Inhibition of TGF-β and NOTCH Signaling by Cutaneous Papillomaviruses

Abstract

Infections with cutaneous papillomaviruses have been linked to cutaneous squamous cell carcinomas that arise in patients who suffer from a rare genetic disorder, epidermodysplasia verruciformis, or those who have experienced long-term, systemic immunosuppression following organ transplantation. The E6 proteins of the prototypical cutaneous human papillomavirus (HPV) 5 and HPV8 inhibit TGF-β and NOTCH signaling. The Mus musculus papillomavirus 1, MmuPV1, infects laboratory mouse strains and causes cutaneous skin warts that can progress to squamous cell carcinomas. MmuPV1 E6 shares biological and biochemical activities with HPV8 E6 including the ability to inhibit TGF-β and NOTCH signaling by binding the SMAD2/SMAD3 and MAML1 transcription factors, respectively. Inhibition of TGF-β and NOTCH signaling is linked to delayed differentiation and sustained proliferation of differentiating keratinocytes. Furthermore, the ability of MmuPV1 E6 to bind MAML1 is necessary for wart and cancer formation in experimentally infected mice. Hence, experimental MmuPV1 infection in mice will be a robust and valuable experimental system to dissect key aspects of cutaneous HPV infection, pathogenesis, and carcinogenesis.

Keywords: epidermodysplasia verruciformis; hit-and-run carcinogenesis; keratinocyte differentiation; squamous cell carcinoma; viral oncogenesis.

FIGURE 1

Subversion of TGF-β signaling by HPV8 and MmuPV1 E6. In normal cells, TGF-β binding to the heterotetrameric serine/threonine kinase receptor complex triggers a phosphorylation cascade resulting in phosphorylation of receptor the SMADs, SMAD2, and SMAD3. Phosphorylated receptor SMADs disassociate from the receptor, form a complex with the co-SMAD, SMAD4, and enter the nucleus. The SMAD2/SMAD3/SMAD4 complex then binds to SMAD binding sites to stimulate target gene transcription (A). HPV8 and MmuPV1 E6 proteins bind SMAD2 and SMAD3 and inhibit TGF-β signaling by preventing the formation of the active DNA bound SMAD2/SMAD3/SMAD4 complex (B).

FIGURE 2

Schematic of NOTCH target gene expression. In the absence of ligand binding, NOTCH responsive genes remain repressed and RBPJ is in a DNA bound, nuclear complex with repressors such as the N-CoR complex (A). Ligand binding leads to NOTCH cleavage and ICN enters the nucleus, displaces repressors and recruits MAML and other co-activators such EP300/CREBBP (B). HPV8 and MmuPV1 E6 proteins inhibit NOTCH signaling by forming a complex with the DNA bound ICN/MAML1 complex thereby preventing formation of a functional transcriptional activator complex (C).

FIGURE 3

Functional consequences of NOTCH and TGF-β signaling by HPV8 E6 and MmuPV1 E6. TGF-β and NOTCH signaling are critical for cell cycle arrest and keratinocyte differentiation that are tightly coupled in normal squamous epithelial cells. Functional uncoupling of these two processes is a necessary prerequisite for papillomavirus progeny formation in terminally differentiated epithelial cells.