A large-scale RNAi screen identifies LCMR1 as a critical regulator of Tspan8-mediated melanoma invasion.

This corrects the article DOI: 10.1038/onc.2016.219.

p53 targets TSPAN8 to prevent invasion in melanoma cells.

Cutaneous melanoma is a very deadly cancer because of its proclivity to metastasize. Despite the recent development of targeted and immune therapies, patient survival remains low. It is therefore crucial to enhance understanding of the molecular mechanisms underlying invasion. We previously identified tetraspanin 8 (TSPAN8) as an important modulator of melanoma invasiveness, and several of its transcriptional regulators, which affect TSPAN8 expression during melanoma progression toward an invasive stage. This study found that TSPAN8 promoter contains consensus-binding sites for p53 transcription factor. We demonstrated that p53 silencing was sufficient to turn on Tspan8 expression in non-invasive melanoma cells and that p53 acts as a direct transcriptional repressor of TSPAN8. We also showed that p53 modulated matrigel invasion in melanoma cells in a TSPAN8-dependent manner. In conclusion, this study reveals p53 as a negative regulator of Tspan8 expression. As TP53 gene is rarely mutated in melanoma, it was hitherto poorly studied but its role in apoptosis and growth suppression in melanoma is increasingly becoming clear. The study highlights the importance of p53 as a regulator of melanoma invasion and the concept that reactivating p53 could provide a strategy for modulating not only proliferative but also invasive capacity in melanoma treatment.

A large-scale RNAi screen identifies LCMR1 as a critical regulator of Tspan8-mediated melanoma invasion.

Melanoma is the deadliest form of skin cancer owing to its proclivity to metastasise, and recently developed therapies have not yielded the expected results, because almost all patients relapse. Therefore, understanding the molecular mechanisms that underlie early invasion by melanoma cells is crucial to improving patient survival. We have previously shown that, whereas the Tetraspanin 8 protein (Tspan8) is undetectable in normal skin and benign lesions, its expression arises with the progression of melanoma and is sufficient to increase cell invasiveness. Therefore, to identify Tspan8 transcriptional regulators that could explain the onset of Tspan8 expression, thereby conferring an invasive phenotype, we performed an innovative RNA interference-based screen, which, for the first time, identified several Tspan8 repressors and activators, such as GSK3ß, PTEN, IQGAP1, TPT1 and LCMR1. LCMR1 is a recently identified protein that is overexpressed in numerous carcinomas; its expression and role, however, had not previously been studied in melanoma. The present study identified Tspan8 as the first LCMR1 target that could explain its function in carcinogenesis. LCMR1 modulation was sufficient to positively regulate endogenous Tspan8 expression, with concomitant in vitro phenotypic changes such as loss of melanoma cell-matrix adherence and increase in invasion, and Tspan8 expression promoted tumourigenicity in vivo. Moreover, LCMR1 and Tspan8 overexpression were shown to correlate in melanoma lesions, and both proteins could be downregulated in vitro by vemurafenib. In conclusion, this study highlights the importance of Tspan8 and its regulators in the control of early melanoma invasion and suggests that they may be promising new therapeutic targets downstream of the RAF-MEK-ERK signalling pathway.

Functional interplay between p63 and p53 controls RUNX1 function in the transition from proliferation to differentiation in human keratinocytes.

The interfollicular epidermis is continuously renewed, thanks to a regulated balance between proliferation and differentiation. The ΔNp63 transcription factor has a key role in the control of this process. It has been shown that ΔNp63 directly regulates Runt-related transcription factor 1 (RUNX1) transcription factor expression in mouse keratinocytes. The present study showed for the first time that RUNX1 is expressed in normal human interfollicular epidermis and that its expression is tightly regulated during the transition from proliferation to differentiation. It demonstrated that ΔNp63 directly binds two different RUNX1 regulatory DNA sequences and modulates RUNX1 expression differentially in proliferative or differentiated human keratinocytes. It also showed that the regulation of RUNX1 expression by ΔNp63 is dependent on p53 and that this coregulation relies on differential binding and activation of RUNX1 regulatory sequences by ΔNp63 and p53. We also found that RUNX1 inhibits keratinocyte proliferation and activates directly the expression of KRT1, a critical actor in early keratinocyte differentiation. Finally, we described that RUNX1 expression, similar to ΔNp63 and p53, was strongly expressed and downregulated in basal cell carcinomas and squamous cell carcinomas respectively. Taken together, these data shed light on the importance of tight control of the functional interplay between ΔNp63 and p53 in regulating RUNX1 transcription factor expression for proper regulation of interfollicular epidermal homeostasis.