N-
acetyl-seryl-aspartyl-lysyl-proline (
Ac-SDKP) is a physiological antifibrotic
peptide that is hydrolysed by
angiotensin I-converting enzyme (ACE). The beneficial antifibrotic effects of
ACE inhibitors have been attributed, in part, to its inhibition of
Ac-SDKP cleavage. There is indirect evidence that the SDK fragment of
Ac-SDKP is the main component required for its antiproliferative action. However, the exact component of the physiological
peptide that is responsible for its antifibrotic effect has yet to be determined.
Ac-SDKP-derived analogues that are resistant to ACE degradation may provide a new avenue for
fibrosis therapy. We tested the antifibrotic potential of various
Ac-SDKP peptide sequences and an analogue resistant to ACE degradation in lung fibroblasts. We investigated the contribution and molecular mechanism of action of the
amino acid residues in the
Ac-SDKP sequence to its antifibrotic effects, and the effects of
Ac-SDKP peptides in the prevention of
collagen deposition in cells. The Ac-DKP fragment moderately inhibited
endothelin-1 (ET-1) mediated
transforming growth factor-β (TGF- β) expression, and could be slowly cleaved by ACE, revealing a different sequence requirement for the antifibrotic action of
Ac-SDKP. The Ac-SDψKP analogue (where the
peptide bond between the
aspartate and
lysine is reduced) inhibited TGF-β/small mother against decapentaplegic (Smad)-3 signalling and
collagen deposition. The
Ac-SDKP peptide, in combination with ACEi, demonstrated a greater inhibition of
hydroxyproline as compared to
Ac-SDKP alone.