THE GENE ENCODING THE P53-REGULATED INHIBITOR OF CDKS (PIC1) IS NOT EXPRESSED IN THE MOLT-4 LEUKEMIA-CELL LINE WITH P53 TRUNCATED AT THE CARBOXYL-TERMINUS, AND HARBORS A NUCLEOTIDE SUBSTITUTION AT CODON-31 IN SEVERAL OTHER CANCER CELL-LINES

Entry into the cell cycle is governed by cyclins, cyclin-dependent kin ases (CDKs) and CDK-inhibitors (CDKIs). The p53-regulated inhibitor of CDKs (PIC1) is a universal CDKI whose gene expression is directly ind uced by the p53 tumor suppressor protein. Reverse transcription and po lymerase chain reaction revealed strong PIC1 gene expression in contro l MRC-5 human embryo lung cells, but relatively weaker bands in A549 l ung carcinoma; Hep3B, Mahlavu, PLC/PRF/5 hepatocellular carcinoma; SiH a, CaSki, HeLa cervical carcinoma; T24 bladder carcinoma; MCF7 breast carcinoma; Raji Burkitt lymphoma; HT-1080 fibrosarcoma; and G-401 Wilm s' tumor cell lines. These data are consistent with other results obta ined by Northern and Western blot and immunoprecipitation techniques, indicating diminished PIC1 expression in cancer cells especially those harboring mutated p53, or human papillomavirus E6 oncoproteins which abrogate p53 activity. PIC1 gene expression was absent in the Molt-4 T -lymphoblastic leukemia cell line with a previously documented alterna tively-spliced p53 transcript translating into p53 protein truncated a t the carboxyl terminus. It is proposed that this aberrant p53 interfe res with the binding of wild-type p53 and other transcription factors to the PIC1 promoter thereby abolishing PIC1 gene expression. This Mol t-4 cell line could serve as a useful experimental system for studying the interaction between p53 and other cellular factors with the PIC1 gene. Single-strand conformation polymorphism and direct cycle DNA seq uencing analyses demonstrated a PIC1 variant (with an AGC to AGA subst itution at codon 31 culminating in a serine to arginine replacement) i n Mahlavu, PLC/PRF/5, SiHa, A549 and Raji cell lines. The higher propo rtion of the PIC1 variant in cancer cell lines (5/13 or 38%) compared with normal individuals (14%), coupled with differences between the pr edicted secondary structures of the normal and variant PIC1 proteins m erit further investigations to elucidate the biological significance o f this variant.