophila. We found that the RasV12 and scrib− mutations can also cause tumours when they affect different adjacent epithelial cells. We show that this interaction between RasV12 and scrib− clones involves JNK signaling propagation and JNK-induced upregulation of JAK/STAT-activating cytokines, a compensatory growth mechanism for tissue homeostasis. The development of RasV12 tumours can also be triggered by tissue damage, a stress condition that activates JNK signaling. Given the conservation of the pathways examined here, similar cooperative mechanisms could play a role in the development of human cancers.

SUMMARY Basement membranes (BMs) are resilient polymer structures that surround organs in all animals. Tissues, however, undergo extensive morphological changes during development. It is not known whether the assembly of BM components plays an active morphogenetic role. To study in vivo the biogenesis and assembly of Collagen IV, the main constituent of BMs, we used a GFP-based RNAi method (iGFPi) designed to knock down any GFP-trapped protein in Drosophila. We found with this method that Collagen IV is synthesized by the fat body, secreted to the hemolymph (insect blood), and continuously incorporated into the BMs of the larva. We also show that incorporation of Collagen IV determines organ shape, first by mechanically constricting cells and second through recruitment of Perlecan, which counters constriction by Collagen IV. Our results uncover incorporation of Collagen IV and Perlecan into BMs as a major determinant of organ shape and animal form.