Naturally Occurring PC1 Inhibitor ProSAAS

ProSAAS was identified by Fricker et al. during an analysis of peptides not properly processed in Cpefat/Cpefat mice lacking carboxypeptidase E activity due to a point mutation in the carboxypeptidase E gene [79, 80]. These mice accumulate peptides with C-terminal Lys and/or Arg extensions. Using an affinity column, peptides with C-terminal basic residues from Cpefat/Cpefat tissues were isolated and analyzed. Five of these peptides were found to be encoded by proSAAS [81]. Subsequent overexpression of proSAAS in endocrine cells revealed its selective inhibitory effect on PC1 [81]. The proSAAS gene is located on the human and mouse chromosome X (Figure 3) and, similarly to 7B2, proSAAS is largely expressed in neuroendocrine cells and its inhibitory domains are located at the C terminus. In contrast to 7B2, which is required for the expression and secretion of active convertase PC2 [82-84], active PC1 can be expressed in cells lacking proSAAS [82-84]. Despite the absence of data on proSAAS null mice, taking together with its inhibitory role on PC1, and similarities to 7B2, proSAAS may be assumed to have other functions such as the control of the body mass blood glucose levels as recently revealed by analysis of transgenic mice expressing proSAAS [85].

3.3 Prosegments and Exogenous Inhibitors

Since the discovery of Furin, many attempts have been made to develop inhibitors to control the activity of the PCs. Initially, taking advantage of the fact that PCs are synthesized as inactive zymogens autocatalytically activated, Anderson et al., demonstrated that the prosegment of Furin, when used as a fusion protein to glutathione S-transferase, exhibits a potent in vitro inhibitory activity on Furin [86]. Previously, we found that purified prosegments and synthetic peptides derived from the prosegments of PC1, PC7 and Furin are potent inhibitors of their corresponding enzymes [87-91]. Using these inhibitors, we were able to intracellularly inhibit the processing of various PC substrates, including PDGF-A [45], PDGF-B [46] VEGF-C [47] and IGF-1 receptor (1.)

In addition to these naturally occurring inhibitors, many exogenous inhibitors were proposed to control the activity of the convertases. Of these molecules, the trypsin inhibitor and the third domain of turkey ovomucoid have been reported to be inhibitors for furin [92]. Subsequently, Garten et al. [93] have shown that acylated peptidyl chloromethane, containing a consensus furin cleavage sequence, decanoyl-Arg-Glu-Lys-Arg-COCH2Cl, that inhibits Furin activity in vitro at low micromolar concentrations to block the cleavage of influenza-virus HA. While these inhibitors were useful for study of the processing of various proteins by Furin, they appear to be unstable and unable to completely block the processing of various PC substrates in vivo due to their inefficiencies and/or decreased capability in entering cells. In 1988, Bathurst et al., and Brennan et al., proposed the use of protein-based inhibitors to control the activity of PCs [94]. They demonstrated that the variant of a1-antitrypsin, called a1-anti-trypsin Pittsburgh (a1-PIT), which has a replacement of the reactive-site Met residue by Arg, inhibits, in vitro, the processing of prealbumin by Kex2p [94]. Subsequently, the group of G. Thomas developed another variant of a:-antitrypsin, called a:-anti-trypsin Portland (arPDX), in which the reactive-site Ala-Ile-Pro-Met has been replaced by Arg-Ile-Pro-Arg. This serpin inhibits Furin in the subnanomolar range, three times lower than that a1-PIT.

Kinetic analysis showed that a portion of bound arPDX operates as a suicide inhibitor [94-97]. Once bound to Furin's active site, arPDX can either undergo proteolysis by Furin or form a kinetically trapped SDS-stable complex with the enzyme. Furthermore, when expressed in cells, arPDX, was shown to be a potent inhibitor of Furin-mediated cleavage of HIV gp160 [97], and subsequently demonstrated to inhibit all PCs involved in processing within the constitutive secretory pathway [1, 97-101]. Inhibition of PCs by arPDX has been shown to reduce the production of the APPa [102] and block the activation of the pore-forming toxin proaerolysin [103], the maturation of infectious pathogens glycoproteins [97], the proteolytic activation of BMP4 [104] and the cleavage of IGF-1R [1, 105], PDGF-A [45], PDGF-B [46] and VEGF-C [47].

In an attempt to produce other PC inhibitors, researchers mutated the bait region of the general protease inhibitor a2-macroglobulin (RVGFYESDVM690 into RVRSKRSLVM690) [106]. This variant was reported to inhibit processing of several Furin substrates including HIV type 1 glycoprotein gp160, von Willebrand factor and TGF-^1 [106]. Other inhibitors were suggested, such as the ovalbumin-type serpin human proteinase inhibitor-8, which contains two instances of the minimal Furin recognition sequence (VVRNSRCSRM343). Although this inhibitor was shown to inhibit Furin in a rapid and tight binding manner, it required the addition of a signal peptide before it could inhibit Furin in vivo [107]. Additionally, the hexa-D-arginine was reported to be a potent and relatively specific Furin inhibitor; however, it showed reduced ability to cross the cell membrane [108].

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