Differential effects of oral and transdermal menopausal hormone therapy on prostacyclin and thromboxane in platelets

Abstract

Abstract Menopausal hormone therapies (MHT) may increase thrombotic risk but modulate endothelial function and reduce development of vascular lesions. This study compared effects of MHT on prostanoid-modulated adenosine triphosphate (ATP) secretion from platelets in relationship with endothelial reactive hyperemia (RH) index and carotid intima medial thickness (CIMT). Participants were healthy, recently menopausal women of the Kronos Early Estrogen Prevention Study (KEEPS) randomized to one of three treatments: oral conjugated equine estrogen (oCEE, 0.45 mg/day), transdermal 17β-estradiol (tE2, 50 μg/day) each with intermittent oral progesterone or placebo pills and patch (PL). Prostacyclin and thromboxane A2 were assessed by quantification of their stable metabolites (6-keto-prostaglandin F1α, 6-k-PGF1α; thromboxane B2, TXB2), using ELISA. Dense granule ATP secretion from activated platelets was determined by bioluminescence; RH and CIMT were determined by fingertip tonometry and ultrasound, respectively. After 48 months of treatment, platelet content of 6-k-PGF1α and TXB2 was significantly lower in oCEE compared to the PL. Inhibition of ATP secretion by exogenous activation of cAMP associated with platelet 6-k-PGF1α (r = -0.41, P = 0.04) and TXB2 (r = 0.71, P = 0.0005) only in the oCEE group. Serum and platelet content of 6-k-PGF1α and TXB2 associated positively in the PL and tE2 groups. Serum 6-k-PGF1α positively associated with RH in the oCEE group (r = 0.73, P = 0.02), while serum TXB2 positively associated with CIMT in the tE2 group (r = 0.64, P = 0.01). Thus, oCEE and tE2 differentially affect prostanoid-mediated platelet secretory pathways but alone would not account for an increased thrombotic risk for oral MHT. Furthermore, platelet-derived prostanoids may contribute to RH and vascular remodeling in healthy menopausal women.

Keywords: Endothelial function; KEEPS; estrogen; estrone.

Figure 1.

Content of stable metabolites of prostacyclin (6‐k‐PGF1_α, left panel), and thromboxane (TXB₂, middle panel), and their ratio (right panel) in platelets of menopausal women treated either with placebo (PL), transdermal 17β‐estradiol (tE2, 50 μg/day), or oral conjugated equine estrogen (oCEE, 0.45 mg/day) for 48 months. Data are shown as means ± SEM, n = unique individuals from whom platelet contents were measured. Asterisk denotes significant difference from PL group, P < 0.05.

Figure 2.

Relationships between serum and platelet levels of prostacyclin (6‐k‐PGF1_α) and thromboxane (TXB₂) in menopausal women treated with either placebo (PL), transdermal 17β‐estradiol (tE2, 50 μg/day), or oral conjugated equine estrogen (oCEE, 0.45 mg/day) for 48 months. Each point represents an individual.

Figure 3.

Percent inhibition of dense granule adenosine triphosphate (ATP) secretion from platelets by PGE₁. Adenosine triphosphate secretion in response to thrombin receptor agonist peptide (TRAP 10 μmol/L) was measured in tandem in paired samples of platelets from each participant in the absence and presence of PGE₁ (~500 nmol/L). Data are shown as the percent change in ATP secretion in the presence of PGE₁. The greater the inhibition suggests greater increase in cAMP by PGE₁. Asterisk denotes significant difference from PL group by Student's t‐test, P < 0.05.

Figure 4.

Associations of inhibition of adenosine triphosphate (ATP) secretion with platelet content of stable metabolites of prostacyclin (6‐k‐PGF1_α) and thromboxane (TXB₂) in platelets of menopausal women treated with either placebo (PL), transdermal 17β‐estradiol (tE2, 50 μg/day), or oral conjugated equine estrogen (oCEE, 0.45 mg/day) for 48 months. Each point represents an individual.

Figure 5.

Associations of serum prostanoids with RH index (upper panels) and CIMT (lower panels) in menopausal women treated with either placebo (PL), transdermal 17β‐estradiol (tE2, 50 μg/day), or oral conjugated equine estrogen (oCEE, 0.45 mg/day) for 48 months. Each point represents an individual.

Figure 6.

Schematic depicting regulation of dense granule ATP secretion by cytosolic prostanoids in platelets. AA, arachidonic acid; ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; COX, cyclooxygenase; PAR, protease‐activated receptor; PGE₁, prostaglandin E₁; PG H2, prostaglandin H2; PGI₂, prostacyclin; TRAP, thrombin receptor agonist peptide; TXA₂, thromboxane; minuses indicate inhibition, pluses indicate activation.