Analysis of mouse mammary glands from various stages of pregnancy indicated that miR-205 may be developmentally regulated.⁴ These studies showed that in the adult virgin miR-205 expression was restricted to the myoepithelial cell layer, which contains both myoepithelial cells and basal stem cells, and expression in both luminal and basal epithelium increased during pregnancy and lactation, with another increase in expression during late involution. Likewise, normal human mammary gland samples showed miR-205 expression in ductal and lobular myoepithelial/basal cell compartments.⁵ Cell culture models of normal mammary epithelial cells also indicated that miR-205 was highly expressed in the “progenitor-like” subpopulation based on FACS purification of Aldefluor^hiSca-1^hi or Sca-1⁺ cells.^6,7 Several groups identified normal mouse mammary stem cell-enriched populations that are capable of repopulating the mammary gland based on CD24 (heat stable antigen), Sca-1 (Stem cell antigen-1; encoded by Ly6a), CD29 (β1-integrin) and CD49f (α6-integrin) cell surface marker expression. These three populations contain a mix of myoepithelial and basal cells, with an estimated 1 in 2000 enrichment for mammary gland stem cells. These stem cell-enriched populations were isolated by the profiles of Lineage(Lin)⁻CD24^+/loSca-1⁻,² Lin⁻CD29^hiCD24⁺,⁸ or Lin⁻CD49f^hiCD24^med (also referred to as the mammary repopulating unit, or MRU).⁹ Analysis of miR-205 in the normal adult mammary gland revealed high expression of miR-205 in these three stem cell-enriched populations, in addition to high expression in the myoepithelial cell population (CD49f^loCD24^lo)⁶ (Fig. 1). These data suggest a role for miR-205 in mouse mammary gland stem cells. While miRNAs important in directing the fate of somatic stem cells of many tissues have been identified,^10–13 the role that miRNAs play in normal mammary gland stem cells is only just starting to be elucidated. Additionally, other studies have found miR-205 expression to be localized specifically to epithelial cells of the developing eye,¹⁴ suggesting an epithelial-specific role for miR-205. Based on miR-205 expression data in mammary stem cell populations⁶ and its regulation during pregnancy and involution,⁴ this miRNA may play a role in mammary epithelial stem cell maintenance and differentiation.

Altered miR-205 expression has been found in many clinical samples of solid tumors compared to normal tissue, as well as many cancer cell lines. High expression of miR-205 was found in head and neck cancer cell lines¹⁷ and squamous cell carcinoma cell lines,¹⁸ as well as clinical samples of metastatic head and neck squamous cell carcinomas.¹⁹ However, other studies demonstrated clinical samples of head and neck squamous cell carcinomas expressing low levels of miR-205 is associated with increased recurrence and poor prognosis.²⁰ In clinical bladder samples, miR-205 was also found to be significantly upregulated compared to normal tissue,²¹ while low expression of miR-205 and members of the miR-200 family was associated with invasive bladder cancers compared to non-invasive bladder cancers.²² Clinical prostate cancer samples also showed down-regulation of miR-205 relative to normal adjacent tissue, and loss of miR-205 was associated with prostate cancer progression.²³ Additionally, miR-205 was highly expressed in cervical cancer cell lines as well as clinical cervical cancer samples compared to normal cervical tissue.²⁴ High expression of miR-205 was found in nearly 65% of the patients studied with non small cell lung cancers.²⁵ Further classification of non small cell lung cancers into squamous cell carcinomas and adenocarcinomas could be achieved based on miR-205 expression, where miR-205 is highly expressed in squamous cell carcinomas but not in adenocarcinomas.^26,27 From these expression studies, it could be hypothesized that miR-205 is expressed at higher levels in cancer subtypes with more epithelial characteristics and a better prognosis and is lower in those that are more invasive with a poorer prognosis.

This last finding is consistent with recent studies indicating that miR-205 is a negative regulator of the epithelial-to-mesenchymal transition (EMT), an early process in metastasis, and expression of miR-205 is lost in mesenchymal breast cancer cell lines.³¹ These studies indicate a negative feedback loop that regulates EMT where miR-205, in addition to members of the miR-200 family, silence the EMT-inducing transcriptional repressors Zeb1 and Zeb2, while induction of EMT via TGFβ results in loss of miR-205 and the miR-200 family members.³¹ Furthermore, a metastatic miRNA expression signature in human breast cancers included decreased expression of miR-205, in addition to mis-expression of miR-200 family members, as well as several other miRNAs, associated with metastasis.³² While miR-205 and the miR-200 family have overlapping functions in the regulation of EMT via Zeb1 and Zeb2, they are not functionally redundant. In both normal mammary gland stem cells (MRUs) and breast cancer stem cells, members of the miR-200 family were significantly downregulated relative to other cell populations,³³ while miR-205 expression was elevated in normal MRUs.⁶ This suggests a balance between the expression miR-200 family members and miR-205 and implies that there must be unique roles for these miRNAs. Interestingly, in normal development miR-205 is observed in the endoderm and ectoderm, but not the mesoderm, during gastrulation of chick embryo, a process that is regulated by EMT.³⁴

Overall, there is likely a tightly controlled balance of miR-205 expression in epithelial cells. Normal mammary epithelial cells would maintain relatively high levels of miR-205, compared to non-epithelial cells. These levels of miR-205 are sufficient to decrease the transcription factors Zeb1 and Zeb2, which would subsequently result in the expression of E-cadherin and cell polarity factors, thus conferring epithelial characteristics.³¹ Regulation of PTEN expression by miR-205 allows for activation of the PI3K/Akt pathway, downstream targets of which include cell survival and proliferation factors.⁶ If miR-205 is lost, however, the Zeb1 and Zeb2 transcription factors are upregulated. The ZEB transcription factors are then able to bind to the E-box elements of the E-cadherin gene, as well as cell polarity genes, resulting in repression E-cadherin, loss of cell polarity, and loss of cell-cell junctions.⁴⁰ These molecular changes can result in an EMT. In corneal epithelial cells, a balance between miR-205 and another miRNA has been shown in a unique mechanism, where miR-184 competes for the binding of miR-205 to the SHIP2 3′UTR to maintain SHIP2 levels required for epithelial characteristics specific to eye function.¹⁸