Mitogen-activated protein kinases mediate changes in gene expression, but not cytoskeletal organization associated with cardiac muscle cell hypertrophy.

Abstract

Abstract. Shortly after birth, cardiac myocytes lose the ability to divide, and, in adult animals, heart muscle grows by a process of cellular hypertrophy where each individual cell gets larger. We have previously shown that activated Ras protein can induce markers of the hypertrophic phenotype, including atrial natriuretic factor (ANF) expression and organization of contractile proteins, and that Ras is at least partially required for the hypertrophic effect of phenylephrine. In the present study, we examine the requirement for the mitogen-activated protein kinases (MAP kinases) in the hypertrophic response induced by phenylephfine. We find that phenylephrine treatment results in the activation of the MAP kinases and that this activity is required for transactivation of the fos, ANF, and MLH promoters. However, inhibition of MAP kinases does not prevent phenylephrine-induced organization of actin. These results suggest that the signal transduction pathways leading to different hypertrophic responses diverge upstream of the MAP kinases but possibly downstream of Ras.

Inhibition of a signaling pathway in cardiac muscle cells by active mitogen-activated protein kinase kinase.

“…These data confirm that expression from all the promoters, including ANF and MLC-2, is inhibited when ERK activity is blocked, although it remains possible that other MAP kinase family members that are also inhibited by CL100 may play a role in activation of these promoters. Combined with the data from the experiments shown in Figure 2 and our previous studies (Thorburn et al, 1994a), we suggest that ERK activation is required for expression from the Fos, ANF, and MLC-2 promoters, and that ERK activation by MKK is sufficient to induce expression from the Fos promoter but not the ANF or MLC-2 promoters.…”

“…Expression of dominant-negative ERK1 or the ERK phosphatase CL100, prevents phenylephrine-induced expression from the ANF or MLC-2 promoters ( Figure 3; Thorburn et al, 1994a). Dominant-negative ERK1 also prevents Raf-l-induced expression (Thorburn et al, 1994b), suggesting that the Raf-1 -> MKK --ERK pathway must be active for expression to occur.…”

“…These data indicate that not all phenotypes associated with active Raf-1 are necessarily also associated with active MKK. (Thorburn et al, 1994a Figure 2 indicate that the ANF and MLC-2 promoters respond differently from the Fos promoter and APl-driven genes to expression of the ERK activator MKK. We therefore wished to use a different approach to confirm the requirement for activity of the ERKs for expression from the cardiac-specific promoters.…”

“…Previously, we found that introduction of active Ras or active Raf-1 were each necessary and sufficient for expression from the c-Fos, ANF, and MLC-2 promoters in phenylephrine-induced hypertrophy (Thorburn et al, 1993;Thorburn et al, 1994b). Active MAP kinase is also required for induction by phenylephrine (Thorburn et al, 1994a) or Raf-1 (Thorburn et al, 1994b). Phenylephrine and other hypertrophic agonists cause biochemical activation of Ras , Raf-1 (Thorburn and Thorburn, unpublished data), MKK (Bogoyevitch et al, 1993;Lazou et al, 1994), and ERKs (Bogoyevitch et al, 1993Yamazaki et al, 1993;Thorburn et al, 1994a).…”

“…Active MAP kinase is also required for induction by phenylephrine (Thorburn et al, 1994a) or Raf-1 (Thorburn et al, 1994b). Phenylephrine and other hypertrophic agonists cause biochemical activation of Ras , Raf-1 (Thorburn and Thorburn, unpublished data), MKK (Bogoyevitch et al, 1993;Lazou et al, 1994), and ERKs (Bogoyevitch et al, 1993Yamazaki et al, 1993;Thorburn et al, 1994a). In addition, dominantnegative Ras or Raf mutants prevent phenylephrineinduced ERK activation (Thorburn, 1994).…”