The human testis-enriched HSPA2 interacts with HIF-1α in epidermal keratinocytes, yet HIF-1α stability and HIF-1-dependent gene expression rely on the HSPA (HSP70) activity.

The Hypoxia-Inducible Factor 1 (HIF-1) is essential for cellular adaptation to reduced oxygen levels. It also facilitates the maintenance and re-establishment of skin homeostasis. Among others, it is involved in regulating keratinocyte differentiation. The stability of the oxygen-liable HIF-1α subunit is regulated by various non-canonical oxygen-independent mechanisms, which among others involve Heat Shock Proteins of the A family (HSPA/HSP70). This group of highly homologous chaperones and proteostasis-controlling factors includes HSPA2, a unique member crucial for spermatogenesis and implicated in the regulation of keratinocyte differentiation. HIF-1 can control the HSPA2 gene expression. In this study, we revealed that HIF-1α is the first confirmed client of HSPA2 in human somatic cells. It colocalises and interacts directly with HSPA2 in the epidermis in situ and immortalised keratinocytes in vitro. Using an in vitro model based on HSPA2-overexpressing and HSPA2-deficient variants of immortalised keratinocytes we showed that changes in HSPA2 levels do not affect the levels and intracellular localisation of HIF-1α or influence the ability of HIF-1 to modulate target gene expression. However, HIF-1α stability in keratinocytes appears critically reliant on HSPAs as a group of functionally overlapping chaperones. In addition to HSPA2, HIF-1α colocalises and forms complexes with HSPA8 and HSPA1, representing housekeeping and stress-inducible HSPA family paralogs, respectively. Chemical inhibition of HSPA activity, but not paralog-specific knockdown of HSPA8 or HSPA1 expression reduced HIF-1α levels and HIF-1-dependent gene expression. These observations suggest that pharmacological targeting of HSPAs could prevent excessive HIF-1 signalling in pathological skin conditions.

Drought stress and re-watering affect the abundance of TIP aquaporin transcripts in barley.

Tonoplast Intrinsic Proteins (TIP) are plant aquaporins that are primarily localized in the tonoplast and play a role in the bidirectional flux of water and other substrates across a membrane. In barley, eleven members of the HvTIP gene subfamily have been identified. Here, we describe the transcription profile of the HvTIP genes in the leaves of barley seedlings being grown under optimal moisture conditions, drought stress and a re-watering phase. The applied drought stress caused a 55% decrease in the relative water content (RWC) in seedlings, while re-watering increased the RWC to 90% of the control. Our analysis showed that all HvTIP genes, except HvTIP3;2, HvTIP4;3 and HvTIP5.1, were expressed in leaves of ten-day-old barley seedlings under optimal water conditions with the transcripts of HvTIP2;3, HvTIP1;2 and HvTIP1;1 being the most abundant. We showed, for the first time in barley, a significant variation in the transcriptional activity between the analysed genes under drought stress. After drought treatment, five HvTIP genes, which are engaged in water transport, were down-regulated to varying degrees, while two, HvTIP3;1 and HvTIP4;1, were up-regulated. The HvTIP3;1 isoform, which is postulated as transporting hydrogen peroxide, expressed the highest increase of activity (ca. 5000x) under drought stress, thus indicating its importance in the response to this stress. Re-hydration caused the return of the expression of many genes to the level that was observed under optimal moisture conditions or, at least, a change in this direction Additionally, we examined the promotor regions of HvTIP and detected the presence of the cis-regulatory elements that are connected with the hormone and stress responses in all of the genes. Overall, our results suggest that 7 of 11 studied HvTIP (HvTIP1;1, HvTIP1;2, HvTIP2;1, HvTIP2;2, HvTIP2;3, HvTIP3;1, HvTIP4;1) have an important function during the adaptation of barley to drought stress conditions. We discuss the identified drought-responsive HvTIP in terms of their function in the adaptation of barley to this stress.