PI 3-kinases, in particular class-IA, are key signalling molecules controlling many cellular processes including growth, proliferation, migration and differentiation. In this Study, we have used a collection of isoform selective PI 3-kinase inhibitors to determine whether attenuation of signalling through class-IA PI 3-kinase isoforms will impact adipocyte differentiation. First, we analysed the expression profiles and found that fibroblastic pre-adipocytes express detectable levels of p110 alpha and p110 delta and that after differentiation, p110 delta levels fall while p110 alpha levels rise, together with C/EBP alpha and PPAR gamma. When using specific inhibitors during the differentiation process, we observed that neither p110 beta nor p110 delta inhibition, had any significant effect. In contrast PIK-75, a selective p110 alpha inhibitor completely abolished adipocyte differentiation as assessed by morphology, transcript and protein levels of adipocyte markets. These results indicate that long term treatment with p110 alpha inhibitors could potentially have a severe impact on fat cell numbers in vivo. (C) 2008 Elsevier Inc. All rights reserved.