We previously reported that about two-thirds of [125I]oestradiol-labelled cytosolic ERs from breast cancer samples eluted as low-molecular-weight isoforms (≤ 37 kDa, size-exclusion fast pressure liquid chromatography [FPLC]). These isoforms failed to adsorb strongly to hydroxylapatite at high ionic strength, a property that was ascribed to receptors devoid of amino-terminal ABC domains. In view of recent data concerning intracellular proteolysis of several transcriptional regulators, the possibility of such behaviour for ER was assessed.
The clinical significance of ER measurement in breast cancer cytosols is well established; approximately 50% of ER-positive cases respond to endocrine therapy. Whether such a poor correlation is related to a high proportion of cleaved ER is a question of prime importance. Failure of routine ER assays to discriminate between full-length and cleaved receptors led us to develop an oestradiol-binding assay based on hydroxylapatite adsorption.
The aims of the present study were to demonstrate that hydroxylapatite adsorption assay easily identifies cleaved cytosolic ER forms and to assess the origin of such ER forms.
Breast cancer cytosols classified as ER-positive according to [3H]oestradiol-binding assay (dextran-coated charcoal [DCC]) were subjected to hydroxylapatite adsorption. ER isoforms covalently labeled with [125I]tamoxifen aziridine (TAZ) released from this matrix with 0.5 mol/l KCl were subsequently immunoprecipitated with a panel of monoclonal antibodies raised against various domains of ER (H222 [E], H226 [C] or ER1D5 [AB]) before being subjected to SDS-gel electrophoresis.
Three approaches were used to identify the origins of the cleaved ER forms: potential truncated ER-α messenger RNAs that may encode ER isoforms of low molecular weights (Northern blot assay) were sought by using ER-α full-length probe; heat treatment of tumour cytosols in the absence or presence of a cocktail of protease inhibitors was performed; and the molecular weight of intracellular ER molecules was determined by in situ [125I]TAZ-labelling, which minimizes ER proteolysis.
Breast cancer samples classified as ER-positive according to both biochemical (cytosolic DCC assay) and histochemical (ER1D5 monoclonal antibody) criteria were labelled with [3H]oestradiol and were subsequently subjected to hydroxylapatite adsorption. Hydroxylapatite extraction index (EI) is defined as a ratio of the specifically bound [3H]oestradiol released from the hydroxylapatite matrix with KCl to the total amount of the specifically bound [3H]oestradiol extracted successively with KCl and ethanol: EI= ([3H]oestradiol) [KCl] × 100/([3H]oestradiol) [KCl] + ([3H]oestradiol) [EtOH]. The EI was calculated for each cytosol in order to evaluate the amount of cleaved ER forms present. Persistence of adsorption ER to hydroxylapatite in the presence of KCl (low EI) and ER1D5 positivity established by immunohistochemistry are two independent criteria for the presence of amino-terminal ABC domains. We therefore assessed whether hydroxylapatite determinations performed on cytosols are related to immuno-histochemistry data.
Cytosol pools labelled with [125I]TAZ gave different electrophoretic patterns depending on the nature of the anti-ER monoclonal antibody used in the immunoprecipitation step preceding electrophoresis. The carboxyl-terminal-specific antibody H222 precipitated all ER isoforms (full-length 67 kDa ER, and cleavage products of 50 and 37-28 kDa), whereas the amino-terminal-specific antibodies H226 and ER1D5 precipitated only the full-length and a partially truncated isoform. Adsorption of this labelled cytosol pool onto hydroxylapatite with subsequent KCl extraction yielded ER isoforms with molecular weights between 37 and 28 kDa when immunoprecipitation of the elutes was carried out using H222. The absence of these isoforms after exposure of the elutes to H226 or ER1D5 demonstrated truncation of these isoforms at a site(s) downstream of ABC domains.
Total RNA from 46 tumours was exposed to ER-α full-length probe (Northern blot). All tumours expressed a full-length 6.6-kb ER mRNA; small-sized isoforms were not recorded. A good correlation resulted when amounts of 6.6-kb ER mRNA estimated by densitometry were compared with corresponding [3H]oestradiol-binding capacities (DCC assay), thereby rejecting the concept that low-molecular-weight isoforms were encoded by truncated ER mRNA.
We next investigated whether such isoforms might be generated by proteolysis. Cytosol samples of a series of breast tumours were labelled with [125I]TAZ in the presence of a cocktail of protease inhibitors. These inhibitors failed to maintain the full-length 67 kDa ER by SDS-PAGE. In situ [125I]TAZ-labelling of receptors associated with a protein extraction procedure minimizing their proteolysis displayed multi-bands electrophoretic patterns, almost identical to those found under conventional methods. Hence, ER molecular heterogeneity appears to result from an intracellular proteolysis. ER1D5 immunostaining scores (ISs) of a series of 15 tumours were significantly correlated with ER levels, as measured by hydroxylapatite assay of corresponding cytosols (total number of binding sites). Sequential extraction of bound [3H]oestradiol from hydroxylapatite with KCl and ethanol revealed an EI of over 30% in the large majority of these cytosols, indicating a high frequency of cleaved ER isoforms. Of note, no significant correlation between IS and EI data was recorded, suggesting that ABC and E domains are separated at high ionic strength, but are apparently held together within the cell nucleus in oligomeric structures.
Endogenous proteolysis is a regulatory mechanism in many cellular processes, such as cell cycle progression and transcriptional regulation. The present data extend this concept to ER. Indeed, proteolysis-generated ER fragments appear to be held together within the cell in oligomeric structures. Because ER proteolysis is probably relevant to several oestrogen target tissues, we suggest that the protein environment, which differs among tissues, may be a factor of major importance in the formation of distinct oligomeric structures, which elicit specific biological responses. The possibility of heterogeneous association between cleaved ER and regulatory proteins might perhaps result in a spectrum of such transcriptional activities. In this context, we propose that a complementary hydroxylapatite extraction assay (EI assessment) should be added to the usual tests to identify ER-positive tumours. Such a complementary test would provide an estimate of the level of cleaved ER forms, which may have biological and/or clinical relevance.
Keywords: estrogen receptor domains, hydroxylapatite, immunohistochemistry, primary breast cancer