Angiogenesis is essential during development or when tissue restoration and oxygenation is required. Limited or excessive formation of blood vessels is a hallmark of several pathologies, and many angiogenesis-related pathways are being studied to highlight potential targets for effective angiogenesis-stimulating or inhibiting therapeutic approaches. A few studies point to the adrenergic system as a significant regulator of angiogenesis, directly or indirectly. Functional adrenergic receptors are expressed on endothelial cells and affect their response to the adrenergic system. The latter can also upregulate the release of growth factors by mural cells of the vessel wall, blood cells or cancer cells, thus subsequently affecting endothelial cell functions and angiogenesis. In the present study we summarize up-to-date literature on the known effects of the adrenergic receptors on physiological and pathological angiogenesis.

Pleiotrophin (PTN) is a secreted factor that regulates endothelial cell migration through protein tyrosine phosphatase receptor zeta 1 (PTPRZ1) and α_vβ₃ integrin. Genetic deletion of Ptprz1 results in enhanced endothelial cell proliferation and migration, due to the decreased expression of β₃ integrin and the subsequent, enhanced cMet phosphorylation. In the present study, we investigated the effect of PTN and PTPRZ1 on activating the mTORC1 kinase and protein synthesis and identified part of the implicated signaling pathway in endothelial cells. PTN or genetic deletion of Ptprz1 activates protein synthesis in a mTORC1-dependent manner, as shown by the enhanced phosphorylation of the mTORC1-downstream targets ribosomal protein S6 kinase 1 (SK61) and 4E-binding protein 1 (4EBP1) and the upregulation of HIF-1α. The cMet tyrosine kinase inhibitor crizotinib abolishes the stimulatory effects of PTN or PTPRZ1 deletion on mTORC1 activation and protein synthesis, suggesting that mTORC1 activation is downstream of cMet. The mTORC1 inhibitor rapamycin abolishes the stimulatory effect of PTN or PTPRZ1 deletion on endothelial cell migration, suggesting that mTORC1 is involved in the PTN/PTPRZ1-dependent cell migration. The α_vβ₃ integrin blocking antibody LM609 and the peptide PTN_112–136, both known to bind to α_νβ₃ and inhibit PTN-induced endothelial cell migration, increase cMet phosphorylation and activate mTORC1, suggesting that cMet and mTORC1 activation are required but are not sufficient to stimulate cell migration. Overall, our data highlight novel aspects of the signaling pathway downstream of the PTN/PTPRZ1 axis that regulates endothelial cell functions.