Forkhead transcription factor HFH-4 and respiratory epithelial cell differentiation.

Abstract

The manuscript by Blatt and colleagues (1) demonstrates the nuclear localization of the winged-helix protein HFH-4 (hepatocyte nuclear factor-3/forkhead homologue-4) in subsets of epithelial cells in various organs, including the respiratory tract. The restricted temporal–spatial pattern of HFH-4 expression, prior to and coincident with ciliogenesis in the developing respiratory tract, strongly infers that HFH-4 plays a unique role in the differentiation and/ or maintenance of ciliated cells. This conclusion is further supported by observations that ciliated cells are lacking in all tissues in HFH-4-gene–targeted mice (2, 3). The HFH4-gene–targeted mice lack cilia and have indeterminant situs, findings similar to the human condition called Kartagener’s Syndrome. Colocalization of b -tubulin IV and HFH-4 in subsets of respiratory epithelial cells during development is consistent with recent findings by Tichelaar and associates (4), demonstrating that staining of HFH-4 was coincident with b -tubulin IV but not with Clara cell secretory protein (CCSP), a selective marker for nonciliated columnar epithelial cells in the respiratory tract. In addition, HFH-4 was present in the respiratory epithelium of thyroid transcription factor-1 (TTF-1) gene–targeted mice (4), demonstrating that HFH-4 marked a lineage of cells distinct from those dependent upon TTF-1, the latter being critical for differentiation of respiratory epithelial cells (5). Finally, ectopic expression of HFH-4 in subsets of peripheral respiratory epithelial cells of transgenic mice produced columnar and occasionally b -tubulin staining cells in the alveoli, a site normally lined solely by cuboidal Type II cells and squamous Type I cells (17). Taken together, these studies support an important role of HFH-4 in the specification, differentiation, and/or maintenance of the ciliated cell phenotype, likely mediated by its action in binding to and regulating various downstream gene targets at the transcriptional level. Findings with HFH-4 provide further support for the important role of various HNF-3 family members in the differentiation and formation of tissues derived from the foregut endoderm, including subsets of respiratory epithelial cells. HNF-3 b and HNF-3 a are expressed in various endodermally derived tissues, including the respiratory epithelium, and function in concert with members of at least two other distinct transcription factor families: homeodomain proteins of the Nkx2 subfamily, such as TTF-1 (or Nkx2.1), and zinc finger proteins, such as GATA-6. Interactions among members of these transcription factor families in subsets of respiratory epithelium likely play a critical role in organogenesis and differentiation of foregut endoderm–derived cells forming the respiratory tract. The winged-helix protein HNF-3 b is necessary for formation of foregut endoderm per se (6, 7). HNF-3 b and TTF-1 (Nkx2.1) are colocalized in the developing respiratory tract, and their pattern of expression is increasingly restricted to peripheral airway cells and alveolar epithelial cells with advancing development (8). HNF-3 b regulates TTF-1 gene transcription and functions in concert with TTF-1 in transcriptional regulation (9). For example, both factors bind to and regulate transcription of several downstream genes expressed selectively in bronchioles and alveolar cells, including surfactant protein B and CCSP (10–12). GATA-6 is transiently expressed at high levels in epithelial cells of the developing respiratory tract and is critical for commitment of subsets of progenitor cells to form bronchioles (13). GATA-6 influences both TTF-1 and surfactant protein gene expression by binding to and activating gene transcription (14). Thus, the precise temporal, spatial, and stoichiometric regulation of members of three families of transcription factors (HNF-3, GATA, and Nkx2) is likely to play a critical role in the formation, differentiation, and function of the complex cell types lining the respiratory tract (Figure 1). Cooperation of these groups of transcription factors appears to be an ancient theme in organogenesis and gene regulation, as it is conserved in such diverse species as Caenorhabditis elegans , mouse, and man. For example, HNF-3, GATA, and Nkx family members play an important regulatory role in the pharynx of C. elegans , where elt-2 (GATA), pha-4 (HNF-3), and ceh-22 (Nkx2) participate in a common transcriptional pathway (15). Distinct cell types that perform unique functions in airway defense, fluid regulation, injury repair, and the reduction of surface tension at the alveolar surface, line the mature respiratory tract. This diversity of function at various sites along the conducting and peripheral airways is achieved through the diversity of cell types that are likely established or influenced by transcription factors binding to and regulating downstream targets. The precise regulation of transcription factors is likely to play a critical role in lung morphogenesis, repair of the lung after injury, and ( Received in original form June 7, 1999 )