Small heat shock proteins HSP27 and HSP20 have been implicated in regulation of contraction and relaxation in smooth muscle. Activation of PKCα promotes contraction by phosphorylation of HSP27 while activation of PKA promotes relaxation by phosphorylation of HSP20 in colonic smooth muscle cells (CSMC). We propose that the balance between the phosphorylation states of HSP27 and HSP20 represents a molecular signaling switch for contraction and relaxation. These molecular signaling switch acts downstream on a molecular mechanical switch (TM) regulating thin filament dynamics. We have examined the role of phosphorylation state(s) of HSP20 on HSP27-mediated thin filament regulation in CSMC. CSMC were transfected with different HSP20 phosphomutants. These transfections had no effect on the integrity of actin cytoskeleton. Cells transfected with 16D-HSP20 (phosphomimic) exhibited inhibition of acetylcholine (Ach)-induced contraction while cell transfected with 16A-HSP20 (non-phosphorylatable) had no effect on Ach-induced contraction. CSMC transfected with 16D-HSP20 cDNA showed significant decreases in (i) phosphorylation of HSP27 (ser78); (ii) phosphorylation of PKCα (ser657); (iii) phosphorylation of TM and CaD (ser789); (iv) Ach-induced phosphorylation of myosin light chain (MLC20); (v) Ach-induced association of tropomyosin (TM) with HSP27 and (vi) Ach-induced dissociation of TM from caldesmon (CaD). We thus propose the crucial physiological relevance of molecular signaling switch (phosphorylation state of HSP27 and HSP20) which dictates (i) the phosphorylation states of TM and CaD, and (ii) their dissociations from each other.
- Copyright © 2009, American Journal of Physiology- Gastrointestinal and Liver Physiology