Purpose: Developing new anticancer therapeutic regimens requires the measurement of tumor cell growth in response to treatment. This is often accomplished by injecting immunocompromised mice with cells from cancer tissue or cell lines. After treating the animals, tumor weight or volume is measured. Such methods are complicated by inaccuracies in measuring tumor mass and often animals must be killed to measure tumor burden. An in vivo tumor model system is presented in which the tumor cell line was stably transfected with a constitutively expressed marker gene: secreted human placental alkaline phosphatase protein (SEAP). The SEAP gene codes for a heat-stable protein that is produced at levels proportional to the amount of tumor cells in the animal. The SEAP protein is detectable in small blood samples so that animals can be repeatedly sampled over the trial period to monitor the course of tumor progression.
Methods: OCC1 ovarian carcinoma cells were stably transfected with pCMV-SEAP. The OCC1-SEAP cells were maintained in vitro to monitor the relationship between cell number and SEAP production. Experiments were performed in vivo to determine whether SEAP levels in blood corresponded to tumor burden. OCC1-SEAP cells were injected s.c. or intraperitoneally into nude mice and tumor volume was measured as well as plasma SEAP levels as the tumors developed.
Results: S.c. tumor volume correlated well with plasma SEAP levels ( R(2)=0.95). OCC1-SEAP cells were also injected intraperitoneally into nude mice and grown as abdominal tumors. After 3 weeks the animals were killed and the tumors were dissected and weighed. SEAP levels in plasma samples from the time of death correlated with intraperitoneal tumor weight ( R(2)=0.87). Experiments were performed to determine whether measuring SEAP levels could be used to monitor ovarian carcinoma cell response to platinum-containing chemotherapeutic drugs. OCC1-SEAP cells cultured in vitro were treated with the platinum-containing drug carboplatin. Carboplatin treatment decreased both cell proliferation and SEAP levels in culture medium. The constitutive rate of SEAP secretion per cell (nanograms SEAP per microgram DNA) was found not to be altered by carboplatin treatment. Therefore changes in SEAP level reflect changes in OCC1 tumor cell number, and not changes in regulation of SEAP secretion due to platinum containing chemotherapeutic drug treatment. OCC1 cells were injected intraperitoneally into nude mice and the mice were treated with the platinum-containing drugs cisplatin or carboplatin. Measurements of plasma SEAP over the treatment period showed OCC1-SEAP ovarian carcinoma growth to be inhibited by cisplatin and carboplatin treatment.
Conclusion: The SEAP marker protein is constitutively expressed by tumor cells and blood levels are correlated with tumor cell number and burden. The results of these studies indicate that SEAP may be used as an in vivo reporter gene in a mouse model to monitor tumor growth and response to therapeutics. Future studies will utilize this model to investigate novel chemotherapeutic approaches to treating ovarian cancer.