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Examination of the Oncogenic Potential of a Tumor-associated Antigen, Intestinal Alkaline Phosphatase, in HeLa x Fibroblast Cell Hybrids¹

Department of Microbiology and Molecular Genetics, College of Medicine, University of California-Irvine, Irvine, California 92717

An exclusive correlation exists between the ectopic expression of the cell-surface marker, intestinal alkaline phosphatase (IAP), and the tumorigenic phenotype of segregants derived from suppressed, nontumorigenic HeLa x fibroblast cell hybrids. This specific association suggests that loss of tumor suppressor function is closely linked to the reexpression of IAP and, therefore, that IAP may be a critical oncogenic factor in these cells. To address this directly, we have used a HeLa IAP cDNA expression vector (pHIAP) to introduce constitutive IAP expression into a nontumorigenic HeLa x fibroblast cell hybrid. Sequence analysis of the HeLa IAP cDNA revealed 5 separate nucleotide alterations when compared to the native IAP cDNA sequence; however, none of these resulted in amino acid substitutions. Four pHIAP transfectants were analyzed for the presence of the intact integrated IAP cDNA and their relative expression levels of exogenous IAP mRNA and protein. The functional integrity of the cDNA-derived IAP product was confirmed by demonstrating proper enzymatic activity and localization to the extracellular membrane. The tumorigenic potentials of the pHIAP transfectants were assayed by s.c. injection into athymic nude mice. No tumors were observed, even after an 11-week incubation in animals. Therefore, expression of IAP in the nontumorigenic HeLa x fibroblast cell hybrid is not sufficient to confer the tumorigenic phenotype. Although ectopic IAP expression is unlikely to be functionally relevant to tumorigenicity in these hybrids, the significance of IAP as a tumor marker is still evident from its apparent strong association with a tumor suppressor locus.

¹ This research was supported by National Cancer Institute Grants CA 19104 and CA 39312.

² To whom requests for reprints should be addressed.

Received 8/28/91. Accepted 11/13/91.

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