Biological Activities of Lasso Peptides and Structure–Activity Relationships - Lasso Peptides: Bacterial Strategies to Make and Maintain Bioactive Entangled Scaffolds - page 39

of the structures provided by nuclear magnetic resonance (NMR) and X-ray crys-
tallography has revealed important differences in conformation, especially in the
C-terminus (Wallace et al. 1995 ). X-ray indicated an α-helix structure in the 9-20
region (Janes et al. 1994 ; Janes and Wallace 1994 ), while NMR showed that the
16-21 C-terminal region has an extended β-structure and is loosely looped back to
the 9-15 α-helix by a turn (Takashima et al. 2004a ). The C-terminus is crucial for
the activity of ET-1 (Kimura et al. 1988 ; Nakajima et al. 1989 ). Several spectro-
scopic studies have indicated that this residue in close proximity to the rings of Tyr
13 and Phe 14 forms a hydrophobic core (Takashima et al. 2004a ; Takashima et al.
2004b ), which could be critical for the mechanism of action.
In the past 20 years, numerous antagonists of ETs have been developed for the
treatment of cardiovascular diseases (Dhaun et al. 2007 ; Kaoukis et al. 2013 ) and
for cancer therapy (Rosano et al. 2013 ). In particular, the mixed ET A/B receptor an-
tagonist bosentan and the selective ET A receptor antagonist sitaxsentan have been
used clinically for the treatment of pulmonary artery hypertension (Anderson and
Nawarskas 2010 ), while the ET A receptor antagonists atrasentan and zibotentan or
the mixed ET A/B receptor antagonist macitentan have demonstrated potential an-
ticancer activity in preclinical and ongoing clinical studies (Rosano et al. 2013 ).
Many antagonists developed have close assembling of their aromatic rings, sug-
gesting that the residues Trp21, Phe13 and Tyr14 of ET-1 define a pharmacophore
(Remuzzi et al. 2002 ; Funk et al. 2004 ; Takashima et al. 2004b ; Fig. 3.1a ).
Selective ET B receptor antagonists appear less promising for therapeutic applica-
tions, although certain positive effects have been reported (Lahav et al. 1999 ). Such
inhibitors provide anyway a very important tool to better understand the physi-
ological and physiopathological role of this receptor (Mazzuca and Khalil 2012 ;
Ohkita et al. 2012 ). Several peptidic and non-peptidic selective antagonists of the
ET B receptor have been described (Mazzuca and Khalil 2012 ; Fig. 3.1b ). In 1994,
Tanaka et al. reported the potent antagonist effect of RES-701-1 (Fig. 3.1c ) on the
ET B receptor (IC 50 10 nM). This effect was measured from competitive experiments
in the presence of 125 I-labelled ET-1 on bovine cerebellar membranes as well as on
membranes from Chinese hamster ovary (CHO) cells expressing the ET B receptor.
RES-701-1 was also shown to block the ET B receptor-mediated responses such as
(1) increase in the intracellular calcium concentration (in COS-7 cells expressing the
ET B receptor) and (2) blood pressure response to exogenously administered ET-1
in anaesthetized rats. By contrast, RES-701-1 did not show any antagonist effect
on the ET A receptor (IC 50 > 5 μM) as well as on various receptors (for adrenaline;
dopamine; histamine; acetylcholine; serotonin; atrial natriuretic peptide (ANP), an-
giotensin II; IC 50 > 1 µM; Tanaka et al. 1994 ). The antagonist effect of RES-701-1
on the ET B receptor was confirmed on different animal models (dog, rabbit, pig,
guinea pig, rat; Tanaka et al. 1995 ). However, the IC 50 value was much weaker in
rats (in the 1 µM range), which rendered this animal model delicate for examining
the role of ET B receptor using RES-701-1 as antagonist. The use of RES-701-1
participated in different advances in the understanding of the physiology of the ET B
receptor (Conrad et al. 1999 ; Miasiro et al. 1999 ; Gandley et al. 2001 ; Yamaguchi
et al. 2003 ; Gardner et al. 2005 ; Cervar-Zivkovic et al. 2011 ; Ji et al. 2013 ). The