Expression of non-muscle myosin isoforms in rabbit myometrium is estrogen-dependent

The putatative effects of different estrogen levels on the expression of non-muscle myosin isoforms in rabbit myometrium have been investigated using three monoclonal anti-platelet myosin heavy chain (MyHC) antibodies (NM-F6, NM-G2, and NM-A9). Western blotting analysis of proteolytic digests of human platelet actomyosin indicates that these antibodies are specific for three distinct epitopes. Comparative immunofluorescence tests on cultered human fibroblasts with polyclonal sequence-specific anti-MyHCA antibody suggest that the patterns of NM-F6, NM-.G2 and NM-A9, although similar, do not overlap with that of type-A MyHC. Distribution of NM myosin isoforms has been studied in indirect immunofluorescence assays using cryosections of tissues from rabbits at various stages of development, pregnancy, or from ovariectomized, 17β-estradiol-treated ovariectomized, and human chorionic gonadotropin-treated animals. Non-muscle myosin antigenicity is still present in the myometrium when the female becomes sexually competent. The immunoreactivity of non-muscle myosin for NM-F6 is steroid-independent, since it does not change with pregnancy or ovariectomy, but that of NM-G2 is estrogen-dependent; the latter disappears during pregnancy and in ovariectomized animals treated with estradiol, whereas it is expressed in ovariectomized rabbits. Although non-muscle myosin immunoreactivity for NM-A9 is detectable under all the experimental conditions, it can assume different patterns of intracellular distribution in vitro (punctate vs filamentous), depending on culture conditions and the presence of estrogens.