Altered Regulation of TGF‐β1 and TGF‐α in Primary Keratinocytes and Papillomas Expressing v‐Ha‐ras

The influence of an oncogenic v‐Ha‐ras gene on the expression of TGF‐β and TGF‐α by mouse keratinocytes and derived tumors has been investigated. Normal mouse keratinocytes cultured as basal cells in 0.05 mM Ca²⁺ secreted low levels of TGF‐β2 peptide, and this increased markedly following culture in 1.4 mM Ca²⁺, retinoic acid, or phorbol esters. In contrast, introduction of a v‐Ha‐ras gene into normal keratinocytes increased basal expression and secretion of TGF‐β1 (rather than TGF‐β2) in response to all three agents. The selective secretion of TGF‐β1 in v‐Ha‐ras keratinocytes in response to 1.4 mM Ca²⁺ occurred even though the four TGF‐β₂ transcripts were induced and the TGF‐β₁ transcript decreased, suggesting that the activated v‐Ha‐ras gene product regulates expression of the TGF‐β isoforms at the posttranscriptional level. Immunohistochemical analysis of papillomas formed following skin grafting of v‐Ha‐ras keratinocytes onto nude mice indicated that TGF‐β1 was abundant in the basal and spinous layers, while there was no expression of TGF‐β1 in normal skin. In contrast, both normal and neoplastic tissues expressed TGF‐β2 and TGF‐β3 in the granular layers. Furthermore, TGF‐α mRNA expression was also elevated fivefold in cultured v‐Ha‐ras keratinocytes, and TGF‐α protein was overexpressed in the grafted papillomas, but there was no detectable expression in normal skin. Elevated expression of both TGF‐β1 and TGF‐α in the basal and spinous layers of benign tumors may be important for the high proliferation rate in these tumors as well as for increased proliferation in the suprabasal layer.