PARATHYROID HORMONE-RELATED PROTEIN GENE-EXPRESSION IN HUMAN SQUAMOUSCARCINOMA-CELLS IS REPRESSED BY MUTANT ISOFORMS OF P53

Parathyroid hormone-related protein (PTHrP) is a normal secretory prod uct of a variety of squamous epithelia, including epidermal keratinocy tes. Only a subset of squamous carcinomas, however, express the gene a t levels sufficient to cause humoral hypercalcemia. In the present stu dy, comparison of PTHrP expression levels with p53 functional status i n a series of squamous carcinoma lines has revealed an association bet ween expression of specific mutant isoforms of p53 and very low levels of PTHrP mRNA. Evaluation of p53 isoforms with mutations in codons 24 8 and 273 showed them to be capable of repressing PTHrP gene expressio n in a high-expressing, p53-negative squamous line by approximately 50 %. Conversely, inactivation of an endogenous mutant p53 with E1B prote ins resulted in an increase in PTHrP expression in a low-expressing ce ll line. Subsequent analysis of promoter-specific PTHrP transcripts in a p53-negative squamous line transfected with mutant p53 isoforms sug gested that down-regulation occurred primarily at the two TATA-based p romoters. Direct testing of a murine PTHrP reporter construct in trans ient transfection assays confirmed the capacity of the 248 and 273 mut ants to repress this TATA-based promoter, although only about half as effectively as wild-type p53.