Transcription factor Six2 induces a stem cell‐like phenotype in renal cell carcinoma cells

Abstract

Renal cell carcinoma (RCC) accounts for 2–3% of adult malignant tumors, and the incidence of RCC worldwide has increased by about 2% over the past two decades. The homeobox protein Six2 has been shown to promote the stemness of breast cancer cells and play a role in kidney development, but its involvement in RCC progression has not previously been investigated. Here, we found that six2 expression was significantly increased in RCC tissues and negatively correlated with the overall survival of patients with RCC. In addition, six2 expression exhibited a remarkably higher level relative to that in normal renal cells. Functional experiments showed that six2 knockdown attenuated the stemness of RCC cells, which was evident by decreased spheroid formation ability and stemness marker (sox2 and nanog) expression. Mechanistic studies indicated that Six2 directly bound to the enhancer of sox2, promoting sox2 expression and downstream effector expression of nanog. Furthermore, overexpression of sox2 rescued the inhibitory effects of six2 on the stemness of RCC cells. Notably, six2 expression is positively correlated with sox2 and nanog expression in RCC tissues. Collectively, our results point toward a six2/sox2 axis responsible for RCC cell stemness.

The SIX Family of Transcription Factors: Common Themes Integrating Developmental and Cancer Biology

“…As one might predict, increased levels of SIX2 are highly correlated with cancers involving the kidney. SIX2 overexpression has been identified in the pediatric cancer Wilms tumor ( Murphy et al, 2012 ; Pierce et al, 2014 ; Walz et al, 2015 ; Wegert et al, 2015 ) as well as in renal cell carcinoma ( Senanayake et al, 2013 ; Cheng et al, 2019 ) and nephroblastoma ( Senanayake et al, 2013 ). Specific point mutations in SIX1 and SIX2 have been detected in Wilms tumor cells where they potentially increase transcriptional activity (mutations denoted in red of Figure 1 ; Wegert et al, 2015 ).…”

“…Disruption or knock-out of SIX gene function in experimental studies has resulted in increased progenitor cell apoptosis concomitant with reduced proliferation in several developing tissues ( Table 1 ). In the context of cancer, ectopic or overexpression of SIX genes has resulted in increased tumor cell proliferation as well as maintenance of cancer stem cells ( Table 2 ; McCoy et al, 2009 ; Farabaugh et al, 2012 ; Cheng et al, 2019 ; Oliphant et al, 2019 ). The one exception may be Six3 where its role in proliferation is not as clear.…”

“…Enhancing cell proliferation by SIX proteins may also occur via crosstalk with other progenitor cell markers such as Sox2. The transcriptional regulator Sox2 has been shown to be directly regulated in developmental and cancer contexts by Six1 ( Zhang T. et al, 2017 ; De Lope et al, 2019 ), Six2 ( Cheng et al, 2019 ; Oliphant et al, 2019 ), Six3 ( Liu et al, 2006 ), and Six6 ( Diacou et al, 2018 ) to further promote stem/progenitor cell phenotypes. SIX family transcription factors seem to promote progenitor cell self-renewal through both direct cell cycle regulation and via indirect mechanisms by activating additional pro-stem cell identity genes.…”