Inhibin, activin and follistatin bind preferentially to the transformed ...

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D J PHILLIPS and others · Hormone binding to transformed α 2 -macroglobulin
radioactivity is recovered from the liver (LaMarre et al.
1991, Philip & O’Connor-McCourt 1991, Crookston
et al. 1993, Niemuller et al. 1995). In the liver, α 2 -M–
proteinase complexes bind to receptors on hepatocytes,
leading to endocytosis and degradation of the ligand
(Gliemann & Davidsen 1986). It is unclear whether
α 2 -M–growth factor complexes targeted to hepatocytes
undergo a similar process of degradation, although recent
evidence suggests that cytokines bound to transformed
α 2 -M may have intracellular actions after being internalized
through the α 2 -M receptor (Borth 1992, Stouffer et al.
1993).
Activin has been shown in this study and previously
(Niemuller et al. 1995) to bind to both native and transformed
α 2 -M. Although activin binds with a lower capacity
to the native conformation, this form is predominant
in the circulation and is present in high abundance. As
hypothesized by others (Schneyer et al. 1992, Krummen
et al. 1993, Vaughan & Vale 1993, Niemuller et al. 1995),
native α 2 -M may play a role as a low-affinity carrier for
activin in the circulation or within tissues where follistatin,
a higher-affinity binding protein, is not abundant. The
affinity of native α 2 -M for activin has been estimated as
510 nM (Niemuller et al. 1995), whereas follistatin has an
affinity for activin in the range 0·01–0·09 nM (Nakamura
et al. 1990, Kogawa et al. 1991, Sugino et al. 1993,
Schneyer et al. 1994). Therefore consideration needs to be
made at the tissue and circulatory level not only of the
affinity of the binding proteins present, but also their
relative abundance. We are currently exploring whether
activin, when bound to α 2 -M, can subsequently be
released under conditions that favour binding to follistatin.
It is of interest that follistatin, a binding protein for
inhibin and activin, was capable of binding to transformed
α 2 -M. We examined two preparations of follistatin,
that purified from bovine follicular fluid and human
recombinant follistatin 288; both were able to bind
α 2 -M with apparently similar capacities. This binding
phenomenon may represent a mechanism whereby excess
follistatin is removed from the circulation or follistatin
is targeted to tissues expressing the α 2 -M receptor. An
intriguing question is whether complexes of activin and
follistatin are able to bind to α 2 -M. This process would be
feasible if the binding domains for α 2 -M on activin and
follistatin are not masked after formation of activin–
follistatin complexes. Although the binding domain for
α 2 -M on activin is almost certainly in an analogous position
to that for TGFβ (Qian et al. 1992), it is unclear if this site
is involved in binding to follistatin. On the other hand, a
binding site for α 2 -M on the follistatin molecule has yet to
be determined, although the regions responsible for binding
to activin have been determined (Inouye et al. 1991,
Sumitomo et al. 1995). It is important for these issues to
be resolved, particularly the implications of whether
follistatin-bound activin can or cannot be cleared via an
α 2 -M binding process.
In summary, inhibin, activin and follistatin were found
to bind to the transformed species of α 2 -M, with activin
also binding to a lesser extent to the native form. The
complexes were reversible, and as such provide potentially
important mechanisms whereby these hormones can be
distributed to various target tissues.
Acknowledgements
These studies were performed with the support of the
National Health and Medical Research Council of
Australia. We thank the National Hormone and Pituitary
Program for the provision of the human recombinant
follistatin 288.
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