**5. Regulation of the expression of hCAT-1**

Regarding the gene organization of CAT transporters, it is known that the *SLC7* family is phylogenetically composed of two subfamilies formed by cationic amino acid transporters (CATs) and glycoprotein-associated amino acid transporters (HATs). The cationic amino acid transporter family is encoded by the *SLC7A* (1–4) genes and corresponds to proteins with 14 transmembrane domains [44]. Specifically, the gene that encodes the hCAT-1 protein corresponds to *SLC7A1* whose open reading frame is formed by 11 exons and 10 introns. The gene is located on chromosome 13q12-13q14 [45].

Among the genes encoding CAT-1 in rat, mouse and human have common characteristics: the promoter region lacks TATA box, and they have multiple binding sites for the transcription factor specific protein 1 (Sp1) and they have an extensive 3′ non-translatable region (3′UTR) that could perform functions in the regulation of mRNA stability or in translation [46–49]. In rats, stress by amino acids deprivation induces an increase in the rCAT-1 mRNA expression by a mechanism related to increased mRNA stability [46]. This increased mRNA stability would be related to the presence of a regulatory region within the 3′UTR sequence of the gene [47]. Subsequent experiments have shown that the effect of amino acids deprivation on rCAT-1 expression would depend on both transcriptional [48] and posttranscriptional mechanisms [50].

In humans, it is known that insulin increases leg blood flow in healthy subjects via stimulation of endothelial NO synthase (eNOS) [51]. Insulin also increases the synthesis and release of NO and release in primary cultures of HUVEC [38, 52]. Biological effects of insulin involve activation of several transcription factors, including Sp1 in several cell types [53, 54]. Insulin increases Sp1 nuclear protein abundance and its binding to a proximal region (−177 and −105 bp from ATG) of the *SLC7A1* promoter containing four consensus sequences for Sp1 [55]. Interestingly, in patients with essential hypertension, a reduction of *SLC7A1* transcriptional activity due to reduced Sp1 activity in the promoter region has been reported [12]. So, the transcriptional regulation of *SLC7A1* is relevant for cardiovascular physiology, and the reduction of the promoter activity of this gene could be associated with cardiovascular disease (CVD).

On the other hand, the first intron of *SLC7A1* may play a bifunctional role in regulating the *SLC7A1* transcriptional activity by the binding of the purine-rich element binding protein A (Pur alpha) in physiological conditions and by activating the transcription factor 4 (ATF4) in endoplasmic reticulum stress or by decreasing the *SLC7A1* transcriptional activity by the C/EBP homologous protein 10 (CHOP) binding in C6 rat glioma cells [56].

For the physiological regulation of hCAT-1 activity, both transcriptional regulation of *SLC7A1* and/or posttranscriptional regulation of *SLC7A1* transcript are relevant for the protein expression and L-arginine transport [55]. Insulin increases the expression of *SLC7A1* gene due to an increased transcriptional activity, most likely due to higher Sp1 activity. So, hCAT-1 expression and activity are regulated by insulin in endothelium, suggesting that in insulin resistance there is a reduction of L-arginine transport and NO synthesis that contributes to endothelial dysfunction and cardiovascular diseases.
