Gestational Changes in the Levels of Transforming Growth Factor-b1 (TGFb1) and TGFb Receptor Types I and II in the Human Myometrium*

As term approaches, a number of key proteins [contraction-associated proteins (CAPs)] are expressed within the human myometrium that are essential for the activation of powerful coordinated contractions during labor. The nature of the signals that switch on the synthesis of CAPs in vivo is not known. The ryanodine-sensitive intracellular Ca release channel (RyR2) is a CAP whose expression in vitro is activated by transforming growth factor-b (TGFb). The present experiments were performed to determine whether TGFb and TGFb receptors are present in the human myometrium at term and to explore the idea that they might form part of a signaling system in vivo. TGFb receptor types I and II, but not III, were demonstrated in myometrial smooth muscle in tissue taken from nonpregnant, pregnant nonlaboring, and spontaneous laboring women. Western blotting was used subsequently to determine the relative expression of TGFb receptor types I and II. Using nonpregnant myometrium as a baseline control the levels of expression of receptor types I and II were significantly increased by 168 6 19% (n 5 6) and 162 6 22% (n 5 7) in pregnant nonlaboring myometrium. In spontaneous laboring myometrium the levels of TGFb receptor type I and II expression were 93 6 12% (n 5 6) and 85 6 11% (n 5 7), respectively, compared to nonpregnant control values and were significantly lower than levels in pregnant nonlaboring tissues. The total TGFb1 levels in the myometrial tissues were 334 6 10, 534 6 73, and 674 6 106 pg/g tissue wet wt in nonpregnant, pregnant nonlaboring, and spontaneous laboring myometrium (n 5 3 in each group), respectively. Thus, the TGFb signaling system appears to be up-regulated in the myometrium before the onset of parturition. The apparent loss of receptors in the spontaneous laboring samples in the presence of elevated total levels of TGFb may be indicative of agonist-induced receptor downregulation. These observations support the idea that cytokines, in particular TGFb1, may play a role in the normal processes that prepare the myometrium for parturition at term. (J Clin Endocrinol Metab 83: 2987–2992, 1998) T ABILITY of the myometrium to contract efficiently at term may be determined by a number of specific genes that encode for key proteins controlling the mechanisms underlying excitation contraction coupling: the contraction-associated proteins (CAPs) (1). By triggering the synthesis of these key proteins, the smooth muscle cells of the uterus can be transferred from a state of overall quiescence into one primed for activation (2). Currently, the list of CAPs includes oxytocin receptors (3), cyclooxygenase enzymes (4), gap junctions (5), ion channels (6, 7), and the ryanodinesensitive intracellular Ca release mechanisms, RyR2 (8). The majority of the work performed to date has focused on the role that each system might play in affecting myometrial contractility. However, the nature of the external influences and cellular processes that trigger and control the activation of these CAP genes is poorly understood. The expression of the gap junction protein connexin 43 (Cx43) has been studied in detail (9–14). Treatment of isolated human myometrial tissue with media containing high estrogen and low progesterone concentrations resulted in an increase in Cx43 expression (11). In addition, activation of human myometrial cells with phorbol ester increased the messenger ribonucleic acid levels of c-fos and c-jun, followed by an increased expression of Cx43 messenger ribonucleic acid, which resulted in a significant increase in Cx43 protein levels 2–4 h later (13). This work has been interpreted to suggest that Cx43 gene expression is regulated by changes in the estrogen/progesterone ratio working through the activation of protein kinase C and the production of specific transcription factors (11–13). Less is known about the activation of other CAP genes. We have reported that the expression of the RyR2 isoform of the ryanodine-sensitive intracellular Ca release channel on the sarcoplasmic reticulum is up-regulated in the human myometrium as term approaches and that this increase in expression can be mimicked in vitro by the cytokine transforming growth factor-b1 (TGFb1) (8). This leads to the idea that TGFb1 could be playing a similar role in activation of the RyR2 gene in the processes of normal preparation of the myometrium in vivo. These observations raise the possibility that cytokines may form part of a signaling system, different from that activating Cx43, responsible for the activation of specific CAP genes