Samalecos A, Gellersen B
Systematic expression analysis and antibody screening do not support the existence of naturally occurring PR-C, PR-M or other truncated progesterone receptor isoforms. [JOURNAL ARTICLE]
Endocrinology 2008 Jul 10.
Functional progesterone withdrawal associated with human parturition has been ascribed to various mechanisms modulating the function of the classical progesterone receptors, PR-B and PR-A, in utero. These include upregulation of the inhibitory PR-C isoform, described as a 60 kDa protein arising from translation initiation at codon 595. Our initial attempts to detect PR-C yielded uninterpretable results. To systematically validate antibodies for immunodetection of PR isoforms, we generated expression vectors for PR variants originating from putative start codons AUG-289, -301, -595, -632 and -692 in addition to those for PR-B and PR-A, and for alternative splice variants PR-T, PR-S and PR-M. All constructs were subjected to in vitro and in vivo translation and immunoblotting with a panel of 13 PR antibodies. While antibodies raised against full-length PR were generally not capable of detecting N-terminally truncated forms, C-terminal antibodies did not or only weakly react with PR-B and PR-A but produced prominent non-specific signals. Thus, immunodetection of N-terminally truncated PR isoforms is prone to artifacts. Proteins of about 64 kDa were expressed from PR-289 and -301, but no corresponding endogenous forms were observed. PR-T, PR-S and PR-M cDNAs yielded no detectable translation products. No protein was translated from AUG-595 in our PR-C expression vector unless a Kozak sequence was introduced, and the product was not 60 but 38 kDa in size. Thus, the 60 kDa protein called PR-C does not originate from AUG-595 and is not a naturally occurring PR isoform.
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