Enforced expression of wild-type p53 curtails the transcription of the O(6)-methylguanine-DNA methyltransferase gene in human tumor cells and enhances their sensitivity to alkylating agents.

We used isogenic human tumor cell lines to investigate the specific and direct effects of wild-type (wt) p53 on the expression of O(6)-methylguanine-DNA methyltransferase (MGMT), a DNA repair protein that confers tumor resistance to many anticancer alkylating agents. A p53-null, MGMT-proficient lung tumor cell line (H1299) was engineered to express wt p53 in a tetracycline-regulated system. High levels of p53 induction achieved by tetracycline withdrawal were accompanied by G(1) cell cycle arrest without significant apoptosis in this cell line. p53 accumulation resulted in a gradual and dramatic loss of MGMT mRNA, protein, and enzyme activity, whose levels were undetectable by day 3 of induction. The loss of MGMT protein was, however, not due to its degradation because the ubiquitin-promoted in vitro degradation of MGMT, which mediates the cellular disposal of the repair protein, was not altered by p53. Run-on transcription assays revealed a significant reduction in the rate of MGMT gene transcription. The negative regulation of MGMT expression by wt p53 was confirmed in two other human isogenic cell lines, namely, the GM47.23 glioblastoma, which contains a dexamethasone-inducible wt p53, and the H460 lung cancer cell line, in which wt p53 had been inactivated by the human papillomavirus E6 protein. Furthermore, a panel of four human tumor cell lines, including gliomas with wt p53 status, displayed markedly lower levels of MGMT gene transcripts than those having p53 mutations. Induction of wt p53 in these models led to a 3- and 2-fold increase in sensitivity to 1,3-bis(2-chloroethyl)-1-nitrosourea and temozolomide, respectively, which generate the MGMT-repairable O(6)-alkyl adducts in DNA. These results demonstrate that p53 is a negative regulator of MGMT gene expression and can create a MGMT-depleted state in human tumors similar to that achieved by O(6)-benzylguanine, a potent inhibitor of MGMT currently undergoing clinical trials. Thus, our study exposes an additional benefit associated with p53 gene therapy and provides a strong biochemical rationale for combining the MGMT-directed alkylators with p53 gene transfer to achieve improved antitumor efficacy.

DNA repair protein O6-alkylguanine-DNA alkyltransferase is phosphorylated by two distinct and novel protein kinases in human brain tumour cells.

We showed recently that human O(6)-alkylguanine-DNA alkyltransferase (AGT), an important target for improving cancer chemotherapy, is a phosphoprotein and that phosphorylation inhibits its activity [Srivenugopal, Mullapudi, Shou, Hazra and Ali-Osman (2000) Cancer Res. 60, 282-287]. In the present study we characterized the cellular kinases that phosphorylate AGT in the human medulloblastoma cell line HBT228. Crude cell extracts used Mg(2+) more efficiently than Mn(2+) for phosphorylating human recombinant AGT (rAGT) protein. Both [gamma-(32)P]ATP and [gamma-(32)P]GTP served as phosphate donors, with the former being twice as efficient. Specific components known to activate protein kinase A, protein kinase C and calmodulin-dependent kinases did not stimulate the phosphorylation of rAGT. Phosphoaminoacid analysis after reaction in vitro with ATP or GTP showed that AGT was modified at the same amino acids (serine, threonine and tyrosine) as in intact HBT228 cells. Although some of these properties pointed to casein kinase II as a candidate enzyme, known inhibitors and activators of casein kinase II did not affect rAGT phosphorylation. Fractionation of the cell extracts on poly(Glu/Tyr)-Sepharose resulted in the adsorption of an AGT kinase that modified the tyrosine residues and the exclusion of a fraction that phosphorylated AGT on serine and threonine residues. In-gel kinase assays after SDS/PAGE and non-denaturing PAGE revealed the presence of two AGT kinases of 75 and 130 kDa in HBT228 cells. The partly purified tyrosine kinase, identified as the 130 kDa enzyme by the same assays, was strongly inhibited by tyrphostin 25 but not by genestein. The tyrosine kinase used ATP or GTP to phosphorylate the AGT protein; this reaction inhibited the DNA repair activity of AGT. Evidence that the kinases might physically associate with AGT in cells was also provided. These results demonstrate that two novel cellular protein kinases, a tyrosine kinase and a serine/threonine kinase, both capable of using GTP as a donor, phosphorylate the AGT protein and affect its function. The new kinases might serve as potential targets for strengthening the biochemical modulation of AGT in human tumours.

Protein phosphorylation is a regulatory mechanism for O6-alkylguanine-DNA alkyltransferase in human brain tumor cells.

The biochemical regulation of human O6-alkylguanine-DNA alkyltransferase (AGT), which determines the susceptibility of normal tissues to methylating carcinogens and resistance of tumor cells to many alkylating agents, is poorly understood. We investigated the regulation of AGT by protein phosphorylation in a human medulloblastoma cell line. Incubation of cell extracts with [gamma-32P]ATP resulted in Mg(2+)-dependent phosphorylation of the endogenous AGT. Immunoprecipitation after exposure of the cells to 32P-labeled inorganic phosphate showed that AGT exists as a phosphoprotein under physiological conditions. Western analysis and chemical stability studies showed the AGT protein to be phosphorylated at tyrosine, threonine, and serine residues. Purified protein kinase A (PKA), casein kinase II (CK II), and protein kinase C (PKC) phosphorylated the recombinant AGT protein with a stoichiometry of 0.15, 0.28, and 0.44 (mol phosphate incorporated/mol protein), respectively. Residual phosphorylation of the endogenous AGT by the PKs present in cell homogenates and phosphorylation of the recombinant AGT by purified serine/threonine kinases, PKA, PKC, and CK II reduced AGT activity by 30-65%. Conversely, dephosphorylation of cell extracts by alkaline phosphatases stimulated AGT activity. We also identified consensus phosphorylation motifs for many cellular kinases, including PKA and CK II in the AGT protein. These data provide the first and conclusive evidence of AGT phosphorylation and suggest that reversible phosphorylation may control the activity of this therapeutically important DNA repair protein in human normal and cancer cells.