Cold-stimulated increase in a regulatory subunit of cAMP-dependent protein kinase in human hepatoblastoma cells.

Although cold-stress responses in bacteria and plants have been well studied and hypothermic conditions are used in clinical treatments, there has been little investigation of cold-stress responses in human cells, and there has been no report on the involvement of signal transduction modulators in the cold-stress response in human cells. We therefore investigated alterations in the expression of genes involved in the signal transduction system and the mechanisms of cold-stimulated increases in the expression of genes in human hepatoblastoma (HepG2) cells. Using a cDNA expression array method, we found that a transcript encoding a regulatory subunit Ibeta (RIbeta) of cyclic AMP-dependent protein kinase (PKA) was increased in cold-stressed cells. Western blot analysis revealed that the amount of PKA RIbeta protein was increased by cold treatment, while that of a PKA catalytic subunit (C) was unchanged. The protein level of PKA RIbeta was increased in cells treated with low concentrations of actinomycin D, whereas that of PKA C was not, implying that the increase was caused by the suppression of transcription at low temperatures. In addition, degradation of the PKA RIbeta protein was not stimulated by cold treatment, unlike that of the PKA C protein. The results suggest that signal transduction through PKA also participates in cold-stress responses in human cells and that multiple mechanisms are involved in the increase in the level of the PKA RIbeta protein.