MOLECULAR MECHANISMS OF REAPPEARANCE OF LUTEINIZING-HORMONE RECEPTOR EXPRESSION AND FUNCTION IN RAT TESTIS AFTER SELECTIVE LEYDIG-CELL DESTRUCTION BY ETHYLENE DIMETHANE SULFONATE

Considering the major role of LH in the control of Leydig cell !LC, de velopment and function. we aimed to characterize further the pattern o f LH receptor (LHR) expression in two experimental paradigms: the rat treated with ethylene dimethane sulfonate (EDS), in which the selectiv e destruction of preexisting mature LCs induces the proliferation and differentiation of newly formed LCs, a process that takes place in the presence of high levels of gonadotropins; and the EDS-rat treated wit h a high dose of testosterone (EDS + T), in which the LH secretion is suppressed, and consequently LC development after EDS arrested. In EDS rats, serum T was suppressed and testicular LHR binding became undete ctable on days 5 and 15 after treatment, The pattern of LHR messenger RNA (mRNA) expression was profoundly modified: only one of the splice variants [1.8-kilobase (kb)] persisted, whereas the others disappeared . On days 20 and 45 after EDS, along with LC repopulation, serum T and LHR binding recovered, and the pattern of LHR mRNA expression gradual ly returned to that resembling controls. In EDS + T rats. a similar dr op in testicular LHR binding and change in the pattern of LHR mRNA exp ression was detected on days 5 and 15 after treatment. However, on day s 20 and 45, no recovery either in LHR binding or in expression of the longer LHR mRNA splice variants was observed, showing that LH is need ed to induce LHR expression in repopulating LCs, at least to a quantit atively significant level. To gain further insight into the mechanism( s) by which LH acts on LC precursors, the translational status of the 1.8-kb LHR transcript, persistently expressed after EDS, was analyzed and compared with that of the 6.8-kb message. In polysome distribution analysis of total testicular RNA, the 6.8-kb LHR message was highly a ssociated with polysomes, whereas the 1.8-kb variant was mainly locali zed to prepolysomal fractions, both in control and EDS testes, thus pr edicting lower translational efficiency. In addition, considering that only LCs express LHRs in the testis, the time course of the reappeara nce of functional receptors was mapped by evaluating testicular respon siveness to human recombinant LH in vitro, No response to LH stimulati on was detected 5 days after EDS. However, cAMP response to LH was obs erved on days 15 and 20, regardless of the presence of high (EDS) or s uppressed (EDS + T) LH in the donor animal. Hence, the appearance of f unctional LHRs, qualitatively, can take place in the absence of measur able LH levels. In EDS-treated rats, the appearance of the cAMP respon se coincided with those of pregnenolone, progesterone, and T. In contr ast, no LH-induced steroid release was observed in EDS + T rats, indic ating that steroidogenic response in developing LC requires LH priming . In conclusion, the appearance of functional LHRs, at a low level of expression. in LC precursors is an LH-independent developmental event, essential for the subsequent LH-dependent maturational steps, includi ng the onset of steroidogenesis and increased LHR expression. In addit ion, our results cast doubt on a major functional role of the truncate d (1.8-kb) form of LHR mRNA which persists after EDS at a high level o f expression, in the early Leydig cell precursors.