Cellular activities of 20K-and 22K-hGH do not necessarily correlate with their binding affinities for rat GH receptor

Even though 20K human growth hormone (20K-hGH) has 3-10% binding affinity f or the rat liver and adipose tissue microsomes as compared to 22K-hGH, it w as also reported that 20K-hGH has the same potency as 22K-hGH in the hypoph ysectomized rat weight gain assay. In order to investigate the reason why s uch controversial data exist, we have studied 20K- and 22K-hGH using the ra t GH receptor extracellular domain (rGHR-ECD) and full-length rGHR. When we examined the complex formation of rGHR-ECD with 20K- and 22K-hGH in gel fi ltration assay, 20K-hGH formed no complex while 22K-hGH formed a 1:1 comple x. Next, rGHR cDNA was introduced into Ba/F3 cells and CHO-K1 cells, and st able transfectants (Ba/F3-rGHR and CHO-rGHR) were established. In the proli feration of Ba/F3-rGHR cells, 20K-hGH had 10-fold lower activity than 22K-h GH, which is consistent with their affinities for rGHR. But surprisingly, i n the Spi2.1 gene promoter activation in CHO-rGHR cells, 20K- and 22K-hGH h ad the same activity, which was found not only in stable CHO-rGHR clones bu t also in CHO-K1 cells transiently expressing rGHR. In conclusion, these re sults indicate that cellular activities of 20K- and 22K-hGH do not necessar ily correlate with their binding affinities for rGHR.