Correlation of HSP110 expression with all-trans retinoic acid-induced apoptosis

In a previous study, we observed the strong expression of a stress protein of the HSP100/Clp family (HSP110) in apoptotic mesectodermal cells during e arly mouse facial development. In the present study, we describe the strong expression of the same HSP110 in mesectodermal cells undergoing apoptosis after all-trans retinoic acid ( RA) administration. We used a teratological model known to increase cell deaths mainly in the f irst and second branchial arches during mammalian cephalogenesis: the treat ment of E9 mouse embryos with all-trans RA, which results in craniofacial m alformations comparable to those that characterize mandibulofacial dysostos is in man. Pregnant NMRI mice were treated with 60 mg/kg body weight of all -trans RA, given orally on day 9 of gestation; embryos were taken 4, 12 or 24 hr after RA administration. The apoptotic pattern of RA-induced cell deaths was confirmed using the dUT P biotin nick-end labeling (TUNEL) method and transmission electron microsc opy (TEM). HSP110 expression was detected using an immunohistochemical approach. The increase in the number of TUNEL-positive cells and HSP110-positive cell s after all-trans RA administration was quantified in the first branchial a rch using a computerized method. Twelve hours after RA administration, the increase in the number of HSP110- positive cells is greater than the increase in the number of TUNEL-positive cells. Twenty-four hours after RA administration, only TUNEL-positive cell s remain strong in number. We suggest that HSP110 expression could represent a biochemical event of ap optotic cell death induced by RA, associated with early stages of the apopt otic process. In order to find out if HSP110 expression resulted from neosynthesis, we pe rformed in situ hybridization, which demonstrated that the expression of HS P110 occurred at the level of mRNA.