Researchers have discovered the means by which the protein ΔNp63α drives cell proliferation in various types of squamous cell carcinoma (SCC), revealing an actionable direction for the development of new drug-based SCC treatments.
The p63 isoform ΔNp63α, a member of the p53 family of transcription factors, is crucial to a developing embryo's epithelial maintenance and epidermal morphogenesis. After this stage of development, it should play no further role. However, if ΔNp63α becomes active in the adult genome, it is a powerful oncogene in various types of SCC, with over half of all SCCs involving excess p63 activity. ΔNp63α overexpression is known as a marker of poor prognosis in SCC, but the exact means by which it promotes malignant cell proliferation and blocks apoptosis have not been satisfactorily identified until now.
Researchers from the University of Colorado Cancer Center hypothesized that inhibiting anti-proliferative signaling pathways is one way that ΔNp63α promotes the proliferation of SCC. They examined lung SCC, one of the SCC types that requires ΔNp63α to proliferate, using a genome-wide CRISPR screen, which facilitates analysis of thousands of genes in a single experiment. They halted ΔNp63α production, causing the cancer's proliferation to cease. Next, they silenced thousands of genes across the genome in order to discover which genes, when inactivated, would permit the lung SCC cells to proliferate. By extension, these genes would be the ones that ΔNp63α suppresses in order to permit the growth of SCC.
The researchers found one pathway that ΔNp63α needed to block in order to promote growth of SCC cells. Two genes in this pathway have major impacts on tumor suppression: RHOA and transforming growth factor beta (TGFβ)2, which induces a cell cycle arrest that depends partially on RHOA. The investigators discovered that when ΔNp63α expression is reduced, RHOA increases in activity, blocking cell division so that cells halt RHOA-dependent proliferation. The researchers note in their paper, published in Cell Reports, that RHOA is "increasingly recognized as a tumor suppressor frequently inactivated in diverse malignancies… but its relationship to ΔNp63α-driven tumorigenesis has not been explored."
The investigators found that an upswing in RHOA activity causes transcriptome changes which suggest increased TGFβ2 signaling. Conversely, they also discovered that adding TGFβ2 to the cell culture medium increases the amount of active RHOA. When ΔNp63α is active, it reduces TGFβ signaling through direct repression of TGFβ2, which then causes depletion of active RHOA, driving cell proliferation in SCC.
"ΔNp63α shuts off TGFβ2 and RHOA to promote cancer progression, and this is clearly a widespread phenomenon in squamous cell carcinomas," stated the study's senior author, Joaqin Espinosa, PhD, professor of pharmacology at the University of Colorado Denver. The researchers state in their paper that genomic data from tumors demonstrate that overexpression of ΔNp63α and inactivation of RHOA and of the TGFβ2 receptor occur in over 80% of lung SCCs.
The study results suggest that the key to inhibiting ΔNp63α is to activate TGFβ2 and RHOA. "If there was a way to deliver something that mimics TGFβ2, perhaps we could stop proliferation of squamous cell carcinoma," remarked Dr. Espinosa. Alternately, he suggested, "If you can find drugs that lock RHOA in its active form, that would shut down cell proliferation as well."
For More Information
Abraham CG, Ludwig MP, Andrysik Z, et al (2018). CRISPR screen identifies RHOA as mediator of proliferation arrest upon ΔNp63α depletion. Cell Reports, 24(12):3224-3236. DOI:10.1016/j.celrep.2018.08.058