CPI-17 was up- and down-regulated in response to SM differentiation and de-differentiation. In contrast, the expression of MYPT1 was constant at any locus and stage, unlike SM-type MLCK, whose promoter is controlled via a myocardin signal (Yin et al. 2006
). The fluctuation of CPI-17 expression causes variation in the ratio of CPI-17 expression to MLCP at each locus. (DELETED) A lack of the expression or the phosphorylation of CPI-17 via Ca2+
-dependent PKC results in the slower force development of SM tissues (Kitazawa et al. 2004
; Dimopoulos et al. 2007
). Thus, the ratio of CPI-17 to MLCP determines the rapidity and magnitude of the agonist-induced SM contraction. It is known that heart rate and blood flow of embryo significantly increase from E10 to E14 (Keller et al. 1996
). Therefore, the histological data suggest that the embryonic arterial SM cells increase the contractility through the expression of CPI-17 in response to a gain of cardiac function and an increase in blood flow. In contrast, CPI-17 is down-regulated in the proliferative phenotype of SM cells, such as cells at neointima and cultured SM cells. It is possible that mechanical stresses and/or agonist signals alter the signal into MLCP regulation upon the phenotype transition, and this will be a subject for further study.
Immunohistochemistry detected significant levels of CPI-17 expression in the embryonic heart, although CPI-17 is negligible in adult rat heart (Woodsome et al. 2001
). The relative expression of CPI-17 was decreased from E10 to E17, suggesting that CPI-17 gene becomes suppressed after the maturation of cardiac muscle. It is consistent with the other SM marker proteins whose expression is turned off in adult heart (Ruzicka and Schwartz. 1988
). MYPT1 was also detected in embryonic heart (), suggesting that CPI-17 may be involved in the regulation of MLCP activity in embryonic cardiac muscle, although the roles of myosin phosphorylation and MLCP in embryonic heart are still unknown. In contrast, embryonic skeletal muscle at E10 does not express CPI-17. Wu et al reported that MYPT1 gene undergoes down-regulation upon the differentiation of myoblasts in parallel to the up-regulation of MYPT2, a close member in the MYPT1 family (Wu et al. 2003
). Indeed, MYPT1 is not detected in adult skeletal muscle (Okubo et al. 1994
). The phosphorylation of myosin II is required for the myotube formation of myoblasts (Wu et al. 2003
). Consistent with this, the expression of MYPT1 was evident in the embryonic skeletal muscle at E10 and E17. CPI-17 seems to be unnecessary for the regulation of MLCP in myotube formation.
As seen in adult animals, CPI-17 is expressed in embryonic neuronal cells. Furthermore, this is the first report that a significant amount of CPI-17 is detected in embryonic epithelial cells. The epithelial expression of CPI-17 in embryonic lung disagrees with the results from the adult tissue (Woodsome et al. 2001
). One possibility is that the condensed embryonic lung emphasizes the CPI-17 staining at epithelium, compared with inflated adult lung. Indeed, a trace amount of CPI-17 was detected in lung epithelium from adult rats (Woodsome et al. 2001
). The role of CPI-17 in epithelial cells is unknown. Recently, a group of tumor cells were reported to express a high amount of CPI-17 that causes hyper-phosphorylation of merlin/NF2 oncogene product and transformation of the cells (Jin et al. 2006
). These tumor cells are derived from epithelial cells. It is possible that CPI-17 functions in the regulation of epithelial cell proliferation. Our findings will provide new avenues for research studying the role of CPI-17 in the regulation of MLCP at non-SM cells.
The most important finding of the present study is that the expression of CPI-17 is reversibly regulated in arterial SM cells. It is similar to other SM marker proteins, such as SM myosin heavy chain, that is regulated upon the phenotype transition of SM via myocardin and MRTFs (Aikawa et al. 1993
; Wang and Olsen 2004
). However, it is unclear whether CPI-17 expression is controlled via myocardin signal. There are significant differences in the expression profiles of CPI-17 and SM α–actin. Dysregulation of CPI-17 expression can cause disorders in the regulation of SM contraction, so that the molecular basis of the gene regulation seems to deserve further investigation.