Enhancement by a synthetic isoprenoid of the toxicity of conjugates of epidermal growth factor with pseudomonas exotoxin

A newly synthesized isoprenoid, N-solanesyl-N,N'-bis(3,4-dimethoxybenzyl)ethylenediamine, has a verapamil-like structure but no calcium channel blocking activity. The isoprenoid enhanced the cytotoxic effect of a conjugate of epidermal growth factor coupled with Pseudomonas exotoxin in human KB cells. By using iodinated epidermal growth factor ([¹²⁵I]EGF), the effect of the isoprenoid on intracellular transport of EGF was examined. The isoprenoid did not affect the binding and uptake of [¹²⁵I]EGF by KB cells. The release of radioactivity associated with [¹²⁵I]EGF into medium was slow in the presence of the isoprenoid. Density gradient fractionation studies using cell homogenates suggest that [¹²⁵I]EGF accumulates in an undegraded form in lysosomes when cells are treated with the isoprenoid. The pH value in lysosomes of KB cells was 5.5, and SDB did not affect significantly the pH value at the concentrations used to potentiate the cytotoxicity of chimeric toxins. Electron microscopy showed an increased number of electron-dense bodies in KB cells grown for 24 hr with 17-51 μg/ml isoprenoid. The potentiating action of chimeric toxins by the isoprenoid is discussed in relation to the altered lysosomal function in treated cells.