A mechanistic role for the intermediate conductance, calcium -activated potassium channel, FIK, in fibroblast proliferation and myogenic differentiation
Potassium channels play a variety of important physiological roles including controlling of electrical excitability, secretion, cell size and growth. The activity of an intermediate conductance, calcium-activated potassium channel class (IK) has been strongly correlated with control of proliferation in non-excitable cells, including lymphocytes, muscle satellite cells, and a variety of fibroblast cell lines. Recent cloning data suggest that the IK channel described in these different cell types is widely expressed in mitogenically active tissues. Further, there is now evidence that supports a functional link between IK channel expression and physiological control of mitogenesis. However, the mechanisms linking IK channel activity to transcriptional regulation of growth control genes have yet to be determined. ^ To understand the role of IK channels as both causal and effector agents in cell growth and differentiation, the focus of my research has been to define the growth control activity of a fibroblast IK channel, FIK, as it affects mitogenesis and differentiation in the 10T1/2-MRF4 myogenic model system. The 10T1/2-MRF4 multipotent fibroblast cell line ectopically overexpresses the muscle specific, basic helix-loop-helix myogenic transcription factor, MRF4, which is activated upon growth factor (bFGF or TGF-β) withdrawal to drive myogenic differentiation. Specifically, in bFGF or TGF-β stimulated cells, MRF4 activity is negatively regulated and muscle differentiation suppressed. Upon growth factor withdrawal, negative regulation of MRF4 is removed, and MRF4-dependent myogenesis is induced as measured by the expression of acetylcholine receptor (AChR) channels and myosin heavy chain, hallmarks of skeletal muscle differentiation. I have demonstrated that bFGF and TGF-β suppression of MRF4 induced myogenesis is dependent on Ras/MEK/ERK upregulation of the FIK channel, and that this suppressive action of the channel is a corollary to its positive mitogenic function. These findings suggest a potentially more complex role for FIK not only as a regulator of mitogenesis, but also as a target of myogenic signaling. Thus, I have established a link between FIK channel activity, proliferation, and the well-defined transcriptional control scenario of MRF4dependent myogenic gene expression. This system can now be exploited to understand the mechanisms linking activity of the IK channel class to transcriptional regulation of cell growth. ^
Major Professor: Stanley G. Rane, Purdue University.
Biology, Neuroscience|Biology, Cell