Involvement of pRB family in TGFβ-dependent epithelial cell hypertrophy

Although renal hypertrophy is often associated with the progressive loss of renal function, the mechanism of hypertrophy is poorly understood. In both primary cultures of rabbit proximal tubules and NRK-52E cells (a renal epithelial cell line), transforming growth factor β1 (TGFβ) converted epidermal growth factor (EGF)-induced hyperplasia into hypertrophy. TGFβ did not affect EGF-induced increases in c-fos mRNA abundance or cyclin E protein abundance, but inhibited EGF-induced entry into S, G₂, and M phases. EGF alone increased the amount of hyperphosphorylated (inactive) pRB; TGFβ blocked EGF-induced pRB phosphorylation, maintaining pRB in the active form. To determine the importance of active pRB in TGFβ-induced hypertrophy, NRK- 52E cells were infected with SV40 large T antigen (which inactivates pRB and related proteins and p53), HPV16 E6 (which degrades p53), HPV16 E7 (which binds and inactivates pRB and related proteins), or both HPV16 E6 and E7. In SV40 large T antigen expressing clones, the magnitude of EGF + TGFβ-induced hypertrophy was inhibited and was inversely related to the magnitude of SV40 large T antigen expression. In the HPV16-infected cells, EGF + TGFβ-induced hypertrophy was inhibited in E7- and E6E7-expressing, but not E6-expressing cells. These results suggest a requirement for active pRB in the development of EGF + TGFβ-induced renal epithelial cell hypertrophy. We suggest a model of renal cell hypertrophy mediated by EGF-induced entry into the cell cycle with TGFβ-induced blockade at G₁/S, the latter due to maintained activity of pRB or a related protein.