Gene regulation by PDGF via the PI3-kinase pathway : focus on HBP1 and stearoyl-coenzyme A desaturase

Abstract

Our laboratory is interested in the study of growth factors and in particular the platelet-derived growth factor (PDGF) and its receptor (PDGFR). Many signaling pathways, including the phosphatidylinositol 3-kinase (PI3K) pathway, are activated downstream of PDGFR, leading to cell proliferation, survival, migration and differentiation. In line with this, the objective of this thesis was to characterize the regulation and the function of two mediators of the PDGFR-PI3K pathway. We focused on the HMG-box protein 1 (HBP1) and the stearoyl-coenzyme A desaturase (SCD). In our first study, we identified the transcription factor HBP1 as a negative target of the growth factor-PI3K-AKT pathway, up-regulated by FOXO via a direct transcriptional regulation. These regulations were observed in different cellular models including normal and cancer cells from human and mice origin. Using small-hairpin RNA (shRNA), we found that HBP1 knockdown increased cell proliferation and potentiated the proliferative effect of growth factors. Finally, we observed that HBP1 and FOXO1 were down-regulated in breast cancer tissues compared to normal breast tissues. In addition, breast tumors with a lower expression of FOXO also presented a lower expression of HBP1, which suggests that, in these tumors, FOXO loss could explain HBP1 down-regulation, at least partially. Our second study started from published data from our laboratory showing that PDGF strongly induces SCD expression via PI3K, mTORC1 and SREBP and stimulates lipid synthesis, in particular unsaturated fatty acids. SCD is a Δ9 desaturase that converts saturated fatty acids (SFA), such as palmitoyl-CoA and stearoyl-CoA, into mono-unsaturated fatty acids (MUFA), such as palmitoleoyl-CoA and oleyl-CoA. By increasing SCD expression, PDGF increased the MUFA / SFA ratio in the total cellular lipids. Using shRNA targeting SCD, we showed that SCD was essential for the proliferation and the survival of human fibroblasts in response to PDGF. The proliferative effect was confirmed with an inhibitor of SCD. Interestingly, the treatment of cells with palmitate, but not oleate, also blocked cell proliferation in response to PDGF, and the addition of oleate to palmitate suppressed this effect. In addition, SCD knockdown and palmitate induced endoplasmic reticulum stress (ER stress), which could explain the observed effects. Together, our results strongly suggest that SCD is necessary for PDGF-mediated cell proliferation by supplying an increased amount of MUFA. It also protects cells from SFA accumulation-induced lipotoxicity and ER stress. To conclude, PDGF activates many signaling pathways that allow the regulation of plenty of genes involved in PDGF functions. Among them, we identified HBP1 and SCD as downstream targets of the PI3K pathway, both involved in the control of cell proliferation by PDGF: HBP1 blocked cell proliferation whereas SCD favored it.(BIFA - Sciences biomédicales et pharmaceutiques) -- UCL, 201