Recombinant
immunotoxins (RIT) are agents being developed for
cancer treatment. They are composed of an Fv that binds to a
cancer cell, fused to a 38-kDa fragment of Pseudomonas
exotoxin A. SS1P is a RIT that targets
mesothelin, a
protein expressed on
mesothelioma as well as pancreatic, ovarian, lung, and other
cancers. Because the
protein tyrosine kinase family regulates a variety of cellular processes and pathways, we hypothesized that
tyrosine kinases might regulate susceptibility to
immunotoxin killing. To investigate their role, we used siRNAs to lower the level of expression of the 88 known
tyrosine kinases. We identified five
tyrosine kinases, INSR, HCK, SRC, PDGFRβ, and BMX that enhance the activity of SS1P when their level of expression is lowered by siRNAs. We further investigated the Src family member HCK in this study. Knocking down of SRC slightly increased SS1P killing in A431/H9 cells, but knocking down HCK substantially enhanced killing by SS1P. We investigated the mechanism of enhancement and found that HCK knockdown enhanced SS1P cleavage by
furin and lowered levels of Mcl-1 and raised Bax. We then found that Src inhibitors mimic the stimulatory effect of HCK knockdown; both
SU6656 and
SKI-606 (
bosutinib) enhanced
immunotoxin killing of
mesothelin-expressing cells by SS1P and CD22-expressing cells by HA22 (
moxetumomab pasudotox).
SU6656 also enhanced the antitumor effects of SS1P and HA22 in mouse xenograft
tumor models. Our data suggest that the combination of
immunotoxin with
tyrosine kinase inhibitors may be an effective way to treat some
cancers.