The expression of LRP was
The expression of LRP1 was found to be sensitive to several factors thought to mediate the pathogenesis of PH. For example, hypoxia-inducible factor-1α (HIF1α) was reported to drive LRP1 expression in vascular SMC cultured under hypoxic conditions . Furthermore, angiotensin II, a peptide hormone known to be markedly increased in the serum of progressive IPAH patients , was described to induce the transcription of the LRP1 gene in the same cell type . Here, we show that PDGF-BB induces LRP1 expression in PASMC in vitro and in vivo. Thus over-activation of the PDGF signaling in PH could be at least in part responsible for elevated LRP1 expression under this pathological condition. Interestingly, none of the conditions tested resulted in changes in LRP1 transcript abundance indicating that LRP1 is predominantly regulated on the protein level in PH. Several mechanisms were found to modulate LRP1 protein synthesis and stability. Leslie et al.  reported the suppression of LRP1 translation by p53-mediated increased expression of miRNA-103/107 in colon cancer λ-Carrageenan and Cal et al.  showed the stabilization of the LRP1 protein through downregulation of CHFR, a RING type E3 ubiquitin ligase, in vascular SMC exposed to aggregated LDL. Our results demonstrate that the increased LRP1 protein stability, but not the enhanced LRP1 protein translation, may account for the high LRP1 protein abundance in PH. Whether other factors/mechanisms, such as the lrp1 natural antisense transcript , additionally contribute to the high LRP1 levels in PH needs further investigation. Next, we tested whether LRP1 regulates PASMC proliferation, migration, and adhesion. Silencing of LRP1 reduced proliferation but potentiated migration and adhesion to fibronectin of PASMC. Given the marked upregulation of LRP1 protein expression following the exposure of PASMC to PDGF-BB, one may speculate that LRP1 propagates pro-proliferative effects of over-activated PDGF signaling in PH [17,32]. A complex interplay between LRP1 and the elements of PDGF signaling is thought to control vessel wall homeostasis. On the one hand, LRP1 was found to suppress PDGFRβ phosphorylation and activation of downstream signaling molecules thus repressing migration and proliferation of vascular SMC [6,20]. On the other hand, an activation of PDGF signaling was reported to induce the formation of the LRP1-PDGFRβ complex and subsequent LRP1 phosphorylation at Tyr63 within a docking site for a number of adaptor/signaling proteins, among others those involved in the propagation of cell pro-mitogenic activities [33,34]. Interestingly, PH is characterized not only by increased growth but also motility of PASMC in response to PDGF receptor ligands . Thus, considering the increased LRP1 protein levels in PASMC exposed to PDGF-BB and its inhibitory role in PDGF-BB-induced SMC migration, LRP1 could eventually limit cell motility in a negative feedback loop manner. Taken together, by demonstrating the role of PDGF signaling in the regulation of LRP1 expression, our study reveals novel possibilities of bidirectional communication between these two pathways in vascular SMC. Our PCR array analysis of LRP1 depleted PASMC revealed a differential regulation of a number of genes involved in PH. For example, LRP1 suppressed the production of THBS2 which was shown to be down-regulated in the lungs of rats subjected to hypoxia . Conversely, LRP1 supported the expression of MMP13 and MMP14, both reported to be present at increased levels in peripheral pulmonary arteries of hypoxia-induced PH in rats  and in PASMC derived from IPAH lungs , respectively. Thus far, several studies have implicated the dysregulated expression and activity of MMPs in PH pathogenesis . An imbalance between MMPs and their inhibitors is thought to contribute to the remodeling of pulmonary arteries by affecting the ECM synthesis and degradation, changing the bioavailability of growth factors, and inducing recruitment of inflammatory cells . Accordingly, the inhibition of MMP activities in the hypoxic rats reduced the abundance of collagen-breakdown products in peripheral pulmonary arteries and decreased muscularization of pulmonary vessels and pulmonary arterial blood pressure thus markedly attenuating PH . Since LRP1 regulates the expression of MMP13 and MMP14 and additionally controls mRNA levels of collagen-encoding genes (COL11A1 and COL15A1), these data suggest that an increased LRP1 production in PASMC could contribute to the dysregulated synthesis and processing of ECM components and hence to de-differentiation of PASMC.