Activation of PPARalpha is followed by its down regulation in murine atrial cell line HL-5

Besides the established role of PPARalpha in modulating lipid metabolism, a growing body of evidence suggests the involvement of this receptor in a wide variety of pathological processes including the failure of contraction resulting from myocardial hypertrophy or ischemia. The present study was aimed at characterizing the pattern of response to PPARalpha stimulation in an experimental model suitable for both genetic manipulation and pharmacological screening. To this aim, we utilized HLS a sub clone originated from HLl, a murine atrial cell line. These cardiomyocytes display spontaneous contraction along with several phenotypic characteristics of adult cardiomyocytes. Transcript characterization by RT-PCR in the HLS subclone showed high levels of mRNA for sarcomeric proteins, such as alpha cardiac actin, TnC, Tnl, TnT and adult isoform of myosin heavy chain (MHCalpha), while the embryonic isoform (MHCbeta) was practically undetectable. Together with metabolic effectors like muscle CPT1, CPT2, GLUT4, UCP2, UCP3 and adaptive factors (ANF, TNFAlpha, HlF alpha and beta), the HLS cell line constitutively expresses to various extents mRNA coding for PPARs (alpha, beta and gamma) and for the obligatory heterodimers RXRs. Biochemical analysis confirmed the translation of the messenger coding for the nuclear receptors (isoforms alpha and gamma). Treating HLS cells with PPARalpha agonists, such as oleate or WYI4643, induced a significant increase in long-chain acyl-CoA dehydrogenase and UCP2 mRNA levels. Similar results have been recently reported for neonatal cardiomyocytes. Interestingly, the incubation of HLS with oleate resulted in a gradual decrease of PPARalpha messenger which was reduced to 25% of untreated cells after 48 hours. In conclusion, the present results suggest the occurrence of a feed-back control capable of down-regulating the receptor following its activation. Finally, HL-S cardiomyocytes appear a suitable model to elucidate the role of PPARalpha in pathophysiological conditions and characterize its pharmacological modulation.