Inhibitors of DNA binding proteins (ID1–ID4) are dominant negative transcriptional regulators expressed in different tissues, including lungs, kidney, cardiovascular tissue, reproductive organs, and neuronal tissues.
56–58 Inhibitors of DNA binding proteins have critical roles in early embryonic development and demonstrate overlapping function during cell cycle progression.
31,32,59 However, IDs promote cell proliferation and regulate cell differentiation depending on cell type and cellular function. For example, IDs inhibit cell differentiation in neural progenitor cells to maintain a neural cell population, while promoting natural killer (NK) cell differentiation.
58,60 Hence, expression and function of IDs are cell- and tissue-specific. Knockdown of both
ID1 and
ID3 genes is lethal for mouse embryos during development due to impaired angiogenesis and neurogenesis.
25,61 During development, the expression of ID1, ID2, and ID3 are predominant in neural crest and neural cells.
31,58 Expression of ID4 has distinctively selective functions in neuronal proliferation and differentiation.
30,62,63 Recent reports suggest that IDs have important roles in retinal development, bipolar cell lineage commitment, and differentiation and fibrotic corneal disease.
33,64,65 However, very little was known about ID expression and their role in human TM cells. To our knowledge, for first time we demonstrated basal expression of ID1, ID2, and ID3 in primary TM cells. Our published microarray data of human TM tissues suggested expression of all four IDs (ID1–4).
66 However, we failed to detect the expression of ID4 in our primary TM cell cultures. In our unpublished data in GTM3 cells, the knockdown of ID1 by siRNA upregulated ID3 expression and vice versa. In addition to our observation, several other studies have demonstrated that expression of ID1 and ID3 is highly correlated.
45,67 Heterozygous ID1/ID3 knockout mice demonstrated suppression of BMP-induced bone formation.
68 Additionally, reports show that ID1 is involved in regulation of fibrosis by inhibiting TGF-β–induced FN and PAI-1 in various cells types. Therefore, we selected ID1 and ID3 to investigate their roles in the TM cell and BMP4 signaling pathway. We demonstrated the expression of ID2 in primary human TM cell strains, and its role in TM cells should be studied further.