Signal transduction involving the dmp1 transcription factor and its alteration in human cancer

Abstract

Dmp1 (cyclin D-interacting myb-like protein 1; also called Dmtf1) is a transcription factor that has been isolated in a yeast two-hybrid screen through its binding property to cyclin D2. Dmp1 directly binds to and activates the Arf promoter and induces Arf-p53-dependent cell cycle arrest in primary cells. D-type cyclins usually inhibit Dmp1-mediated transcription in a Cdk-independent fashion; however, Dmp1 shows synergistic effects with D-cyclins on the Arf promoter. Ras or Myc oncogene-induced tumor formation is accelerated in both Dmp1(+/-) and Dmp1(-/-) mice with no significant differences between Dmp1(+/-) and Dmp1(-/-). Thus, Dmp1 is haplo-insufficient for tumor suppression. Tumors from Dmp1(-/-) or Dmp1(+/-) mice often retain wild-type Arf and p53, suggesting that Dmp1 is a physiological regulator of the Arf-p53 pathway. The Dmp1 promoter is activated by oncogenic Ras-Raf signaling, while it is repressed by physiological mitogenic stimuli, overexpression of E2F proteins, and genotoxic stimuli mediated by NF-κB. The human DMP1 gene (hDMP1) is located on chromosome 7q21 and is hemizygously deleted in approximately 40% of human lung cancers, especially those that retain normal INK4a/ARF and P53 loci. Thus, hDMP1 is clearly involved in human carcinogenesis, and tumors with hDMP1 deletion may constitute a discrete disease entity.

Keywords: Arf; Dmp1; Ras; haplo-insufficiency; lung cancer; p53.

Figure 1. Structure and protein interacting surfaces of Dmp1 and cyclin D1

Top. The structure of the Dmp1 transcription factor. Dmp1 consists of 761 amino acids in mice and 760 amino acids in humans, and undergoes extensive posttranslational modification. It has a central DNA-binding domain with three Myb-like repeats that are essential for DNA binding. Mutation of lysine at 319 to glutamic acid abolishes its DNA binding. Bottom. Cyclin D1 binds to its catalytic partner Cdk4 through the cyclin box (orange box, amino acid residues 56 to 152). Mutation of lysine at 112 or 114 to glutamic acid abolishes its binding property to Cdks. Cyclin D1 binds to Dmp1 through its carboxyl-terminal half that overlaps the region for estrogen receptor interaction (Inoue and Sherr, 1998; Zwijsen et al. 1997). Cyclin D1 also interacts with TAF_II250 (Adnane et al. 1999) and androgen receptors (Reutens et al. 2001). Abbreviations: ER: estrogen receptor; SRC: steroid receptor coactivator; TAF_II250: TATA-binding protein-associated factor; AR: androgen receptor.

Figure 2. Tumors found in Dmp1-deficient mice

A: mandibular carcinosarcoma (Dmp1^−/−, untreated, 76 weeks); B: T-cell leukemia/lymphoma (Dmp1^+/−, untreated, 48 weeks); C: lung adenocarcinoma (Dmp1^−/−, untreated, 86 weeks); D: ovarian granulose cell tumor (Dmp1^−/−, DMBA-treated, 26 weeks); E: hepatocellular carcinoma (Dmp1^+/−, irradiated, 61 weeks); and F: malignant melanoma (Dmp1^−/−, DMBA-treated, 39 weeks). These tumors were not observed in the Dmp1^+/+ littermate controls of the same age (Inoue et al. 2000, 2001).

Figure 3. Signaling pathways involving Dmp1

Arf is induced by potentially oncogenic signals stemming from overexpression of oncogenes such as c-Myc, E2F-1, and activated Ras. This induction quenches inappropriate mitogenic signaling by diverting incipient cancer cells to undergo p53-dependent and -independent growth arrest or cell death. Arf expression is repressed by a number of nuclear proteins, such as Bmi1, Twist, Tbx2/3, and Pokemon. Dmp1 is unique in that it directly binds and activates the Arf promoter and induces cell cycle arrest in an Arf-dependent fashion. Both Dmp1-null and heterozygous mice show hypersensitivity to develop tumors in response to carcinogen DMBA and γ-irradiation. This phenotype could be explained by the inactivation of the Arf-Mdm2-p53 pathway in the absence of the functional Dmp1 protein, although it is possible that Dmp1 has targets other than Arf. D-type cyclins inhibit Dmp1’s transcriptional activity in a Cdk-independent fashion when E2Fs do not bind to the same promoter; however, D-cyclins cooperate with Dmp1 to activate the Arf promoter. The Dmp1 promoter is efficiently activated by the oncogenic Ras-Raf-MEK-ERK-Jun pathway but is repressed by overexpressed c-Myc, E2Fs, and by physiological mitogenic signaling. The induction of Arf by oncogenic Ras is largely dependent on Dmp1. We recently reported that the Dmp1-Arf pathway was inhibited by NF-κB proteins in response to genotoxic stress signaling (Taneja et al. 2007).

Figure 4. Deletion of hDMP1 is a new category of human lung cancer

Fifty-one cases of human non-small cell carcinoma (NSCLC) were studied for loss of heterozygosity (LOH) with 6 sets of PCR primers (2 sets for the hDMP1 locus, 2 sets for the INK4a/ARF locus, and 2 sets for the P53 locus) (Mallakin et al. 2007). The numbers show the percentage of lung cancer samples that showed LOH for each tumor suppressor locus with our relaxed criteria, i.e. the LOH values showed >2.0 or <0.5 with one of the two sets of primers. The numbers in parenthesis show the percentages of LOH cases that do not overlap LOH of other loci. Eighty-seven percent of NSCLC showed LOH with at least one of these sets of primers. LOH of hDMP1 occurred in a mutually exclusive fashion with LOH of INK4a/ARF or that of P53 in most cases. On the other hand, a significant percentage of samples showed LOH for both INK4a/ARF and P53.