The tumor suppressor p53 inhibits tumor growth primarily through its ability to induce apoptosis. Mutations in p53 occur in at least 50% of human tumors. We hypothesized that reactivation of mutant p53 in such tumors should trigger massive apoptosis and eliminate the tumor cells. To test this, we screened a library of low-molecular-weight compounds in order to identify compounds that can restore wild-type function to mutant p53. We found one compound capable of inducing apoptosis in human tumor cells through restoration of the transcriptional transactivation function to mutant p53. This molecule, named PRIMA-1, restored sequence-specific DNA binding and the active conformation to mutant p53 proteins in vitro and in living cells. PRIMA-1 rescued both DNA contact and structural p53 mutants. In vivo studies in mice revealed an antitumor effect with no apparent toxicity. This molecule may serve as a lead compound for the development of anticancer drugs targeting mutant p53.

Restoration of wild-type p53 expression triggers cell death and eliminates tumors in vivo. The identification of mutant p53-reactivating small molecules such as PRIMA-1 opens possibilities for the development of more efficient anticancer drugs. Although the biological effects of PRIMA-1 are well demonstrated, little is known about its molecular mechanism of action. We show here that PRIMA-1 is converted to compounds that form adducts with thiols in mutant p53. Covalent modification of mutant p53 per se is sufficient to induce apoptosis in tumor cells. These findings might facilitate the design of more potent and specific mutant p53-targeting anticancer drugs.

Dioxins and other polycyclic aromatic compounds formed during the combustion of waste and fossil fuels represent a risk to human health, as well as to the well being of our environment. The primary sequence and structure of AhR 3 in different species have been highly conserved during the course of evolution, suggesting that this receptor, which belongs to the family of basic helix-loop-helix nuclear transcription factors, plays an important physiological role(s) in homeostatic processes (1, 2). However, despite intensive studies, this physiological role and the endogenous activators of the AhR remain to be elucidated (3). At the cellular level, activated AhR interacts with various signal transduction pathways; induces biotransformation enzymes; alters the cell cycle, cell adhesion, and migration; and causes apoptosis or aberrant cell growth (4 -7). In vivo, the AhR plays significant roles in connection with development, immunological and reproductive functions, and adaptive responses to light and xenobiotics (8 -11). Compounds of this nature exert carcinogenic and endocrinedisrupting effects in experimental animals by binding to the orphan aryl hydrocarbon receptor (AhR). Understanding the mechanism of action of these pollutants, as well as the physiological role(s) of the AhR, requires identification of the endogenous ligand(s) of this receptor. We reported earlier that activation of AhR by ultraviolet radiation is mediated by the chromophoric amino acid tryptophan (Trp), and we suggested that a new class of compounds derived fromThe AhR protein has been shown to bind the compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as well as similar highly lipophilic halogenated and non-halogenated hydrocarbons leading to cardiovascular, carcinogenic, and endocrine effects (12-16). Consequently, most studies designed to explore the functions of this receptor have been performed with TCDD. One remarkable feature of activation of AhR by dioxinlike compounds is sustained induction of both cytochromes P450 and other metabolizing enzymes, whereas other agonists cause only transient induction of these enzymes. It seems likely that for purposes of regulation, endogenous ligands of AhR are metabolized rapidly, so that the use of persistent xenobiotics such as dioxins to investigate this receptor might be inappropriate. A striking discrepancy between the effects of different types of AhR activators was made evident in two recent studies (17,18). The authors showed that FICZ, the suggested physiologic AhR ligand, boosted T H 17-cell differentiation and worsened the experimentally induced autoimmune encephalomyelitis, whereas TCDD increased levels of T reg and suppressed the pathological effects in myelin-immunized mice.* This work was supported by Swedish Research Council (Formas), the Swedish Radiation Safety Authority, Karolinska Institutet, and the Sven and Lily Lawskis Fund. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertiseme...

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Mutant p53-carrying tumors are often more resistant to chemotherapeutical drugs. We demonstrate here that the mutant p53-reactivating compound PRIMA-1 MET acts synergistically with several chemotherapeutic drugs to inhibit tumor cell growth. Combined treatment with cisplatin and PRIMA-1 MET resulted in a synergistic induction of tumor cell apoptosis and inhibition of human tumor xenograft growth in vivo in SCID mice. The induction of mutant p53 levels by chemotherapeutic drugs is likely to increase the sensitivity of tumor cells to PRIMA-1 MET . Thus, the combination of PRIMA-1 MET with currently used chemotherapeutic drugs may represent a novel and more efficient therapeutic strategy for treatment of mutant p53-carrying tumors.