Activation of the autocrine transforming growth factor alpha pathway in human squamous carcinoma cells

Transforming growth factor alpha is an autocrine mitogen for nonneoplastic keratinocytes, which exerts its function by binding to the receptor for epidermal growth factor. In order to determine whether this autocrine pathway is activated in squamous carcinoma cells, we analyzed the production of transforming growth factor alpha as well as the expression and regulation of epidermal growth factor receptors in a panel of human squamous carcinoma cell lines. Immunoreactive transforming growth factor alpha was detectable in squamous carcinoma cells as well as in quiescent nonneoplastic keratinocytes. However, in the absence of exogenous mitogens, only the squamous carcinoma cells secreted the growth factor into the medium, whereas untransformed keratinocytes did not. Each of the squamous carcinoma cell lines expressed significantly greater numbers of cell surface epidermal growth factor receptors than normal keratinocytes. The epidermal growth factor receptor gene was amplified and overexpressed in three of the squamous carcinoma cell lines (A431, CaSki, SqCC/Y1). Two of the squamous carcinoma cell lines (C4-1 and CE-48) displayed a relative inability to down-regulate epidermal growth factor receptors in response to epidermal growth factor. The mechanism of receptor overexpression in the remaining three cell lines (A253, CaLu-1, FaDu) is unexplained. Thus, human squamous carcinoma cell lines frequently exhibit a combination of the constitutive secretion of transforming growth factor alpha and the overexpression of epidermal growth factor receptors. Treatment of these tumor cells with an antibody directed against the ligand-binding domain of the epidermal growth factor receptor inhibited their growth by approximately 50%. These findings suggest that designing strategies to interrupt the transforming growth factor alpha autocrine pathway might lead to new modalities to treat this class of malignant tumors.