Effect of 17β-estradiol on chondrocyte membrane fluidity and phospholipid metabolism is membrane-specific, sex-specific, and cell maturation-dependent

In this study we examined the hypothesis that 17β-estradiol exerts both rapid and direct, nongenomic effects on cells in the endochondral pathway. To do this, we used a cell culture model in which chondrocytes at two distinct stages of cell maturation are isolated from the costochondral cartilage of male and female rats, and examined the short-term effect of 17α- and 17β-estradiol on [[¹⁴C]arachidonic acid turnover in the cell layer and phospholipase A₂ specific activity in plasma membranes and extracellular matrix vesicles isolated from similarly prepared cultures. In addition, the effect of 17α- and 17β-estradiol on plasma membrane and matrix vesicle membrane fluidity was assessed. The effect of hormone on arachidonic acid turnover was rapid, time- and concentration-dependent, stereo-specific, and cell maturation-specific. Only resting zone cells from female rats were affected, and only 17β-estradiol elicited a response. Similarly, only female rat resting zone chondrocytes exhibited a change in phospholipase A₂ activity after a 24 h exposure to hormone, causing an increase in enzyme activity in the matrix vesicles, but not plasma membranes. When isolated membranes were incubated directly with hormone, membrane fluidity was decreased in both plasma membranes and matrix vesicles isolated from female rat resting zone chondrocyte cultures. This nongenomic effect was dose-dependent and stereo-specific and differentially expressed in the two membrane fractions with respect to time course and magnitude of response. These results support the hypothesis that 17β-estradiol has a rapid action on chondrocyte membrane lipid metabolism and suggest that specific membrane components, characteristic of a particular sex and state of cell maturation, are involved in the nongenomic effects of this sex hormone on isolated matrix vesicles and plasma membranes.