Inhibitor of differentiation 4 (ID4): From development to cancer

Abstract

Highly conserved Inhibitor of DNA Binding (ID1-4) genes encode multi-functional proteins whose transcriptional activity is based on dominant negative inhibition of basic helix-loop-helix (bHLH) transcription factors. Initial animal models indicated a degree of compensatory overlap between ID genes such that deletion of multiple ID genes was required to generate easily recognizable phenotypes. More recently, new model systems have revealed alterations in mice harboring deletions in single ID genes suggesting complex gene and tissue specific functions for members of the ID gene family. Because ID genes are highly expressed during development and their function is associated with a primitive, proliferative cellular phenotype there has been significant interest in understanding their potential roles in neoplasia. Indeed, numerous studies indicate an oncogenic function for ID1, 2 and 3. In contrast, the inhibitor of differentiation 4 (ID4) presents a paradigm shift in context of well-established role of ID1, 2 and 3 in development and cancer. Apart from some degree of functional redundancy such as HLH dependent interactions with bHLH protein E2A, many of the functions of ID4 are distinct from ID1, 2 and 3: ID4 proteins a) regulate distinct developmental processes and tissue expression in the adult, b) promotes stem cell survival, differentiation and/or timing of differentiation, c) epigenetic inactivation/loss of expression in several advanced stage cancers and d) increased expression in some cancers such as those arising in the breast and ovary. Thus, in spite of sharing the conserved HLH domain, ID4 defies the established model of ID protein function and expression. The underlying molecular mechanism responsible for the unique role of ID4 as compared to other ID proteins still remains largely un-explored. This review will focus on the current understanding of ID4 in context of development and cancer.

Induction of Lumen Formation in a Three-dimensional Model of Mammary Morphogenesis by Transcriptional Regulator ID4

“…Because ID4 has the general structural motif of bHLH transcription factors, but lacks a DNA binding domain, it has been hypothesized that it functions by forming inactive heterodimers with bHLH transcription factors (9). Although several targets of ID4 have been identified (9), including ID1-3 (10), the question arose in our study, how does CEACAM1 up-regulate ID4?…”

“…In addition, mutation of two residues (Thr-457 and Ser-459) to alanine (double alanine or DA mutant) in the cytoplasmic domain of the short isoform abrogated its lumen formation function (7). Among the various up-regulated genes that were differentially expressed between the short isoform, wild type CEACAM1-transfected MCF7 cells (SW) and the double mutant, nonlumen forming CEACAM1-transfected cells (DA), ID4 was identified for further study due to its postulated role as a tumor suppressor in several cancers, including breast cancer (9). When ID4 was overexpressed in parental MCF7 cells, lumen formation was observed, whereas when ID4 expression was inhibited with RNAi in SW cells, lumen formation was inhibited, thus, confirming ID4 as a viable target for further study in lumenogenesis.…”

“…on its down-regulation in many types of cancers, ID4 has been proposed to play a role as a tumor suppressor gene (9). In this regard, whereas normal breast cells express both CEACAM1 and ID4, breast cancer cells fail to express either.…”

“…Strikingly, ID1-3 have ascribed oncogenic properties, although ID4 is a putative tumor suppressor gene, due to inhibition of bHLH transcription factors, and perhaps due to its formation of heterodimers with ID1-3 (10). Moreover, ID4 has been shown to play an important role in breast, prostate, and colon (9) cancers, all tissues that show high CEACAM1 expression in normal and low expression in malignant tissue. On the other hand, ID4 is expressed in a subset of mammary basal cells as a marker of stemness, acting upstream of Notch signaling (11).…”

“…, known as inhibitor of differentiation 4 (9), is a member of a gene family including ID1-4, all of which resemble basic helix-loop-helix (bHLH) transcription factors, but lack a DNA binding domain, thus suggesting their activity is based on a dominant-negative effect where they form inactivating heterodimers with functional bHLH transcription factors (9). Strikingly, ID1-3 have ascribed oncogenic properties, although ID4 is a putative tumor suppressor gene, due to inhibition of bHLH transcription factors, and perhaps due to its formation of heterodimers with ID1-3 (10).…”