Description:
Prostaglandin F2α, pharmaceutically termed carboprost, is a naturally occurring prostaglandin used in medicine to induce labor and as an abortifacient. In domestic mammals, it is produced by the uterus when stimulated by oxytocin, in the event that there has been no implantation during the luteal phase. It acts on the corpus luteum to cause luteolysis, forming a corpus albicans and stopping the production of progesterone. Action of PGF2α is dependent on the number of receptors on the corpus luteum membrane. The PGF2α isoform 8-iso-PGF2α was found in significantly increased amounts in patients with endometriosis, thus being a potential causative link in endometriosis-associated oxidative stress.
Uses:
When injected into the body or amniotic sac, PGF2α can either induce labor or cause an abortion depending on the concentration used. In small doses (1–4 mg/day), PGF2α acts to stimulate uterine muscle contractions, which aids in the birth process. However, during the first trimester and in higher concentrations (40 mg/day), PGF2α can cause an abortion by degrading the corpus luteum, which nourishes the fetus in the womb. Since the fetus is not viable outside the womb by this time, the lack of sustenance starves and aborts the fetus after a day or two.
Uses in bodybuilding:
Skeletal muscle growth requires multiple steps to form large multinucleated muscle cells. Molecules that stimulate muscle growth may be therapeutic for muscle loss associated with aging, injury, or disease. However, few factors are known to increase muscle cell size. We demonstrate that prostaglandin F2α (PGF2α) as well as two analogues augment muscle cell size in vitro. This increased myotube size is not due to PGF2α-enhancing cell fusion that initially forms myotubes, but rather to PGF2α recruiting the fusion of cells with preexisting multinucleated cells. This growth is mediated through the PGF2α receptor (FP receptor). As the FP receptor can increase levels of intracellular calcium, the involvement of the calcium-regulated transcription factor nuclear factor of activated T cells (NFAT) in mediating PGF2α-enhanced cell growth was examined. We show that NFAT is activated by PGF2α, and the isoform NFATC2 is required for PGF2α-induced muscle cell growth and nuclear accretion, demonstrating the first intersection between prostaglandin receptor activation and NFAT signaling. Given this novel role for PGF2α in skeletal muscle cell growth, these studies raise caution that extended use of drugs that inhibit PG production, such as nonsteroidal anti-inflammatory drugs, may be deleterious for muscle growth.
Skeletal myogenesis follows an ordered set of cellular events involving cell cycle exit of myoblasts, their subsequent differentiation, and fusion to form multinucleated myofibers in vivo or myotubes in vitro. In most cases, mammalian muscle growth requires the fusion of differentiated muscle cells with the growing multinucleated muscle cell.1 By adding additional nuclei to muscle cells during growth, an increased number of nuclei are contained within one cytoplasm, allowing each nucleus to regulate more cytoplasm.2 These fusion events allow increased protein synthesis and increases in cell size. Understanding the molecular pathways that regulate muscle growth are important for treating muscle disorders and loss of muscle mass during aging. However, few molecules are known to stimulate increased muscle cell fusion and skeletal muscle growth.
Prostaglandins (PGs) are paracrine signaling molecules that are synthesized from arachidonic acid in response to cytokines, cell injury, or growth factors.3 The synthesis of PGs involves the metabolism of arachidonic acid by cyclooxygenase enzymes into an intermediate PG. Specific PG synthases convert this intermediate PG into the primary PG molecules (PGE2, PGF2α, PGI2, and PGD2). Once produced, PGs are secreted and mediate signaling through G protein–coupled receptors that are distinct for each PG. Activation of PG receptors leads to an array of effects in a range of cell and tissue types, including skeletal muscle.
PGs have been implicated in skeletal muscle growth. For skeletal muscle to grow, a population of myoblasts must be available to differentiate and fuse with the myofiber. Different PGs can control proliferation4, differentiation5, as well as fusion of myoblasts.6 Once myotubes are formed, muscle cell size continues to increase through enhanced protein synthesis. PGs regulate this stage of growth by altering both protein degradation and protein synthesis within myotubes.7 Consistent with a general role for PGs in skeletal muscle growth, inhibition of PG production blocks growth of myofibers in vivo.8 These data suggest that PGs regulate muscle cell growth by influencing multiple steps of myogenesis.
Signaling pathways that are activated by calcium are important for skeletal muscle growth.9 PGs have been shown to activate increases in intracellular calcium within a variety of muscle cell types.10 Specifically, PGF2α and PGE2 can activate increases in intracellular calcium through their receptors, PGF2α receptor (FP) and EP1/EP3, respectively.11 One calcium-regulated pathway involved in skeletal muscle growth is the family of transcription factors, nuclear factor of activated T cells.12 Several NFAT isoforms are expressed in skeletal muscle, and the regulation of individual NFAT isoforms appears to occur at the level of nuclear translocation.13 For instance, the NFATC2 isoform is activated only in newly formed or nascent myotubes but not at other stages of myogenesis.14 Previously, we have shown that the NFATC2 isoform is important for skeletal muscle growth15, but upstream activators of this pathway have not been elucidated.
Because PGF2α can increase intracellular calcium and calcium signaling pathways are important for numerous stages of myogenesis that contribute to muscle growth, we hypothesized that PGF2α may regulate skeletal muscle growth. Although PGF2α can regulate the final stages of muscle growth by inducing protein synthesis16, we sought to investigate the role of PGF2α in other steps of myogenesis that require calcium, such as differentiation17 and fusion.18 We show that PGF2α enhances myonuclear accretion after the initial formation of myotubes, leading to increases in myotube size. Furthermore, the growth induced by PGF2α occurs through an NFAT-dependent pathway. These data implicate not only a novel function for PGF2α in skeletal muscle growth but also a novel intersection between prostaglandin and NFAT signaling pathways.
To test the hypothesis that PGF2α has a role in the growth of skeletal muscle cells, differentiating primary muscle cultures were treated with different doses of PGF2α or with a stable synthetic analogue of PGF2α, 17-phenyl trinor PGF2α (17-phPGF2α). After 24 h, the majority of cells are differentiated and have formed a few multinucleated cells. Although no difference is apparent between the vehicle- and drug-treated groups at 24 h, after 48 h, drug-treated myotubes are larger in size as compared with vehicle, suggesting that PGF2α can regulate muscle growth.
The formation of a multinucleated cell requires multiple cellular processes including the formation of an adequate number of myoblasts through cell proliferation, their differentiation, and subsequent membrane fusion. To determine if PGF2α increases cell proliferation and/or cell survival in our assay, the DNA content was quantified. No difference exists in the DNA content between PGF2α and vehicle-treated cells. Differentiation was assessed in vehicle and PGF2α-treated cultures at 24 and 48 h by immunostaining the cultures with embryonic myosin heavy chain (EMyHC), a marker of differentiation, and counting the number of nuclei contained within EMyHC-positive cells. The percentage of differentiated cells does not differ between vehicle- or PGF2α-treated cultures at 24 or 48 h. In addition, the fusion index was determined. The percentage of nuclei in myotubes is not different between vehicle- or PGF2α-treated cultures. These data suggest that PGF2α does not affect myoblast proliferation or survival, differentiation, or fusion to lead to muscle growth.
After the initial fusion of myoblasts that forms a multinucleated cell, cell growth occurs through the fusion of differentiated muscle cells with the nascent myotube to increase myonuclear number and cell size.19 Although the fusion index determines the percentage of the total cell population that has fused within muscle cultures, it is not a measure of the number of myonuclei within individual myotubes. By analyzing the myonuclear number within individual myotubes, cell fusion that contributes to muscle growth can be determined. To determine if PGF2α increases muscle cell size by enhancing addition of myonuclei to existing myotubes, the number of nuclei in individual myotubes was determined in cultures treated with vehicle and PGF2α for 48 h. With vehicle treatment, an equal percentage of myotubes are present with two to four nuclei as those with five or more nuclei. However, with 10−6 M PGF2α treatment, a significant increase occurs in the percentage of myotubes with five or more nuclei with a parallel decrease in the percentage of myotubes with two to four nuclei. This dose of PGF2α has been shown to give maximal effects in assays using cardiac and smooth muscle cells.20 Other doses of PGF2α tested do not significantly differ from vehicle. Similarly, treatment of cells with 17-phPGF2α also increases myonuclear number to the same extent as PGF2α but at lower doses, which is consistent with its higher affinity and greater metabolic stability.21 These data suggest that PGF2α increases cell fusion with myotubes to facilitate increases in muscle cell size.
To further study the effect of PGF2α on increases in myonuclear number, differentiating muscle cells were treated with PGF2α at different stages of fusion. To determine if PGF2α can act at the initial stages of cell fusion, cells were only treated with PGF2α at the onset of differentiation at 0 h in differentiation media (DM). To determine if PGF2α acts during later fusion events, cells were only treated at 24 h, a time when cells are beginning to fuse and a few multinucleated muscle cells are present. In both cases, the nuclear number of individual myotubes was analyzed at 48 h. When PGF2α is administered at the onset of differentiation (0 h), no significant difference exists in the percentage of myotubes with five or more nuclei as compared with vehicle-treated cultures. However, when PGF2α is administered at 24 h, the percentage of myotubes with five or more nuclei is significantly higher than vehicle-treated cells. This difference is comparable to the increase in nuclear number when PGF2α is added at both 0 and 24 h. These data further confirm that PGF2α acts at later stages of muscle cell fusion to allow an increase in muscle cell size.
Side Effects:
In 17 patients with cardiac extrasystoles, prostaglandin (PG) F2a (= alpha) administered intravenously at consecutive infusion rates of 25 to 100 micrograms/min showed the following effects in a dose-dependent manner: prolongation of total electromechanical systole, pre-ejection period, and left ventricular ejection time, indicating a negative inotropic activity, increase in systolic and diastolic blood pressure, and incidence of side effects. An additional 2 patients were excluded from the uniform evaluation because the PGF2a dosage was changed owing to a clear antiarrhythmic action in one and serious side effects in the other. The results supplement clinical-pharmacological findings with PGF2a.
1 Darr and Schultz, 1989; Rosenblatt and Parry, 1992; Phelan and Gonyea, 1997; Barton-Davis et al., 1999; Horsley et al., 2001; Mitchell and Pavlath, 2001
2 Allen et al., 1999
3 Funk, 2001
4 Zalin, 1987
5 Schutzle et al., 1984
6 Zalin, 1977; David and Higginbotham, 1981; Entwistle et al., 1986; Rossi et al., 1989
7 Rodemann and Goldberg, 1982; Palmer, 1990; Vandenburgh et al., 1990
8 Templeton et al., 1986; McLennan, 1987
9 Abbott et al., 1998; Dunn et al., 1999; Musaro et al., 1999; Semsarian et al., 1999; Delling et al., 2000; Friday et al., 2000; Horsley et al., 2001; Mitchell et al., 2002
10 Asboth et al., 1996; Chen et al., 1997; Yew et al., 1998; Yousufzai and Abdel-Latif, 1998
11 Breyer et al., 2001
12 NFAT; Horsley and Pavlath, 2002
13 Abbott et al., 1998
14 Abbott et al., 1998
15 Horsley et al., 2001
16 Vandenburgh et al., 1990
17 Shainberg et al., 1969; Morris and Cole, 1979
18 Shainberg et al., 1969; Knudsen and Horwitz, 1977
19 Bate, 1990; Horsley et al., 2001
20 Adams et al., 1996; Griffin et al., 1998; Kunapuli et al., 1998; Yew et al., 1998; Katsuyama et al., 2002
21 Lake et al., 1994; Pierce et al., 1997
