Establishment and characterization of a conditionally immortalized smooth muscle/myometrial-like cell line

A novel smooth muscle/myometrial-like cell line, SMU1-10, has been generated from the uterus of a H-2k(b)-tsA58 transgenic mouse carrying a thermolabile SV40 large T-antigen gene. These cells grow continuously when maintained at the permissive temperature (33°C) for the SV40 large T- antigen but stop dividing when placed at the non-permissive temperature (39°C) and ultimately die within 3 weeks. All of the SMU1-10 cells produce smooth muscle α-actin (SMAA) at both 33°C and 39°C. A subset of the cells also contain smooth muscle γ-actin (SMGA), a hallmark of smooth muscle differentiation, and the fraction of cells staining for this actin increases from about 1% when maintained for three days at 33°C to as much as 30% at 39°C over the same length of time. However, the appearance of SMGA in SMU1- 10 cells appears to be regulated mainly at a post-transcriptional level since in situ hybridization indicates that all cells contain SMGA mRNA at both 33°C and 39°C. SMU1-10 cultures also contain smooth muscle myosin heavy chain (SM-MHC) and SM22α, both of which are only found in smooth muscle of the adult mouse. Three additional smooth muscle (myometrium)-related markers, connexin 43, the thromboxane A₂ receptor, and the progesterone receptor also are present in these cells. At the nonpermissive temperature for SV40 large T-antigen, the both level of SMGA mRNA and the number of cells staining for this actin are significantly increased in the presence of progesterone, a process that is similar to the upregulation of SMGA in the myometrium late in pregnancy. Overall, SMU1-10 cells provides a potentially useful in vitro model system to study smooth muscle/myometrial differentiation.