Mitogen-induced up-regulation of non-smooth muscle isoform of alpha-tropomyosin in rat aortic smooth muscle cells

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

C2-ceramide induces vasodilation in phenylephrine-induced pre-contracted rat thoracic aorta: role of RhoA/Rho-kinase and intracellular Ca2+ concentration

It is known that ceramide may play an important regulatory role in vascular tone although its effect on vascular tone and the mechanisms involved are controversial. The present study was designed to investigate the effects of ceramide and its key initial regulators, TNF-alpha and neutral sphingomyelinase (SMase), on vascular tone of isolated rat thoracic aortic rings and elucidate the mechanisms involved in the changes in vascular tone induced by ceramide. Contractile responses and Fura-2 Ca2+ signals were measured in rat thoracic aortic rings or strips. 10(-5) M C2-ceramide, 0.1 U/ml neutral sphingomyelinase (SMase), and 5x10(-7) g/ml TNF-alpha had no effect on resting tone in rat thoracic aortic rings. However, in phenylephrine-induced pre-contracted rings, treatment with ceramide, SMase, and TNF-alpha evoked a gradual but sustained vasodilation. Vasodilation effect in response to 10(-5) M C2-ceramide was not significantly changed by the absence or presence of endothelium, a cyclooxygenase pathway inhibitor (10(-6) M indomethacin), or PKC inhibitors (10(-5) M H-7 & 5x10(-7) M calphostin-C). Pretreatment with 1 microM Y-27632, a RhoA/Rho-kinase inhibitor, significantly inhibited the phenylephrine-induced contraction itself as well as the C2-ceramide-induced vasodilation. Pre-treatment with 10(-5) M C2-ceramide had no effect on phasic rise in [Ca2+]i and tension evoked by stimulation with 10(-8) M phenylephrine, but post-treatment of C2-ceramide significantly reduced the phenylephrine-induced secondary tonic [Ca2+]i and tension plateau. Our results indicate that C2-ceramide induces vasodilation in phenylephrine-induced pre-contracted rat thoracic aorta. Furthermore, inhibition of phenylephrine-induced activation of RhoA/Rho-kinase pathway as well as phenylephrine-induced elevations in [Ca2+]i are clearly a key factors in C2-ceramide-induced vasodilation.

Intracellular alkalinization induces Ca2+ influx via non-voltage-operated Ca2+ channels in rat aortic smooth muscle cells

In smooth muscle, the cytosolic Ca2+ concentration ([Ca2+](i)) is the primary determinant of contraction, and the intracellular pH (pH(i)) modulates contractility. Using fura-2 and 2',7'-biscarboxyethyl-5(6) carboxyfluorescein (BCECF) fluorometry and rat aortic smooth muscle cells in primary culture, we investigated the effect of the increase in pH(i) on [Ca2+](i). The application of the NH(4)Cl induced concentration-dependent increases in both pH(i) and [Ca2+](i). The extent of [Ca2+](i) elevation induced by 20mM NH(4)Cl was approximately 50% of that obtained with 100mM K(+)-depolarization. The NH(4)Cl-induced elevation of [Ca2+](i) was completely abolished by the removal of extracellular Ca2+ or the addition of extracellular Ni2+. The 100mM K(+)-induced [Ca2+](i) elevation was markedly inhibited by a voltage-operated Ca2+ channel blocker, diltiazem, and partly inhibited by a non-voltage-operated Ca2+ channel blocker, SKF96365. On the other hand, the NH(4)Cl-induced [Ca2+](i) elevation was resistant to diltiazem, but was markedly inhibited by SKF96365. It is thus concluded that intracellular alkalinization activates the Ca2+ influx via non-voltage-operated Ca2+ channels and thereby increases [Ca2+](i) in the vascular smooth muscle cells. The alkalinization-induced Ca2+ influx may therefore contribute to the enhancement of contraction.

Mechanism of down-regulation of L-type Ca(2+) channel in the proliferating smooth muscle cells of rat aorta

The mechanism of down-regulation of L-type Ca(2+) channel (L-VOC) was investigated in rat aortic smooth muscle cells in primary culture. On culture days 3-5, the cells actively incorporated the 5-bromo-2'-deoxy-uridine (BrdU), and did not respond to K(+) depolarization nor express alpha(1C) subunit of L-VOC. At confluence on day 8, BrdU incorporation decreased, and the cells up-regulated alpha(1C) subunit mRNA, expressed alpha(1C) subunit protein at cell periphery, and responded to K(+) depolarization. Treating the proliferating cells on day 3 with serum-free media or 10 microM PD98059, a MAP kinase kinase inhibitor, for 2 days induced the expression of alpha(1C) subunit protein and the responsiveness to K(+) depolarization. However, the serum starvation, but not PD98059, decreased the BrdU incorporation and increased the alpha(1C) subunit mRNA. It is concluded that the expression of L-VOC is substantially suppressed in the proliferating cells due to two mechanisms; a MAP kinase-mediated post-transcriptional down-regulation and the transcriptional down-regulation by additional mitogenic signals.