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SOX2 and PI3K Cooperate to Induce and Stabilize a Squamous-Committed Stem Cell Injury State during Lung Squamous Cell Carcinoma Pathogenesis

View Article: PubMed Central - PubMed

ABSTRACT

Although cancers are considered stem cell diseases, mechanisms involving stem cell alterations are poorly understood. Squamous cell carcinoma (SQCC) is the second most common lung cancer, and its pathogenesis appears to hinge on changes in the stem cell behavior of basal cells in the bronchial airways. Basal cells are normally quiescent and differentiate into mucociliary epithelia. Smoking triggers a hyperproliferative response resulting in progressive premalignant epithelial changes ranging from squamous metaplasia to dysplasia. These changes can regress naturally, even with chronic smoking. However, for unknown reasons, dysplasias have higher progression rates than earlier stages. We used primary human tracheobronchial basal cells to investigate how copy number gains in SOX2 and PIK3CA at 3q26-28, which co-occur in dysplasia and are observed in 94% of SQCCs, may promote progression. We find that SOX2 cooperates with PI3K signaling, which is activated by smoking, to initiate the squamous injury response in basal cells. This response involves SOX9 repression, and, accordingly, SOX2 and PI3K signaling levels are high during dysplasia, while SOX9 is not expressed. By contrast, during regeneration of mucociliary epithelia, PI3K signaling is low and basal cells transiently enter a SOX2LoSOX9Hi state, with SOX9 promoting proliferation and preventing squamous differentiation. Transient reduction in SOX2 is necessary for ciliogenesis, although SOX2 expression later rises and drives mucinous differentiation, as SOX9 levels decline. Frequent coamplification of SOX2 and PIK3CA in dysplasia may, thus, promote progression by locking basal cells in a SOX2HiSOX9Lo state with active PI3K signaling, which sustains the squamous injury response while precluding normal mucociliary differentiation. Surprisingly, we find that, although later in invasive carcinoma SOX9 is generally expressed at low levels, its expression is higher in a subset of SQCCs with less squamous identity and worse clinical outcome. We propose that early pathogenesis of most SQCCs involves stabilization of the squamous injury state in stem cells through copy number gains at 3q, with the pro-proliferative activity of SOX9 possibly being exploited in a subset of SQCCs in later stages.

No MeSH data available.


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SOX2, SOX9, and phospho-S6 (P-S6) expression during SQCC pathogenesis.Stage 2A lung resection from a 45 packs/year smoker showing normal respiratory epithelium, squamous metaplasia, high grade dysplasia, and invasive squamous carcinoma. Met = squamous metaplasia, Dys = high grade dysplasia, Ca = carcinoma. Arrows point to representative basal cells (Ba). Insets show magnified areas within the dashed boxes. In squamous metaplasia (including basal cells), the percentage of P-S6-positive cells and the intensity of staining per cell increased towards areas of dysplasia. All scale bars are 50 μm.
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pbio.1002581.g011: SOX2, SOX9, and phospho-S6 (P-S6) expression during SQCC pathogenesis.Stage 2A lung resection from a 45 packs/year smoker showing normal respiratory epithelium, squamous metaplasia, high grade dysplasia, and invasive squamous carcinoma. Met = squamous metaplasia, Dys = high grade dysplasia, Ca = carcinoma. Arrows point to representative basal cells (Ba). Insets show magnified areas within the dashed boxes. In squamous metaplasia (including basal cells), the percentage of P-S6-positive cells and the intensity of staining per cell increased towards areas of dysplasia. All scale bars are 50 μm.

Mentions: To further investigate if the mechanism regulating squamous differentiation in stem cells might contribute to progression through early stages of SQCC pathogenesis, we characterized SOX2, SOX9, and P-S6 expression during preneoplasia (Fig 11). We analyzed an SQCC resection from a smoker that included normal respiratory epithelium, squamous metaplasia, and high-grade dysplasia, in addition to frank carcinoma. As expected, most cells in the normal mucociliary epithelium expressed SOX2, while SOX9 was not detected, and P-S6 expression was confined to some columnar, but not basal cells (Fig 11). In squamous metaplasia, SOX2 was expressed in most cells, but heterogeneously in the basal and suprabasal layers. The heterogeneity in basal cells would be consistent with ongoing squamous differentiation of some basal cells (SOX2Hi cells), while other basal cells are poised to regenerate columnar cells during later regression (SOX2Lo cells). SOX9 was not expressed in most basal cells of squamous metaplasia, which supports these cells either being squamous-committed or in an early stage of transitioning to columnar fates. Surprisingly, however, SOX9 was heterogeneously expressed in suprabasal cells, possibly signifying a non-stem cell function for SOX9 in some squamous differentiated progeny (see also S7 Fig). P-S6 staining was intense in the uppermost layers of squamous metaplasia but varied in the basal cell compartment. In the centermost basal region, P-S6 staining was low, while its intensity increased in basal cells as they approached areas of dysplasia (lowermost panels). By contrast, in high-grade dysplasia, expression of SOX2, SOX9, and P-S6 was uniform. SOX2 and P-S6 were expressed at high levels, while SOX9 expression was lost (see also S7 Fig), supporting most dysplastic cells being stem cell-like and committed to the squamous fate. The dysplastic expression patterns were maintained in the invasive carcinoma (S7 Fig).


SOX2 and PI3K Cooperate to Induce and Stabilize a Squamous-Committed Stem Cell Injury State during Lung Squamous Cell Carcinoma Pathogenesis
SOX2, SOX9, and phospho-S6 (P-S6) expression during SQCC pathogenesis.Stage 2A lung resection from a 45 packs/year smoker showing normal respiratory epithelium, squamous metaplasia, high grade dysplasia, and invasive squamous carcinoma. Met = squamous metaplasia, Dys = high grade dysplasia, Ca = carcinoma. Arrows point to representative basal cells (Ba). Insets show magnified areas within the dashed boxes. In squamous metaplasia (including basal cells), the percentage of P-S6-positive cells and the intensity of staining per cell increased towards areas of dysplasia. All scale bars are 50 μm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5120804&req=5

pbio.1002581.g011: SOX2, SOX9, and phospho-S6 (P-S6) expression during SQCC pathogenesis.Stage 2A lung resection from a 45 packs/year smoker showing normal respiratory epithelium, squamous metaplasia, high grade dysplasia, and invasive squamous carcinoma. Met = squamous metaplasia, Dys = high grade dysplasia, Ca = carcinoma. Arrows point to representative basal cells (Ba). Insets show magnified areas within the dashed boxes. In squamous metaplasia (including basal cells), the percentage of P-S6-positive cells and the intensity of staining per cell increased towards areas of dysplasia. All scale bars are 50 μm.
Mentions: To further investigate if the mechanism regulating squamous differentiation in stem cells might contribute to progression through early stages of SQCC pathogenesis, we characterized SOX2, SOX9, and P-S6 expression during preneoplasia (Fig 11). We analyzed an SQCC resection from a smoker that included normal respiratory epithelium, squamous metaplasia, and high-grade dysplasia, in addition to frank carcinoma. As expected, most cells in the normal mucociliary epithelium expressed SOX2, while SOX9 was not detected, and P-S6 expression was confined to some columnar, but not basal cells (Fig 11). In squamous metaplasia, SOX2 was expressed in most cells, but heterogeneously in the basal and suprabasal layers. The heterogeneity in basal cells would be consistent with ongoing squamous differentiation of some basal cells (SOX2Hi cells), while other basal cells are poised to regenerate columnar cells during later regression (SOX2Lo cells). SOX9 was not expressed in most basal cells of squamous metaplasia, which supports these cells either being squamous-committed or in an early stage of transitioning to columnar fates. Surprisingly, however, SOX9 was heterogeneously expressed in suprabasal cells, possibly signifying a non-stem cell function for SOX9 in some squamous differentiated progeny (see also S7 Fig). P-S6 staining was intense in the uppermost layers of squamous metaplasia but varied in the basal cell compartment. In the centermost basal region, P-S6 staining was low, while its intensity increased in basal cells as they approached areas of dysplasia (lowermost panels). By contrast, in high-grade dysplasia, expression of SOX2, SOX9, and P-S6 was uniform. SOX2 and P-S6 were expressed at high levels, while SOX9 expression was lost (see also S7 Fig), supporting most dysplastic cells being stem cell-like and committed to the squamous fate. The dysplastic expression patterns were maintained in the invasive carcinoma (S7 Fig).

View Article: PubMed Central - PubMed

ABSTRACT

Although cancers are considered stem cell diseases, mechanisms involving stem cell alterations are poorly understood. Squamous cell carcinoma (SQCC) is the second most common lung cancer, and its pathogenesis appears to hinge on changes in the stem cell behavior of basal cells in the bronchial airways. Basal cells are normally quiescent and differentiate into mucociliary epithelia. Smoking triggers a hyperproliferative response resulting in progressive premalignant epithelial changes ranging from squamous metaplasia to dysplasia. These changes can regress naturally, even with chronic smoking. However, for unknown reasons, dysplasias have higher progression rates than earlier stages. We used primary human tracheobronchial basal cells to investigate how copy number gains in SOX2 and PIK3CA at 3q26-28, which co-occur in dysplasia and are observed in 94% of SQCCs, may promote progression. We find that SOX2 cooperates with PI3K signaling, which is activated by smoking, to initiate the squamous injury response in basal cells. This response involves SOX9 repression, and, accordingly, SOX2 and PI3K signaling levels are high during dysplasia, while SOX9 is not expressed. By contrast, during regeneration of mucociliary epithelia, PI3K signaling is low and basal cells transiently enter a SOX2LoSOX9Hi state, with SOX9 promoting proliferation and preventing squamous differentiation. Transient reduction in SOX2 is necessary for ciliogenesis, although SOX2 expression later rises and drives mucinous differentiation, as SOX9 levels decline. Frequent coamplification of SOX2 and PIK3CA in dysplasia may, thus, promote progression by locking basal cells in a SOX2HiSOX9Lo state with active PI3K signaling, which sustains the squamous injury response while precluding normal mucociliary differentiation. Surprisingly, we find that, although later in invasive carcinoma SOX9 is generally expressed at low levels, its expression is higher in a subset of SQCCs with less squamous identity and worse clinical outcome. We propose that early pathogenesis of most SQCCs involves stabilization of the squamous injury state in stem cells through copy number gains at 3q, with the pro-proliferative activity of SOX9 possibly being exploited in a subset of SQCCs in later stages.

No MeSH data available.


Related in: MedlinePlus