Expression of SIK1 and SIK3 in postnatal rat retina

Abstract

Salt-inducible kinases (SIKs) have been shown to be expressed in various cells and organs including adrenocortical tissue, adipocytes and liver. They were implicated in adipocyte differentiation, steroidogenic gene expression and pathogenesis of diabetes. SIK2 was observed in all cell layers and in post-natal developmental stages of rat retina. Two FGF signaling pathway related proteins GAB1 and A-Raf were found to be phosphorylated in vitro by SIK2. This suggested that SIKs may have a role in FGF signal transduction pathway in retina which is important for differentiation and survival of Müller glial cells. In this study, physical interaction of SIK2 with its candidate substrate, GAB1 was investigated using a proteinprotein interaction approach. We found no evidence of interaction. This may be due to several reasons including insufficiency of the fragments used, transient nature of enzyme-substrate interaction and/or difference in conformation of recombinant protein fragments. Also, the expression of SIK family members, SIK1 and SIK3, in the context of rat retina and MIO-M1 cells were analyzed by RT-PCR. We showed the presence of SIK1 and SIK3 transcripts in retina for the first time. A comparative developmental analysis of SIK1 and SIK3 levels using postnatal (P0 to P30) rat retina RNA samples revealed that while steady state levels of SIK1 transcripts did not change significantly, that of SIK3 increased 1.5-fold at P14 relative to P10 and peaked at P30. In MIO-M1 cells expression of SIK3 but not SIK1 was detected. Upon FGF2 treatment a significant decrease of SIK3 levels was observed in MIO-M1 cells by 2 and 5 hours of treatment while high levels of glucose did not have a significant effect. These results extend the findings in our laboratory regarding the presence of SIK family members in retinal context and provide a framework for future studies directed to understand SIK function in retina.