Abstract
Human squamous carcinoma (COLO-16) cells synthesize and secrete hepatocyte-stimulating factor-III (HSF-III), a glycoprotein with Mr = 39,000, which stimulates the synthesis of several acute phase plasma proteins in human hepatoma (HepG2) cells. The qualitative response of HepG2 cells to HSF-III is essentially the same as that elicited by human recombinant interleukin-6 (IL-6). Although similar in hepatocyte-stimulating activity, HSF-III and IL-6 are distinct molecules which differ not only in size and charge but also in immunologic properties: no cross-recognition of HSF-III and IL-6 occurs using neutralizing antibodies against IL-6 and HSF-III, respectively. In addition, Northern blot hybridization of IL-6 cDNA to mRNA from COLO-16 cells revealed no detectable IL-6 message. HSF-III does not compete for binding to the IL-6 receptors suggesting that HepG2 cells carry receptors specific for each hormone. Both receptor types may trigger similar intracellular processes explaining the identical regulation of acute phase protein expression.
Highlights
Human Hepatocyte-stimulating Factor-I11 andInterleukin-6 Are Structurally and ImmunologicallyDistinct but Regulate the Production of the Same AcutePhase Plasma Proteins*
We show by Northern blot hybridization of IL-6 cDNA to mRNA using specific antibodies that HSF-I11 and IL-6 are different from COLO-16 cells revealed no detectable IL-6 message
We provide evidence that HSF-111 and IL-6 do not utilize common receptors in HepG2 cells for stimulating the expression of acute phase proteins
Summary
HCl, pH 6.8, for 3 h, brought to 5% glycerol and 0.1% SDS, and separated on an 11% nondenaturing SDS gel. HSF-I11 migrating as a single band with apparent molecular weight of 36,000 was either eluted from the gel in MEMcontaining 10% fetal calf serum for measuring HSF activity, or was subjected to a second electrophoretic separation. The lZ5I-IL- fold diluted eluate was assayed for HSF activity on HepG2 cells. The cells were incubated in 500 p1 of binding medium (MEM, 1%bovine serum albumin, mM Hepes, pH 7.2) with the appropriate concentration of lz5I-IL-6and competing ligands for 4 h at 11"C. These conditions were sufficient to reach binding equilibrium of the showed a single silver-stained protein band witMh , = 39,000 (Fig. 1). The accuracy of the HSFassay in this cell system was verified by using COS cell-derived
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