Regulation of Id-HLH transcription factor function in third messenger signalling

Developmental expression patterns and localization of DNA-binding protein inhibitor (Id3) in the mouse retina

Id3 (inhibitor of DNA binding/differentiation), a member of the Id helix-loop-helix protein family, has long been studied as a positive regulator of proliferation and a negative regulator of differentiation. In this study, we examined the expression pattern and cellular phenotypes of Id3 in postnatal and adult mouse retina. Id3 was mainly expressed in the early postnatal inner retina. From the late postnatal development towards adulthood, Id3 expression was confined to the ganglion cell layer and the inner nuclear layer. Colocalization analysis showed that Id3 positive cells were identified as retinal ganglion cells and amacrine cells. The differential expression profiles of Id3 provide the groundwork for the elucidation of its possible role in retinal development.

Helix-loop-helix proteins in mammary gland development and breast cancer

The basic helix-loop-helix (bHLH) family of transcription factors functions in the coordinated regulation of gene expression, cell lineage commitment, and cell differentiation in most mammalian tissues. Helix-loop-helix Id (Inhibitor of DNA binding) proteins are distinct from bHLH transcription factors in that they lack the basic domain necessary for DNA binding. Id proteins thus function as dominant negative regulators of bHLH transcription factors. The inhibition of bHLH factor activity by forced constitutive expression of Id proteins is closely associated with the inhibition of differentiation in a number of different cell types, including mammary epithelial cells. Moreover, recent literature suggests important roles of HLH proteins in many normal and transformed tissues, including mammary gland. Therefore, future directions for prognosis or therapeutic treatments of breast cancer may be able to exploit bHLH and Id genes as useful molecular targets. The purpose of this review is to summarize the evidence implicating HLH proteins in the regulation of normal and transformed mammary epithelial cell phenotypes.

Expression of the helix-loop-helix protein ID1 in keratinocytes is upregulated by loss of cell-matrix contact

To analyze the inhibitor of DNA-binding type 1 (ID1) in the human epidermis and in cultured keratinocytes we generated and characterized ID1-specific monoclonal antibodies. Immunohistological studies on human skin biopsies revealed that ID1 is not detectable in normal human epidermis but in lesional epidermis of bullous pemphigoid. In the latter case we found ID1 in the cytoplasm of basal and proximal suprabasal keratinocytes. Cultured normal human epidermal keratinocytes displayed ID1 in the cytoplasm; upon differentiation into a multilayered keratinocyte sheet, ID1 was no longer detectable. It was reexpressed after dispase-mediated detachment of the keratinocyte cultures from the growth substratum. In this case ID1 was localized to the cytoplasm and the nucleus. Our data indicate that after epidermal injury-in our case loss of cell-matrix contact-ID1 is upregulated in affected keratinocytes. In view of the ID1 function in other cell types, we speculate that ID1 facilitates the transition from the resting to the migrating and proliferating keratinocyte required for efficient repair of epidermal lesions by reepithelialization. Taken together we suggest that ID1 is an important player in epidermal (patho-)physiology.

Lymphoid-specific expression of the Id3 gene in hematopoietic cells. Selective antagonism of E2A basic helix-loop-helix protein associated with Id3-induced differentiation of erythroleukemia cells

Accumulating evidence implicates functions of the Id family of helix-loop-helix proteins in the regulation of cell growth and differentiation in metazoa. Within the mammalian hematopoietic organ, expression of the Id3 gene is restricted to the lymphoid cell compartment. We show here that in non-lymphoid hematopoietic cells, repression of transcription is correlated with hypermethylation of sequences in the vicinity of the upstream regulatory region of the Id3 gene, suggestive of a strict developmental control of expression of this gene in lymphoid versus non-lymphoid hematopoietic cells. Enforced ectopic expression of Id3 in K562 erythroid progenitor cells promotes erythroid differentiation and is correlated with a quantitative/qualitative shift in the profile of interacting TAL1 and E protein heterodimers that bind to a consensus E box sequence in in vitro band shift assays, consistent with selective targeting of E2A E protein(s) by Id3 and suggesting a possible mechanism involving TAL1-mediated differentiation. By using a Gal 4-VP16 two-hybrid competition assay and an E box-dependent reporter assay, we demonstrate directly that the E2A protein E47 preferentially associates with Id3 in vivo. These observations provide a paradigm for understanding how overlapping but distinct specificities of individual Id proteins may constitute a developmentally regulated program underlying cell determination in diverse lineages.