bud tips are induced to condense into a pre-tubular aggregate, and
undergo MET to form the primitive epithelial renal vesicle, which
then elongates and fuses to the ureteric bud tip to form the nascent
nephron [12]. Another example of developmental MET occurs
during mammary gland morphogenesis in puberty (Fig. 2, bottom
panel). Mammary epithelial stem/progenitor cells residing at the
tips of terminal end buds (TEBs) acquire migratory ability through
partial-EMT, transiently losing apical-basal polarity and reducing
intercellular adhesive junctions to invade the surrounding fat pad,
and subsequently revert to an epithelial state through MET to drive
ductal morphogenesis [13, 14].
4 Establishment and Maintenance of the Epithelial Transcriptional Program
Epithelial cells are widely believed to precede the mesenchymal
state in both evolutionary and ontological terms [15]. Accordingly,
the epithelial state has been hypothesized to represent a “default”
state [10]. Early work on the E-cadherin promoter elucidated
regulatory control consistent with a default state of transcriptional
activation. The CDH1 promoter lacks a TATA-box, usually associated with tissue-specific gene expression [16], but possesses discrete positive regulatory components comprising a CCAAT-box, a
GC-rich region, a CE-box, and an enhancer in the first intron [17–
21]. These regions are recognized by ubiquitous transcription
factors (TFs) including AP-2, c-Myc, RB, and Sp1 [22, 23]. Notably, overexpression of AP-2α was sufficient to restore E-cadherin
expression in mesenchymal cells by directly binding to and activating the CDH1 promoter [24]. Moreover, AP-2 activated expression of the epidermal-specific gene KRT14 in keratinocytes [25],
and AP-2γ directly controlled expression of apical-basal polarity
determinant Pardb6 and TJ component CLDN4 [26], suggesting
that ubiquitous TFs like AP-2 may control expression of general
epithelial features. Later studies using transgenic reporter mice
revealed that the proximal promoter was insufficient to drive
epithelial-specific CDH1 expression during embryogenesis, and
additional cis-regulatory elements in the second intron were
required [27, 28], likely due to a requirement for epigenetic remodeling to enhance chromatin accessibility at the CDH1 locus [29].
The CDH1 promoter also contains E-boxes that mediate transcriptional repression in non-epithelial cell types [17, 30]. E-boxes
within the CDH1 gene are recognized by specific TFs that function
as transcriptional repressors. These include the zinc-finger TFs
SNAIL1/Snail [31, 32] and SNAIL2/Slug [33, 34], the
two-handed zinc-finger TFs delta EF1/ZEB1 [35, 36] and
SIP1/ZEB2 [37], and the basic helix-loop-helix TFs Twist [38]
and E12 and E47 [38]. These TFs recruit co-repressors with chromatin modifying ability to induce genome-wide epigenetic changes
Mechanisms of MET in Development and Cancer
47
undergo MET to form the primitive epithelial renal vesicle, which
then elongates and fuses to the ureteric bud tip to form the nascent
nephron [12]. Another example of developmental MET occurs
during mammary gland morphogenesis in puberty (Fig. 2, bottom
panel). Mammary epithelial stem/progenitor cells residing at the
tips of terminal end buds (TEBs) acquire migratory ability through
partial-EMT, transiently losing apical-basal polarity and reducing
intercellular adhesive junctions to invade the surrounding fat pad,
and subsequently revert to an epithelial state through MET to drive
ductal morphogenesis [13, 14].
4 Establishment and Maintenance of the Epithelial Transcriptional Program
Epithelial cells are widely believed to precede the mesenchymal
state in both evolutionary and ontological terms [15]. Accordingly,
the epithelial state has been hypothesized to represent a “default”
state [10]. Early work on the E-cadherin promoter elucidated
regulatory control consistent with a default state of transcriptional
activation. The CDH1 promoter lacks a TATA-box, usually associated with tissue-specific gene expression [16], but possesses discrete positive regulatory components comprising a CCAAT-box, a
GC-rich region, a CE-box, and an enhancer in the first intron [17–
21]. These regions are recognized by ubiquitous transcription
factors (TFs) including AP-2, c-Myc, RB, and Sp1 [22, 23]. Notably, overexpression of AP-2α was sufficient to restore E-cadherin
expression in mesenchymal cells by directly binding to and activating the CDH1 promoter [24]. Moreover, AP-2 activated expression of the epidermal-specific gene KRT14 in keratinocytes [25],
and AP-2γ directly controlled expression of apical-basal polarity
determinant Pardb6 and TJ component CLDN4 [26], suggesting
that ubiquitous TFs like AP-2 may control expression of general
epithelial features. Later studies using transgenic reporter mice
revealed that the proximal promoter was insufficient to drive
epithelial-specific CDH1 expression during embryogenesis, and
additional cis-regulatory elements in the second intron were
required [27, 28], likely due to a requirement for epigenetic remodeling to enhance chromatin accessibility at the CDH1 locus [29].
The CDH1 promoter also contains E-boxes that mediate transcriptional repression in non-epithelial cell types [17, 30]. E-boxes
within the CDH1 gene are recognized by specific TFs that function
as transcriptional repressors. These include the zinc-finger TFs
SNAIL1/Snail [31, 32] and SNAIL2/Slug [33, 34], the
two-handed zinc-finger TFs delta EF1/ZEB1 [35, 36] and
SIP1/ZEB2 [37], and the basic helix-loop-helix TFs Twist [38]
and E12 and E47 [38]. These TFs recruit co-repressors with chromatin modifying ability to induce genome-wide epigenetic changes
Mechanisms of MET in Development and Cancer
47