Inhibition of protein tyrosine kinase alters the effect of serum basic protein I on triacylglycerols and cholesterol differently in normal and hyperapoB fibroblasts

We studied whether the stimulatory effect of human serum basic protein I (BP I) on the formation of cell triacylglycerols and cholesterol may be mediated through protein tyrosine kinase in normal fibroblasts, and whether there was a deficiency in such a process in cells from subjects with hyperapobetalipoproteinemia hyperapoB). Genistein, a highly specific inhibitor of tyrosine kinase phosphorylation, was used as a probe. When BP I (428.0 nmol/L) alone was added to F-12 medium without genistein, the mean mass of cell triacylglycerols doubled in six normal cell lines from healthy subjects, an effect that was decreased by 50% in six cell lines from subjects with hyperapoB (P=.0007). The addition of genistein with BP I to normal cells decreased the stimulation of triacylglycerol formation by BP I by about 50% (P=.008), whereas genistein had little effect in the BP I-treated hyperapoB cells. The effect of genistein on the stimulation of triglyceride and cholesterol production by BP I was shown to be both time and concentration (92.5 nmol/mL medium nadir) dependent. In normal fibroblasts, BP I stimulated the rate of incorporation of both [¹⁴C]acetate (P=.0001) and [³H]mevalonolactone (P=.002) into unesterified cholesterol, an effect that was markedly deficient in the hyperapoB cells (P=.0001 for [¹⁴C]acetate and P=.0002 for [³H]mevalonolactone). In normal but not hyperapoB cells, genistein inhibited the significant stimulation by BP I of the rates of both [¹⁴C]acetate (P=.0001) and [³H]mevalonolactone (P=.04) incorporation into unesterified cholesterol. There was also a significantly greater stimulation by BP I of the rate of [¹⁴C]acetate incorporation into cell esterified cholesterol in normal cells than in hyperapoB cells (P=.003), an effect that was inhibited by genistein in both normal (P=.0009) and hyperapoB (P=.01) cells. BP I also stimulated to a greater extent the mass of total cholesterol (P=.0009) and unesterified cholesterol (P=.015), but to a lesser degree that a esterified cholesterol (P=.44), in normal cells than in hyperapoB cells. Herbimycin A and tyrphostin A47, two other inhibition of protein tyrosine kinase, also significantly inhibited the effects of BP I on triacylglycerol and cholesterol mass in normal cells but not in hyperapoB cells. The effect of BP I on triacylglycerols and cholesterol formation in normal cells appeared to be mediated through a tyrosine kinase-dependent process that was deficient in hyperapoB cells.