
IN VIVO FIBROSARCOMA GENE TARGETING BY GRP78 PROMOTER
izations were performed with [35S]UTP-labeled "antisense" RNA
probes able to detect endogenous grp78 and neo mRNA. To monitor
for background hybridizations, equivalent sections were hybridized
with the corresponding "sense" probes labeled to equal specific ac
tivities. Previously, with the use of RNA blots, it has been shown that
grp78 mRNA levels are elevated in radiation-induced fibrosarcoma
tumors, correlating with the size of the tumors (8). Here, using in situ
hybridization, we observed strong hybridization of the grp78 probe to
the tumor sections, in contrast to the uniform low background ob
tained with the control probe (Fig. 3A, compare sections 4 and 5). The
level of grp78 was much lower in the muscle sections surrounding the
tumor (data not shown). Upon a higher stringency wash, we detected
pockets of higher intensity grp78 mRNA levels in localized, center
regions of the tumor (Fig. 3A, Sections 1-3). Similar results were
observed with the neo probe in tumors derived from pHa-
MAGRP.neo-transduced cells, neo expression was much enhanced at
or near the center of the fibrosarcoma, and the overall neo level was
higher than that of the background control (Fig. 3B), suggesting that
the internal grp78 promoter in the retroviral vector was effective in
conferring similar inducibility to the reporter gene. The neo mRNA
levels in cells transduced with the SV40 internal promoter were
uniformly low. Their levels were comparable to, or at best, minimally
higher than that of the background control (Fig. 3C), showing low
SV40 promoter activity in these tumor environments. Microscopic
examination revealed that the fibrosarcomas derived from
vHaMAGRP.neo and vHaMASV.neo tumors were similar in appear
ance (data not shown). Within the central portion of the tumors were
areas of necrosis associated with polymorphonuclear leukocytes.
Thus, the enhanced neo expression in these regions in the
vHaMAGRP.neo tumors strongly suggests that the cells were expe
riencing a stress response resulting in the specific activation of the
grp78 promoter but not of the SV40 promoter.
These combined results demonstrate that a 600-base pair subfrag
ment of the grp78 promoter, as an internal promoter within a HaMSV/
MDR\ retroviral vector, is able to confer high level expression of a
reporter gene in a murine fibrosarcoma in vivo. Unlike other viral or
cellular promoters which express constitutively in normal and malig
nant cells, the stress-inducible grp78 promoter offers a novel approach
for targeted gene therapy for tumors in vivo. The truncated grp78
promoter described here may be useful to express a wide range of
therapeutic reagents in retroviral vectors or other gene delivery sys
tems in a variety of malignantly transformed cells and tumors which
exhibit properties of increased glucose utilization and anaerobic gly-
colysis. Possible applications could include the expression of tumor
suppressor genes, foreign MHC genes, or cytokines (28, 29) to induce
cell death or inflammation at the site of a progressively growing
tumor, leading to systemic immunity against metastasized tumor cells.
Since most anticancer agents are extremely toxic when expressed at
high levels in normal cells, the discovery of a candidate promoter,
such as that of grp78, with stringently enhanced expression in a tumor
environment, could advance cancer gene therapy. By coupling the
grp78 promoter to a delivery system which can effectively target
tumor cells, the cumbersome and expensive requirement for near
absolute targeted tumor delivery will be alleviated.
Acknowledgments
We thank Dr. GüntherDennert for helpful discussions and assistance in s.c.
injection. Colin Jamora and Mariane Metz provided excellent technical assist
ance. We are grateful to Xiaomei Cui and Dr. Charles Schuler for tumor
sectioning. We thank Drs. W. French Anderson and GüntherDennert for
critical review of the manuscript.
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