High expression of the peroxisome proliferator-activated receptor ␣ (PPAR␣) differentiates brown fat from white, and is related to its high capacity of lipid oxidation. We analyzed the effects of PPAR␣ activation on expression of the brown fat-specific uncoupling protein-1 (ucp-1) gene. Activators of PPAR␣ increased UCP-1 mRNA levels severalfold both in primary brown adipocytes and in brown fat in vivo. Transient transfection assays indicated that the (؊4551)UCP1-CAT construct, containing the 5-regulatory region of the rat ucp-1 gene, was activated by PPAR␣ co-transfection in a dose-dependent manner and this activation was potentiated by Wy 14,643 and retinoid X receptor ␣. The coactivators CBP and PPAR␥-coactivator-1 (PGC-1), which is highly expressed in brown fat, also enhanced the PPAR␣-dependent regulation of the ucp-1 gene. Deletion and point-mutation mapping analysis indicated that the PPAR␣-responsive element was located in the upstream enhancer region of the ucp-1 gene. This ؊2485/؊2458 element bound PPAR␣ and PPAR␥ from brown fat nuclei. Moreover, this element behaved as a promiscuous responsive site to either PPAR␣ or PPAR␥ activation, and we propose that it mediates ucp-1 gene up-regulation associated with adipogenic differentiation (via PPAR␥) or in coordination with gene expression for the fatty acid oxidation machinery required for active thermogenesis (via PPAR␣).

Cyclin D1 (Ccnd1) together with its binding partner Cdk4 act as a transcriptional regulator to control cell proliferation and migration, and abnormal Ccnd1·Cdk4 expression promotes tumour growth and metastasis. While different nuclear Ccnd1·Cdk4 targets participating in cell proliferation and tissue development have been identified, little is known about how Ccnd1·Cdk4 controls cell adherence and invasion. Here, we show that the focal adhesion component paxillin is a cytoplasmic substrate of Ccnd1·Cdk4. This complex phosphorylates a fraction of paxillin specifically associated to the cell membrane, and promotes Rac1 activation, thereby triggering membrane ruffling and cell invasion in both normal fibroblasts and tumour cells. Our results demonstrate that localization of Ccnd1·Cdk4 to the cytoplasm does not simply act to restrain cell proliferation, but constitutes a functionally relevant mechanism operating under normal and pathological conditions to control cell adhesion, migration and metastasis through activation of a Ccnd1·Cdk4-paxillin-Rac1 axis.

Uncoupling protein-3 (UCP3) gene is a member of the mitochondrial carrier superfamily preferentially expressed in skeletal muscle and up-regulated by fatty acids. Peroxisome proliferator-activated receptor (PPAR)alpha and PPARdelta (also known as PPARbeta) mediate human UCP3 gene regulation by fatty acids through a direct-repeat (DR-1) element in the promoter. DR-1 mutation renders UCP3 promoter unresponsive to PPAR ligand in vitro and consistently blocks gene induction by fatty acids in vivo. Although they act through separate sites in the promoter, MyoD and PPAR-dependent regulatory pathways are functionally connected: only in the presence of MyoD, does UCP3 become sensitive to PPAR ligand-dependent regulation. MyoD controls UCP3 promoter activity through a noncanonical Ebox site located in the proximal region, close to transcription initiation site. Moreover, acetylation processes play a crucial role in the control of UCP3 gene regulation. The coactivator p300 protein enhances PPAR ligand-mediated regulation whereas a mutant form devoid of histone acetylase activity blocks the response of the promoter to fatty acids. Conversely, histone deacetylase-1 blunts MyoD-dependent expression of the UCP3 promoter and reduces PPAR-dependent responsiveness. A mutated form of MyoD unable to be acetylated has a lower transactivation capacity on the human UCP3 promoter with respect to wild-type MyoD. It is concluded that MyoD and PPAR-dependent pathways mediate human UCP3 gene regulation and that acetylase activity elicited by coregulators is implicated in the functional interaction between these regulatory pathways. Therefore the convergence of MyoD and PPAR-dependent pathways provides a molecular mechanism for skeletal muscle specificity and fatty acid regulation of human UCP3 gene.