Twist, Epithelial-to-Mesenchymal Transition, and Stem Cells

The article by Battula et al. published in the June 22, 2010 issue of Stem Cells demonstrates that immortalized human mammary epithelial cells (HMECs) induced to undergo an epithelial-to-mesenchymal transition (EMT) via forced overexpression of Twist, Snail, or transforming growth factor beta (TGF-b) adopt characteristics of bone marrow-derived mesenchymal stem cells (BM-MSCs), for example, express a similar repertoire of cell surface receptors and differentiate into adipocytes, chondrocytes, and osteoblasts in vitro [1]. Although the classification of EMT-derived cells as MSC-like based on these properties is consistent with standards in the field [2], the authors appear to have overlooked a critical difference between these cell populations as it relates to Twist function. Specifically, the EMT-derived cells described by Battula et al. [1] acquire the capacity to differentiate into connective tissue cell lineages due to the constitutive overexpression of Twist protein. In contrast, Isenmann et al. [3] recently showed that ectopic overexpression of Twist in BM-MSCs blocked their osteogenic and chondrogenic differentiation in vitro and inhibited their ability to form heterotopic osseous tissue in vivo. The latter studies are consistent with an established role of Twist as a negative regulator of mesenchymal cell specification during development [4–7] and the protein’s ability to block differentiation of connective tissue progenitors in vitro [8–11]. One explanation for the disparate outcomes between these two studies may be related to gene dosage as the composition of Twist homodimers versus heterodimers is known to influence its regulatory functions [7, 12]. Alternatively, the fact that the engineered HMECs were immortalized may also be critical as Twist is known to interact with tumor suppressor proteins, such as p53 [13]. Nevertheless, it remains unclear whey Twist blocks multilineage differentiation of human MSCs but is permissive for the differentiation of EMTderived cells. Consequently, the function of Twist in the two cell populations does not appear to be equivalent and suggests that the protein plays distinct roles in transitional mesenchyme and adult MSCs. The latter may reflect the existence of independent pathways governing cell fate specification and EMT during gastrulation [14]. In support of this, our recent data indicate that twist is expressed at significantly higher levels in multipotent versus bipotent and unipotent human MSC clones and that loss of Twist expression correlates with loss in potency [15] (D.G.P., personal communication). These findings implicate Twist in regulating self-maintenance and/or self-renewal of MSCs. Therefore, although EMT-derived and BM-MSC populations appear functionally similar, a more rigorous analysis of the former is required to determine whether they recapitulate the functional complexity of MSC populations with respect to stem/progenitor composition [15] and stromal-like function [16], both of which may be modulated by Twist. Moreover, the suggestion by Battula et al. that epithelium may represent a more abundant source of MSCs for cell therapy is unwarranted based on the fact that various groups have developed methods to generate large numbers (>10) of MSCs for experimental and clinical use [17, 18].