Independent activities of FSH and LH structurally confined in a single polypeptide: Selective modification of the relative potencies of the hormones

The human glycoprotein hormones CG, LH, FSH, and TSH are heterodimers compo sed of a common a subunit noncovalently associated with a hormone-specific P subunit. Recently, it was reported that a covalently fused triple-domain gonadotropin analog containing FSH beta, CG beta, and a subunits was dually active because it bound to both FSH and human CG (hCG)/LH receptors. Howev er, it is not known whether both activities can be uncoupled from each othe r or whether they change in tandem when modifications are made in the molec ule. To address this point, we constructed a triple-domain analog containin g FSH beta, LH beta, and ce subunits, and variants of this analog differing in the carboxyl-terminal region of LH beta. All of the analogs exhibited b ifunctional action, i.e. they bound to both LH/hCG and human FSH receptors. FSH binding and signal transduction were similar for all variants and diff ered less than 2-fold from that of the heterodimer. In contrast, the triple -domain variants manifested distinct individual differences in LH activity. Binding affinity of the longest variant was 30-fold lower than that of the heterodimer. Shortening the length of the LH beta carboxyl-terminal region resulted in decreasing affinities between 210- and more than 480-fold. The potency of adenylate cyclase activation for LH/hCG also decreased as the c arboxyl length of LH beta subunit decreased. Thus, while minimally affectin g the FSH activity, truncating the carboxyl end of the LH beta subunit in t he triple-domain analogs alters the alignment of the LH beta-alpha domains, presumably at the junction between the subunits, and perturbs epitopes req uired for receptor binding. These data imply that the relative potencies of the two gonadotropin components of a triple-domain structure are independe nt from each other and can be selectively modified. Because there is a stro ng rationale for FSH/LH combinations for clinical protocols and patients ex hibit variations in metabolic responses in the ratio of FSH/LH, the ability to vary the individual activities represents a potential addition to the t herapeutic repertoire for treating infertility.