Prostaglandin E2 receptor heterogeneity and dysfunction in mammary tumor cells

Abstract

We have reported previously that murine mammary tumor cell subpopulations isolated from one spontaneous adenocarcinoma are heterogenous in terms of prostaglandin E₂ (PGE₂) syntnetic capacity. We have also shown that tumor-PGE₂ contributes to the ability of these cells to grow and metastasize in vivo (Fulton and Heppner: Cancer Research 45:4779–4784, 1985). In the present study, we have asked whether exogenous PGE₂ has direct effects on the proliferation of these cells in vitro and if such responses can be attributed to the capacity of these cells to (1) bind PGE₂ and (2) activate adenylate cyclase via the PGE₂ receptor. We report that PGE₂, at concentrations below 1 × 10⁻⁵ M, does not affect the proliferation rate of these cells. This unresponsiveness is not due to the absence of receptors for PGE₂. However, marked heterogeneity in receptor binding and function was detected in these closely related cell lines. Two metastatic lines (66 and 410.4) have high-affinity receptors for PGE₂ (average Kd = 4.3 × 10⁻⁹ M/L and 4.2 × 10⁻⁹ M/L, respectively) and similar binding capacities (4.1 × 10⁴ and 2.9 × 10⁴ binding sites, respectively). Two nonmetastatic lines, 410 and 67, have receptors with lower affinity (Kd = 8.3 × 10⁻⁹ M/L and 1.6 × 10⁻⁷ M/L, respectively) and binding capacities of 2.8 × 10⁵/410 cell or 7.3 × 10⁴/67 cell. A third nonmetastatic line (168) exhibits no specific binding. PGE₂ receptor stimulation leads to elevated intracellular cAMP in lines 66, 410, and 67. Line 410.4 cells appear to have a functional lesion in the PGE₂ receptor resulting in a failure to elevate cAMP in response to receptor occupancy. Adenylate cyclase can, however, be activated in these cells by cholera toxin, NaF, or forskolin. In comparison to the other cell lines, line 168 cells respond poorly to all cAMP-stimulating agents. Thus, we have found that PGE₂ binding is a heterogenous property for these cells, and, in addition, we have identified an apparent uncoupling of PGE₂ receptor to the adenylate cyclase system in one cell line.