The Myb-p300 Interaction Is a Novel Molecular Pharmacologic Target.

There are large and growing numbers of molecularly targeted agents under study in oncology. Yet, the nature of the pathways targeted in the cancer clinic should be broadened (1). Success stories include the discoveries of antagonists that affect growth factor receptors and enzymes that are either overexpressed or mutated relative to their normal counterparts. These proteins function as oncogenic changes that drive the growth, survival, or abnormal differentiation state of a specific cancer. Clinical examples of this includeHER2-targeted treatments of breast cancer (2), inhibitors of BRAF in melanoma (3), or antagonists of the epidermal growth factor receptor (EGFR) and its tyrosine kinase in lung cancer (4), to cite a few of many possible ones. However, most cancers diagnosed inpatients donot harbor such alterations. Hence, other biochemical pathways that interfere with aberrant growth of cancers must be found. One way to expand the spectrum of such targets is to disrupt protein–protein interactions. This is the case when cancers depend on them for their growth, maturation, or survival. Promising antineoplastic strategies would then be uncovered. Indeed, targets once considered intractablewouldbecomeexposed. This is why the study by Uttarkar and colleagues (5) is noteworthy. They revealed a way to interfere with the oncogenic effects of one of the earliest known oncogenes,Myb (6). This oncogenewas previously considered an unattractive pharmacologic target. They accomplished the feat of showing that Myb is amenable to interference by exploiting a critical c-Myb protein–protein interaction. This builds on prior work that showed Myb-dependent gene expression was inhibited by the sesquiterpene lactone mexicanin-1 (7). Myb functions as a transcription factor via binding to its genomic binding site t/cAACt/gG (8). Myb also interacts with the coactivators CBP or p300 to regulate gene expression (9, 10). The KIX domain of p300 interacts with a LXXLL motif in the transactivation domain of c-Myb (11). More than 80 Myb target genes exist and are placed into three functional groups. These include housekeeping genes such as MAT2A and GSTM1, cell lineage and differentiation gene products as, for instance, ELA2, MIM1, CD4, and PTCRA as well as species involved in the carcinogenesis process such as MYC, Cyclin A1, Cyclin E, KIT, BCL2, HSPA5, and GATA3 (12, 13). Myb plays critical roles in lineage determination, stem and progenitor cell proliferation, as well as in controlling differentiation (6). It is a major species in determining hematopoietic lineage in lymphoid and erythroid cells (14). Immature or progenitor-like cells typically have high levels of Myb, whereas differentiated cells have low levels (15). Myb is a key regulator of the adult colonic crypt. Loss or repression of Myb in the colon results in reduced crypt size and proliferation, increased goblet cells, and decreased enterocytes as well as enteroendocrine cells (16). Myb was initially identified as a retroviral oncogene (v-Myb) of avian myeloblastosis virus (17). It has vital functional roles in diverse malignancies. For example, Myb is highly expressed in hematopoietic progenitor cells, and its forced overexpression blocks differentiation and promotes leukemic cell transformation (15). Leukemic cells often depend on high basal c-Myb protein expression for their survival (18). Myb also has a major role in the development of several solid tumors. It is overexpressed in more than 80% of colorectal cancers (19). This expression profile predicts an unfavorable clinical outcome (20). Myb is detected in subsets of breast cancers and is associated with expression of the estrogen receptor-a (ERa; ref. 21) in part because Myb is a direct ERa target (22). Myb oncogene addiction in leukemia and its expression in other cancers provided a rational basis for targeting Myb in cancer therapeutics. Naphthol AS-E phosphate is an organophosphate. It was first used in histochemical experiments to quantify alkaline and acid phosphatase activities in polyacrylamide membrane model systems (23). Its biologic activity was found when naphthol AS-E phosphate was determined to disrupt the interaction between the CREB and CBP complex and attenuate target gene expression in response to exposure to a cAMP agonist (24). Naphthol AS-E phosphate is now shown to interfere with binding between the KID domain of CREB and the KIX domain of CBP (5). This intriguing work established that a small molecule is able to disrupt this protein–protein interaction in the nucleus. This interferes with subsequent signaling cascades. In turn, this establishes a biochemical basis for targeting interactions between Myb and p300, because the KIX domain of p300 complexes with Myb in hematopoietic cells (25). Uttarkar and colleagues (5) used the bacterial autodisplay assay to discern an interaction between Myb and the KIX domain of p300. They built on this experimental approach by showing that naphthol AS-E phosphate antagonizes this association and by this represses Myb transcriptional activity. Using microscale thermophoresis, these investigators determined the dissociation constant for the Myb–KIX association ( 2.5 mmol/L) and IC50 for inhibition of the Myb–KIX interaction by naphthol AS-E phosphate ( 30 mmol/L). These findings were extended by studying the humanmyeloid leukemia cell lines HL60, U937, and NB4. These studies determined that treatment with naphthol AS-E phosphate is able to trigger differentiation and to reduce expression of c-Myc Department of Thoracic/Head and Neck Medical Oncology, The University of TexasMDAndersonCancer Center, Houston,Texas. Department of Cancer Biology,The University of Texas MDAnderson Cancer Center, Houston, Texas.