Defective cleavage of membrane bound TGF alpha leads to enhanced activation of the EGF receptor in malignant cells

Transforming growth factor alpha (TGF alpha) is widely expressed in maligna nt as well as normal cells and is involved in regulating cell growth and di fferentiation. Although processing of TGF alpha has been extensively studie d in normal cells, there is little information regarding TGF alpha cleavage in malignant cells. Therefore, we compared the processing of TGF alpha in two human colon carcinoma cell lines. We found that there was a defective c leavage pattern for the TGF alpha precursor resulting in retention of parti ally processed TGF alpha on the cell surface of both the HCT116a2 alpha S3 and CBS4 alpha S2 cell lines. This raised the possibility that signaling fr om the resulting defective cleavage species could differ from that of solub le TGF alpha, The membrane-associated TGF alpha induced higher phosphorylat ion of EGFR on the cell surface of adjacent cells than equivalent levels of mature TGF alpha. The interaction of membrane bound TGF alpha precursor wi th the EGFR caused a slower internalization of activated EGFR relative to t he internalization of the soluble TGF alpha/EGFR complexes. In addition, th e tethered TGF alpha was resistant to the ability of protein-tyrosine phosp hatases (PTPs) to reduce EGFR tyrosine phosphorylation, also contributing t o higher activation of EGFR, The enhanced activation of EGFR by the tethere d form of TGF alpha was reflected by higher activation of Grb2, SHC and Erk downstream mediators of EGF receptor signaling. The higher activation of E GFR by membrane tethered TGF alpha indicates that defective TGF alpha proce ssing provides a mechanism whereby malignant cells can obtain a growth adva ntage over normal cells.