The human papillomavirus type 16 E5 protein modulates phospholipase C-gamma-1 activity and phosphatidyl inositol turnover in mouse fibroblasts

The human papillomavirus type 16 E5 (HPV16-E5) protein is a membrane protei n that has been associated with malignant growth. The protein affects growt h factor-mediated signal transduction in a ligand-dependent manner. We show now that E5 expression in A31 fibroblasts results in an increased level of diacylglycerol (DAG) and inositol phosphates. Immunoprecipitation of phosp holipase C-gamma-1 (PLC-gamma-1) with specific antibodies and immunoblottin g with anti-phosphotyrosine antibodies reveal a large increase in tyrosine phosphorylation of the enzyme in ES-expressing cells compared to control ve ctor-transfected cells. This activation of tyrosine phosphorylation is grow th factor independent. In addition, an enhanced formation of phosphatidic a cid (PA) was observed in E5 cells. This increase did not result from activa tion of phospholipase D (PLD), although the enzyme was activatable by treat ment with phorbol ester. Thus, a phosphohydrolase-mediated DAG synthesis fr om PLD-produced PA can be excluded. The observed effects were not further e nhanced by EGF showing that the presence of the growth factor is not necess ary for maintaining permanent activation of PLC-gamma-1 in E5-expressing ce lls. The DAG- and inositol phosphate-mediated signal cascade within the cel ls is thus effectively uncoupled from external control via EGF and its rece ptor in the presence of E5 protein.