Tumor promoter induces high mobility group HMG-Y protein expression in transformation-sensitive but not -resistant cells

Elevated levels of high mobility group (HMG) nonhistone chromosomal proteins I and Y, alternatively spliced members of the HMG-I(Y) family of architectural transcription factors, have been linked with human cancer and with neo-plastic and metastatic phenotypes in model systems. To investigate whether HMG-I(Y) proteins may influence susceptibility to neoplastic transformation, HMG-I(Y) mRNA and protein levels were compared in the JB6 murine model of neoplastic progression. HMG-I(Y) mRNAs were expressed at very low levels in preneoplastic, transformation-resistant (P−) cell lines and were constitutively expressed at much higher levels in both transformation-sensitive (P+) and transformed (Tx) tumorigenic cell lines. HMG-I(Y) mRNAs were induced to higher levels by the tumor promoter 12- O -tetradecanoylphorbol acetate (TPA) and were sustained longer in P+ than in P− cells. Nevertheless, in both P− and P+ cells, primer extension analysis revealed that the same four major HMG-I(Y) gene transcription start sites were utilized with or without TPA treatment. RT–PCR revealed that there was always slightly more Y than I form mRNA present in all of the variant JB6 cell lines. Immunoblotting indicated that both HMG-I and -Y proteins increased in P+ cells in response to TPA treatment. Remarkably, in P− cells treated with TPA, only HMG-I (and not HMG-Y) protein levels increased. This unique differential TPA-induction of the HMG-Y protein in JB6 variants suggests a role for HMG-Y in mediating tumor promoter-induced neoplastic transformation. Furthermore, these results demonstrate that HMG-I and Y protein translation and/or stability is differently regulated in JB6 P− cells and provide the first indication that I and Y proteins may have different functions.