Matriptase expression and the subsequent shedding of active matriptase can be enhanced by the expression of very low levels of HAI-1 or HAI-2 in Daudi cells. (A) Daudi cells transduced with the tetracycline transactivator protein alone (TA, lanes 1), Teton HAI-1 cells (HAI-1, lanes 2), and Teton HAI-2 cells (HAI-2, lanes 3) were grown in the absence of doxycycline. The cell lysates prepared from an equal number of cells were analyzed by immunoblot assay for HAI-1, HAI-2, and matriptase, as indicated. GAPDH was used as a loading control and to estimate the ratios of matriptase expression in Teton HAI-1 cells and Teton HAI-2 cells relative to the TA control cells. The ratios are indicated. (B) These cells at the same number were induced to activate matriptase by transiently exposing the cells to a pH 6.0 buffer. The conditioned buffers were collected and assayed for matriptase proteolytic activity using the fluorogenic substrate. The data presented are representative of those obtained in more than three independent experiments. ∗∗: P < 0.01; ∗∗∗: P < 0.001.