| Infection of airway epithelial cells by HPVs types 6 and 11 causes papillomas that are defective in terminal differentiation. The HPVs express multiple proteins, which alter cell functions. Papilloma cells overexpress the epidermal growth factor receptor (EGFR), which regulates multiple functions including proliferation, gene expression, survival/apoptosis, and differentiation. Papilloma cells have constitutively active MAPK and enhanced PI3K activity. However, activation of the downstream effector of PI3K Akt, is reduced due to elevated expression of the tumor suppressor PTEN. We hypothesized that MAPK and PI3K/Akt signaling contributes to abnormal differentiation of papillomas, and therefore, that one or more of these pathways is required for differentiation of normal cells. We further asked whether HPV E6 and/or E7 proteins contribute to the altered differentiation in papilloma cells. Interfering with PI3K synthesis or function in normal laryngeal cells inhibited differentiation, and blocking the MAPK pathway enhanced differentiation. However, reduction of Akt protein levels using siRNA did not inhibit differentiation of normal laryngeal cells, and transfection of constitutively active Akt constructs into papilloma cells did not induce differentiation, suggesting that Akt or its downstream mediators are not responsible for the abnormal differentiation observed in papillomas. I therefore looked at two intermediates upstream of Akt activation and downstream of PI3K, and found that reduction of ILK1 or PDK1 did not inhibit differentiation, suggesting that they, too, are not essential for this process. Both E6 and E7 proteins inhibited differentiation. I concluded that PI3K, but not Akt, is a key regulator of early phase differentiation in human mucosal keratinocytes, and that HPV proteins alter PI3K signaling through an undefined and possibly novel pathway, resulting in the differentiation block. |
