In vitro effects of stimulators and inhibitors of erythropoietin synthesis on thrombopoietin production

The hemopoietic hormones erythropoietin (Epo) and thrombopoietin (Tpo) show similarities with respect to their structure and their production sites. Herein, we compared the effects of physiological modulators of Epo synthesis on the formation of Epo and Tpo in human hepatoma cell lines (Hep3B, HepG2). Tpo and Epo were measured by ELISA and radioimmunoassay, respectively, in the supernatants of confluent cultures (24-well dishes, 0.5 ml/cmz) after 24 h incubation periods. The results were qualitatively very similar when HepG2 and HepSB cultures were compared. Epo production was stimulated by thyroid hormones (T3/T4) and retinol (e.g., 146 ±4 U Epo/g cellular protein with retinol 4 ug/ml versus 70 ±8 U/g in untreated Hep3B cultures incubated for 24 h; P < 0.05, n = 4, mean ±3D). In contrast, Tpo production was not stimulated by these compounds (e.g., 229 ±13 ng Tpo/g cellular protein with retinol versus 221 ±20 ng Tpo/g in untreated Hep3B cultures). Likewise, Tpo production was unaltered when compounds were added that suppressed Epo production, such as proinflammatory cytokines (e.g., 70 ±6 U Epo/g cellular protein with TNFa 10 ng/ml and 105 ±U Epo/g with IL-1 300 pg/ml versus 185 ±22 U Epo/g in untreated HepG2 cultures; P < 0.05, n = 4). The corresponding 24 h-rates of Tpo production were 247 ±8 ng/g cellular protein with TNFa, 258 ±26 ng/g with IL-1, and 246 ±23 ng/g in untreated HepG2 cultures. These findings support the concept that hepatocytes elaborate both Epo and Tpo. While Epo synthesis is controlled by several humoral and paracrine mediators, Tpo appears to be produced more constitutively in hepatic cells.