Design, construction, and intracellular activation of an intramolecularly self-silenced signal transduction inhibitor

Article abstract of DOI:10.1002/anie.200462004

(Figure Presented) Ricin, diphtheria toxin, and other cytotoxic proteins are two-component systems which contain a carrier protein that biochemically silences the activity of the appended "warhead" protein until it is delivered into the cell. A cell-penet

Full text of DOI:10.1002/anie.200462004

Communications
It occurred to us that the AB protein toxin mechanism by
which a highly toxic, but membrane-impermeable, protein is
delivered to a cell in an inert form might prove applicable to
other inhibitory agents as well. We describe herein a strategy
that employs a cell-penetrating peptide (CPP)[2] to 1) sup-
press the activity of an appended inhibitor, 2) deliver the
membrane-impermeable inhibitor to the intracellular envi-
ronment, and, once inside the cell, 3) release the inhibitor in
its active form.
We previously described the protein tyrosine phosphatase
(PTP) 1B inhibitor 1.[3] This species is the most potent
Drug Delivery
Design, Construction, and Intracellular Activation
of an Intramolecularly Self-Silenced Signal
Transduction Inhibitor**
Seung-Yub Lee, Fubo Liang, Xiao-Ling Guo,
Laiping Xie, Sean M. Cahill, Michael Blumenstein,
Heyi Yang, David S. Lawrence,* and Zhong-Yin Zhang*
(inhibitory constant, Ki = 2.4 nm) and selective PTP1B inhib-
The AB protein toxins are among the most potent and lethal
cell death inducing agents known. This protein family
includes such notorious cytotoxins as ricin, and the cholera,
diphtheria, tetanus, and botulinum toxins.[1] These, and
related family members, share a variety of structural and
mechanistic motifs. The A polypeptide acts on a specific
intracellular target in a fashion that compromises cellular
viability. By contrast, the B moiety serves as a transporter that
delivers the A component to the intracellular environment. In
addition, the B polypeptide is typically conjoined to the A
moiety through a disulfide bridge, which suppresses the
biological activity of the latter. However, upon intracellular
reduction of the disulfide, the A protein is released into its
fully activated state.
itor (1000–10000-fold versus a panel of phosphatases)
reported to date. The negatively charged aryldifluorophos-
phonate functional groups participate in key active-site and
near-active-site interactions that are responsible for the high
PTP1B potency displayed by 1.[4] Not surprisingly, the
negatively charged inhibitor is not cell-permeable.[5] We
anticipated that a cell-permeable derivative of 1 could be
prepared by coupling it to the CPP ((d)-Arg)8 moiety to
generate 2 (Scheme 1). Polycationic CPPs are widely used to
transport cargo through cell membranes.[2] However, we also
expected that electrostatic interactions between the positively
charged CPP and the negatively charged cargo might have the
desirable effect of suppressing inhibitory efficacy. Conse-
quently, effective inhibition of PTP1B, a cytoplasmically
oriented endoplasmic reticulum (ER)-embedded protein,[6]
should only transpire upon release of the cargo from the CPP
and only if release occurs within a PTP1B-available region of
the cell. By contrast, a corresponding construct that lacks the
disulfide bridge (5) should be unable to effectively inhibit
PTP1B.
[*] Dr. S.-Y. Lee, Dr. S. M. Cahill, Prof. D. S. Lawrence
Department of Biochemistry
Albert Einstein College of Medicine
1300 Morris Park Avenue, Bronx, NY 10461 (USA)
Fax: (+1)718-430-8565
Compound 2 was directly prepared on the Rink resin by
sequential coupling of eight (d)-Fmoc-Arg(Pbf) residues
(Pbf = 2,2,4,6,7-pentamethyldihydrobenzofuran-5-sulfonyl),
FmocNH(CH2)2SS(CH2)2CO2H, compound 3, Fmoc-Asp-
(tBu)OH, and compound 4 (Scheme 1).[7] Simultaneous
deprotection and cleavage mediated by CF3CO2H furnished
2. Compound 2 is an extraordinarily ineffective inhibitor of
PTP1B, with a Ki value (8.2 mm) that is three orders of
magnitude lower than that of 1. This appears to be a
consequence of intramolecular interactions between the
PTP1B-directed portion of 2 and its disulfide-linked ((d)-
Arg)8 appendage. NMR experiments under aqueous condi-
tions confirm that the key active-site-directed aryldifluoro-
phosphonate moiety is engaged in through-space interactions
with the ((d)-Arg)8 cell-permeabilizing sequence (see Sup-
porting Information). Furthermore, the intramolecular nature
E-mail: dlawrenc@aecom.yu.edu
Dr. F. Liang, X.-L. Guo, Dr. L. Xie, Dr. H. Yang, Prof. Z.-Y. Zhang
Department of Molecular Pharmacology
Albert Einstein College of Medicine
1300 Morris Park Avenue, Bronx, NY 10461 (USA)
Fax: (+1)718-430-8922
E-mail: zyzhang@aecom.yu.edu
Dr. M. Blumenstein
Department of Chemistry
Hunter College and
the Graduate School of the City University of New York
New York, NY 10021 (USA)
[**] This work was supported by NIH Grants DK68447 and CA095019,
and the G. Harold and Leila Y. Mathers Charitable Foundation.
Z.-Y.Z. is an Irma T. Hirschl Career Scientist.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200462004
Angew. Chem. Int. Ed. 2005, 44, 4242 –4244

Angewandte
Chemie
enhancements in IRb and IRS-1 phosphotyrosine levels
have been reported with a PTP1B inhibitor.[5]
We performed an analogous study using a Chinese
hamster oocyte (CHO) cell line transfected with an expres-
sion plasmid encoding human IR (CHO/HIRc). We previ-
ously found that compound 1 has no effect on IRb and IRS-1
phosphotyrosine levels in this cell line, since its five negative
charges render it membrane-impermeable.[5] The CHO/HIRc
cell line was exposed to compounds 2 and 5 over a selected
range of concentrations for 1 h and subsequently treated
either with or without insulin (10 nm) for 10 min. Cell lysates
were then resolved by SDS-PAGE, electrotransferred to
nitrocellulose membranes, and probed with anti-pTyr anti-
body (Figure 1). Compound 5 has no effect on IRb or IRS-1
Scheme 1. Solid-phase synthesis of the CPP-S-S-(PTB1-B inhibitor)
constructs 2 and 5. Peptides were prepared on the Rink amide resin by
using 9-fluorenylmethoxycarbonyl (Fmoc)-protected amino acids and a
standard coupling protocol involving 1-hydroxybenzotriazole/2-(1H-
benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate/
N-methylmorpholine for the activation of acids.
Figure 1. Effect of PTP1B inhibitors 5 (upper gel) and 2 (lower gel) on
the tyrosine phosphorylation status of IRS-1 and IRb. Lane a (no inhibi-
tor, without insulin); lane b (no inhibitor, with insulin); lane c (5-nm 5
and 2, without insulin); lane d (5-nm 5 and 2, with insulin); lane e (20-
nm 5 and 2, without insulin); lane f (20-nm 5 and 2, with insulin).
of this interaction is confirmed by the fact that the inhibitory
efficacy of free 1 is not affected by added ((d)-Arg)8 (at
concentrations up to 20 mm). Finally, the masking effect of the
positively charged permeabilizing sequence is due to electro-
static interactions, since a corresponding cell-permeable fatty-
acid-derivatized PTP1B inhibitor is fully active.[5]
phosphotyrosine levels, even at concentrations as high as
100 nm (data not shown). This is not surprising given that the
latter concentration is 90 times lower than the in vitro Ki of 5.
By contrast, even at 5 nm, compound 2 enhances the
phosphotyrosine content of IRb and IRS-1 1.7- and 1.4-fold,
respectively. An even more pronounced effect is observed at
20 nm (2.4- and 2.2-fold, respectively). These concentrations
are nearly three orders of magnitude lower than the Ki value
displayed by 2 under nonreducing conditions in vitro. These
results are consistent with the notion that the disulfide bridge
linking the PTP1B inhibitory component (Ki = 2.4 nm) with
the CPP has been cleaved and that the liberated inhibitor has
ready access to the cytoplasmically oriented protein target.
In summary, we have shown that a CPP module can be
used to biochemically silence the biological activity of
appended cargo.[9] This property, which is a characteristic
feature of AB protein toxins, should restrict the desired
biological effect to the targeted intracellular environment. We
are currently investigating the scope of this approach as well
as the utility of compound 2 for the treatment of diabetes and
obesity.[10]
The disulfide bridge present in 2 is absent in the otherwise
structurally equivalent derivative 5. Compound 5 was synthe-
sized in a fashion analogous to that outlined in Scheme 1 with
the exception that FmocNH(CH2)5CO2H was employed in
step 1. As expected, both compounds 2 and 5 (Ki = 9.0 mm) are
equally poor PTP1B inhibitors under nonreducing conditions.
We determined the cellular effects of compounds 2 and 5
on insulin signaling to assess the efficacy of the delivery
strategy mimicking AB protein toxins. Insulin-mediated
activation of the insulin receptor (IR) produces a wide
array of cellular responses, including enhanced phosphoryla-
tion of the insulin receptor b (IRb) subunit and the insulin
receptor substrate-1 (IRS-1). Previous studies have demon-
strated that PTP1B serves as a negative regulator of the
insulin-activated signaling pathways by catalyzing the dephos-
phorylation of IRb and IRS-1.[5] For example, the antisense-
mediated reduction of PTP1B in rat hepatoma cells generates
a 2.3-fold and 1.5-fold increase in IRb and IRS-1 phospho-
tyrosine levels, respectively.[8] In addition, significant
Received: September 15, 2004
Revised: March 7, 2005
Published online: June 2, 2005
Angew. Chem. Int. Ed. 2005, 44, 4242 –4244
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4243

Communications
Keywords: drug delivery · inhibitors · peptides · prodrugs ·
.
signal transduction
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[7] See Supporting Information for details concerning the synthesis
and characterization of compounds 2 and 5, IRb and IRS-1
phosphorylation status, and cell-culture studies.
[8] J. E. Clampit, J. L. Meuth, H. T. Smith, R. M. Reilly, M. R.
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[9] While this manuscript was under review, a paper appeared
describing a similar strategy: T. Jiang, E. S. Olson, Q. T. Nguyen,
M. Roy, P. A. Jennings, R. Y. Tsien, Proc. Natl. Acad. Sci. USA
2004, 101, 17867.
[10] Z. Y. Zhang, S. Y. Lee, Expert Opin. Invest. Drugs 2003, 12, 223.
4244
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 4242 –4244